Ecology and Management of Brome Grass ( Bromus Rigidus Roth and Bromus Diandrus Roth) in Cropping Sustems of Southern Australia
Ecology and Management of Brome Grass (Bromus rigidus
Roth and Bromus diandrus Roth) in Cropping Systems of
Southern Australia
SAMUEL GEORGE LLOYD KLEEMANN
Bachelor of Agricultural Science (General)
A thesis by prior publications submitted to The University of Adelaide, South Australia
In the fulfilment of the degree of DOCTOR OF PHILOSOPHY
School of Agriculture, Food and Wine
Faculty of Sciences
I
TABLE OF CONTENTS ABSTRACT ...... IV DECLARATION ...... VI PUBLICATIONS ARISING FROM THIS THESIS ...... VIII ACKNOWLEDGMENTS ...... IX ACRONYMS ...... X
CHAPTER 1 REVIEW OF LITERATURE ...... 1 1. Introduction ...... 1 2. Taxonomy ...... 3 3. Ecology and Biology...... 6 3.1. Habitat, Distribution and Occurrence...... 6 3.2. Growth and Development ...... 8 3.2.1. Seed dormancy and germination ...... 8 3.2.2. Establishment...... 11 3.2.3. Growth ...... 12 3.3. Reproduction ...... 13 3.3.1. Flowering ...... 13 3.3.2. Seed production ...... 14 3.4. Population Dynamics ...... 16 4. Detrimental Importance ...... 18 4.1. Competition ...... 18 4.2. Contamination ...... 19 5. Management ...... 19 6. Summary and Knowledge Gaps...... 22 7. Outline of the Thesis ...... 23 8. References ...... 23
II
CHAPTER 2 Differences in the distribution and seed dormancy of populations of Bromus rigidus and Bromus diandrus in southern Australia: adaptations to habitat and implications for management ...... 34
CHAPTER 3 Seed dormancy and seedling emergence in ripgut brome (Bromus diandrus Roth) populations in southern Australia ...... 43
CHAPTER 4 Population ecology and management of rigid brome (Bromus rigidus) in Australian cropping systems ...... 53
CHAPTER 5 Applications of metribuzin for the control of rigid brome (Bromus rigidus) in no-till barley crops of southern Australia ...... 61
CHAPTER 6 The role of imidazolinone herbicides for the control of Bromus rigidus (rigid brome) in wheat in southern Australia ...... 67
CHAPTER 7 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS...... 73
III
ABSTRACT
Brome grass species Bromus rigidus (rigid brome) and B. diandrus (great brome) are winter annual grasses that have proliferated in recent years to become serious weeds of crops and
pastures in southern Australia. Until recently there had been few studies on the population
ecology of B. rigidus and B. diandrus and the research that had been done tended to focus on
populations that had naturalised in Western Australia. Increased knowledge of the behaviour
of B. rigidus and B. diandrus under current farming systems in southern Australia and the
impact of management strategies on population ecology and seedbank dynamics would
facilitate development of more effective weed control programs.
A field survey was undertaken in 2003 on the Yorke and Eyre Peninsulas of South Australia
to determine relative distribution of these two brome grass species. Bromus rigidus was found
more frequently and at higher densities in South Australian crops than B. diandrus. Field
populations of both spp. were shown to possess much longer seed dormancy than what had
been previously reported in Australian literature. Germination of dormant seeds of B. rigidus
and B. diandrus was overcome with the addition of gibberellic acid (0.001 M GA3) rather
than upon removal of the husk (i.e. lemma and palea) protecting the seed; indicating that
dormancy is most likely under hormonal control within the embryo. Dormant populations of
B. diandrus from cropping fields were highly responsive to cold stratification (i.e. chilling), a process which has been shown to increase GA synthesis within the seed. Populations of B. diandrus from cropping fields also showed much longer seed dormancy than those collected from adjacent fence-lines. The large differences in germination pattern between these cropping and adjacent fence-line populations provide some evidence to suggest that management practices being used by growers in crop production are selecting for increased dormancy in B. diandrus. In cropping fields, there could be considerable adaptive value of dormancy mechanisms (i.e. cold stratification requirement, light inhibition) that delay
IV germination and seedling emergence until after pre-sowing weed control tactics have been used. Dormant populations of both B. rigidus and B. diandrus also showed strong inhibition
of seed germination when exposed to light. This is the first Australian study to report the
inhibitory effect of light on seed germination of Bromus spp. and provides a possible
explanation for their increasing prevalence in southern Australia since the adoption of no-till
farming.
Selection of greater dormancy in Bromus spp. is likely to contribute to the development of a
more persistent seedbank. A 3-year field experiment undertaken at Lock on the Eyre
Peninsula of South Australia from 2003 to 2005 showed that about 20% of B. rigidus seedbank can persist from one season to the next. In this field study, management strategies that combined effective herbicides (ACCase-inhibitors and imidazolinone Clearfield technology) and crop competition over consecutive years provided effective control of B. rigidus population and depleted its seedbank to low levels (from 1748 to <5 seeds m-2) within
3 years.
In field studies, metribuzin, and metribuzin plus pendimethalin incorporated by sowing
provided safe and effective control of B. rigidus (>75%) in barley. Post-emergent applications of imidazolinone herbicides, imazapyr, imazapyr plus imazapic and imazapyr plus imazamox, to imidazolinone-tolerant wheat (Clearfield™) also provided consistent and high levels of B. rigidus control (≥87%). Effective management of Bromus spp. will require a major change in cropping systems used by growers in southern Australia and will involve combining effective herbicide technologies (i.e. Clearfield) with more competitive crops and more diverse rotations, where possible.
V
DECLARATION
This work contains no material that has been accepted for the award of any other degree or diploma in any university or other institution to Samuel George Lloyd Kleemann and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text.
I give consent to this copy of my thesis when deposited in the University Library, being made available for loan and photocopying, subject to the provision of Copyright Act 1968.
The author acknowledges that copyright of the following published works contained within the thesis resides with the copyright holder(s) of those works.
1. S.G.L. Kleemann and G.S. Gill (2006). Differences in the distribution and seed
germination behaviour of populations of Bromus rigidus and Bromus diandrus in South
Australia: adaptations to habitat and implications for weed management. Australian
Journal of Agricultural Research 57, 213-219.
2. Samuel G.L. Kleemann and Gurjeet S. Gill (2013). Seed dormancy and seedling
emergence in ripgut brome (Bromus diandrus Roth) populations in southern Australia.
Weed Science 61, 222-229.
3. Samuel G.L. Kleemann and Gurjeet S. Gill (2009). Population ecology and management
of rigid brome (Bromus rigidus) in Australian cropping systems. Weed Science 57, 202-
207.
4. Samuel G.L. Kleemann and Gurjeet S. Gill (2008). Applications of metribuzin for the
control of rigid brome (Bromus rigidus) in no-till barley crops of southern Australia.
Weed Technology 22, 34-37.
VI
PUBLICATIONS ARISING FROM THIS THESIS
• S.G.L. Kleemann and G.S. Gill (2006). Differences in the distribution and seed
germination behaviour of populations of Bromus rigidus and Bromus diandrus in
South Australia: adaptations to habitat and implications for weed management.
Australian Journal of Agricultural Research 57, 213-219.
• Samuel G.L. Kleemann and Gurjeet S. Gill (2013). Seed dormancy and seedling
emergence in ripgut brome (Bromus diandrus Roth) populations in southern Australia.
Weed Science 61, 222-229.
• Samuel G.L. Kleemann and Gurjeet S. Gill (2009). Population ecology and
management of rigid brome (Bromus rigidus) in Australian cropping systems. Weed
Science 57, 202-207.
• Samuel G.L. Kleemann and Gurjeet S. Gill (2008). Applications of metribuzin for the
control of rigid brome (Bromus rigidus) in no-till barley crops of southern Australia.
Weed Technology 22, 34-37.
• Samuel G.L. Kleemann and Gurjeet S. Gill (2009). The role of imidazolinone
herbicides for the control of Bromus rigidus (rigid brome) in wheat in southern
Australia. Crop Protection 28, 913-916.
VIII
ACKNOWLEDGMENTS
I would like to thank my supervisors Associate Professor Gurjeet Gill and Dr. Art Diggle for
their advice and guidance throughout the course of this project. I am indebted to my principal
supervisor Gurjeet for his constant support, insightful direction, encouragement and
friendship.
In particular, I would like to thank Andrew and Jenny Polkinghorne, and Allen and Mark
Edwards for their hospitality, friendship and for providing experimental sites. I am especially
grateful to Daniel Radulovic, Michael Burdett and Malinee Thongmee for field and technical
assistance. I would also like to thank staff from the University’s School of Agriculture, Food and Wine and from the Minnipa Agricultural Centre for their assistance during the project.
I would like to offer a special note of appreciation to Professor David Coventry for his
encouragement, friendship and kind reminders to stay the course and complete.
I am obliged to the Grains Research and Development Corporation for funding the research.
Finally and most sincerely I wish to deeply thank my family, for their constant patience, support, encouragement and understanding throughout. Dad, thanks for everything, you’re
the greatest. To my beautiful wife Sarah and son Doshie I can’t thank you guys enough for
your continual love and support, you’re my inspiration.
IX
ACRONYMS
ANOVA Analysis of variance
AUS Australia
C Celsius
CLF Clearfield
CS Cropping systems d Days
DAP Diammonium phosphate
EP Eyre Peninsula
GA Gibberellic acid
GSR Growing season rainfall h Hours
IBS Incorporated by sowing kPa Kilopascals
LSD Least significant difference
M Molar mo Months
POST Post-emergence
SA South Australia
SEM Standard error of mean
VIC Victoria
WAP Weeks after planting wks Weeks
YP Yorke Peninsula
X
CHAPTER 1
REVIEW OF LITERATURE
1. Introduction
Brome grass, the common name given to several species of grasses belonging to the genus
Bromus can be found throughout the temperate world, with origins in the Mediterranean and
European region (Kon and Blacklow 1995). Bromus species that have naturalised in Australia
include B. rigidus, B. diandrus, B. rubens, B. madritensis, B. tectorum and B. sterilis (Kon
and Blacklow 1995). However, B. rigidus and B. diandrus with the accepted common names
of rigid brome and ripgut brome (Williams 1986) are noted in Australian agriculture as
serious weeds of cereal crops and pastures. Both species are of similar vegetative appearance
and are often found together (Gill and Carstairs 1988). Consequently, there has been some
confusion in Australia concerning the identification and taxonomy of B. rigidus and B.
diandrus (Williams 1986; Gill and Carstairs 1988; Kon and Blacklow 1988). However, it is
now widely accepted that both species are distributed across regions of southern Australia
characterised by winter rainfall and hot, dry summers (Kon and Blacklow 1988).
In recent years B. rigidus and B. diandrus have proliferated as weeds in cereal crops of
southern Australia, because of an increased frequency of cropping, early seeding made
possible by the adoption of conservation tillage and lack of effective herbicides for selective
control in wheat (Gill et al. 1987). Gill et al. (1987) showed that B. diandrus was an
extremely aggressive weed in wheat (Triticum aestivum L.), significantly reducing crop
growth and yield. Studies (Gill and Blacklow 1984) showed that the reduction in the growth
of wheat due to competition with B. diandrus was not a result of competition for light but the
ability of this weed to compete aggressively for nitrogen and phosphorus during the early life
of the crop. Although the competitive effect of B. rigidus has not been published it is likely to
1 be similar to B. diandrus (Kon and Blacklow 1995). Bromus rigidus and B. diandrus are also regarded as contaminants of grain (Mock and Amor 1982; Michael et al. 2010) and hosts for cereal diseases (Kon and Blacklow 1995). In pastures, the seeds of B. rigidus and B. diandrus are responsible for reducing animal productivity with seeds penetrating soft tissue, and causing injury to livestock (Kon and Blacklow 1988).
The annual lifecycle of B. rigidus and B. diandrus is typical of many annual grasses across southern Australia, with most germination occurring over the winter months, rapid growth during spring and seed production complete by early summer (Kon and Blacklow 1995).
Seeds persist and remain viable over the dry hot months of summer (Cheam 1988), with seeds of B. diandrus usually germinable when conditions become favourable at the break of the season (Gill and Blacklow 1985). However, some seeds remain ungerminated and dormant during the wetter months of winter contributing to a persistent seedbank (Gill and
Blacklow 1985). Previous research has shown that the germination behaviour of B. rigidus and B. diandrus is quite dissimilar, with seed of B. rigidus showing greater dormancy and later germination (Gill and Blacklow 1985; Kon and Blacklow 1988). Gill and Carstairs
(1988) suggested that seed dormancy of B. rigidus could result in the development of a persistent seedbank, which would make it a more troublesome species to control. In contrast,
Cheam (1986) suggested that the seedbank of B. diandrus is essentially transient in nature, with small carry-over of seeds to successive years. Furthermore, rapid seedling establishment shown by B. diandrus at the break of the season enables more effective control of the species by a single, timely cultivation or spray application.
Considering the competitiveness of B. rigidus and B. diandrus in cereal crops of southern
Australia and the increasing adoption of minimum tillage by growers in the region (D’Emden and Llewellyn 2004), these weed species pose a serious threat to productivity and
2
sustainability of current farming systems. Although research previously undertaken has
considerably improved our understanding of the ecology and management of brome grass
(Gill and Blacklow 1988, Kon and Blacklow 1988), much of the research and literature
tended to focus on populations from Western Australia. Furthermore, significant knowledge
gaps still remain concerning seed biology (dormancy and germination) and seedbank
dynamics of B. rigidus and B. diandrus. At present there is limited information in Australian literature on seedbank decline, population build-up rates and persistence of the seedbank of
Bromus spp. Better understanding of the population dynamics and ecology of both B. rigidus
and B. diandrus is likely to be critical for the development of effective management programs
for these troublesome weeds.
2. Taxonomy
In the seedling and vegetative stages of growth, B. rigidus and B. diandrus are
morphologically very similar in appearance (Smith 1980). Even in the later stages of
development the species have often been misidentified (Williams 1986; Gill and Carstairs
1988; Kon and Blacklow 1988). This has resulted in considerable confusion regarding the
taxonomy of B. rigidus and B. diandrus in southern Australia to the extent that most
taxonomists and weed researchers between 1966 and 1986 believed that B. rigidus did not
exist in Australia (Kon and Blacklow 1995). According to Kon and Blacklow (1995) this was
disproved upon Williams’ (1986) revisitation of the Bromus spp., identifying the presence of
B. rigidus in southern Australia.
3
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 1.1 Mature plants of (a) B. diandrus, (b) B. rigidus, short-awned biotype and
(c) B. rigidus, long-awned biotype. (Reproduced from Kon and Blacklow 1995)
Bromus rigidus has been differentiated from B. diandrus on the basis of panicle and seed characteristics and chromosome number (Table 1.1) (Gill and Carstairs 1988, Kon and
Blacklow 1988). Kon and Blacklow (1988) showed that B. rigidus differed from B. diandrus in having more compact and erect panicles with shorter spikelet branches (Figure 1.1). They were also able to differentiate B. rigidus into a long and short-awned biotype. Furthermore,
Gill and Carstairs (1988) showed that there were marked differences in the shape of the callus-scar on the caryopses (seeds) of the two species, with B. rigidus having a more acute or elliptical scar, whereas with B. diandrus the scar was circular or rounded in shape (Figure
1.2). Somatic chromosome counts in cells of root meristem, although tedious, is also an effective means of differentiating between B. rigidus (2n=6×=42) a hexaploid and B. diandrus (2n=8×=56) an octaploid (Gill and Carstairs 1988, Kon and Blacklow 1988).
4
Table 1.1 Discrimination between Western Australian populations of B. rigidus and
B. diandrus (Kon and Blacklow 1988)
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Recent molecular and phylogenetic analysis of populations of B. rigidus and B. diandrus collected from France, have reaffirmed how genetically similar the species are, suggesting they share a common maternal genome. At this stage however, the exact paternal background for these species remains unclear (Fortune et al. 2008).
5
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 1.2 Abscission scars and lemma calluses of (a) B. diandrus, (b) B. rigidus, short-awned biotype and (c) B. rigidus, long-awned biotype. (i) ventral, (ii) dorsal, (iii) lateral views. Symbols: lemma, l; palea, pa; rachilla, ra; abscission scar, as; lemma callus, lc. Measurement bars are 1mm long. (Reproduced from Kon and Blacklow 1995)
3. Ecology and Biology
3.1. Habitat, Distribution and Occurrence
In southern Australia B. rigidus and B. diandrus have been collected from a diverse range of habitats including croplands, pastures, wastelands, roadsides, hill tops, coastal sand dunes, national parks and reserves (Kon and Blacklow 1995). Kon and Blacklow (1995) reported that B. rigidus and B. diandrus are distributed between latitudes 23° and 44°S, in areas that receive greater than 250 mm of mean annual rainfall, a growing season period of at least four months and a mean July temperature of at least 15°C. Bromus diandrus can be found across southern Australia from far eastern Queensland to south-west Western Australia, while the distribution of B. rigidus is more limited (Figure 1.3). Using herbarium records, Kon and
Blacklow (1995) isolated B. rigidus to regions in New South Wales, Victoria, South Australia and Western Australia.
6
A NOTE: This figure/table/image has been removed to comply with copyright regulations. It is included in the print copy of the thesis held by the University of Adelaide Library.
Figure 1.3 Estimated distribution of B. rigidus and B. diandrus based on herbarium specimens between 1875 and 1988 and local information. Earliest records of B. rigidus
(R) and of B. diandrus (D). Symbols are B. rigidus (black); B. diandrus (dots).
Isophytochrone (solid lines), 4 or 5 months growing season; isotherm (broken line),
15°C mean July temperature. (Reproduced from Kon and Blacklow 1995)
Bromus diandrus tolerates a wide range of climates and grows on acidic and alkaline sandy
or loamy soils, particularly soils plentiful in nitrogen and phosphorus. In contrast, B. rigidus appears to show greater preference for sandy calcareous soil types, typically found along coastal fringes (Kon and Blacklow 1995).
Weed composition is known to change in response to agricultural practices and environmental factors (Webster and Coble 1997). Since the 1980s, the grain-belt of southern
Australia has seen widespread adoption of reduced tillage practices and an increase in cropping intensity at the expense of grazed pastures (D’Emden et al. 2008). Such major changes in the management of agricultural land would be expected to change the weed flora.
A survey of weed composition in Western Australia in 1997 and repeated in 2008, reported a
7 noticeable decrease in the incidence of B. diandrus (-20%), and this was consistent with farmers’ perceptions (Borger et al. 2012). Environmental factors (i.e. drier climate) and improved management were given as a possible explanation for the reduction in B. diandrus
(Borger et al. 2012). However, in stark contrast to these findings, many farmers across southeastern Australia perceive Bromus spp. to be increasing in prevalence under current farming practices.
The present status of B. rigidus and B. diandrus under current farming practices in southern
Australia is unclear, and requires attention.
3.2. Growth and Development
3.2.1. Seed dormancy and germination
Seeds of B. diandrus show innate dormancy following seed shed, but with after-ripening in the field become readily germinable upon opening rains in autumn, when field conditions are favourable for germination (Gill and Blacklow 1985, Cheam 1986, Harradine 1986, Gill and
Carstairs 1988, Kon and Blacklow 1988). However, Cheam (1986) reported differences in the rate of loss of dormancy in B. diandrus that had naturalised in Western Australia, with accessions from the northern wheat belt losing dormancy more rapidly than those from the south. Cheam (1986) concluded that higher seed dormancy in the southern accessions is an adaptive feature that ensures minimal wastage in a region where un-seasonal rainfall events are more common. Gill and Blacklow (1985) showed that the duration of seed dormancy varied among different seed accessions of B. diandrus from Western Australia, even when produced at a common field site at Perth, suggesting that variation was genetically controlled.
Furthermore, they found that the environmental conditions (heat units, rainfall, light, etc.) under which the seeds were stored did not affect the rate of breakdown of seed dormancy or the time required for seeds to become germinable. Gill and Blacklow (1985) concluded that
8 populations of B. diandrus that had naturalised in Western Australia had adapted genetically to the climate of southern Australia.
Seeds of B. diandrus also exhibit enforced dormancy when exposed to high temperatures and low soil moisture (Gill and Blacklow 1985, Cheam 1986, Harradine 1986). Gill and Blacklow
(1985) showed that 20°C is the optimum temperature for germination of B. diandrus, with germination inhibited at high temperatures (30°C). Furthermore, Cheam (1986) showed that as populations of B. diandrus were coming out of innate dormancy, germination was higher in darkness than in light, irrespective of test temperature. Cheam (1986) suggested that temperatures of 20/10°C in complete darkness were best for giving more rapid and complete germination of B. diandrus. Recent studies from Spain (Del Monte and Dorado 2011) have also shown that B. diandrus appears to be photosensitive, with seeds showing some dormancy under light conditions and requiring burial to initiate germination. In contrast,
Harradine (1986) reported a small stimulatory effect of light on the germination of B. diandrus.
In studies overseas, the influence of light on inhibiting germination and emergence of B. sterilis and B. tectorum is well documented (Pollard 1982; Peters et al. 2000; Andersson et al.
2002). Andersson et al. (2002) concluded that light inhibition would be of ecological benefit, allowing seeds on the soil surface to remain ungerminated until sowing of the crop, preventing weed seedlings from being killed during seedbed preparation. Gill (1985) suggested that low germination of B. diandrus seeds from the soil surface was due to a lack of imbibition rather than dormancy imposed by light.
The effect of light on germination of B. rigidus is yet to be investigated; however, several studies have reported strong expression of innate dormancy in this species (Gill and Carstairs
1988; Kon and Blacklow 1988; Serrano et al. 1992). A short-awned population of B. rigidus
9
collected from East Chapman, Western Australia, showed incomplete germination with 30%
of seed remaining ungerminated and dormant at the start of the growing season in April (Kon
and Blacklow 1988). Gill and Blacklow (1985) also reported strong innate dormancy in a
population of B. rigidus collected from Geraldton, Western Australia. Further research showed that strong innate dormancy in this population of B. rigidus was alleviated upon the
removal of the palea and lemma from the caryopses and by treatment with gibberellic acid
(Gill and Carstairs 1988). Gill and Carstairs (1988) concluded that the cause of strong innate
dormancy was not hard-seededness, but inhibition in the embryo. These findings suggest that
the duration of innate dormancy is considerably different between populations of B. diandrus
and B. rigidus.
The depth at which weed seeds are buried in soil strongly influences seed viability and
longevity. Cheam (1987) reported that the most effective depth for enhancing loss of viability
of seed of B. diandrus was 1 cm. Furthermore, Cheam (1987) showed that ungerminated
seeds of B. diandrus can remain viable in the field, on the soil surface, for between 2 to 3
years, and that carry-over to successive years was directly related to low soil moisture. Poor
germination from the soil surface would be expected with seeds imbibing then drying, but not
germinating, with seed remaining viable (Cheam 1987). Similarly, Harradine (1986) found
that seed of B. diandrus remained viable for 2 years, however, only 1% of seeds buried to a
depth of 15 cm were viable after 3 months. In contrast, Burnside et al. (1996) found that B.
tectorum seed persisted for 5 to 9 years in North America, when buried 20 cm deep. Cheam
(1987) and Wicks (1997) concluded that shallow soil burial of weed seed by cultivation to
promote uniform germination was more likely to ensure rapid exhaustion of the seedbank,
preventing persistence of brome species. Addition of nitrogen in the form of KNO3 has also
been shown to promote seed germination in B. tectorum (Evans and Young 1975), and was
suggested as a means of depleting seed reserves in the soil.
10
Given the low levels of seed dormancy in B. diandrus, this species is more susceptible to pre- sowing control with either cultivation or knock-down herbicides (Gill and Blacklow 1985). In contrast, B. rigidus which shows greater innate dormancy than B. diandrus (Gill and Carstairs
1988, Kon and Blacklow 1988) is more likely to exhibit seed carry-over from one season to the next, making it a more troublesome weed to manage.
3.2.2. Establishment
In many grass weeds, several cohorts establish due to protracted germination and because emergence can be from variable depths with buried seeds. Harradine (1986) showed that B. diandrus germination was greatest from a depth of 50 mm, with 97% of total seedling emergence occurring within 1 month of burial. Hulbert (1955) showed that most seedlings of
B. tectorum emerged from the soil surface and to a depth of 2 cm. Furthermore, Thill et al.
(1979) showed that emergence of B. tectorum seedlings was greatly inhibited by increasing soil compaction. Evans and Young (1984) reported that B. tectorum establishment was greatly improved with more favourable moisture levels and temperatures from coverage with stubble residue, seed burial and soil aggregation. The establishment of B. diandrus was shown to be more rapid when seeds were covered by stubble residue or buried by shallow cultivation as compared with germination from a bare soil surface (Cheam 1986). Similarly,
Gleichsner and Appleby (1989) reported that seed of B. rigidus covered by soil germinated more rapidly, with less than 10% of the initial population ungerminated after 1 month, regardless of burial depth. Consequently, timely cultivation, which provides favourable germination conditions, may reduce seed survival and seedling density of B. rigidus and B. diandrus in the field (Cheam 1986, Gleichsner and Appleby 1989). However, staggered germination and establishment, although more limited in B. diandrus is important for the survival of the Bromus spp., particularly in areas prone to false starts.
11
3.2.3. Growth
The growth habit of the Bromus spp. is typical of most annual grasses. When nutrient supply is high and plant densities low, a succession of tillers can be produced from each plant (Kon and Blacklow 1995). During early vegetative growth, the tillers of B. diandrus tend to be prostrate until culm elongation in the spring; in contrast, the growth habit of B. rigidus is more erect (Kon and Blacklow 1995). Development in B. diandrus differs between biotypes, with flowering responsive to vernalisation and dependent on the length of the growing season
(Gill and Blacklow 1985, Kon and Blacklow 1988). Furthermore, at low plant densities B. rigidus and B. diandrus can produce more than 50 tillers per plant when moisture and nutrients are not limiting (Kon and Blacklow 1988). Uresk et al. (1979) reported that the growth rate of B. tectorum was strongly correlated to soil temperature and plant tissue nitrogen.
As with many other grasses, the root systems of the Bromus spp. are fibrous and concentrated in the upper soil profile. Hulbert (1955) found that B. tectorum had a typical depth of rooting of around 30 cm; however, in some cases it was as long as 1.5 m. The depth to which B. tectorum roots elongated was suggested to be highly dependent on the soil type, as well as the prevailing climate (Hulbert 1955). Gill (1985) showed that B. diandrus was more effective at developing a stronger rooting system in comparison to wheat, partitioning more growth beneath the ground. In Western Australia, the proliferation of B. diandrus on deep sands was attributed to the species’ capacity to extract nitrogen from these deficient soils (Gladstone and Loneragan 1975) and was likely related to its efficient rooting system. Although the root systems of B. diandrus and B. rigidus are assumed to be similar, there is currently no information available on the rooting architecture of B. rigidus.
12
3.3. Reproduction
3.3.1. Flowering
B. rigidus and B. diandrus have a requirement of vernalisation to initiate flowering and the
reproductive phase, however, B. rigidus generally flowers earlier than B. diandrus (Gill and
Blacklow 1985, Kon and Blacklow 1988, Gleichsner and Appleby 1996). Flowering in B.
rigidus and B. diandrus usually occurs between September and November in southern
Australia (Williams 1986). Requirements of vernalisation for flowering have been reported in
other Bromus spp., most notably B. tectorum (Finnerty and Klingman 1962). However,
Hulbert (1955) reported that B. rubens was atypical of other annual bromes requiring little
vernalisation to initiate flowering. The vernalisation requirement of B. diandrus was
suggested by Gill and Blacklow (1985) to be an adaptation suited to the cold climate of
Europe from which it originates. Furthermore, Gill and Blacklow (1985) observed
differences in the timing of flowering between populations of B. diandrus and concluded that
this variability was strongly correlated to the length of growing season to which the
population was adapted. For instance, populations sourced from drier environments were
found to flower earlier to avoid harsh conditions in the later part of the growing season.
Comparatively, populations adapted to longer and higher rainfall growing seasons showed
delayed maturity to benefit from the prevailing conditions (Gill and Blacklow 1985). Kon and
Blacklow (1988) reported that variation in the time of panicle emergence between seasons for
a population of B. rigidus was related to temperature, and that the heat-sum from establishment to panicle emergence was 1450°C days. Furthermore, Kon and Blacklow
(1988) found that enough heritable variation exists amongst populations of both B. rigidus and B. diandrus in timing of panicle emergence, to enable adaptation to new or changing environments.
13
Generally, B. diandrus requires long photoperiods to flower, and is considered to be a long-
day plant (Ashby and Hellmers 1959). Kon and Blacklow (1995) suggested that the flowering
requirements of B. rigidus and B. diandrus, that is cold temperatures and/or short photoperiods followed by long photoperiods are likely the main reasons these species are localised to southern Australia.
Although the species are generally considered self-compatible with florets usually cleistogamous, occasionally the anthers are exerted from chasmogamous florets to enable outcrossing (Kon and Blacklow, 1990). However, studies undertaken by Kon and Blacklow
(1990) showed that outcrossing in B. diandrus was generally <1%, and considering the similar floral biology of B. rigidus the breeding system would be much the same as B. diandrus. Wu and Jain (1978), similarly described B. rubens as self-compatible with an outcrossing rate of between 0-1% and that 73-88% of natural variation was non-genetic. Gill and Blacklow (1988) suggested that the ability of B. diandrus to naturalise across Western
Australia was not only a result of genetic adaptation but also due to the phenotypic plasticity of the species.
3.3.2. Seed production
Seed production in B. rigidus and B. diandrus is highly variable and is dependent on the phenotypic plasticity of tillers per plant (Kon and Blacklow 1988). Consequently, Kon and
Blacklow (1988) showed that the number of seeds per plant of both species was highly variable between and within populations, with seed production ranging from 1160 to 2910 seeds per plant for B. rigidus and 660 to 3380 seeds for B. diandrus. Kon and Blacklow
(1988) also showed that the duration of seed maturation for three populations of B. rigidus and B. diandrus was 26 days from anthesis to seed shedding. Mitch (1999) described B. tectorum seed production as highly variable and dependent on the plants ability to tiller under
14
adequate soil moisture and sunlight, with production ranging from 25 to 5000 seeds per plant.
Richardson et al. (1989) reported that seed production in B. tectorum was strongly affected
by different moisture levels in the soil, with seed production decreasing in a curvilinear
fashion from higher to lower moisture contents, with the number of seeds per panicle
significantly reduced with moisture stress.
Seed mass was shown to differ between the species, with B. rigidus (13-15 mg/seed)
producing on average heavier seeds than B. diandrus (10–13 mg/seed) (Kon and Blacklow
1988). Similarly, Burghardt and Froud-Williams (1997) showed that different populations of
B. sterilis, B. diandrus, B. hordeaceus and B. commutatus grown at a single location in the
United Kingdom, had considerably different 1000-seed weights (2.57-15.16 g). Furthermore,
Burghardt and Froud-Williams (1997) concluded that populations collected from southern
UK allocated more resources to reproductive weight than northern accessions. Several studies
(Steadman et al. 2004; Peters 1982; Sexsmith 1969) in other annual grasses, notably, annual ryegrass (Lolium rigidum) and wild oats (Avena fatua), have shown that environmental conditions (moisture and temperature) during the reproductive and seed filling stages can have a significant impact on seed weight. Furthermore, Steadman et al. (2004) showed that milder and wetter conditions during seed development of annual ryegrass, produced seeds with deeper and longer dormancy. To date, there are no Australian studies on the impact of environmental constraints (moisture and temperature) on seed production and dormancy status for either B. rigidus or B. diandrus, and is an area requiring attention.
Seeds of Bromus spp. are easily dispersed by animals, clothes and machines (Kon and
Blacklow 1995), with the lemma callus at the tip of the seed enabling attachment, and transportation over long distances. The seeds of the Bromus spp. can also be difficult to clean from crop samples, aiding transportation as seed impurities (Mock 1987). Seeds of Bromus
15
spp. were found to be one of the most common weed seed contaminants of grain in Western
Australia and seeding contaminated crop seed was suggested to be enhancing weed dispersal
(Michael et al. 2010).
3.4. Population Dynamics
In southern Australia, B. rigidus and B. diandrus follow a typical cycle for annual grasses,
with most germination occurring over the winter months, rapid growth during spring and seed
production complete by early summer (Kon and Blacklow 1995). Seeds persist and remain
viable over the dry hot months of summer (Cheam 1988), with seeds of B. diandrus usually germinable when conditions become favourable at the break of the season (Gill and Blacklow
1988). However, some seeds remain ungerminated and dormant during the wetter months of winter contributing to a persistent seedbank (Gill and Blacklow 1985). Cheam (1986) suggested that germination of non-dormant seeds over the summer months is minimised because of high temperatures and low soil moisture content.
Several studies from Western Australia have shown however, that the germination behaviour of B. rigidus and B. diandrus are quite dissimilar, with seed of B. rigidus showing greater dormancy and reluctance to germinate (Gill and Carstairs 1988; Kon and Blacklow 1988).
Gill and Carstairs (1988) suggested that seed dormancy of B. rigidus was likely to contribute
to a persistent seedbank and make it a more troublesome species to control. In contrast,
Cheam (1986) suggested that the seedbank of B. diandrus is essentially transient in nature, with small carry-over of seeds to successive years (<1%). Furthermore it was suggested that low seed dormancy and rapid germination of B. diandrus are features likely to make this species more susceptible to pre-sowing control with either cultivation or knockdown herbicides (Cheam 1987; Gill and Carstairs 1988). Cheam (1988) showed that with effective crop rotations and herbicide management to prevent fresh seed input, the seedbank of B.
16
diandrus could be depleted within two years. In non-cropping situations more than two consecutive years of seed-shed prevention were required for seedbank depletion (Cheam
1987).
Crop rotations of either lupins (Lupinus angustifolius L.) or pasture with wheat, where selective in-crop herbicides could be applied, were shown to be effective in reducing brome
grass populations in Western Australia (Cheam et al. 1992). Modelling also showed that
seedbanks of brome grass could be significantly lowered following lupins, if effective grass
selective herbicides were used (Zaicou et al. 1993). However, despite the use of effective
crop rotations and other cultural practices, infestations of Bromus spp. in the cereal phase are
quite common (Cheam and Lee 1991). Cheam (1988) showed that the persistence of B.
diandrus as a weed of cropping systems was related more to survivors shedding seeds than to
the persistence of seed in the soil. Given the fecundity of B. rigidus and B. diandrus (Kon and
Blacklow 1988), it is likely that only a small number of plants surviving control could
replenish the seedbank.
To date, limited information in Australia exists on the seedbank persistence and behaviour of
B. rigidus under various environmental and management regimes. In order to develop
effective long-term management strategies for Bromus spp. more information on the
seedbank dynamics under current farming systems in southern Australia is required.
Consequently, seedbank dynamics is an important research area requiring immediate
attention to ascertain the period of control needed, as long-term management of the species
ultimately depends upon the exhaustion of seedbank in the soil.
17
4. Detrimental Importance
4.1. Competition
The Bromus spp. compete strongly for moisture, light and nutrients with winter crops and
pastures. Poole and Gill (1987) showed that B. diandrus at densities of 100 plants m-2 could result in yield losses as high as 30% in wheat. Overseas studies have also reported similar levels of yield loss (25-35%) in wheat and barley with moderate densities of B. diandrus
(Dastgheib et al. 2003). Gill et al. (1987) showed that regardless of soil type, rainfall and
other growing conditions the relationship between B. diandrus and reduction in wheat yield
was remarkably consistent across several seasons and experimental sites. Studies conducted
by Gill and Blacklow (1984) on competition for nitrogen and phosphorus between B.
diandrus and wheat, showed that brome was able to compete strongly for these elements
during the early life of the crop. Therefore, suggesting that any reduction in the growth of
wheat due to competition with brome was not likely a result of competition for light (Gill and
Blacklow 1984). Furthermore, there was evidence from this experiment for competition for
water during the grain-filling phase of wheat, which was reflected in a significant reduction
in mean grain weight of wheat (Gill and Blacklow 1984). Studies in Western Australia also
showed that B. diandrus competes strongly with lupins, particularly when soil nutrient levels
are high (Suspasilapa et al. 1992).
To date the competitive nature of B. rigidus has not been researched in Australia; however,
overseas studies showed that the species could reduce durum wheat (Triticum turgidum)
yields by as much as 57 to 81% (Hamal et al. 2001). According to Hamal et al. (2000) B.
rigidus competitiveness could be due to its aggressive root system. A similar summation for
the competitiveness of B. diandrus was given by Gill (1985).
18
4.2. Contamination
Not only are the Bromus spp. detrimental to crop productivity through competition, the seeds
of the species are often found as contaminants in grain samples, resulting in dockage upon
delivery at grain handling facilities. Mock and Amor (1982), reported that between 1977/78
and 1985/86 dockage of grain deliveries due to contamination of seeds of B. diandrus had
increased in Victoria from 6% to 26%, respectively. Contaminated samples of barley in some
cases had more than 30 seeds of B. diandrus per 100 g of grain sample. Of the 11 different
weed spp. identified as contaminants of the grain of field crops in Western Australia, Bromus
spp. ranked third behind annual ryegrass and wild radish (Michael et al. 2010). Michael et al.
(2010) reported contamination in as many as 29% of grain samples with an average of 9 brome seeds 10 kg-1 grain.
5. Management
Considering the detrimental impact of the Bromus spp. on crop yields and grain
contamination, serious efforts to control these species has been thwarted by the limited
number of useful management techniques. Prior to crop sowing, either cultivation or non-
residual knockdown herbicides including the non-translocating bipyridiles (paraquat and
diquat) and translocating glyphosate provide seedling control of brome (Kon and Blacklow
1995). Cheam (1988), showed that early germinating B. diandrus that had escaped from a
paraquat and diquat treatment prior to sowing wheat, had the potential to strongly compete
with the crop and produce large amounts of seed (5000 seeds m-2).
Prior to the availability of acetolactate synthase inhibiting herbicides (group B), growers had to use trifluralin alone or in combination with other herbicides but these were relatively ineffective in controlling Bromus spp. Herbicide options currently available for post-
emergent control of brome in wheat include sulfonlyurea herbicides, sulfosulfuron (Monza) 19
and mesosulfuron-methyl (Atlantis), sulfonamide herbicide pyroxsulam (Crusader®) and imidazolinones, imazapic plus imazapyr (Midas) and imazamox plus imazapyr (Intervix®).
The imidazolinone chemistry however, can only be used in “Clearfield” wheat and canola varieties (Geier et al. 2004). Imidazolinone tolerant wheat was released to Australian growers in 2001 (Hashem et al. 2006) and has the potential to provide effective control of Bromus spp. with good crop safety. The imidazolinone herbicides have been shown to control a range of annual grass weeds, including jointed goatgrass (Aegilops cylindrical) (Ball and Walenta
1997; Geier et al. 2004), downy brome (Bromus tectorum) (Ball and Walenta 1997; Gamroth et al. 1997; Miller and Alford 2001), wild oat (Avena fatua) (Wille et al. 1998), Italian ryegrass (Lolium multiflorum) (Brewster et al. 1997) and others (Gamroth et al. 1997).
However, issues concerning persistence of imidazolinone herbicides and rotational restrictions may limit the adoption of the Clearfield technology. Tanji (2001) showed that applications of sulfosulfuron at 27 g/ha could effectively provide 78-90% control of B. rigidus when applied to brome plants between one and four leaf growth stages. Similarly,
Blackshaw and Hamman (1998) reported effective control of B. tectorum in North American wheat crops with post-emergent applications of sulfosulfuron. At present there is limited information available on the efficacy of post-emergent herbicides, Atlantis, Midas and
Intervix® for controlling the Bromus spp. in wheat and warrants further research. Herbicide
ethyl-metribuzin, which has been available for the past two decades has been shown to
provide selective control of the Bromus spp. in both barley and tolerant wheat cultivars (Gill
and Bowran 1990; Matic and Black 1990; Zaicou and Gill 1992). This herbicide can be
particularly effective when applied IBS (Incorporated By Sowing) with either trifluralin or
pendimethalin (Gill and Bowran 1990, Zaicou and Gill 1992). Grower adoption of ethyl-
metribuzin for use in either barley or wheat has been limited however, because of issues
concerning crop selectivity. To date, limited information in Australia exists on the
20
performance and safety of ethyl-metribuzin, particularly under conservation tillage systems.
Consequently, this is an area that requires attention given ethyl-metribuzin is one of only a
few herbicides of different mode of action (group C) with activity on Bromus spp. and
provides an alternative to resistance prone group A and B herbicides.
In Western Australia, simazine (triazine) has been traditionally used to selectively control
Bromus spp. in lupins. However, development of triazine tolerant crops including canola has
enabled growers to safely apply simazine prior to and after sowing to control a broad-
spectrum of weeds. Several acetyl-coenzyme A carboxylase (ACCase) inhibiting post-
emergent herbicides are available for selective grass control in legume crops (peas, beans and
lupins) and pasture phases and include, fluazifop-butyl, haloxyfop-methyl and quizalopfop- ethyl (Wall and Phimister 1987). Furthermore, a pasture phase presents the opportunity for late use of non-selective herbicides such as paraquat, diquat and glyphosate, to prevent seed production of Bromus spp. in a process referred to as pasture topping (Cheam 1985,
Harradine 1986). The pasture phase also presents an opportunity to utilise cultural control methods of grazing, mowing or slashing to limit seed production of Bromus spp. (Kon and
Blacklow 1995). In continuous wheat rotation, combination of stubble burning and cultivation was shown to maintain great brome populations at low levels (Heenan et al.
1990). However, growers across southern Australia are reluctant to cultivate and burn stubbles due to high risk of soil erosion. Consequently, there has been rapidly increasing adoption of no-till and zero-till with stubble retention.
Considering the heavy reliance growers place on herbicides to control Bromus spp. in
southern Australia, coupled with confirmed resistance in B. rigidus and B. diandrus to group
A and B herbicides (Boutsalis and Preston 2006; Boutsalis et al. 2012; Owen et al. 2012),
greater research effort is needed to develop alternative management strategies. However,
21
development of sound integrated weed management strategies requires fundamental
understanding of the behaviour of these Bromus spp.
6. Summary and Knowledge Gaps
As highlighted in the review, B. rigidus and B. diandrus are problematic weeds across the southern Australian wheat belt. The biological attributes of both species have allowed for successful colonisation and weediness across a diverse range of habitats. Therefore, an integrated weed management programme based on a sound understanding of the ecology and seedbank dynamics of these weed species is required. To date, and as evident from the review, there have been few studies on the population ecology and management of B. rigidus and B. diandrus and the research that had been done tended to focus on populations in
Western Australia. Consequently, increased knowledge on the ecology of B. rigidus and B.
diandrus under current farming practices in southern Australia is required to facilitate development of effective management strategies for these important weed species. Therefore,
the work presented in this thesis is designed to address the following objectives:
i. To determine the distribution and frequency of B. rigidus and B. diandrus infesting
South Australian cereal crops.
ii. Assess variation in seed dormancy, mechanisms of dormancy, and seedling
emergence behaviour of populations of B. rigidus and B. diandrus from southern
Australia.
iii. Investigate seedbank persistence of B. rigidus and the influence of different
management strategies on population ecology and dynamics of the weed seedbank.
iv. Evaluate herbicide options for the selective control of B. rigidus in barley and wheat.
22
7. Outline of the Thesis
The starting point of this study was to determine the relative distribution of Bromus spp. in
South Australian cereal crops. For this purpose a field survey of Bromus spp. was undertaken on the Yorke and Eyre Peninsula of South Australia. The next chapter (Chapter 2) details the survey and methodology involved in identifying populations as either B. rigidus or B. diandrus (i.e. based on seed morphology, panicle structure and cytological discrimination) and provides some initial information on seed dormancy of weed populations. Seed dormancy and seedling emergence behaviour of different populations of B. diandrus is covered further in Chapter 3. Chapter 4 then presents results from a 3-year field experiment assessing impact of different strategies on population management of B. rigidus, and provides details of the changes to its seedbank under different cropping systems. The results of herbicide evaluations comparing metribuzin in barley and imidazolinone herbicides in imi- tolerant wheat (Clearfield) for selective control of B. rigidus are discussed in Chapters 5 and 6. The thesis concludes in Chapter 7 where the main findings are summarised, and the conclusions from the research and its practical implications are presented, followed by recommendations for future research.
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angustifolius L.) and great brome (Bromus diandrus Roth.): development of leaf area,
light interception and yields. Australian Journal of Experimental Agriculture 32: 71-81.
Tanji, A. (2001) Response of ripgut brome (Bromus rigidus) and foxtail brome (Bromus
rubens) to MON 37500. Weed Technology 15: 642-646.
31
Thill, D.C., Schirman, R.D., and Appleby, A.P. (1979) Influence of soil moisture,
temperature, and compaction on the germination of downy brome (Bromus tectorum).
Weed Science 27: 625-630.
Uresk, D.W., Cline, J.F., and Richard, W.H. (1979) Growth rates of a cheat grass community
and some associated factors. Journal of Rangeland Management 32: 168-170.
Wall, H., and Phimister, J.R. (1987) Control of annual grasses in lupins. Proceedings of the
8th Australian Weeds Conference, Sydney: 221-224.
Webster, T.M., and Coble, H.D. (1997) Changes in the weed species composition of the
southern United States: 1974 to 1995. Weed Technology 11: 308-317.
Wicks, G.A. (1997) Survival of downy brome (Bromus tectorum) seed in four environments.
Weed Science 45: 225-228.
Wille, M.J., Thill, D.C., and Price, W.J. (1998) Wild oat (Avena fatua) seed production in
spring barley (Hordeum vulgare) is affected by the interaction of wild oat density and
herbicide rate. Weed Science 46: 336-343.
Williams, L.D. (1986) Bromeae. In: Flora of South Australia (ed. J.P. Jessop and H.R.
Toelken). South Australian Printing Division, Adelaide, part IV: 1878-1882.
Wu, K.K., and Jain, S.K. (1978) Genetic and plastic responses in geographic differentiation
of Bromus rubens populations. Canadian Journal of Botany 56: 873-879.
32
Zaicou, C.M., and Gill, G.S. (1992) Effect of crop seeding rates and chemical control of
brome grass on crop yield and brome grass seed output. Proceedings of the 1st
International Weed Control Congress, Melbourne, Australia: 589-591.
Zaicou, C.M., Martin, R.J., and Gill, G.S. (1993) Modelling the fate of brome grass in the
lupin/wheat rotation. Proceedings of the 10th Australian Weeds Conference/14th Asian-
Pacific Weed Science Society Conference, Brisbane, Australia: 247-250.
33
CHAPTER 2
DIFFERENCES IN THE DISTRIBUTION AND SEED DORMANCY OF
POPULATIONS OF BROMUS RIGIDUS AND BROMUS DIANDRUS IN SOUTHERN
AUSTRALIA: ADAPTATIONS TO HABITAT AND IMPLICATIONS FOR WEED
MANAGEMENT
S.G.L. Kleemann and G.S. Gill
The University of Adelaide, Roseworthy, South Australia
Australian Journal of Agricultural Research 57, 213-219.
(With permission from CSIRO PUBLISHING)
34
A Kleemann, S.G.L. & Gill, G.S. (2006) Differences in the distribution and seed germination behaviour of populations of Bromus rigidus and Bromus diandrus in South Australia: adaptations to habitat and implications for weed management. Australian Journal of Agricultural Research, v. 57(2), pp. 213-219
NOTE: This publication is included on pages 36-42 in the print copy of the thesis held in the University of Adelaide Library.
It is also available online to authorised users at:
http://dx.doi.org/10.1071/AR05200
CHAPTER 3
SEED DORMANCY AND SEEDLING EMERGENCE IN RIPGUT BROME
(BROMUS DIANDRUS ROTH) POPULATIONS IN SOUTHERN AUSTRALIA
Samuel G.L. Kleemann and Gurjeet S. Gill
The University of Adelaide, Waite Campus, South Australia
Weed Science 61, 222-229.
(With permission from the Weed Science Society of America and Allen Press Publishing
Services)
43
A Kleemann, S.G.L. & Gill, G.S. (2013) Seed dormancy and seedling emergence in Ripgut Brome (Bromus diandrus) populations in Southern Australia. Weed Science, v. 61(2), pp. 222-229
NOTE: This publication is included on pages 45-52 in the print copy of the thesis held in the University of Adelaide Library.
It is also available online to authorised users at:
http://dx.doi.org/10.1614/WS-D-12-00083.1
CHAPTER 4
POPULATION ECOLOGY AND MANAGEMENT OF RIGID BROME (BROMUS
RIGIDUS) IN AUSTRALIAN CROPPING SYSTEMS
Samuel G.L. Kleemann and Gurjeet S. Gill
The University of Adelaide, South Australia
Weed Science 57, 202-207.
(With permission from the Weed Science Society of America and Allen Press Publishing
Services)
53
A Kleemann, S.G.L. & Gill, G.S. (2009) Population ecology and management of Rigid Brome (Bromus rigidus) in Australian cropping systems. Weed Science, v. 57(2), pp. 202-207
NOTE: This publication is included on pages 55-60 in the print copy of the thesis held in the University of Adelaide Library.
It is also available online to authorised users at:
http://dx.doi.org/10.1614/WS-08-121.1
CHAPTER 5
APPLICATIONS OF METRIBUZIN FOR THE CONTROL OF RIGID BROME
(BROMUS RIGIDUS) IN NO-TILL BARLEY CROPS OF SOUTHERN AUSTRALIA
Samuel G.L. Kleemann and Gurjeet S. Gill
The University of Adelaide, South Australia
Weed Technology 22(1), 34-37.
(With permission from the Weed Science Society of America and Allen Press Publishing
Services)
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A Kleemann, S.G.L. & Gill, G.S. (2008) Applications of Metribuzin for the control of Rigid Brome (Bromus rigidus) in no-till barley crops of Southern Australia. Weed Technology, v. 22(1), pp. 34-37
NOTE: This publication is included on pages 63-66 in the print copy of the thesis held in the University of Adelaide Library.
It is also available online to authorised users at:
http://dx.doi.org/10.1614/WT-07-017.1
CHAPTER 6
THE ROLE OF IMIDAZOLINONE HERBICIDES FOR THE CONTROL OF
BROMUS RIGIDUS (RIGID BROME) IN WHEAT OF SOUTHERN AUSTRALIA
Samuel G.L. Kleemann and Gurjeet S. Gill
The University of Adelaide, South Australia
Crop Protection 28(11), 913-916.
(With permission from Elsevier)
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A Kleemann, S.G.L. & Gill, G.S. (2009) The role of imidazolinone herbicides for the control of Bromus rigidus (rigid brome) in wheat in Southern Australia. Crop Protection, v. 28(11), pp. 913-916
NOTE: This publication is included on pages 69-72 in the print copy of the thesis held in the University of Adelaide Library.
It is also available online to authorised users at:
http://dx.doi.org/10.1016/j.cropro.2009.07.005
CHAPTER 7
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
Brome grass is a winter annual weed of crops and pastures, which has naturalised across
areas of southern Australia characterised by winter rainfall and hot, dry summers. The two
most common species found are Bromus rigidus and B. diandrus, which have proliferated in
the absence of effective herbicides for weed control in cereals and with the introduction of
conservation tillage. However, the status of both spp. under current farming practices in
southern Australia was not clear, and little Australian literature was available concerning the
impact of management strategies on population ecology and seedbank dynamics.
A survey of cereal crops on the Yorke and Eyre Peninsula of South Australia found greater frequency of B. rigidus than B. diandrus, which showed preference for undisturbed fence-line margins. However, both B. rigidus and B. diandrus were found to co-habit the fence-lines. In some instances B. rigidus was found infesting crops at densities as high as 300 plants m-2,
where it would be expected to have a large impact on crop growth and yield.
As the two Bromus spp. are morphologically very similar in appearance, mature panicles of
brome grass were collected from each location surveyed, and used for species identification.
This was initially done by assessing the morphology of the callus-scar of the caryopsis as
well as the structure of the panicle. Cytological discrimination was also undertaken by
counting somatic chromosome number in the cells of root tips. There was consistent
agreement between these approaches used for species identification indicating that the
morphological features can be used with confidence to discriminate between B. rigidus and
B. diandrus in the field.
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Initial studies on seed biology of B. rigidus and B. diandrus showed that there were large
differences in seed dormancy between the species. Populations of B. diandrus collected from
fence-lines exhibited a much faster loss of dormancy than B. rigidus from neighbouring
cropped areas. This was particularly evident for populations of B. rigidus collected from the lower Yorke Peninsula, which remained almost completely dormant (<15% germination in complete darkness) until well after the start of the next growing season. In addition, populations of B. rigidus from both the Yorke and Eyre Peninsula showed strong inhibition of seed germination when exposed to light. Photosensitivity was also shown in populations of
B. diandrus collected from cropping areas of Mid North of South Australia and Victorian
Mallee. Seeds of these populations showed lower germination when exposed to light. Strong
photo-inhibition of germination of both Bromus spp. is likely to aid survival in the field by
enabling seeds to remain ungerminated on the soil surface until sowing of the crop, thus
preventing seedlings from being killed during seed-bed preparation. This feature of brome
grass ecology is of great current relevance, given the widespread adoption of no-till systems by growers in southern Australia over the past 10 years. In the no-till system, a large proportion of the weed seedbank remains unburied on the soil surface until after the sowing operation. Presence of photo-inhibition in Bromus spp. appears to be important in explaining increasing prevalence of B. rigidus and B. diandrus in southern Australia since the adoption of no-till farming.
There were large differences in rate of dormancy loss between different populations of B. rigidus. Even in the absence of light, populations of B. rigidus from the Yorke Peninsula showed a much slower loss of dormancy than populations collected from the Eyre Peninsula.
Studies on the mechanisms of seed dormancy showed that dormancy was under strong hormonal control (gibberellins) within the seed embryo. It is quite feasible that more favourable growing conditions often experienced during seed development on the Yorke
74
Peninsula may have contributed to development of greater dormancy in B. rigidus than those
from the Eyre Peninsula where growing season rainfall is much lower. Given the greater
incidence of summer and early autumn rains on the Yorke Peninsula, development of
stronger dormancy may have survival value in this environment by helping to prevent out of
season germination of B. rigidus. Furthermore, protracted germination of B. rigidus observed
in this study would enable seedling emergence to coincide with crop establishment where few
herbicide options are available for selective control in cereals.
Previous studies on B. diandrus populations found them to possess low levels of seed
dormancy, with most seeds on or near the soil surface germinating rapidly after autumn rains.
Such attributes were considered to make B. diandrus susceptible to pre-sowing control with either cultivation or knockdown herbicides. Previous research also showed only a small carry-over of dormant seed from one season to the next, which would make this weed species relatively easy to manage. However, recent increase in the prevalence of B. diandrus across several districts of South Australia and Victoria has raised the possibility of a major change in the emergence behaviour of this weed species. Studies were therefore undertaken to assess variation in seed dormancy, mechanisms of seed dormancy and seedling emergence behaviour of several populations of B. diandrus collected from within cropping fields and adjacent fence-line habitats. Comparison of results from this study with previous research
(e.g. Cheam 1986; Gill and Blacklow 1985) on B. diandrus ecology clearly indicated a major change in dormancy behaviour. Bromus diandrus populations from cropping fields were found to possess much longer seed dormancy than that reported previously in literature. In addition, these populations from cropping fields showed much longer seed dormancy than those collected from adjacent non-cropped fence-lines. These non-dormant fence-line populations showed high seedling establishment (>80%) during autumn, which coincides with the planting time of winter crops in southern Australia. In contrast, populations from
75
cropping fields showed a much lower seedling establishment (3 to 17%) during autumn to early winter. Such large differences in the germination behaviour between fence-line and cropped populations provides evidence that management practices used by the growers in crop production has selected for increased dormancy in B. diandrus. Given most weed species tend to possess polymorphism for seed dormancy, it is feasible that increased selection pressure from cropping systems has selected for mechanisms that increase expression of seed dormancy.
Longer seed dormancy in B. diandrus populations from cropping fields was overcome with addition of GA3, particularly when the lemma and palea had been removed. In contrast to B. rigidus, the lemma and palea covering the caryopsis of dormant populations of B. diandrus appeared to impair the ability of GA to penetrate and reach the embryo. Furthermore, these highly dormant populations of B. diandrus from cropping fields were also responsive to cold stratification (i.e. chilling), with germination increasing with chilling exposure up to 14 days.
Requirement for short chilling periods to overcome dormancy would be expected to delay germination until late autumn to early winter, when ambient temperatures have declined to low levels. Such delays in germination and seedling establishment in populations from cropping fields is likely to enable them to evade more effective pre-seeding weed control tactics. In addition, these dormant populations of B. diandrus showed a more persistent seedbank at the end of the growing season (up to 33%), which is in stark contrast to the results of previous Australian research. Selection for greater seed dormancy that allows B. diandrus to evade pre-sowing weed control and develop more persistent seedbanks would make it a far more difficult weed to manage. Hence, the findings of this study go some way to explaining the increase in prevalence of B. diandrus in cropping systems across southern
Australia.
76
The impact of management strategies on population ecology and seedbank dynamics of
brome grass has not been extensively reported in Australia literature. Hence, a 3-year field
experiment was undertaken at Lock on the Eyre Peninsula of South Australia from 2003 to
2005 to investigate seedbank persistence and influence of different management strategies on
B. rigidus population. Changes in weed seedbank, persistence, in-crop weed infestations and seed production of B rigidus were monitored under different cropping systems. In fixed sampling rings, where fresh seed input was excluded, seeds of B. rigidus were shown to persist in the soil for up to 3 years with 18% of the seedbank persisting from one season to the next. This result of seed longevity was also supported by the relationship between pre- sowing and residual seedbanks, which was assessed in treatment plots in the autumn and spring prior to any new seed set. Consistent differences were recorded over the 3 growing seasons between the two parameters (residual Vs pre-sowing seedbank) which indicated that
22 to 27% of the seedbank of B. rigidus persisted from one season to the next. The consistency between two independent methods of assessment of persistence of B. rigidus
(sampling rings Vs treatment plots) is reassuring and has practical implications for management of the species. Given that B. rigidus can produce large quantities of seed (e.g.
1160 to 2910 seeds plant-1) these results indicate that a single year of effective control is
unlikely to deplete the weed seedbank to levels which prevent reinfestation in the following
season. Seedbank carry-over of the magnitude reported from this study (>20%) could be an
important factor in the proliferation of B. rigidus where crop rotations have provided only a
single year’s intervention (i.e. wheat-lupin rotation in WA) or under cereal monoculture where few effective control options have been available in the past. Fortunately, recent
introduction of imidazolinone-tolerant wheat (Clearfield) has widened grower options for
the management of Bromus spp. in the wheat cropping phase. Rotations that utilised a break
crop of vetch or pasture with Clearfield wheat, where a combination of effective herbicide
77
options (ACCase-inhibitors and imidazolinone) were used over consecutive years, was shown
to significantly reduce brome density and seed production in the final year of a 3-year
cropping sequence. These management strategies were able to significantly reduce the initial
seedbank of brome to manageable levels (from 1748 to <5 seeds m-2). Conversely, B. rigidus
seedbank increased 3 to 4-fold when ineffective management practices still used by many
local growers (i.e. trifluralin plus trisulfuron) were implemented. Prior to this study, growers
were under the perception that prevention of seed set in a single year was enough to maintain
Bromus spp. at manageable levels. This study however, has clearly shown that a minimum of
two consecutive years of effective management is required to deplete brome seedbank to
levels below the economic threshold.
Cropping sequences utilising effective herbicide options and more competitive crops (i.e.
barley) can play an important role in the effective integrated management of this weed
species. However, overreliance on ALS-inhibiting herbicides for the control of brome, could
potentially lead to development of widespread herbicide resistance. Metribuzin, a triazinone
herbicide (photosystem II inhibitor) that has been available for many years can be effective
for controlling Bromus spp. in barley. However, little is known about the behaviour of this soil residual herbicide under no-till cropping systems in southern Australia. Studies undertaken at Warooka and Rudall on the Yorke and Eyre Peninsula of South Australia during 2004 and 2005, clearly showed that metribuzin applied IBS (Incorporated By Sowing) at rates of 203 and 270 g a.i. ha-1 were far more effective (~80% weed control) than post-
emergence applications at controlling B. rigidus. However, high rates of metribuzin were
phytotoxic to barley, particularly on light sandy textured soils low in organic matter and clay
content. Metribuzin mixtures with pendimethalin were however, effective on B. rigidus and
safer on barley crop. Although, metribuzin provides growers with an opportunity to
78
selectively control Bromus spp. in no-till barley, high rates (>200 g a.i. ha-1) of this herbicide
on sandy textured soils can result in significant crop damage.
Until recently there were few effective herbicide options available for the control of Bromus
spp. in wheat. However, development and release of imidazolinone-tolerant wheat to
Australian growers as part of the Clearfield system, provides a new opportunity to
selectively control these troublesome weeds in wheat with good crop safety. Five field
experiments were undertaken at different locations in South Australia from 2003 to 2005 to
evaluate the efficacy of imidazolinone herbicides against B. rigidus. Imidazolinone
herbicides, imazapyr, imazapyr plus imazapic (Midas®) and imazapyr plus imazamox
(Intervix®) applied post-emergence to imi-wheat (cvs. CLF-Janz and CLF-Stiletto) at the 4- leaf stage, provided consistent and high levels of B. rigidus control (>87%). In contrast, post- emergent application of alternate herbicide mesosulfuron (Atlantis®) provided limited and
variable control of B. rigidus (11 to 67%). In addition, imidazolinone herbicides were
extremely effective in reducing the seed production of B. rigidus (<6 seeds m-2) as compared
with mesosulfuron (461 to 3983 seeds m-2) and the untreated control (2257 to 11865 seeds m-
2). The residual soil activity of these herbicides may also provide beneficial control of late
emerging cohorts, which could be particularly important given the staggered emergence of both B. rigidus and B. diandrus. Importantly, higher crop productivity was achieved where
imidazolinone herbicides were used to control B. rigidus. Recent development and release of
double gene imidazolinone-tolerant wheat cultivars with superior agronomic performance
and herbicide tolerance is expected to increase the adoption of this technology for the
management of Bromus spp. in southern Australia.
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Recommendations
Based on the findings of this thesis, the following areas for further research are recommended.
i. The influence of seasonal conditions, particularly during seed formation, on the
expression of seed dormancy in B. rigidus and B. diandrus
As growing season rainfall can be highly variable in southern Australia, further
research is required to investigate whether seasonal conditions during seed
development can modify dormancy expression in Bromus spp. In other grass spp.
milder and wetter conditions during seed development have been shown to result in
seeds with longer dormancy. Based on climatic averages, spring weather conditions
on the Yorke Peninsula are relatively milder and wetter than that of the Upper Eyre
Peninsula, and may have contributed to production of more dormant seeds. It is also
feasible that these brome populations have resulted from separate introduction events
during early settlement. Improving our understanding of the influence of seasonal
conditions on seed dormancy expression may allow us to more accurately predict the
germination behaviour of weed populations leading into the next season and the
relative ease of their management.
ii. Is inhibition of germination by light an adaptation found only in in-crop
populations of B. rigidus and B. diandrus? What is the impact of photo-inhibition
of germination on seedling recruitment and establishment under zero-tillage
systems (i.e. discs)?
Studies undertaken as part of this thesis clearly demonstrated that seed germination of
in-crop populations of both B. rigidus and B. diandrus were strongly inhibited by
80
light. This is the first Australian report of light-mediated dormancy for either Bromus
spp. Given this important finding, further investigation is required to clarify whether
this adaptation is found only in in-crop populations of B. diandrus or is it a general feature of this species. Such studies will improve our understanding of the development of such adaptations and the potential influence of current farming practices and management on the selection process. Since much of the earlier research was undertaken on Bromus spp. in the mid 1980s, there has been significant adoption of conservation tillage by growers. It is quite feasible that adoption of reduced tillage may have played an important role in selection of light-mediated dormancy in Bromus spp. Such mechanisms would allow seed to remain ungerminated on the soil surface until burial with the sowing pass, enabling it to evade pre-sowing weed control tactics and persist in this environment. However, little is known of the exact field response of both B. rigidus and B. diandrus to changes in tillage system. Furthermore, there has been growing interest and adoption of disc tillage systems (i.e. zero-till) amongst
Australian farmers. These systems create little soil surface disturbance at sowing relative to standard farmer practice using knife-point tines and press wheels. Under disc systems, more seeds would be expected to remain on the soil surface and exposed to light even after sowing, which could further delay weed germination. Such responses are likely to provide the crop with a competitive advantage. Field experiments investigating the influence of tillage systems on germination behaviour and seedling recruitment of Bromus spp. would contribute important information on changes in population ecology after the adoption of zero-till systems.
81
iii. Investigate impact of management strategies of crop production systems on
germination behaviour of B. rigidus and B. diandrus
In stark contrast to previous research on B. diandrus ecology, studies undertaken as
part of this thesis clearly indicated a major shift in seed dormancy of this species.
Populations of B. diandrus collected from cropping fields showed much stronger
dormancy as compared with seeds from adjacent fence-lines. These large differences
in germination pattern between populations from the same farm provide some
evidence that management practices being used by the growers in crop production are
selecting for increased seed dormancy in B. diandrus. However, the control tactics
involved in selection of greater seed dormancy and their use patterns required to shift
germination behaviour of B. diandrus is unclear and warrants further research.
Furthermore it is unclear whether strong seed dormancy in B. rigidus is an inherent
characteristic of the species, or a consequence of selection pressures shaped by
grower management. A larger number of paired cropping and adjacent fence-line
populations of brome is required to help identify potential associations between
management and seed dormancy. Such studies will further improve our understanding
of the influence of grower management on seed biology of Bromus spp.
iv. Investigate persistence of seedbanks of highly dormant cropping populations of
B. diandrus
As previously reported in Australian literature, low levels of seed dormancy and rapid
emergence was considered typical behaviour of B. diandrus. These inherent
characteristics made it susceptible to pre-sowing weed control (i.e. cultivation or
knockdown herbicides), resulting in very small carry-over of dormant seeds to the
82
next year (<10%) (Cheam 1987). Such rapid exhaustion of the seedbank made it
relatively easy to achieve long-term control of this weed species. However, selection
for increased seed dormancy and delayed seedling establishment in the field could
result in a more persistent seedbank. This was indeed confirmed in a pot study in the
field which showed that some cropping populations of B. diandrus had >25%
persistent seedbank. In contrast there were other populations in which seeds that
failed to germinate and recruit seedlings had decayed. B. diandrus populations with
seedbank persistence similar to that found in this pot study (25%) would have serious
practical implications for the management of this weed species. Furthermore, given
that B. diandrus can produce large quantities of seed, it is unlikely that a single year
of effective control would provide adequate population management. Therefore,
studies established to investigate changes in B. diandrus seedbank and its longevity
under field conditions and grower management would provide information required
for the development of more effective longer-term control strategies.
v. Investigate the effectiveness of late seed set preventative measures for managing
dormant populations of B. rigidus and B. diandrus
Major changes in seed dormancy of Bromus spp. which promote more staggered
germination and establishment, enabling it to thwart effective pre-sowing control
attempts, is likely to make it a far more troublesome weed to manage than previously.
Although it is difficult to prevent seed production from species that establish and
produce seed over an extended period, more emphasis on averting fresh seed inputs
into the seedbank is required. At present there is limited Australian literature detailing
the effectiveness of late seed set preventative measures on management of either B.
rigidus or B. diandrus. Spray-topping pastures with non-selective herbicides, such as
83
paraquat, diquat and glyphosate, can be an effective but highly variable management
practice for reducing seed production of Bromus spp. However, potential use and
benefits of such herbicides in early crop-topping of pulses (i.e. field peas, lupins and
lentils), cereals (wheat and barley) and hay freezing have not been fully realised and
warrant further investigation. Furthermore, some growers across southern Australia
have adopted chaff carts to capture weed seeds at harvest. Research advancements in
harvest weed seed control, has recently resulted in commercial release of the
Harrington weed seed destructor, a trailer-mounted device attached behind the grain
harvester which incorporates a high capacity cage mill to process chaff residue
sufficiently to destroy weed seeds. At present little is known of the efficiency of both
chaff carts and the Harrington seed destructor in preventing the return of brome seeds
to the field. The ability of these systems to work effectively depends entirely on the
capacity of the harvester to intercept and capture weed seeds in the first instance.
However, in the case of both B. rigidus and B. diandrus seed maturation often occurs
well before the crop, and seeds are shed before crop harvest. Large differences in the
timing of seed maturation between the crop and brome are likely to reduce the
effectiveness of seed capture at harvest against these weed species. Hence, there is an
urgent need to determine the effectiveness of techniques to prevent replenishment of
the seedbank.
vi. Implications of wide-spread adoption of imidazolinone-tolerant crops on
resistance development to ALS-inhibiting herbicides in B. rigidus and B.
diandrus
Until recently sulfonylurea herbicides (SUs) sulfosulfuron (Monza®) and
mesosulfuron (Atlantis®), and sulfonamide herbicide pyroxsulam (Crusader®) were
84
the only options available for the selective control of Bromus spp. in wheat. However,
recent development and release of imidazolinone-tolerant wheat into Australia has
provided growers an effective new option for control of Bromus spp. with good crop
safety. In addition to wheat, imidazolinone tolerance has been developed in canola,
and more recently in barley and lentils, which are available to Australian growers as
part of the Clearfield management system. Imidazolinone group of herbicides (i.e.
imazapyr, imazamox, imazapic and imazethapyr) are members of the larger group B
herbicide family which inhibit the enzyme acetolactate synthase in plants (ALS-
inhibitors). However, the ALS-inhibiting herbicides are also extremely prone to
resistance development and their continued use has lead to widespread evolution of
resistance in many weed spp. Resistance in Bromus spp. to SU herbicides but not
imidazolinone herbicides was known in Australia; however the first case of
imidazolinone resistance has now been reported. In order to prevent or slow the onset
of further resistance in Bromus spp. to imidazolinone herbicides, information on the
resistance status of Bromus populations and current use-patterns of this new
technology amongst growers is required. Such information would be beneficial in
developing cropping systems that promote herbicide rotation and integrated weed
management.
vii. The impact of herbicide residues of imidazolinone herbicides in low rainfall
environments on longer-term cropping performance and productivity
As is the case with sulfonylurea herbicides, imidazolinone herbicides can show high
levels of soil persistence, particularly in low rainfall environments where degradation
(i.e. microbial and hydrolysis) is often slow. Herbicide label recommendations
highlight this issue, suggesting that growers adhere to long plant-back restrictions for
85
susceptible crops when <250 mm rainfall is received between herbicide application
and sowing of the next year’s crop. The frequency of such low rainfall is a common
reality for many growers using these herbicides to control Bromus spp. Given the
availability of several crop types possessing tolerance to imidazolinone herbicides and
the propensity to use these herbicides, growers are at risk of crop damage from the
residues of these herbicides in the soil. At present there is limited published literature
in Australia on the impact imidazolinone herbicides in low rainfall environments on
longer-term cropping performance and productivity and is an area worthy of future
research.
viii. Identification of new sources of herbicide tolerance as alternatives to
imidazolinone herbicides for controlling Bromus spp. in wheat and barley
Although effective post-emergent herbicide options are now available to growers for
the selective control of Bromus spp. in wheat (i.e. SUs, sulfonamide and Imi-
herbicides), these herbicides are prone to rapid evolution of resistance in weeds.
Hence, there is urgent need to develop crop cultivars with tolerance to alternative
modes of action. To date research has shown that the rates of metribuzin required to
provide consistent control of Bromus spp. often result in crop phytotoxicity in wheat
and to a lesser extent in barley. If genetic tolerance to this photosystem II inhibitor
could be identified, it would provide another useful option for controlling Bromus
spp. Furthermore, metribuzin could be rotated with other herbicides of different mode
of action to delay resistance evolution to imidazolinone herbicides.
86