Instantaneous intake rate in horses of different body sizes: Influence of sward biomass and fibrousness

The functional response, that is the relationship between the food intake rate of a forager and the availability of food items, has been subject to numerous investigations in ruminants. In horses however, the functional response has been poorly studied despite of the importance of grazed forage in horse nutrition and the increasing role of horses in the management of grasslands in Europe. Large differences in body size can be found in adult horses of different breeds and intake rates are commonly affected by body size and mouth morphology in mammalian herbivores. This study describes the functional response of horses using, for the first time, natural swards offering a wide range of biomass (from 82 to 513 gDM m −2, i.e. heights from 3 cm to 63 cm) and varying in quality (NDF: 53–68%), and animals of different body sizes (ponies: 253 kg, saddle horses: 602 kg, heavy horses: 953 kg). Instantaneous intake rate was the product of bite size measurements on sward trays and bite rate values obtained on the same swards while horses were grazing at pasture. Using the Spalinger and Hobbs [Spalinger, D.E., Hobbs, N.T., 1992. Mechanisms of foraging in mammalian herbivores: new model of functional response. Am. Nat. 140, 325–347] model developed for mammalian herbivores, we found that Type II functional responses (i.e. asymptotic curve) provided a satisfactory fit to the intake rate by horses of contrasting body sizes on the range of biomass tested. As has been found in ruminants, handling time (i.e. the time needed to crop and to process a mouthful) increased linearly with bite size in the three sizes of horses. The maximum processing rate increased with body size, indicating that smaller horses are more constrained when bite size increases. Taking into account the fibrousness of swards did not improve the estimation of handling time which means that horses were relatively unaffected by the range of fibrousness in our study.