EFFECTS OF SWARD HEIGHT AND BULK-DENSITY ON BITE DIMENSIONS OF CATTLE GRAZING HOMOGENEOUS SWARDS

Effects of sward height and density on the dimensions and weights of bites taken by cattle were examined. In one experiment, swards of dallisgrass lamina were constructed by hand in a factorial combination of four heights (80, 150, 180 and 300 mm) and three densities (c. 700, 1500 and 2700 g m-3). In the other, swards of lucerne were constructed in a factorial combination of three heights (70, 150 and 250 mm) and three densities (1500, 2800 and 5900 g m-3). Treatments were replicated on three steers of 750 kg average weight. The first six bites taken from the sward were monitored, and functional relationships between sward characteristics and bite dimensions derived. Results from both experiments were similar. Average bite area was not constant as often assumed, but decreased linearly with density and increased quadratically with height, with slope negatively affected by density and height. In tall swards, bite area reached a plateau of c. 170 cm-2, determined by the sweep of the tongue. In contrast with the widely used model, bite depth increased linearly with height, with slope negatively affected by density. Response of bite dimensions was explained by the mechanics of the interaction between tongue and jaw movements, and sward structure. Bite weight varied less than bite dimensions, because of compensatory effects between bite area, bite depth and density. Animals obtained heavier bites in tall sparse swards than on short dense ones of equal mass/area. Even in homogeneous swards, both density and height are necessary to predict bite weight.