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Influence of sward height on diet selection by horses
Applied Animal Behaviour Science 90 (2005) 49–63 www.elsevier.com/locate/applanim
Influence of sward height on diet selection by horses A. Naujecka,*, J. Hilla,1, M.J. Gibbb a
Faculty of Applied Science and Technology, Writtle College, Lordship Road, Writtle, Chelmsford, CM1 3RR, UK b Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK Accepted 12 August 2004 Available online 25 September 2004
Abstract Foraging herbivores are often faced with spatial and temporal heterogeneity within the vegetation they have available to graze and therefore have to make decisions where and when to graze. The study reported in this paper investigated the influence of sward height on diet selection by horses grazing perennial rye-grass swards. The study comprised two experiments. In Experiment 1, perennial ryegrass paddocks were mown to four sward heights (heights: 3.5, 4.5, 7.5 and 15 cm) to create a patchy environment. Within each paddock one horse grazed for a period of 1 h during which residence time, number of bites and frequency of visits per patch were recorded. This was replicated with all seven horses used in the experiment. The same experiment was repeated in Experiment 2, but without mowing the field and allowing 1 week of re-growth for each paddock. During both experiments horses entered equally often but resided significantly longer on patches with long grass (15 cm) than on those with short grass (below 4.5 cm; P < 0.05). Grazing time and number of bites on a patch were highly correlated. The number of bites on patches with the highest sward height was greater than that on short patches (P < 0.05). Horses behaved as selective grazers, feeding mainly on grass taller than 7 cm. In Experiment 2, re-growth of the sward reduced the difference in sward height between the patches. Time spent per patch and total numbers of bites taken were less affected by sward height than in Experiment 1. It is suggested that horses behaved as energy maximisers (residing longer periods on patches and increasing number of bites taken). These data complement previous findings that bite dimension and bite mass increase with increasing sward height. When grazing, a horse rarely resided * Corresponding author. Tel.: +44 1245 424200; fax: +44 1245 420456. E-mail address: [email protected] (A. Naujeck). 1 Current address: Institute of Land and Food Resources, University of Melbourne, Victoria 3010, Australia. 0168-1591/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2004.08.001
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on a preferred patch for a long duration of time (maximum 305 s, median 79 s), but moved on after a few minutes. They sampled their environment continuously, but almost exclusively returned to long patches for feeding. # 2004 Elsevier B.V. All rights reserved. Keywords: Horses; Grazing behaviour; Diet selection; Selective grazing; Exploration; Energy maximisation
1. Introduction Diet selection is defined as an animal’s choice of food from a range of different food items (Norbury and Sanson, 1992) influenced by many factors such as nutritional demands, toxic plant compounds, forage availability, social interactions and predator risks (Krebs and Davis, 1993). Selecting the ‘right’ food is a trade-off between costs and benefits and has a short- and long-term effect on the animal’s fitness (Krebs and Davis, 1997). Foraging herbivores are often faced with spatial and temporal heterogeneity within the vegetation they have available to graze (Senft et al., 1987) and therefore have to make decisions where and when to graze. Optimal foraging models (Belovsky, 1986; Stephens and Krebs, 1986) predict that animals forage in ways that optimise the rate of food intake. Patch models like the marginal value theorem (Charnov, 1976) even suggest that ‘when the intake rate, in any patch, drops to the average rate of the habitat, the animal should move on to another patch’. If animals do behave in such a way, they must be able to gather and interpret information about their environment in order to modify their grazing behaviour to increase the rate of reward (Bailey et al., 1996). Studies on grazing behaviour by sheep, cattle and horses (Black and Kenney, 1984; Laca et al., 1992; Ungar and Ravid, 1999; Ungar et al., 2001; Illius et al., 1992; Naujeck and Hill, 2003) showed that they discern differences in grass height and that they increased bite dimensions with increasing grass height. To maximize voluntary intake, foraging models suggest that herbivores should select from tall, rather than short, grass. However the feeding site may be characterized by intermediate biomass in order to maximize intake of digestible nutrients as a result of the ‘trade-off’ between herbage quality and quantity (forage maturation hypothesis; Durant et al., 2004). Such behaviour was confirmed in studies on sheep and cattle (Bazely, 1990; Diestel et al., 1995), but little is known about diet selection by horses (Duncan, 1992; Fleurance et al., 2001; Putman et al., 1987; Gordon, 1989). The present study aimed to investigate the influence of sward height on diet selection by horses grazing perennial rye-grass swards. For this purpose, patchy paddocks were created by mowing. Behaviour such as time spent on patches, number of bites per patch and frequencies of visits per patch were recorded. 2. Materials and method The study was conducted at Writtle College (latitude 518440 N, longitude 08260 E, 32.5 m OD), UK in August 2002. It comprised two experiments. Experiment 1 investigated the grazing behaviour by horses on a heterogeneous paddock in which patches were cut to four
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different sward heights the day before measurements were made. In Experiment 2 grazing behaviour by horses was recorded on two of the same paddocks used in Experiment 1, but after one week of sward re-growth. 2.1. Experiment 1 Seven paddocks of perennial rye-grass (Lolium perenne) pasture measuring 16 m 16 m were prepared, each containing 16 squares (patches). To avoid any influence from previous horse grazing, the site chosen had never been stocked with horses. Each experimental paddock was cut with a lawn mower (ETESIA, PSE) the day before it was used for the single behaviour studies. Sixteen 4 m 4 m square patches of four different sward heights (3.5, 4.5, 7.5, 15 cm (uncut)) were prepared. Patches of the same sward height were allocated such that they were not adjacent. Each patch was marked with a fencing post and a letter. The same patch layout was used for each paddock as shown in Fig. 1. Seven horses, three males (geldings) and four mares were used in this study (horse details; Table 1). All horses were managed in one group on a perennial rye-grass pasture from April to October 2002. To minimise isolation stress for the single animals, the behaviour observations were performed with the study animal separated from, but still within sight of the rest of the group. In addition, a paddock with a companion animal was set up adjacent to the experimental paddock (Fig. 1). To familiarise the animals with the experimental paddock, two horses were moved to the companion field the day before the experiment. The next day, prior to moving one of the horses onto the experimental paddock, grass height was measured with a ruler (five measurements per patch, randomly, undisturbed height). Dry matter mass (g/m2) was determined by cutting the herbage to ground level within two randomly placed 10 cm 10 cm quadrates within each patch and by drying the cut material at 80 8C for 24 h. After herbage sampling, one of the two horses was moved to the experimental paddock. The focal horse was allowed to graze within this paddock for 1 h. Continuous recording was conducted using ‘The Observer’ mobile and software (Noldus, NL, Version 3.0). Location, bites and behaviours that were not associated with grazing such as idling, lying, alert and drinking were recorded. After 1 h, both horses were moved to a new companion paddock and the experimental paddock for the next day was prepared accordingly. The same procedure was repeated with the rest of the group. 2.2. Experiment 2 After 1 week of re-growth two of the seven paddocks used in Experiment 1 were used in Experiment 2 again. The experimental procedure and data analysis were similar to Experiment 1, but the grass was not cut before the experiment. Only six horses were used. Three horses successively grazed one paddock, each for 1 h. As three horses were successively grazing the same paddock for 1 h a change in sward conditions for the second and third horse in comparison to the first would occur. However, depletion of sward height and biomass after 1 h of grazing on a 16 m 16 m paddock was expected to be of little consequence.
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Fig. 1. Field and patch layout.
2.3. Data analysis Five different behaviour parameters were studied in the experiment. The number of visits per patch was recorded by continuous observation of the location of the focal animal. The data was analysed using chi-square with the assumption that all patches would be visited with an equal frequency. Time spent on each patch grazing was calculated from all behaviour observed during the total time an animal resided on a patch. As with number of
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Table 1 Characteristics of horses used Horse
Sex
Date of birth
Breed Sire Dam
Height (cm)
Weight (kg)
1 2 3 4 5 6 7
Female Female Male Male Male Female Female
24.02.2000 12.06.2000 16.02.2000 14.04.2000 24.04.2000 29.04.2001 02.2001
TB Cob Hann British warmblood Welsh TB Welsh TB Hann British warmblood Hann British warmblood ? TB
164 164 163 165 163 158 155
525 518 461 489 435 402 428
visits per patch, the data was analysed using chi-square with the assumption that the residence time would be equal regardless of the location within the paddock. A time-bite correlation (Spearmans rank correlation) was applied to investigate the interaction between residence time and number of bites taken during the period an animal was on the patch. Any individual animal differences for proportion of bites taken or time resided on a patch were assessed using the Kruskal–Wallis statistic with the assumption that all animals would have similar behaviour on all patches. The total number of bites during 1 h was calculated for each horse and the percentage of bites on each sward height determined. The differences in number of bites in each sward height were assessed using the Kruskal–Wallis statistic. Any differences between numbers of bites on individual sward heights were identified using Dunn’s test for multiple comparisons (Heath, 1995). All statistical procedures were conducted using SPSS Version 10.0 (SPSS Inc., 1999).
3. Results 3.1. Vegetation The data for sward height and dry matter mass are shown in Tables 2 and 3 (Experiments 1 and 2, respectively). The variability associated with sward height and dry matter yield are typical of that of perennial rye-grass swards after cutting to a series of heights (Experiment 1; Table 2). After 1 week of re-growth, there was a considerable increase in the variability of sward height and dry matter yield, reflecting an increase in heterogeneity in herbage production (Experiment 2; Table 3). Table 2 Sward height and dry matter mass Patches
Sward height (cm)
S.D.
% cv
Dry matter (g/m2)
S.D.
% cv
C, E, K, M D, F, L, N A, G, I, O B, H, J, P
3.6 5.3 8.2 15.5
0.30 0.24 0.27 1.45
8.3 4.5 3.3 9.4
72 103 178 242
12.7 15.7 30.0 37.7
17.6 15.3 16.9 15.6
S.D.: standard deviation, % cv: coefficient of the variation of the different patches (A–P) on the experimental paddock the day after cutting (N = 56 measurements per sward height).
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Table 3 Sward height and dry matter mass Patches
Sward height (cm)
S.D.
% cv
Dry matter (g/m2)
S.D.
% cv
C, E, K, M D, F, L, N A, G, I, O B, H, J, P
7.0 8.9 10.5 13.6
2.57 2.81 2.95 3.88
36.7 31.6 28.1 28.5
62 73 217 257
26.5 31.8 74.9 126.2
42.7 43.5 34.5 49.1
S.D.: standard deviation, % cv: coefficient of the variation of the different patches (A–P) in Experiment 2 (N = 56 measurements per sward height).
3.2. Number of visits per patch The number of visits per patch of each horse in Experiment 1 is presented in Fig. 2a. There were numerical differences (P > 0.05) in the number of visits per patch for each horse and each horse visited each patch equally often. In Experiment 2, a greater
Fig. 2. (a) Number of visits on any of the 16 patches in Experiment 1. Each graph shows the mean number of visits on any patch, the standard deviation and extreme values (empty dots) for each individual horse. (b) Number of visits on any of the 16 patches in Experiment 2. Each graph shows the mean number of visits on any patch, the standard deviation and extreme values (empty dots) for each individual horse.
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uniformity in the mean number of visits per patch was observed compared with Experiment 1 (for all animals the number of visits per patch ranged from 5 to 6; Fig. 2b). Four of the six horses had a similar incidence of visits per patch, however there was a significant increase in the number of visits per patch by horse 6 (visited E (16 times) and I (15 times); P < 0.01) and horse 1 (visited patch O 12 times; P < 0.05). 3.3. Time spent grazing per patch In the first experiment, considerable variation between time spent grazing on different patches (P < 0.01) was observed. Horses resided for longer periods of time on patches B, H, J and P than on other patches (Fig. 3a) and the residence time on a patch varied within a range of 1–353 s (Table 4). In Experiment 2 each horse resided for different durations on different patches (P < 0.01). The proportion of time horses grazed on patches A, B, H, I, J, O and P was often greater than on the other patches (Fig. 3b). Residence time on a patch varied within a range of 1–305 s (Table 4). 3.4. Time/bite correlation The correlation between time spent on a patch and the number of bites taken by each animal in Experiment 1 is shown in Table 5. The different sward height classes (Figs. 4a and 5a) demonstrate that when the horses visited short patches (3.6 and 5.3 cm) they only took a few bites from them. Similar results to Experiment 1 were observed in Experiment 2 (Table 5, Figs. 4b and 5b). 3.5. Individual differences in selection of swards of different height No differences between individual horses in the percentage of bites and the percentage of time spent on the four different sward heights (P > 0.05) were observed in Experiments 1 and 2. However, the comparison of Experiments 1 and 2 demonstrated that the proportion of time spent per patch, as well as the proportion of bites taken per patch was less variable in Experiment 2 than in Experiment 1 (Figs. 4a, b and 5a, b). 3.6. Number of bites from swards of different height The proportions of bites per sward height in Experiment 1 are in Fig. 6a and b. There was a significant difference in the proportion of bites on different sward heights (P < 0.01). Subsequently, multiple comparisons (Dunn’s test) demonstrated that more bites (P < 0.05) were performed on 15.5 cm than on the 3.6 and 5.3 cm patches. Further, more bites were made on 8.2 cm patches than on the 3.6 cm patches. The situation was different in Experiment 2. The proportions of bites on each sward height class are presented in Fig. 6b. A lower degree of variation between different sward height classes was observed compared to Experiment 1 (Fig. 6a). A difference in the proportion of bites on different sward heights (P < 0.01) was observed. Multiple comparison analysis (Dunn’s test) suggested that the number of bites was significantly greater (P < 0.05) only on the 13.7 cm patches compared with the 7 cm patches (Fig. 6b).
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Fig. 3. (a) Time spent grazing on each of the 16 patches (letter A–P) by each horse in Experiment 1. (b) Time spent grazing on each of the 16 patches (letter A–P) by each horse in Experiment 2.
Table 4 Mean, median and range of residence time (seconds) on a patch during a single visit of a horse for Experiments 1 and 2 Experiment 1 Grass height (cm) Mean Median S.D. Range
3.6 5 3 4.9 1–35
5.3 5 3 7.7 1–60
Experiment 2 8.2 19 6 27.7 1–159
15.5 83 70 65.0 3–353
7 19 8 24.7 1–136
8.9 27 15 30.8 1–168
10.5 37 27 34.6 1–188
13.6 64 51 60.0 3–305
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Table 5 Correlation between time spent on a patch and the number of bites Horse
Experiment 1 rs
Experiment 2 rs
1 2 3 4 5 6 7
0.937 0.907 0.892 0.865 0.729 0.958 0.947
0.986 0.985 0.996 0.970 0.991 0.976
Correlations were significant at the P < 0.01 level in Experiments 1 and 2.
4. Discussion The results of this study suggest that horses behave as selective grazers when offered a choice in grass height. Previous studies on the influence of sward height on bite dimensions of horses showed bite depth, weight, volumes and bite area were not a fixed size, but increased with sward height (Naujeck and Hill, 2003). The selection for a particular sward height in a patchy environment would therefore have an influence on bite dimensions and total intake of herbage by the horse. In the first experiment horses visited each patch with equal incidence. This behaviour was similar to those recorded by Diestel et al. (1995) in a study on diet selection by cattle that were provided with a choice of four different types of patches. Visiting each patch equally often could be explained by the animal’s motivation to explore the area. When moved to a new area, animals tend to walk along the boundary (personal observation; Fraser and Broom, 1990; Waran, 2001) for the first few minutes as to check the new surrounding. The horses in the experiments reported here investigated their surroundings but not always walk along the boundary of the paddock. Visual observations and subsequent examination of video recordings (data not presented here) showed that the horses tended to go straight to the tall sward patches, take a few bites, lift their head, move on and sample another patch. Furthermore there was no evidence that the foraging process was influenced by the position of the companion animal or the group in either experiment. The mean number of visits per patch was similar for all horses in Experiment 2 but the individual differences were less than in those noted in Experiment 1. As the differences in sward height between patches were greater in Experiment 1 than in Experiment 2 some horses may have had a greater motivation to explore the area more often than if the paddock looked more homogenous (as in Experiment 2). Differences in behaviour of each individual horse seemed to become more obvious in a more diverse environment. However it is not clear if the diversity within the pasture or site of initial foraging contribute to apparent disparity in the behavioural process of individual animals. In Experiment 1 the processes involved with foraging lead to horses residing longer on patches with long grass than on patches with short grass (Fig. 3a). The residence time spent on each patch was highly correlated with the number of bites taken. Similar behaviour has been demonstrated in sheep and cattle. For instance when sheep were provided a choice between tall (15.1–21.5 cm) and short grass patches (6.5–11.8 cm) Bazely (1990) reported
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Fig. 4. (a) Amount of bites (%) taken by each individual horse from four different sward heights during the first hour in Experiment 1. (b) Amount of bites (%) taken by each individual horse from four different sward heights in Experiment 2. Each of the six horses was observed for 1 h.
that sheep mainly selected herbage from tall patches. Diestel et al. (1995) further demonstrated that cattle had a longer residence time on patches with long grass than on those with short grass. Like cattle, horses in this trial visited short patches of 3.6 and 5.3 cm sward height only briefly whereas patches of 15.5 cm sward height were highly selected, followed by patches of 8.2 cm sward height. These observations confirm that horses, like other herbivores (Diestel et al., 1995; Bazely, 1990) are able to respond to differences in the vegetation height and biomass and adjust their behaviour according to their needs. However it also poses two questions: When given the choice between different sward
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Fig. 5. (a) Amount of time (%) each individual horse spent grazing on each of the four different sward heights during the first hour in Experiment 1. (b) Amount of time (%) each individual horse spent grazing on each of the four different sward heights in Experiment 2. Each horse was observed for 1 h.
heights, why do horses select tall swards? And, when selecting one sward height, why do horses not deplete the patch totally before moving on to the next one? Potentially, one of the important influences on grazing behaviour of horses is the requirement to optimise energy intake per bite within time. As bite dimensions in various herbivores increase with sward height (Black and Kenney, 1984; Laca et al., 1992; Naujeck and Hill, 2003), horses seemingly maximised their intake per bite by selecting long grass. However selection of tall swards may be more complicated than the optimal intake model
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Fig. 6. (a) Percentage of bites taken by all horses (N = 7) on four different sward heights during Experiment 1. (b) Percentage of bites taken by all horses (N = 6) on four different sward heights in Experiment 2.
suggests. The horses may aim to avoid herbage contaminated with soil (generally higher levels of soil contamination occur in the lower horizons of pastures; Healy, 1967; Hodgson, 1990). In addition, a paddock with different patches of grass, some evenly mown and some left uncut was a novel environment for the horses used in the experiments. Patches of different sward height established by mowing may be perceived differently by an animal compared with those developed naturally. As a result the animal may focused their attention on the uncut, taller patches with the more natural appearance. Furthermore, the cutting of pasture can alter the sensory input component of foraging (change the texture of the grazing surface. While eating, touch receptors on the nose and lips enable a horse to sense the food to be gathered (Frape, 1998) changes in sensory input may increase aversion to cut surfaces (Provenza, 1996).
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The horses never resided on a patch for a long period but moved on to the next patch after a few minutes (Table 4) and returned later. There was therefore no complete depletion of a patch during one visit. The concepts developed by the marginal value theorem (Charnov, 1976) are that ‘when the intake rate in any patch drops to the average rate for the habitat, the animal should move on to another patch’. Besides the main criticism as to whether animals are able to measure the instantaneous rate of gain in a patch (Stephens and Krebs, 1986; Illius and Gordon, 1990) the horses in the present study always returned to patches with long grass they had previously been grazing. However their decision to move after a short period of time grazing patches of low sward height compared to those of higher sward height suggests that they responded to grass height rather than herbage allowance. These observations are qualitatively consistent with the marginal value theorem (especially in Experiment 1) but the relationship is more complex in Experiment 2. Diestel et al. (1995) suggested that cattle may be able ‘to perceive herbage intake as a reward and have a feedback mechanism that enhances behaviour to increase the rate of reward’. Possibly the horses in the experiment reported here ‘measured’ the average intake rate of the habitat to optimise their intake but the pattern of foraging did not allow to predict that an animal would return to patches that provide the greatest possible intake rate. These observations are similar to those of Wilmshurst et al. (1995) who suggested wapiti would prefer patches with intermediate herbage availability. However the application of the marginal value theorem depends on identification of appropriate scale and trade-off within a heterogenous environment (Bailey et al., 1996). The motivation to move from one patch to another is not clear. Motivation to move during foraging can be related to dissatisfaction or exploration behaviour (Senft et al., 1987; Provenza, 1996; Provenza et al., 2003). These strategies allow vegetation to recover, but in addition such behaviour will possibly give the animal an important picture of the environment. In a short- or long-term it may memorise sites providing suitable herbage (either areas of rapidly digestible nutrients or sites of aversion) and may (or not) return to them. With horses on a managed pasture, a similar behaviour was observed which might explain why they did not reside on a patch for a long period. When the animal accessed a managed pasture, a horse continuously walks while grazing and the sward is grazed down homogeneously (Naujeck and Hill, 2001) until patches develop due to defecation (Frape, 1998). It is possible that this behaviour reflects the fine line between satiety and surfeit, and thus aversion as the latter has been suggested as involuntary and not a result of a conscious decision by the animal (Provenza, 1996). The situation is possibly different in heterogenous pastures (Experiment 2). In the second experiment, horses not only spent considerable periods of time on shorter patches suggesting the animal may either optimise their intake in patches of intermediate herbage availability, they may not be able to differentiate between the different sward heights or aversion may be occurring in relation to previous defoliation and defaecation (Provenza et al., 2003). Furthermore horses accessed patches that may not seem to be optimal in relation to the marginal value theorem and they also foraged from them, whereas in Experiment 1 they seemed to investigate patches without eating from them. During Experiment 1 the horses visited patches with short swards as often as the ones with long grass, but rarely took any bites from them. It seems that they sampled their environment on a regular basis, always returning to long
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patches for eating. This behaviour might also explain why the time/bite correlation was more obvious in Experiment 2 than 1. Finally, Gordon (1989) noted, that diet selection of ponies seemed to be influenced by the quality of food (plant based variable) rather than the rate of energy intake (animal process). He observed that when the abundance of live material of the preferred high quality Agrostis–Festuca pastures declined in the winter, cattle moved to long Molina dominated pastures that had a lower quality. Ponies on the other hand stayed on the short Agrostis–Festuca areas. Similar behaviour was observed in Experiment 2. The appearance of the short patches changed due to emergence and elongation of new leaves. New tissue growth was less apparent in the taller patches. Sward height varied from 7.0 to 13.6 cm and the horizon that was cut to an even height the week before was now heterogeneous with variable tiller heights. On these paddocks, horses still showed a preference for the longest grass, but they also grazed from all the other patches (Fig. 6b). In Experiment 1 the horses took only few single bites from vegetation of the 3.6 or 5.6 cm tall patches (Fig. 6a and b), whereas in Experiment 2 the horses took 11% of the total number of bites from 7 cm patches. Interestingly the amount of biomass on short patches was very similar in both experiments, but the bulk density of the upper horizon of a 7 cm sward must have been less than that of a 3.6 or 5.6 cm sward. The fact that the horses grazed from the 7 cm patches in Experiment 2 therefore underlines the assumption that sward height as well as the quality of the young re-grown grass had an influence on diet selection of horses.
Acknowledgement The authors wish to thank Writtle College for funding this study, which is a part of a wider investigation on grazing behaviour of horses.
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Frape, D., 1998. Equine Nutrition and Feeding, 2nd ed. Blackwell Science, Oxford. Fraser, A.F., Broom, D.M., 1990. Farm Animal Behaviour and Welfare, 3rd ed. Bailliere Tindall, London. Gordon, I.J., 1989. Vegetation community selection by ungulates on the Isle of Rhum III. Determinants of vegetation community selection. J. Appl. Ecol. 26, 65–79. Healy, W.B., 1967. Ingestion of soil by sheep. Proc. NZ Soc. Anim. Prod. 27, 109–120. Heath, D., 1995. An Introduction to Experimental Design and Statistics for Biology. UCL Press, London. Hodgson, J., 1990. Grazing Management. Longman Handbooks in Agriculture, Hong Kong. Illius, A.W., Gordon, I.J., 1990. Constraints on diet selection and foraging behaviour in mammalian herbivores. In: Hughes, R.N. (Ed.), Behavioural Mechanisms of Food Selection. NATO ASI Series, vol. G20. SpringerVerlag, Berlin, pp. 369–393. Illius, A.W., Clark, D.A., Hodgson, J., 1992. Discrimination and patch choice by sheep grazing grass-clover swards. J. Anim. Ecol. 61, 183–194. Krebs, J.R., Davis, N.B., 1993. An Introduction to Behavioural Ecology, 3rd ed. Blackwell Science, Oxford. Krebs, J.R., Davis, N.B., 1997. Behavioural Ecology, 4th ed. Blackwell Science, Oxford. Laca, E.A., Ungar, E.D., Seligman, N.G., Demment, M.W., 1992. Effects of sward height and bulk density on bite dimensions of cattle grazing homogeneous swards. Grass For. Sci. 47, 91–102. Naujeck, A., Hill, J., 2001. The impact of social and grazing behaviour of horses on patch development in heavily grazed perennial ryegrass swards. Interfacing behaviour with other disciplines. In: Proceedings of the ASAB Summer conference, September, 18–20, University of Glasgow, UK, . Naujeck, A., Hill, J., 2003. The influence of sward height on bite dimensions of horses. Anim. Sci. 77, 95–100. Norbury, G.L., Sanson, G.D., 1992. Problems with measuring diet selection of terrestrial mammalian herbivores. Aus. J. Ecol. 17, 1–7. Provenza, F.D., 1996. Acquired aversions as the basis for varied diets of ruminants foraging on rangelands. J. Anim. Sci. 74, 2010–2020. Provenza, F.D., Villalba, J.J., Dziba, L.E., Atwood, S.B., Banner, R.E., 2003. Linking herbivore experience, varied diets and plant biochemical diversity. Small Rumin. Res. 49, 257–274. Putman, R.J., Pratt, R.M., Ekins, J.R., Edwards, P.J., 1987. Food and feeding behaviour of cattle and ponies in the New Forest, Hampshire. J. Appl. Ecol. 24, 369–380. Senft, R.L., Coughenour, M.B., Bailey, D.W., Rittenhouse, L.R., Sala, O.E., Swift, D.M., 1987. Large herbivore foraging and ecological hierarchies. Bioscience 37, 700–789. Stephens, D.W., Krebs, J.R., 1986. Foraging Theory. Princeton University Press, Princeton, NJ. Ungar, E.D., Ravid, N., 1999. Bite horizons and dimensions for cattle grazing herbage to high levels of depletion. Grass For. Sci. 54, 357–364. Ungar, E.D., Ravid, N., Bruckental, I., 2001. Bite dimensions for cattle grazing herbage at low levels of depletion. Grass For. Sci. 56, 35–45. Waran, N.K., 2001. The social behaviour of horses. In: Keeling, L.J., Gonyou, H.W. (Eds.), Social Behaviour of Farm Animals. CABI Publishing, Wallingford. Wilmshurst, J.F., Fryxell, J.M., Hudson, R.J., 1995. Forage quality and patch choice by Wapiti (Cervus elaphus). Behav. Ecol. 6, 209–217.
Influence of sward height on diet selection by horses A. Naujecka,*, J. Hilla,1, M.J. Gibbb a
Faculty of Applied Science and Technology, Writtle College, Lordship Road, Writtle, Chelmsford, CM1 3RR, UK b Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK Accepted 12 August 2004 Available online 25 September 2004
Abstract Foraging herbivores are often faced with spatial and temporal heterogeneity within the vegetation they have available to graze and therefore have to make decisions where and when to graze. The study reported in this paper investigated the influence of sward height on diet selection by horses grazing perennial rye-grass swards. The study comprised two experiments. In Experiment 1, perennial ryegrass paddocks were mown to four sward heights (heights: 3.5, 4.5, 7.5 and 15 cm) to create a patchy environment. Within each paddock one horse grazed for a period of 1 h during which residence time, number of bites and frequency of visits per patch were recorded. This was replicated with all seven horses used in the experiment. The same experiment was repeated in Experiment 2, but without mowing the field and allowing 1 week of re-growth for each paddock. During both experiments horses entered equally often but resided significantly longer on patches with long grass (15 cm) than on those with short grass (below 4.5 cm; P < 0.05). Grazing time and number of bites on a patch were highly correlated. The number of bites on patches with the highest sward height was greater than that on short patches (P < 0.05). Horses behaved as selective grazers, feeding mainly on grass taller than 7 cm. In Experiment 2, re-growth of the sward reduced the difference in sward height between the patches. Time spent per patch and total numbers of bites taken were less affected by sward height than in Experiment 1. It is suggested that horses behaved as energy maximisers (residing longer periods on patches and increasing number of bites taken). These data complement previous findings that bite dimension and bite mass increase with increasing sward height. When grazing, a horse rarely resided * Corresponding author. Tel.: +44 1245 424200; fax: +44 1245 420456. E-mail address: [email protected] (A. Naujeck). 1 Current address: Institute of Land and Food Resources, University of Melbourne, Victoria 3010, Australia. 0168-1591/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2004.08.001
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on a preferred patch for a long duration of time (maximum 305 s, median 79 s), but moved on after a few minutes. They sampled their environment continuously, but almost exclusively returned to long patches for feeding. # 2004 Elsevier B.V. All rights reserved. Keywords: Horses; Grazing behaviour; Diet selection; Selective grazing; Exploration; Energy maximisation
1. Introduction Diet selection is defined as an animal’s choice of food from a range of different food items (Norbury and Sanson, 1992) influenced by many factors such as nutritional demands, toxic plant compounds, forage availability, social interactions and predator risks (Krebs and Davis, 1993). Selecting the ‘right’ food is a trade-off between costs and benefits and has a short- and long-term effect on the animal’s fitness (Krebs and Davis, 1997). Foraging herbivores are often faced with spatial and temporal heterogeneity within the vegetation they have available to graze (Senft et al., 1987) and therefore have to make decisions where and when to graze. Optimal foraging models (Belovsky, 1986; Stephens and Krebs, 1986) predict that animals forage in ways that optimise the rate of food intake. Patch models like the marginal value theorem (Charnov, 1976) even suggest that ‘when the intake rate, in any patch, drops to the average rate of the habitat, the animal should move on to another patch’. If animals do behave in such a way, they must be able to gather and interpret information about their environment in order to modify their grazing behaviour to increase the rate of reward (Bailey et al., 1996). Studies on grazing behaviour by sheep, cattle and horses (Black and Kenney, 1984; Laca et al., 1992; Ungar and Ravid, 1999; Ungar et al., 2001; Illius et al., 1992; Naujeck and Hill, 2003) showed that they discern differences in grass height and that they increased bite dimensions with increasing grass height. To maximize voluntary intake, foraging models suggest that herbivores should select from tall, rather than short, grass. However the feeding site may be characterized by intermediate biomass in order to maximize intake of digestible nutrients as a result of the ‘trade-off’ between herbage quality and quantity (forage maturation hypothesis; Durant et al., 2004). Such behaviour was confirmed in studies on sheep and cattle (Bazely, 1990; Diestel et al., 1995), but little is known about diet selection by horses (Duncan, 1992; Fleurance et al., 2001; Putman et al., 1987; Gordon, 1989). The present study aimed to investigate the influence of sward height on diet selection by horses grazing perennial rye-grass swards. For this purpose, patchy paddocks were created by mowing. Behaviour such as time spent on patches, number of bites per patch and frequencies of visits per patch were recorded. 2. Materials and method The study was conducted at Writtle College (latitude 518440 N, longitude 08260 E, 32.5 m OD), UK in August 2002. It comprised two experiments. Experiment 1 investigated the grazing behaviour by horses on a heterogeneous paddock in which patches were cut to four
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different sward heights the day before measurements were made. In Experiment 2 grazing behaviour by horses was recorded on two of the same paddocks used in Experiment 1, but after one week of sward re-growth. 2.1. Experiment 1 Seven paddocks of perennial rye-grass (Lolium perenne) pasture measuring 16 m 16 m were prepared, each containing 16 squares (patches). To avoid any influence from previous horse grazing, the site chosen had never been stocked with horses. Each experimental paddock was cut with a lawn mower (ETESIA, PSE) the day before it was used for the single behaviour studies. Sixteen 4 m 4 m square patches of four different sward heights (3.5, 4.5, 7.5, 15 cm (uncut)) were prepared. Patches of the same sward height were allocated such that they were not adjacent. Each patch was marked with a fencing post and a letter. The same patch layout was used for each paddock as shown in Fig. 1. Seven horses, three males (geldings) and four mares were used in this study (horse details; Table 1). All horses were managed in one group on a perennial rye-grass pasture from April to October 2002. To minimise isolation stress for the single animals, the behaviour observations were performed with the study animal separated from, but still within sight of the rest of the group. In addition, a paddock with a companion animal was set up adjacent to the experimental paddock (Fig. 1). To familiarise the animals with the experimental paddock, two horses were moved to the companion field the day before the experiment. The next day, prior to moving one of the horses onto the experimental paddock, grass height was measured with a ruler (five measurements per patch, randomly, undisturbed height). Dry matter mass (g/m2) was determined by cutting the herbage to ground level within two randomly placed 10 cm 10 cm quadrates within each patch and by drying the cut material at 80 8C for 24 h. After herbage sampling, one of the two horses was moved to the experimental paddock. The focal horse was allowed to graze within this paddock for 1 h. Continuous recording was conducted using ‘The Observer’ mobile and software (Noldus, NL, Version 3.0). Location, bites and behaviours that were not associated with grazing such as idling, lying, alert and drinking were recorded. After 1 h, both horses were moved to a new companion paddock and the experimental paddock for the next day was prepared accordingly. The same procedure was repeated with the rest of the group. 2.2. Experiment 2 After 1 week of re-growth two of the seven paddocks used in Experiment 1 were used in Experiment 2 again. The experimental procedure and data analysis were similar to Experiment 1, but the grass was not cut before the experiment. Only six horses were used. Three horses successively grazed one paddock, each for 1 h. As three horses were successively grazing the same paddock for 1 h a change in sward conditions for the second and third horse in comparison to the first would occur. However, depletion of sward height and biomass after 1 h of grazing on a 16 m 16 m paddock was expected to be of little consequence.
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Fig. 1. Field and patch layout.
2.3. Data analysis Five different behaviour parameters were studied in the experiment. The number of visits per patch was recorded by continuous observation of the location of the focal animal. The data was analysed using chi-square with the assumption that all patches would be visited with an equal frequency. Time spent on each patch grazing was calculated from all behaviour observed during the total time an animal resided on a patch. As with number of
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Table 1 Characteristics of horses used Horse
Sex
Date of birth
Breed Sire Dam
Height (cm)
Weight (kg)
1 2 3 4 5 6 7
Female Female Male Male Male Female Female
24.02.2000 12.06.2000 16.02.2000 14.04.2000 24.04.2000 29.04.2001 02.2001
TB Cob Hann British warmblood Welsh TB Welsh TB Hann British warmblood Hann British warmblood ? TB
164 164 163 165 163 158 155
525 518 461 489 435 402 428
visits per patch, the data was analysed using chi-square with the assumption that the residence time would be equal regardless of the location within the paddock. A time-bite correlation (Spearmans rank correlation) was applied to investigate the interaction between residence time and number of bites taken during the period an animal was on the patch. Any individual animal differences for proportion of bites taken or time resided on a patch were assessed using the Kruskal–Wallis statistic with the assumption that all animals would have similar behaviour on all patches. The total number of bites during 1 h was calculated for each horse and the percentage of bites on each sward height determined. The differences in number of bites in each sward height were assessed using the Kruskal–Wallis statistic. Any differences between numbers of bites on individual sward heights were identified using Dunn’s test for multiple comparisons (Heath, 1995). All statistical procedures were conducted using SPSS Version 10.0 (SPSS Inc., 1999).
3. Results 3.1. Vegetation The data for sward height and dry matter mass are shown in Tables 2 and 3 (Experiments 1 and 2, respectively). The variability associated with sward height and dry matter yield are typical of that of perennial rye-grass swards after cutting to a series of heights (Experiment 1; Table 2). After 1 week of re-growth, there was a considerable increase in the variability of sward height and dry matter yield, reflecting an increase in heterogeneity in herbage production (Experiment 2; Table 3). Table 2 Sward height and dry matter mass Patches
Sward height (cm)
S.D.
% cv
Dry matter (g/m2)
S.D.
% cv
C, E, K, M D, F, L, N A, G, I, O B, H, J, P
3.6 5.3 8.2 15.5
0.30 0.24 0.27 1.45
8.3 4.5 3.3 9.4
72 103 178 242
12.7 15.7 30.0 37.7
17.6 15.3 16.9 15.6
S.D.: standard deviation, % cv: coefficient of the variation of the different patches (A–P) on the experimental paddock the day after cutting (N = 56 measurements per sward height).
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Table 3 Sward height and dry matter mass Patches
Sward height (cm)
S.D.
% cv
Dry matter (g/m2)
S.D.
% cv
C, E, K, M D, F, L, N A, G, I, O B, H, J, P
7.0 8.9 10.5 13.6
2.57 2.81 2.95 3.88
36.7 31.6 28.1 28.5
62 73 217 257
26.5 31.8 74.9 126.2
42.7 43.5 34.5 49.1
S.D.: standard deviation, % cv: coefficient of the variation of the different patches (A–P) in Experiment 2 (N = 56 measurements per sward height).
3.2. Number of visits per patch The number of visits per patch of each horse in Experiment 1 is presented in Fig. 2a. There were numerical differences (P > 0.05) in the number of visits per patch for each horse and each horse visited each patch equally often. In Experiment 2, a greater
Fig. 2. (a) Number of visits on any of the 16 patches in Experiment 1. Each graph shows the mean number of visits on any patch, the standard deviation and extreme values (empty dots) for each individual horse. (b) Number of visits on any of the 16 patches in Experiment 2. Each graph shows the mean number of visits on any patch, the standard deviation and extreme values (empty dots) for each individual horse.
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uniformity in the mean number of visits per patch was observed compared with Experiment 1 (for all animals the number of visits per patch ranged from 5 to 6; Fig. 2b). Four of the six horses had a similar incidence of visits per patch, however there was a significant increase in the number of visits per patch by horse 6 (visited E (16 times) and I (15 times); P < 0.01) and horse 1 (visited patch O 12 times; P < 0.05). 3.3. Time spent grazing per patch In the first experiment, considerable variation between time spent grazing on different patches (P < 0.01) was observed. Horses resided for longer periods of time on patches B, H, J and P than on other patches (Fig. 3a) and the residence time on a patch varied within a range of 1–353 s (Table 4). In Experiment 2 each horse resided for different durations on different patches (P < 0.01). The proportion of time horses grazed on patches A, B, H, I, J, O and P was often greater than on the other patches (Fig. 3b). Residence time on a patch varied within a range of 1–305 s (Table 4). 3.4. Time/bite correlation The correlation between time spent on a patch and the number of bites taken by each animal in Experiment 1 is shown in Table 5. The different sward height classes (Figs. 4a and 5a) demonstrate that when the horses visited short patches (3.6 and 5.3 cm) they only took a few bites from them. Similar results to Experiment 1 were observed in Experiment 2 (Table 5, Figs. 4b and 5b). 3.5. Individual differences in selection of swards of different height No differences between individual horses in the percentage of bites and the percentage of time spent on the four different sward heights (P > 0.05) were observed in Experiments 1 and 2. However, the comparison of Experiments 1 and 2 demonstrated that the proportion of time spent per patch, as well as the proportion of bites taken per patch was less variable in Experiment 2 than in Experiment 1 (Figs. 4a, b and 5a, b). 3.6. Number of bites from swards of different height The proportions of bites per sward height in Experiment 1 are in Fig. 6a and b. There was a significant difference in the proportion of bites on different sward heights (P < 0.01). Subsequently, multiple comparisons (Dunn’s test) demonstrated that more bites (P < 0.05) were performed on 15.5 cm than on the 3.6 and 5.3 cm patches. Further, more bites were made on 8.2 cm patches than on the 3.6 cm patches. The situation was different in Experiment 2. The proportions of bites on each sward height class are presented in Fig. 6b. A lower degree of variation between different sward height classes was observed compared to Experiment 1 (Fig. 6a). A difference in the proportion of bites on different sward heights (P < 0.01) was observed. Multiple comparison analysis (Dunn’s test) suggested that the number of bites was significantly greater (P < 0.05) only on the 13.7 cm patches compared with the 7 cm patches (Fig. 6b).
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Fig. 3. (a) Time spent grazing on each of the 16 patches (letter A–P) by each horse in Experiment 1. (b) Time spent grazing on each of the 16 patches (letter A–P) by each horse in Experiment 2.
Table 4 Mean, median and range of residence time (seconds) on a patch during a single visit of a horse for Experiments 1 and 2 Experiment 1 Grass height (cm) Mean Median S.D. Range
3.6 5 3 4.9 1–35
5.3 5 3 7.7 1–60
Experiment 2 8.2 19 6 27.7 1–159
15.5 83 70 65.0 3–353
7 19 8 24.7 1–136
8.9 27 15 30.8 1–168
10.5 37 27 34.6 1–188
13.6 64 51 60.0 3–305
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Table 5 Correlation between time spent on a patch and the number of bites Horse
Experiment 1 rs
Experiment 2 rs
1 2 3 4 5 6 7
0.937 0.907 0.892 0.865 0.729 0.958 0.947
0.986 0.985 0.996 0.970 0.991 0.976
Correlations were significant at the P < 0.01 level in Experiments 1 and 2.
4. Discussion The results of this study suggest that horses behave as selective grazers when offered a choice in grass height. Previous studies on the influence of sward height on bite dimensions of horses showed bite depth, weight, volumes and bite area were not a fixed size, but increased with sward height (Naujeck and Hill, 2003). The selection for a particular sward height in a patchy environment would therefore have an influence on bite dimensions and total intake of herbage by the horse. In the first experiment horses visited each patch with equal incidence. This behaviour was similar to those recorded by Diestel et al. (1995) in a study on diet selection by cattle that were provided with a choice of four different types of patches. Visiting each patch equally often could be explained by the animal’s motivation to explore the area. When moved to a new area, animals tend to walk along the boundary (personal observation; Fraser and Broom, 1990; Waran, 2001) for the first few minutes as to check the new surrounding. The horses in the experiments reported here investigated their surroundings but not always walk along the boundary of the paddock. Visual observations and subsequent examination of video recordings (data not presented here) showed that the horses tended to go straight to the tall sward patches, take a few bites, lift their head, move on and sample another patch. Furthermore there was no evidence that the foraging process was influenced by the position of the companion animal or the group in either experiment. The mean number of visits per patch was similar for all horses in Experiment 2 but the individual differences were less than in those noted in Experiment 1. As the differences in sward height between patches were greater in Experiment 1 than in Experiment 2 some horses may have had a greater motivation to explore the area more often than if the paddock looked more homogenous (as in Experiment 2). Differences in behaviour of each individual horse seemed to become more obvious in a more diverse environment. However it is not clear if the diversity within the pasture or site of initial foraging contribute to apparent disparity in the behavioural process of individual animals. In Experiment 1 the processes involved with foraging lead to horses residing longer on patches with long grass than on patches with short grass (Fig. 3a). The residence time spent on each patch was highly correlated with the number of bites taken. Similar behaviour has been demonstrated in sheep and cattle. For instance when sheep were provided a choice between tall (15.1–21.5 cm) and short grass patches (6.5–11.8 cm) Bazely (1990) reported
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Fig. 4. (a) Amount of bites (%) taken by each individual horse from four different sward heights during the first hour in Experiment 1. (b) Amount of bites (%) taken by each individual horse from four different sward heights in Experiment 2. Each of the six horses was observed for 1 h.
that sheep mainly selected herbage from tall patches. Diestel et al. (1995) further demonstrated that cattle had a longer residence time on patches with long grass than on those with short grass. Like cattle, horses in this trial visited short patches of 3.6 and 5.3 cm sward height only briefly whereas patches of 15.5 cm sward height were highly selected, followed by patches of 8.2 cm sward height. These observations confirm that horses, like other herbivores (Diestel et al., 1995; Bazely, 1990) are able to respond to differences in the vegetation height and biomass and adjust their behaviour according to their needs. However it also poses two questions: When given the choice between different sward
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Fig. 5. (a) Amount of time (%) each individual horse spent grazing on each of the four different sward heights during the first hour in Experiment 1. (b) Amount of time (%) each individual horse spent grazing on each of the four different sward heights in Experiment 2. Each horse was observed for 1 h.
heights, why do horses select tall swards? And, when selecting one sward height, why do horses not deplete the patch totally before moving on to the next one? Potentially, one of the important influences on grazing behaviour of horses is the requirement to optimise energy intake per bite within time. As bite dimensions in various herbivores increase with sward height (Black and Kenney, 1984; Laca et al., 1992; Naujeck and Hill, 2003), horses seemingly maximised their intake per bite by selecting long grass. However selection of tall swards may be more complicated than the optimal intake model
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Fig. 6. (a) Percentage of bites taken by all horses (N = 7) on four different sward heights during Experiment 1. (b) Percentage of bites taken by all horses (N = 6) on four different sward heights in Experiment 2.
suggests. The horses may aim to avoid herbage contaminated with soil (generally higher levels of soil contamination occur in the lower horizons of pastures; Healy, 1967; Hodgson, 1990). In addition, a paddock with different patches of grass, some evenly mown and some left uncut was a novel environment for the horses used in the experiments. Patches of different sward height established by mowing may be perceived differently by an animal compared with those developed naturally. As a result the animal may focused their attention on the uncut, taller patches with the more natural appearance. Furthermore, the cutting of pasture can alter the sensory input component of foraging (change the texture of the grazing surface. While eating, touch receptors on the nose and lips enable a horse to sense the food to be gathered (Frape, 1998) changes in sensory input may increase aversion to cut surfaces (Provenza, 1996).
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The horses never resided on a patch for a long period but moved on to the next patch after a few minutes (Table 4) and returned later. There was therefore no complete depletion of a patch during one visit. The concepts developed by the marginal value theorem (Charnov, 1976) are that ‘when the intake rate in any patch drops to the average rate for the habitat, the animal should move on to another patch’. Besides the main criticism as to whether animals are able to measure the instantaneous rate of gain in a patch (Stephens and Krebs, 1986; Illius and Gordon, 1990) the horses in the present study always returned to patches with long grass they had previously been grazing. However their decision to move after a short period of time grazing patches of low sward height compared to those of higher sward height suggests that they responded to grass height rather than herbage allowance. These observations are qualitatively consistent with the marginal value theorem (especially in Experiment 1) but the relationship is more complex in Experiment 2. Diestel et al. (1995) suggested that cattle may be able ‘to perceive herbage intake as a reward and have a feedback mechanism that enhances behaviour to increase the rate of reward’. Possibly the horses in the experiment reported here ‘measured’ the average intake rate of the habitat to optimise their intake but the pattern of foraging did not allow to predict that an animal would return to patches that provide the greatest possible intake rate. These observations are similar to those of Wilmshurst et al. (1995) who suggested wapiti would prefer patches with intermediate herbage availability. However the application of the marginal value theorem depends on identification of appropriate scale and trade-off within a heterogenous environment (Bailey et al., 1996). The motivation to move from one patch to another is not clear. Motivation to move during foraging can be related to dissatisfaction or exploration behaviour (Senft et al., 1987; Provenza, 1996; Provenza et al., 2003). These strategies allow vegetation to recover, but in addition such behaviour will possibly give the animal an important picture of the environment. In a short- or long-term it may memorise sites providing suitable herbage (either areas of rapidly digestible nutrients or sites of aversion) and may (or not) return to them. With horses on a managed pasture, a similar behaviour was observed which might explain why they did not reside on a patch for a long period. When the animal accessed a managed pasture, a horse continuously walks while grazing and the sward is grazed down homogeneously (Naujeck and Hill, 2001) until patches develop due to defecation (Frape, 1998). It is possible that this behaviour reflects the fine line between satiety and surfeit, and thus aversion as the latter has been suggested as involuntary and not a result of a conscious decision by the animal (Provenza, 1996). The situation is possibly different in heterogenous pastures (Experiment 2). In the second experiment, horses not only spent considerable periods of time on shorter patches suggesting the animal may either optimise their intake in patches of intermediate herbage availability, they may not be able to differentiate between the different sward heights or aversion may be occurring in relation to previous defoliation and defaecation (Provenza et al., 2003). Furthermore horses accessed patches that may not seem to be optimal in relation to the marginal value theorem and they also foraged from them, whereas in Experiment 1 they seemed to investigate patches without eating from them. During Experiment 1 the horses visited patches with short swards as often as the ones with long grass, but rarely took any bites from them. It seems that they sampled their environment on a regular basis, always returning to long
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patches for eating. This behaviour might also explain why the time/bite correlation was more obvious in Experiment 2 than 1. Finally, Gordon (1989) noted, that diet selection of ponies seemed to be influenced by the quality of food (plant based variable) rather than the rate of energy intake (animal process). He observed that when the abundance of live material of the preferred high quality Agrostis–Festuca pastures declined in the winter, cattle moved to long Molina dominated pastures that had a lower quality. Ponies on the other hand stayed on the short Agrostis–Festuca areas. Similar behaviour was observed in Experiment 2. The appearance of the short patches changed due to emergence and elongation of new leaves. New tissue growth was less apparent in the taller patches. Sward height varied from 7.0 to 13.6 cm and the horizon that was cut to an even height the week before was now heterogeneous with variable tiller heights. On these paddocks, horses still showed a preference for the longest grass, but they also grazed from all the other patches (Fig. 6b). In Experiment 1 the horses took only few single bites from vegetation of the 3.6 or 5.6 cm tall patches (Fig. 6a and b), whereas in Experiment 2 the horses took 11% of the total number of bites from 7 cm patches. Interestingly the amount of biomass on short patches was very similar in both experiments, but the bulk density of the upper horizon of a 7 cm sward must have been less than that of a 3.6 or 5.6 cm sward. The fact that the horses grazed from the 7 cm patches in Experiment 2 therefore underlines the assumption that sward height as well as the quality of the young re-grown grass had an influence on diet selection of horses.
Acknowledgement The authors wish to thank Writtle College for funding this study, which is a part of a wider investigation on grazing behaviour of horses.
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