Influence of sward height on diet selection by horses
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.
Section snippets
Materials and method
The study was conducted at Writtle College (latitude 51°44′N, longitude 0°26′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 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.
Vegetation
The data for sward height and dry matter mass are shown in Table 2, Table 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).
Number of visits per patch
The
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
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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Current address: Institute of Land and Food Resources, University of Melbourne, Victoria 3010, Australia.