Bite Dimensions and Grazing Movements by Sheep and Cattle Grazing Homogeneous Perennial Ryegrass Swards A.J

Applied Animal Behaviour Science 88 (2004) 227–242

Bite dimensions and grazing movements by sheep and cattle grazing homogeneous perennial ryegrass swards A.J. Rook∗, A. Harvey, A.J. Parsons1, R.J. Orr, S.M. Rutter Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon EX20 2SB, UK Received 3 September 2003; received in revised form 5 February 2004; accepted 5 March 2004

Abstract

Pairs of ewes or heifers were allowed to graze for short periods from previously prepared ‘homogeneous’ areas of ryegrass, in order to study their bite dimensions and movement patterns and to investigate the role of different species in initiating spatial heterogeneity. Apparent bite area was calculated from number of bites and total bitten area when ewes or heifers took 10–15 bites from an 80 cm×80 cm area in a homogeneous Lolium perenne sward. Apparent bite depth was calculated from sward surface height (SSH) within and adjacent to bitten areas. Apparent bite mass was calculated from grazed stratum bulk density. Incisor arcade breadth was measured. Animals also grazed 20 m × 40 m plots for 45 min. Position and activity were recorded and distance travelled calculated. Number of bites and steps, number and duration of grazing bouts, feeding station dimensions and horizontal head sweep were recorded. Bites per bout, per feeding station and per m forward movement, bite rate, inter-bout interval, and distance travelled, speed of movement, number of steps, step length and step rate within and between bouts were calculated. Distribution of grazing bout and inter-bout durations and the animals’ foraging paths were tested for randomness. Apparent bite area for heifers was 2.2 times that for ewes and incisor arcade breadth 1.8 times that for ewes. Apparent bite depths were similar at 0.35 of SSH. SSH distribution within bitten areas was non-normal and differed between animal species. Heifers’ bite mass was 2.1 times that for ewes. Heifers had longer grazing bouts, more feeding stations per bout, moved farther per bout but had similar movement speed while grazing. Distance between feeding stations was similar between species but heifers’ feeding station residence time was twice as long. Heifers’ biting rate was lower but they took more bites per bout, per feeding station and per metre of forward movement. Mean head sweep distance was greater for heifers. Both species moved a similar distance between grazing bouts but ewes moved more slowly with more, shorter steps. For both species, grazing bout and inter-bout

∗ Corresponding author. Tel.: +44-1837-883548; fax: +44-1837-82139. E-mail address: [email protected] (A.J. Rook). 1 Present address: AgResearch, Private Bag 11008, Palmerston North, New Zealand.

0168-1591/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2004.03.006 228 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 durations were not randomly distributed. At a 1 m2 scale for both species and a 5 m2 scale for heifers, movement was non-random with a strong propensity to walk in a straight line but at a 5 m2 scale sheep movement was uniformly distributed on the circle. © 2004 Elsevier B.V. All rights reserved.

Keywords: Grazing behaviour; Cattle; Sheep; Spatial distribution

1. Introduction

Grazing animals can both create and respond to heterogeneity in vegetation (Adler et al., 2001; Farnsworth and Beecham, 1997, 1999). There has been substantial effort in studying how animals respond to heterogeneity once it is present (Edwards et al., 1996a,b, 1997; Dumont and Petit, 1998; Dumont et al., 2000, 2002), but much less on how animals might intrinsically distribute their grazing behaviours in the absence of heterogeneity, including the mechanisms by which different animal species might create and sustain different spatial patterns. Grazed swards generally exhibit a higher degree of horizontal heterogeneity than cut swards (see Edwards et al., 1996a,b) as, in contrast to a mower, grazing animals do not defoliate the entire sward to an even height at one pass due to both positive selection of desirable areas and avoidance of undesirable areas. For example, in cattle swards, heterogeneity is often enhanced because animals avoid grazing close to dung pats, with the result that sward surface height in cattle grazed swards often shows a double normal distribution (Gibb and Ridout, 1986). Sward heterogeneity is important for a number of reasons: it will affect subsequent utilisation by grazing animals which may in turn affect patterns of nutrient return and treading damage, it will affect inter-plant competition and hence potentially the species composition of the sward in the long term, and it affects the habitat niches available for invertebrate species (Marriott and Carrère, 1998; Morris, 2000). Mathematical models have been proposed to explain how fundamentally different spatial and temporal distributions of defoliation (biting) could affect the nature of spatial heterogeneity and the potential consequences of this for yield and stability (Schwinning and Parsons, 1999; and review by Parsons and Dumont, 2003 and references therein). However, these models are difficult to parameterise and validate, largely as a result of the difficulties of making detailed measurements of spatial aspects of grazing behaviour, such as bite dimensions, bite placement and animal movements, in field conditions (see review by Gordon, 1995). Previous studies have measured bite depth and area using ar- tificial hand-constructed swards (e.g. Black and Kenney, 1984; Laca et al., 1992)orby confining sheep in cages (e.g. Burlinson et al., 1991; Roguet et al., 1998). Results of such studies may not be applicable in field conditions. Others have studied feeding station behaviour in the field (e.g. Ruyle and Dwyer, 1985; Roguet and Prache, 1995). Edwards et al. (1995) measured bite dimensions in field conditions with sheep. However, we are not aware of previous studies that have integrated these aspects. In this study, the bite dimensions and movement patterns of cattle and sheep grazing a spatially homogeneous grass sward were compared to investigate the role of different species in initiating spatial heterogeneity. A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 229

2. Materials and methods

2.1. Swards

The swards used for this experiment were sown in 1982 with perennial ryegrass (Lolium perenne L. cv. Melle) and white clover (Trifolium repens L. cv. Grasslands Huia). By April 1994 the swards contained less than 5% clover. Swards were sprayed with ‘Field Marshal’ (160 g/l MCPA-MTM Agrochemicals Ltd.) on 15 April 1994 to remove the remaining clover. From April 1994 until September 1994, the swards were managed to create a high level of horizontal homogeneity. The 90 kg N ha−1 was applied in April with a further 10 applications each of 30 kg N ha−1 at 14-day intervals between May and September, giving a total application of 390 kg N ha−1. Between May and September the swards were cut at frequent intervals (mean 4.1 days) using a gang mower (Ransomes Ltd., Ipswich, UK). The mean sward surface height (SSH), measured using a sward stick (Hill Farming Research Organisation, 1986), was 6.1 cm prior to cutting, and 4.0 cm post-cutting.

2.2. Animals

Twelve Scottish Halfbred (Border Leicester × Cheviot) ewes (97 ± 3.9 kg) and 12 Holstein-Friesian heifers (312 ± 8.3 kg) were used. For 10 days prior to the start of the experiment the ewes and heifers grazed separately on areas that had received the same nitrogen fertiliser applications as the experimental area but had not been subject to regular cutting. During this period the animals were trained to become accustomed to the pres- ence of observers, using a 65 m × 29 m area which had been managed in a similar manner to the main experimental area. In total, the animals had grazed in this training area for approximately 30 min prior to the experimental observations.

2.3. Experimental design

The experiment was conducted in two phases (‘bites’ and ‘movement’). Within each phase of the experiment there were six replicate paddocks for each animal species with two different animals in each paddock (one focal measurement animal and one companion animal). The same animals were used in both phases of the experiment and the focal animals remained the same. Treatments and animals were allocated at random to paddocks. Measurements were made for both animal species on the same day, but each replicate was measured on a different day. The time of day was randomised in order to allow for diurnal effects on grazing behaviour. The swards were cut to ensure that all measurements were made within 5–8 days of the previous cut.

2.4. Sward measurements

On the day prior to commencement of ‘bite’ and ‘movement’ measurements, sward surface heights were recorded at 10 cm intervals along two 50 m transects placed across the experimental areas before ﬁnal fencing into paddocks. 230 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242

Fig. 1. Example map of bite area.

2.5. Bite dimensions phase

Paddocks for this phase of the experiment measured 10 m × 12 m. Each pair of animals was introduced to a paddock and allowed to graze. Each focal animal was then observed by the same observers until it had taken 10–15 bites from within an area of approximately 80 cm×80 cm. Two separate areas were observed for each focal animal. The number of bites taken by the focal animal was recorded. An 80 cm × 80 cm wire mesh grid with 5 cm grid intervals, raised on legs above the sward, was immediately placed over the grazed area. The bitten areas within this grid were mapped visually onto paper by two observers (the same people throughout the trial) working together (Fig. 1). The bitten areas were then estimated by planimetry. Bite area was calculated on the assumption that bites did not overlap and is thus referred to as ‘apparent’ bite area. Thirty sward surface height measurements were taken within the bitten areas identiﬁed by the observers and a further 30 immediately adjacent to the bitten area. These were used to estimate mean apparent bite depth. Herbage was cut from the grazed stratum (i.e. down to the mean bite depth) within one 15.5cm× 15.5 cm quadrat per paddock. that was placed on an ungrazed area adjacent to a bitten area. The herbage was subsequently dried at 85 ◦C for 24 h to determine dry matter concentration. These data were used to calculate grazed stratum bulk density and hence apparent bite mass. The incisor arcade breadth of the animals was measured as follows. The animal was restrained in a crush and the mouth held open with a veterinary gag. A cast of the incisors was A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 231 made onto modelling clay, held in a shallow tin, by pressing this onto the teeth. Subsequently, the impression of the teeth was traced onto clear acetate sheets and the incisor arcade breadth measured.

2.6. Grazing movement patterns phase

For the second (movement) phase of the experiment each pair of animals was allowed to graze undisturbed for approximately 45 min in 20 m×40 m paddocks. Markers were placed at 1 m intervals around the perimeter of the paddock along with pegs within the plot on a 5 m grid. Two observers positioned in an elevated hide recorded the position and activity of the mouth of the focal animal on a scaled map of the plot (Fig. 2). A digital map measurer was used on these maps to calculate distance travelled. The mean of the two recordings was used in analyses. Two video recordings were also made; one taken from the long side and one from the short side of the plot. These were subsequently used to count bites and steps and to record the times of the start and end of grazing bouts by the focal animal. The following deﬁnitions were used. A feeding station was deﬁned as the area from which an animal was able to graze without moving its forelegs (Ruyle and Dwyer, 1985). A grazing bout was considered to start when the focal animal had stopped walking, lowered its head to the sward and was about to take a bite. It was considered to end when the focal animal stopped biting the sward, lifted its head from the sward and walked at least two steps with one foreleg without grazing. If the animal stood still without grazing, this time was included in the duration of the bout or inter-bout interval and in all derived variables. These observations allowed duration of grazing bouts and inter-bout intervals to be calculated. Within grazing bouts, bites per bout, bites per feeding station, bites per m forward movement and bite rate were calculated. For inter-bout intervals, distance travelled, speed of movement, number of steps, step length and step rate were calculated. The mean sweep of the animal’s head (i.e. the grazing path width) was measured from the video recordings. Scaling from the screen image was achieved by utilising detailed measurements that had been made previously of the animal’s head (e.g. distance from eye to nostril). Head sweep measurements were made within feeding stations when it was possible to view the animal head-on.

2.7. Statistical analysis

All analyses were carried out using GENSTAT (Lawes Agricultural Trust, 2000). Differ- ences between mean values of variables for cattle and sheep were analysed using completely- randomised, one-way analysis of variance. In these analyses, between-animal variation was used as the error term. Differences in distribution of SSH were analysed using the χ2-test. SSH distributions were also analysed for departure from normality using the χ2-test. The distribution of grazing bout and inter-bout durations were checked for randomness by comparison with an exponential distribution (i.e. with the expectation given a continuous Poisson process) using the χ2-test. In order to test whether the animal’s foraging path was random, we calculated the angle of entry to each new 1 m2 cell in the paddock relative to the direction of travel as the animal entered the previously occupied cell. These angles were combined into eight bins and 232 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242

Fig. 2. Example maps of animal movement patterns. A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 233

Fig. 3. Rose diagrams of direction of movement of animals on leaving a ‘cell’ within the paddock relative to their direction of entry (entry was in the positive y-axis direction): (a) sheep at a 1 m2 scale, (b) heifers at a 1 m2 scale, (c) sheep at a 5 m2 scale, and (d) heifers at a 5 m2 scale. compared with a uniform distribution on the circle using a χ2-test. This was repeated using 5m2 cells as non-random movement at a small scale does not exclude random movement at a larger scale. However, at this scale only movements at 0◦,90◦, 180◦ and 270◦ were used in the test as there was relatively little diagonal movement between cells at this scale (see Fig. 3).

3. Results

Sward surface height across the experimental area during the experimental period was highly homogeneous, being normally distributed (χ2 = 45.39 on 39 d.f.) with a mean of 7.7 cm and standard deviation 1.54 cm. 234 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242

Table 1 Mean bite geometry and movement parameters for ewes and heifers Ewe Heifer F S.E.D.a Apparent bite area (cm2) 16.7 36.4 <0.001 2.66 Apparent bite depth (mm) 26.2 28.3 0.406 2.46 Incisor arcade breadth (mm) 40.0 71.5 <0.001 2.22 Apparent bite mass (mg DM per bite) 69.3 146.0 0.015 25.8 Grazing bout duration (s) 44 179 <0.001 27.4 Speed while grazing (m min−1) 2.9 2.3 0.298 0.61 Distance moved during grazing bout (m) 1.7 5.8 0.006 1.20 Number of feeding stations per grazing bout 7.2 17.9 0.003 2.82 Step length while grazing (m) 0.23 0.28 0.212 0.036 Feeding station residence time (s) 5.3 10.4 0.066 2.51 Biting rate within bout (min−1) 87 68 0.067 9.46 Number of bites per feeding station 7.1 9.2 0.070 1.05 Number of bites/m forward movement within grazing bouts 35.8 44.2 0.348 8.54 Number of bites per grazing bout 57 200 0.005 39.5 Head sweep (grazing path width) (cm) 25 59 <0.001 5.71 Inter-bout distance (m) 1.94 1.80 0.854 0.736 Inter-bout speed (m min−1) 14.2 31.0 0.027 6.52 Number of steps between grazing bouts 6.7 3.6 0.178 2.17 Inter-bout step length (m) 0.26 0.50 <0.001 0.043 Inter-bout duration (s) 14.5 5.5 0.024 3.41 a Standard error of differences between treatment means.

Bite geometry and movement parameters for ewes and heifers are shown in Table 1. Apparent bite area for heifers was approximately 2.2 times that for ewes but incisor arcade breadth was only 1.8 times that for ewes. Mean apparent bite depth was similar. Both species removed approximately 0.35 of SSH. However, the distribution of SSH within the bitten area (Fig. 4) was different for heifers and ewes (χ2 = 36.19 on 20 d.f., P = 0.015) and for neither species was it normally distributed (ewes χ2 = 36.2 on 20 d.f., heifers χ2 = 45.7 on 18 d.f.). Mean calculated bite mass for heifers was 2.1 times that for ewes. Heifers had significantly longer grazing bouts than did ewes but moved at a similar walking speed while grazing. Thus, heifers moved a significantly greater distance in each bout. Heifers also had more feeding stations per bout. The step length while grazing (i.e. the distance moved between feeding stations) was similar for both species but the feeding station residence time was almost twice as long for heifers as for ewes. Biting rate during grazing was lower for heifers than ewes (P = 0.067) but the longer feeding station residence time meant that heifers took approximately two more bites per feeding station than ewes and over six more bites per metre of forward movement, though neither of these differences was statistically significant. As a result of this and the greater distance moved, heifers took significantly more bites per bout. Mean grazing path width was significantly greater for heifers than for ewes. This was two times the mean width of the head (ear base to ear base) (12.4 cm) for ewes and 2.2 times for cattle (26.4 cm). Both heifers and ewes moved a similar distance between grazing bouts but ewes moved significantly more slowly than heifers and took more, but shorter, steps. A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 235

Fig. 4. Distributions of sward heights in bitten area.

The distributions of grazing bout duration and inter-bout duration were signiﬁcantly different from the exponential distribution for both cattle (χ2 = 20.96 on 8 d.f. and 1478 on 8 d.f., respectively) and sheep (χ2 = 27.05 on 8 d.f. and 2194 on 8 d.f., respectively), indicating that bout and inter-bout duration were not the result of a random Poisson process. 236 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242

The direction of entry to a cell in the paddock relative to entry to the previously occupied cell at 1 or 5 m2 scales is shown in Fig. 4.Ata1m2 scale the movement of both sheep and cattle was non-random (χ2 = 878 and 923, respectively, on 7 d.f., P<0.001) with a strong propensity for animals to continue in a straight line (0.43 and 0.47 of between cell movements for sheep and cattle, respectively). At a 5 m2 scale, the movement of cattle was also non-random (χ2 = 30 on 3 d.f., P = 0.007) but that of sheep did not differ signiﬁcantly from a uniform distribution (χ2 = 3 on 3 d.f., P = 0.296).

4. Discussion

4.1. Methodological issues

Prior to this experiment the technique for estimating apparent bite area had been evaluated using seven different observers (Harvey and Orr, 1998). Nine sample grids were set up in an area grazed by heifers as described above. Each observer estimated the bite area for each sample. The mean bite area recorded across the nine samples ranged from 377 to 755 cm2 depending on observer. Different observers in that experiment appeared to be consistently recording high or low estimates of bite area. Thus while the absolute bite area recorded in the present trial must be treated with some caution, there should not be bias between treatments, provided as in this case, the same observers were used throughout. Although the sward was managed to be as homogeneous as possible, at the scale of individual leaves it is clear that the sward height would have been heterogeneous. It thus follows that it would be expected that the variance of the mean sward surface height of bite sized patches (16.7 cm2 for sheep and 36.4 cm2 for heifers) would have been lower than the variance in sward surface height measured from the transects which are based on a sample area of 2 cm2 (the ‘window’ of the sward stick). In other words, at the functional scale that affects the animal (the bite) the sward is likely to have been even more homogeneous than the transect data suggest. Thus heterogeneity in SSH between bites is unlikely to be an explanation of bite placement and the failure to exploit all possible bites within a feeding station.

4.2. Foraging styles of sheep and cattle

As bite depth for the two species was similar, differences in amount of biomass removed at any one point during the first pass over the sward (the defoliation severity) do not appear to contribute to differences in heterogeneity which develop in response to grazing by cattle or sheep. The estimates of bite mass assume that herbage bulk density is constant throughout the grazed horizon. Thus bite mass is probably over-estimated since bulk density is higher in the lower horizons of the sward (Barthram et al., 2000). Further, the frequency distribution of sward height in the bitten areas was not normal but significantly skewed with more low heights than would be expected although this skewness was only significant for heifers (Fig. 3). A bite actually removes a saucer shaped volume (Laca et al., 1993) as confirmed in this experiment by the distribution of amount of material removed from the different horizons, which differed for cattle and sheep despite the similar mean value, reflecting A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 237 the different biting styles of the two species. This is also apparent in the smaller relative difference between species in incisor arcade breadths than between apparent bite areas, which is probably due to the use of the tongue by cattle to gather herbage into the mouth and thus increase the effective bite width. Despite their smaller body size and hence shorter step length, which appears to lead to a slower inter-bout walking speed, the overall walking speed (both within and between bouts) was greater for sheep (3.73 m min−1) than for heifers (2.47 m min−1). Rind and Phillips (1999) reported overall walking speeds for grazing dairy cows of 3.0–3.3 m min−1, while Phillips et al. (1999) found walking speeds for 130 kg steers of 1.3–2.3 m min−1 depending on the species being grazed. These studies appear to have included both within and between bout movement as defined here. Animals in this experiment were grazing in pairs and both sheep (Penning et al., 1993) and cattle (Rind and Phillips, 1999) have been reported as showing behaviour consistent with greater vigilance when grazing in small groups. The species differences may result from the smaller body size of sheep exposing them to a greater perceived predation risk than cattle, because a wider range of predators would be able to successfully attack them, and thus a greater requirement for vigilance than cattle. Because of their smaller body and mouth size, sheep are both able to feed more selectively than cattle and need to do so in order to compensate for lower rumen residence time and hence lower digestibility (Demment and Van Soest, 1985). In the present study, sheep showed shorter grazing bouts and lower numbers of bites per feeding station, than cattle. This, combined with a greater overall walking speed in sheep is consistent with an evolved behavioural pattern for sheep to move on more quickly than cattle in search of better quality material, with these patterns manifested even when they were grazing homogeneous swards (the animals had no prior knowledge of these areas though they had grazed similar swards previously). For sheep (Penning et al., 1991), but not for cattle (Laca et al., 1994), bites and chews are mutually exclusive, with sheep spending greater time masticating (9.6 s/g bite mass in sheep versus <2.0 s/g in cattle; Gibb et al., 1997; Parsons and Chapman, 1998 from data of Penning et al., 1991). Sheep thus need to pause from biting, and so terminate the grazing bout, in order to process the harvested food and appear to use this time to move on to a new area. Consistent with this, biting rate within a feeding station was similar but bites per grazing bout and duration of bouts were less for sheep than for cattle.

4.3. Spatial foraging strategies

Many previous studies have focussed on the response by animals to existing heterogeneity and for this animals are often exposed to experimentally established ‘patches’, for example of tall vegetation, generally shaped as circles or squares. These are often relatively large with a pre-set distance between them. In contrast to such patterns, spatial foraging in the present study by both animal species was characterised by grazing a ‘swath’ of defoliation, biting more or less contiguously along a relatively narrow ‘path’, and did not create large, broad and distinct patches. The duration of foraging bouts and inter-bout intervals and the distance covered between and within bouts were non-random for both sheep and heifers. This suggests that, even in these highly homogeneous swards, both species had an innate spatial method for exploring 238 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 and exploiting the vegetation. The inter-bout distances were similar enough, for sheep and cattle, to argue that both could have been following the same broad spatial strategy, but with some aspects scaled by body size. The distance travelled during grazing bouts, and the ‘width’ of the defoliated ‘swath’ were much smaller for sheep than for heifers. Hence at a single pass, sheep created a smaller scale of imprint on the vegetation, comprising smaller very elongated ‘patches’ of shorter vegetation, compared to cattle; albeit separated by similar interpatch distances. Neither ewes nor heifers grazed all the resource available to them within a feeding station. Multiplying the mean step length by the mean head sweep gives an available feeding station area of 1652 cm2 for heifers and 575 cm2 for ewes. Multiplying the apparent bite area by the number of bites per feeding station give actual areas grazed per feeding station of 335 cm2 for heifers and 119 cm2 for ewes. Thus the proportion of the available area utilised was 0.20 for heifers and 0.21 for ewes. This is a conservative estimate based on mean rather than maximum head sweep so the true proportional utilisation would be even lower. Head sweep widths for 130 kg steers of 60–80 cm were reported by Phillips et al. (1999) using a slightly different method which is comparable to the mean value of 59 cm for heifers reported here. Our animals were newly introduced to the experimental areas and may thus have had a high drive to sample the whole area, a strategy that may have evolved to deal with heterogeneous environments where there is always the possibility of greater rewards elsewhere. There may also be an element of anti-predator activity involved since, by reducing the time standing still at a single feeding station, animals would make the predator’s task more difﬁcult. It is instructive to compare some of the implications of the results for the proportion of sward defoliated in a day with values obtained in the literature by other methods. Assume, for example, that a 300 kg heifer eats approximately 7.0 kg dry matter (DM) per day (Rutter et al., 2002) and a 90 kg dry ewe eats 1.5 kg DM per day (Penning et al., 1995). We can calculate from the present data that 3.84 (heifer) or 1.09 (sheep) g DM are eaten per meter forward movement (overall including movement between bouts), and thus we can calculate that the animal needs to walk 1823 (heifer) or 1376 (sheep) m per day (within meals) to meet its needs. Calculations based on the walking speeds and grazing times of the 130 kg steers studied by Phillips et al. (1999) suggest that those animals walked 1265 m per day while grazing, while similar calculations for dairy cows grazing in small groups (based on Rind and Phillips, 1999) give a distance walked per day while grazing of 1509 m. We have calculated also that, respectively, heifers and ewes graze from an area of 970 or 261 cm2 m−1 overall forward movement and thus from 177 or 36 m2 per day. If we assume a mean herbage net growth rate of 60 kg ha−1 per day (Orr et al., 1988) then a stocking rate of 8.6 heifers or 40 ewes ha−1 is needed to achieve equilibrium between growth and consumption. The 8.6 heifers or 40 ewes would defoliate 1522 or 1440 m2 per day, i.e. 0.152 or 0.144 of the available area. Parsons and Chapman (1998) calculated from tissue turnover dynamics that about 0.025 of the standing mass should be removed daily in a sward at equilibrium. On the assumption that animals remove 0.35 of the mass at any one point (based on the 0.35 of SSH removed reported by Wade, 1991) they thus calculated that only some 7% of the area would be grazed on any 1 day. However, the ﬁgure of 0.35 is based on a uniform distribution of mass with sward height and thus over-estimates mass removed at each point and under-estimates the area grazed. In fact, results such as those of Barthram et al. (2000) suggest that in grazed swards the top 0.67 of the sward contains at most about A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 239

0.36 of the mass and under some circumstances as little as 0.007 of the mass. Thus our estimate of area grazed made from animal behavioural observations is not unreasonable, given the assumptions made.

4.4. Implications for foraging models

Information on the grazing behaviour variables studied here is of crucial importance in informing and parameterising models of grazing behaviour, herbage intake and the impact of spatial patterns of grazing on sward heterogeneity and productivity and in particular in allowing scaling-up from bite scale to field scale processes (Schwinning and Parsons, 1999; Parsons et al., 2001). In most applied situations the animal is confronted with a highly heterogeneous sward and it is not possible to determine, experimentally, the relative contribution of heterogeneity in vegetation factors (e.g. replacement rate) and the role of the animal per se in creating and sustaining that heterogeneity. The dynamic consequences of different spatial and temporal patterns of grazing are very complex. Parsons et al. (2000) proposed three fundamentally different theoretical ‘types’ of grazing process at the bite scale: sequential (type 1), random (type 2) and selective (type 3). These are concerned not only with the grazing patterns that animals use over small time scales, such as those considered here, but as much with the spatial and temporal pattern by which animals revisit bite sized patches. Taking the conceptually simplest grazing type (‘random’-type 2) first, it is clear from the present study that animals did not place bites at random at the bite scale, and at the feeding station scale there was a strong bias to move straight ahead or to turn at only a shallow angle. This was despite the experimental swards having achieved the intended homogeneity, and so the lack of cues to direct a foraging response. Bites were not only taken along a narrow ‘path’, but the distribution of bites in space and time along that path was non-random. Despite the propensity to move straight ahead, animals turned sufficiently to be faced, within the time scale (45 min) of this experiment, with areas they had only recently defoliated (see Fig. 2 for an example). The complexity of the foraging paths of individual animals and the fact that in practice there would normally be a number of animals in the same pasture each following its own path, suggests that the probability of re-encounter with patches (frequency distribution in time) could be random, even if the subsequent response of the animals to the material thus encountered was not. The proposed ‘type 3-selective’ grazing, considers the implications when animals encounter bite sized areas at random but select whether or not to bite from them. The present experiment cannot provide information on animals response to patches with distinctly different height or state, but it is clear from the literature that both sheep and cattle are capable of making selective foraging decisions. The specific contribution of this study is to iden- tify and quantify intrinsic differences between sheep and cattle in the spatial imprint they impose on the sward which could lead to the differences in heterogeneity arising under sheep or cattle grazing. Our results suggest that type 3 grazing (random encounter but selective decision) is likely to be a valid case, potentially more so for sheep, given that their foraging patterns are not significantly different from random at a 5 m2 scale and that in practice many more individual sheep than cattle would be grazing a hectare of pasture. 240 A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242

The essence of ‘type 1-sequential’ defoliation is that animals would always eat from the next most rewarding (e.g. tallest) area (sensu Ungar, 1996). This need not be contiguous with the last area grazed or even in the immediate vicinity, but whatever sequence patches were eaten from when they were first encountered, this sequence would need to be fairly strictly repeated for subsequent defoliations. The critical consequence of this process is that there is much less variance in the residual state from which patches regrow, or in defoliation intervals, which has pronounced impacts on the rate of replacement of vegetation (Parsons et al., 2000, 2001). The probability of animals being able to return to feeding stations in sequence would increase either if each station were large enough, and uniformly defoliated enough, to enable animals to more readily rank their state, or alternatively if animals could use memory to retrace the sequence. Where experiments have been conducted using pre-established distinct and large patches within a single grazed pasture, there is some evidence that animals are capable of this (see, e.g. Dumont et al., 2002). But to suggest that animals regularly use ‘type 1-sequential’ grazing would require initial spatial foraging behaviour that could lead to the initiation of such distinguishable larger scale patches. Spatial models of foraging readily show how marked heterogeneity can arise at a fine scale, from both type 2 (random re-encounter: random biting) foraging, and from type 3 (random re-encounter: selective biting) foraging strategies (Parsons et al., 2000). But the analyses stress that, for larger scale patterns to emerge (i.e. depleted areas that are wider or larger than the path grazed by a single animal as seen here) there would need to be some process to amalgamate foraging effects. Even if the animal foraged using a random walk, leaving a narrow grazed path, larger scale patterns can be shown to emerge by pure chance, as ‘patterns in randomness’. However, these would be short lived. The behaviour adopted by animals in the present study would mean that any larger scale patches that arose by chance would be larger and more distinct for cattle grazing than for sheep grazing (foraging stations were larger and bites were more contiguous for cattle). This provides greater opportunity for cattle to subsequently reinforce any large scale pattern by deliberately moving to and placing bites in a previously visited feeding station after a finite period of regrowth. But of the three foraging definitions, we propose that type 1-sequential is the least relevant. We conclude there is sufficient evidence here of innate differences in bite dimensions and movement patterns between sheep and cattle to explain how marked differences in sward structure and heterogeneity are initiated and can be sustained. The differences are in keeping with the widely observed greater heterogeneity found in pastures grazed (notably continuously grazed) by cattle compared to those grazed by sheep (Gibb et al., 1997).

Acknowledgements

We thank R.A. Champion, C. Raine, C.L. Kelly and Maryanne White for assistance with ﬁeld work and P.D. Penning for helpful discussions. This work formed part of a commission from the UK Ministry of Agriculture, Fisheries and Food, and was also supported by a grant from the UK Biotechnology and Biological Sciences Research Council’s Joint Agricultural and Environmental Programme. Use of experimental animals at IGER is subject to approval by the Institute’s Ethical Review Process. A.J. Rook et al. / Applied Animal Behaviour Science 88 (2004) 227–242 241

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