The grazing behavior of large herbivores is influenced by many factors. However, the processes of voluntary intake during grazing events are important in the management of pastures and for maintaining an adequate supply of energy and protein. Surprisingly little is known about the processes controlling voluntary intake of herbage by horses, especially regarding the dimensions of the bite. Bite dimensions (depth, area and volume) have been measured in cattle, sheep and other grazing ruminants, but little research has been performed with horses. This paper examines a novel method of measuring the bite dimensions of horses offered herbage cut to four grazing planes.

Method
Laca et al. [1] developed a method to investigate bite dimensions by heifers offered swards of known tiller density. However, swards were constructed for their study by growing and presenting individual grass plants arranged in a discrete area and pattern. In the present experiment, the method used relied on the production of swards on an artificial growth medium. Perennial rye grass swards were established and grown on artificial media (40 x 90 cm) for a period of five months before the grazing experiment. The swards developed a tiller density of 3000/m2. The four grazing planes cut were 3, 8, 15 and 19 cm mean sward height. Eight horses (2 to 4 years of age; mean body weight 512 kg, s.e. 24.7 kg) were each offered individual swards for a period of time and allowed to take six bites. The swards were then removed and bite dimensions were measured. The bite dimensions recorded were depth, area, weight and volume. During the measurement period, the horses’ behavior was recorded using a monochrome CCTV camera, and the films were analysed using time lapse editing.

Results and Discussion
The mean dimensions of bite are presented in Table 1. The dimensions of bite depth were not fixed in relation to sward height, suggesting that the horses perceived differences in sward height and adjusted their ingestive behavior by modifying their grazing process. Similar behavior has been observed in sheep and cattle [1,2]. However, the mechanisms associated with the modification of grazing are poorly understood. The horses also showed a degree of objectivity during grazing. By analysing the CCTV films, the time taken for six bites, overlap and single bites were recorded (Table 2). The median time needed to take six bites of herbage varied little between horses, but the range of time taken was greater at either very short swards (3 cm) or very tall swards (19 cm). Overlap of bites increased with sward height, suggesting a reduction in selectivity.

Table 1. Dimensions of bite (s.d.).

Table 2. Objectivity and time for six bites.

References

1. Laca, E.A.; Unger, E.D.; Seligman, N.G.; Ramey, M.R.; Demment, M.W. (1992). Grass and Forage Science, 47, 81-90.
2. Milne, J.A.; Hodgson, J.; Thompson, R.; Souter, W.G.; Barthram, G.T. (1982). Grass and Forage Science, 37, 209-218.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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