Influence of sward height on diet selection by horses

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 rye-grass 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 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.

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.