SWARD HEIGHT AND VERTICAL MORPHOLOGICAL-DIFFERENTIATION DETERMINE CATTLE BITE DIMENSIONS

To identify and quantify the influence of sward structure on the intake of grazing animals requires an understanding of sward-animal interactions at the bite level. We tested the hypothesis that bite dimensions on vertically heterogeneous swards are determined by structural characteristics of the top leaf stratum, independent of those of a bottom layer of pseudostem or stem. Dallisgrass (Paspalum dilatatam Poir.) microswards were constructed by hand in a factorial combination of two lamina lengths (5 and 8 cm) and two sward heights (8 and 16 cm). Tillers consisted of a top segment of lamina and a bottom one of pseudostem (Exp. 1) or stem (Exp. 2) of length equal to the difference between sward height and lamina length. Treatments were replicated over three steers (Bos taurus) of 750 kg avg. body weight. Bite area and depth increased with sward height in swards with pseudostem but were not affected by lamina length. Pseudostems were not a barrier to defoliation and did not affect bite dimensions. Steers restricted grazing of tillers with stem to the upper lamina horizon. Bite depth was limited by stems only when lamina length was less than half of sward height. Bite area was reduced by the presence of stems because tillers bent at ligule height, restricting the animals to gather fewer tillers in each bite than when tillers bent at the base. Bite weight was reduced severely in swards with stem, relative to those with pseudostem. Results confirmed the importance of stem horizons and their relative positions in the sward as determinants of bite weight.