Does sward height affect feeding patch choice and voluntary intake in horses?

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 from swards is essential for themanagement 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. Theaimof this studywas todeterminewhether thebehaviourof 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 horseswould select the taller sward, i.e. that allowing a higher reward in termsof drymatter intake rate. Three groups of three 2-year-old saddle horses were grazed on a semi-natura pasture thatwasmanaged toproduce threecontrasting swardheightsat6,11and17 cm, ina Latin-square design. The instantaneous intake ratewas determined frombite ratemeasured 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 from faecal nitrogen Short-term (first 30 min) and daily preferences were assessed from the time spent grazing each sward offered inpair-wise tests at pasture. The results show that daily voluntary intake (an average of 21 g DM kg LW 1 day ) and total grazing time (an average of 14 h day ) were independent of sward height and of the choice of patches offered. In choice situations the animals spentmore time grazing on the taller sward, both during the first 30 min and at the daily scale. These results show that horses choose between vegetative patches of a good andsimilar quality according to thepredictions fromoptimal foragingmodels, and select the one that they can ingest faster. Further researchwill nowhave to explore how thehorseswil adapt their feeding behaviourwhen they face a trade-off between sward height and quality 2009 Elsevier B.V. All rights reserved * Corresponding author at: INRA, UR1213 Herbivores, 63122 St-GenèsChampanelle, France. Tel.: +33 4 73 62 46 52; fax: +33 4 73 62 41 18. E-mail address: [email protected] (G. Fleurance). 0168-1591/$ – see front matter 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2009.03.017