Rotational Constraints for Pulse Crops Following the Use of Aminopyralid, Clopyralid and Picloram Herbicides

NATIONAL FACT SHEET JUNE 2019 Rotational constraints for pulse crops following the use of aminopyralid, clopyralid and picloram herbicides

Herbicides from the Group I Mode of Action are effective knockdown THERE ARE SEVERAL HERBICIDE SUB-CLASSES herbicides against many broadleaf WITHIN THE GROUP I MODE OF ACTION GROUP. weeds. However, care must be taken as several have significant soil Sub-class Examples of herbicides residual properties with implications for Arylpicolinate halauxifen (e.g. Arylex® active herbicides) rotational crops. florpyrauxifen (e.g. Ubeniq®) The potential for residual effects Benzoic acids dicamba (e.g. Kamba®) causing crop injury to following pulse/ legume crops in the crop sequence is Phenoxys 2,4-D, 2,4-DB, MCPA, MCPB particularly high with some herbicides Pyridines aminopyralid (e.g. Hotshot®, also contains fluroxypyr) from the pyridine sub class of Group I clopyralid (e.g. Lontrel®) herbicides. Their capacity to persist fluroxypyr (e.g. Starane®) and damage subsequent crops is picloram (e.g. Tordon®) complicated by how these herbicides triclopyr (e.g. Garlon®) persist in crop residues. How do Group I herbicides work within the plant? Group I herbicides mimic the naturally occurring plant hormone indole-3-acetic acid (IAA).

PHOTO: ROHAN BRILL, NSW DPI PHOTO: IAA plays a critical role in managing division, differentiation and elongation of plant cells. Changes in concentration within a cell signalsfor a growth response in roots and particularly shoots. In addition, changes in IAA concentration also plays a role in controlling seedling morphology, apical dominance, leaf senescence and many processes at a whole plant level including abscission, flowering and fruit production (Congreve & Cameron 2018). Concentration of IAA within the cell must be highly regulated by the plant. For example, when cell elongation is required, the concentration of IAA within the cell will increase. After the required elongation has occurred the plant will downregulate (switch off) IAA production, Sub-lethal picloram damage to faba beans appears as upwards cupping of leaves. causing the concentration to fall.

Level 4, 4 National Circuit, Barton ACT 2600 | PO Box 5367, Kingston ACT 2604 P grdc.com.au 1 T +61 2 6166 4500 F +61 2 6166 4599 E [email protected] PHOTO: MARK CONGREVE Group I herbicides mimic this growth regulation function of IAA, although they are not able to be downregulated by the plant, as is the case with naturally occurring IAA. Following exposure to Group I herbicides, a cascade of unregulated growth occurs within susceptible plants, leading to rapid stem and leaf distortion and eventually plant death. At sub-lethal concentrations, plant symptoms in susceptible species typically appear as cupping of the leaf margins and leaf vein thickening, especially on new growth, with downward twisting, curling and corky growth in stems. Why use pyridine herbicides in grain cropping? At labelled crop timing and application rates, pyridine herbicides have acceptable post-emergent tolerance to winter cereal crops, allowing selective, in-crop weed control of many broadleaf Retaining stubble or concentrating stubble for chaff lining may see accumulation of aminopyralid, clopyralid weed species. In addition, clopyralid is or picloram residues which may adversely affect following sensitive crops such as pulses. also able to be used post-emergent in microbial degradation (although Applications to advanced tillering crops canola without damage. plant-back periods do exist for certain may capture a high percentage of the These herbicides can be used be crops). Dry soil can result in significantly herbicide, with less herbicide reaching used for broadleaf weed control in fallow longer safe replanting times. the soil. situations, when appropriate plant-back Herbicides containing aminopyralid, periods are implemented. clopyralid or picloram are known to How pyridine herbicides Depending upon application rate, persist under certain circumstances, carry over in soils aminopyralid, clopyralid and picloram with implications for rotational crops – in Herbicide in the soil near the surface will provide some level of extended particular pulse crops, lucerne and annual is degraded by soil microbes, providing residual persistence in the soil and pasture legumes (e.g. clovers, medics). there is adequate soil moisture over the may provide control of subsequent Labelled plant-back constraints warmer months. Where environmental weed germinations. This extended typically require several months before and soil conditions do not support residual control is often a key reason degradation, herbicides containing why these pyridine herbicides may be susceptible crops can be planted. More aminopyralid, clopyralid or picloram can chosen over another Group I herbicide. recent labels include minimum rainfall persist for many months and possibly into For example, for residual control of requirements within this time period. the next cropping phase. Extensive weeds such as fleabane. Under heavy rainfall, some herbicide may be lost due to runoff or leaching plant-back directions are included on the How are pyridine deeper into the soil profile, although most up to date product labels. herbicides degraded? this should not be relied upon as a These herbicides can be mobile in The primary pathway for degradation of dissipation method. Losses from volatility the soil, especially in sandy or light soils these herbicides upon reaching the soil or photodegradation are negligible. with low organic matter. Following is via microbial activity. Little microbial When these herbicides are applied rainfall, some herbicide may move activity occurs during cold winter months. post-emergent to an established crop deeper into the soil profile. Herbicide Microbial populations near the soil (e.g. applications in winter cereals), some that has moved deeper in the soil profile surface build rapidly as temperatures of the applied herbicide will be taken may take significantly longer to dissipate start to rise in spring, provided there up by the crop and weeds and may be as there is often less microbial activity is adequate soil moisture. Without subsequently metabolised. The amount deeper in the profile. adequate soil moisture in the soil profile of herbicide intercepted by the crop Where these herbicides have moved (in particular in the top 10cm) there will be will depend upon the ground cover of deeper in the profile, the following crops little, or no, microbial degradation. the crop at the time of application. Early may establish adequately as microbial Fluroxypyr, triclopyr, dicamba and applications (i.e. just prior, or at the start, degradation has occurred in the topsoil, the phenoxys have relatively short soil of tillering) may result in most of the with herbicide symptoms only appearing persistence over the summer months, herbicide contacting the soil, as there later in the crop when roots have reached provided the soil is moist to support is likely to be minimum ground cover. herbicide present at depth.

2 grdc.com.au Soils with sub-soil constraints at depth is slowly metabolised over time. It is Further degradation of active herbicide (e.g. plough pans, substantial change possible that all herbicide within the crop in the stubble following harvest will be in soil horizon etc.) may see herbicide is not fully metabolised before the crop insignificant until the stubble decomposes accumulate above these barriers. These hays off, and metabolism ceases. The in following seasons, at which time the soils can be particularly problematic. concentration of pyridine herbicide within herbicide is released back into the soil the winter cereal at the commencement where microbes can access it. This How pyridine herbicides of senescence will be a factor of release of herbicide back into the soil carry over in crop residues application rate, crop size at application following stubble decomposition is often (stubble) (amount of herbicide capture), length of a contributing factor in damage to pulse When applied post-emergent in winter time since application, seasonal growing crops that are grown in the year following cereals, herbicide taken up by the crop conditions and growth dilution. pyridine applications in cereals.

Flaxleaf fleabane(Conyza To provide robust knockdown and metabolised before the crop hays off. bonariensis) – A case study optimise the length of residual control This results in higher levels of herbicide into spring, the application rate for present in stubble and the potential In recent years, fleabane has become fleabane control is generally at, or for more impact on sensitive rotational a widespread weed of many cropping close to, the maximum registered rate crops as the stubble decomposes. paddocks across Australia. Fleabane is for in-crop use. For clopyralid, the full Trials have shown that applications a surface germinator, well adapted to label rate registered for cereals is applied earlier (i.e. early tillering zero till farming systems. It germinates recommended, which is higher than where there is a lower percentage in large numbers in spring and autumn rates typically used in common tank of crop cover) often results in better in many regions and hence can mixes. High application rates are likely long-term control. establish in the winter crop phase. to increase the chance of rotational A preferred strategy is generally Fleabane that has not been crop issues where susceptible crops to apply earlier in the crop (i.e. early controlled in the winter crop is likely to are grown the following season. tillering) where the crop and weeds develop an extensive tap root and will Users seeking to maximise residual are small. This results in more effective be very difficult to kill with knockdown control into the spring may wish to knockdown of existing weeds and herbicides after harvest, especially to delay application until the latest better residual control, as more where the plant tops are removed registered crop growth stage, on herbicide reaches the soil surface during the harvest operation. the assumption that this will result where it is required. The herbicide Several Group I herbicides can in the longest period of residual intercepted by the crop will also have provide effective knockdown control control. However, this is often not longer to be metabolised and be of fleabane in winter cereals. Pyridines the case. Applying these herbicides subject to substantial crop growth with extended residual control (i.e. later results in more of the herbicide dilution. Residues in the stubble are herbicides containing aminopyralid, being intercepted by the crop or likely to be considerably less following clopyralid or picloram) can also provide weeds present at application, and less early application. some extended control of spring herbicide reaching the soil, where it is If a pulse crop is likely to be planted fleabane germinations, so the use of required for residual weed control. the following year then stubble should these herbicides has become very These later applications result in also be removed or incorporated soon popular in recent years (Brill et. al. 2012) more herbicide entering the cereal crop after harvest. (Welsh and Plater 2014). and less time for the herbicide to be The use of residual pyridine herbicides to achieve control of spring germinating fleabane can be Fleabane control using Tordon® 242 (1 L/ha) + metsulfuron (5 g/ha) associated with increased damage applied at two application timings in wheat. (Northern Grown Alliance 2012). to pulse crops grown the following year where conditions have not been % control v % control v Application Crop growth adequate for herbicide breakdown. Location (trial no.) untreated (in-crop untreated date stage measurement) (after harvest) Application rate, timing and TI 7/8/12 GS 30-33 100% 100% Pilliga (AM1208) length of residual control T2 25/9/12 GS 39 84% 85% Maximum application rates of T1 24/7/12 GS 31 99% 97% aminopyralid, clopyralid or picloram Bowenville (RD1205) T2 7/8/12 GS 32 91% 62% herbicides used in winter cereals have T1 15/6/12 GS 12-13 82% been established to ensure acceptable North Star (RM1217) trial abandoned due to drought crop selectivity. T2 24/7/12 GS 31 68%

grdc.com.au 3 Techniques that help to reduce In situations where aminopyralid, this risk are recommended on several clopyralid or picloram herbicides are to FURTHER RESOURCES product labels. More recent product be used in the winter cereal it is likely that labels for aminopyralid, clopyralid and a pulse crop may be grown the following season, it is recommended to apply the Brill, R.; Street, M. & Monroe, J. picloram herbicides contain directions in (2012). Management of fleabane in relation to stubble managemen, to avoid pyridine herbicide as early as practical crop rotations. (GRDC Update stubble from treated crops containing in the winter cereal crop, so as to allow paper). https://www.dpi.nsw. the maximum length of time for herbicide gov.au/__data/assets/pdf_ herbicide residues from affecting metabolism and growth dilution. In file/0007/431269/Fleabane-manage- subsequent susceptible crops. addition, remove or incorporate stubble ment-in-crop-rotations.pdf immediately after harvest, and monitor Dow AgroSciences (2014). Lontrel® For example, the Lontrel® spring/summer rainfall to ensure this is Advanced. Potential herbicide Advanced label states: carry-over issues. http://msdssearch. adequate for microbial decomposition. dow.com/PublishedLiteratureDAS/

Stubble from treated crops – Some labels suggest conducting a dh_08fc/0901b803808fcc40.pdf?- ensure that harvesters effectively field or pot bioassay, whereby seeds filepath=au/pdf spread crop straw and do not leave of the rotational crop are planted into Congreve, M. and Cameron, J. (eds) a heavy ‘header trail’ after soil suspected to contain herbicide (2019). Rotational crop constraints harvest. Burn (if legal in the area), residues. Bioassays can help determine for herbicides used in Australian the likelihood of successful emergence, farming systems. GRDC publication, bale and remove, slash or Australia. www.grdc.com.au/rota- incorporate stubble as soon as however care should be taken as tional-crop-constraints bioassays may not adequately address practical after harvest and as Congreve, M. and Cameron, J. (eds) long as possible before planting herbicide residues located deeper in (2018). Soil behaviour of pre-emer- next year to allow microbial the soil profile or may not account for gent herbicides in Australian farming systems – a national reference man- breakdown of any residues in herbicide being subsequently released from degrading stubble. ual for advisers. 2nd Edition. GRDC straw. Heavy stubble loads may Instructions for conducting a pot publication, Australia. https://grdc. carry more residues into the com.auSoilBehaviourPreEmergentH- bioassay can be found here https://www. erbicides following season. agronomo.com.au/latest-news/https/ Northern Grower Alliance (2012). wwwagronomocomau/latest-news-2 Fleabane control in wheat. (Trial Strategies that help summaries) https://www.nga.org.au/ results-and-publications/browse/79/ avoid carry-over issues MORE INFORMATION trial-summary-sheets-2012-winter to sensitive crops Storrie, A. (2019) Is it safe to Extensive plant-back information can plant that next crop? Agronomo. be found on individual product labels Mark Congreve Online. https://www.agronomo. Independent Consultants which can be accessed from the APVMA com.au/latest-news/https/ Australia Network PubCRIS website https://portal.apvma. wwwagronomocomau/latest-news-2 0427 209 234 Wells, G. (2019). Safe use of gov.au/pubcris [email protected] The GRDC publication Lontrel® Advanced in Southeast Rotational Australian crop rotations. GRDC crop constraints for herbicides used Update paper. https://grdc.com. in Australian farming systems also au/resources-and-publications/ provides detailed information on plant- grdc-update-papers/tab-content/ back constraints www.grdc.com.au/ grdc-update-papers/2019/02/ rotational-crop-constraints-for-herbicides safe-use-of-lontrel-advanced-in- southeast-australian-crop-rotations Where a winter cereal crop was treated with an aminopyralid, clopyralid Welsh, M. and Plater, C. (2014) Clopyralid for knockdown and or picloram based herbicide and stubble residual control of flaxleaf fleabane is retained and/or there has not been in broadacre cropping and adequate soil moisture over the warmer fallows. 19th Australasian Weeds months for microbial degradation, the Conference. http://caws.org.nz/old- safest approach is to grow a cereal or site/awc/2014/awc201411171.pdf canola crop the following year.

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