Influence of sward height on diet selection by horses

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.