Does sward height affect feeding patch choice and voluntary intake in horses?
Résumé
The numbers of horses grazing at pasture are increasing in developed countries, so a proper understanding of their feeding selectivity and of the tactics they use for extracting nutrients fromswards is essential for the management of horses and grasslands. Resource acquisition in herbivores can be optimised through the modulation of their intake and patch selection, both being strongly dependent on the characteristics of swards. However, the principles by which horses adjust their grazing behaviour in response to variations in sward features are not completelyunderstood. The aimof this studywasto determinewhether thebehaviour of horses conforms to optimal foraging models. We hypothesized that, faced with binary choices between vegetative swards of a good and similar quality at three different heights, horses would select the taller sward, i.e. that allowing a higher reward in termsof dry matter intake rate. Three groups of three 2-year-old saddle horses were grazed on a semi-natural pasture thatwasmanaged to produce three contrasting sward heights at 6,11 and 17 cm, in a Latin-square design. The instantaneous intake rate was determined frombite rate measured at pasture on the three sward heights, and bite mass estimated from measurements using swards offered indoors in experimental trays. Daily dry matter intake was estimated individually by total faecal collection and an estimation of digestibility fromfaecal nitrogen. Short-term (first 30 min) and daily preferences were assessed from the time spent grazing each sward offered in pair-wise tests at pasture. The results showthat daily voluntary intake (an average of 21 g DM kg LW1 day1) and total grazing time (an average of 14 h day1) were independent of sward height and of the choice of patches offered. In choice situations, the animals spent more time grazing on the taller sward, both during the first 30 min and at the daily scale. These results showthat horses choose between vegetative patches of a good and similar quality according to the predictions fromoptimal foragingmodels, and select the one that they can ingest faster. Further research willnowhave to explore howthe horses will adapt their feeding behaviour when they face a trade-off between sward height and quality.