SWARD HEIGHT AND VERTICAL MORPHOLOGICAL-DIFFERENTIATION DETERMINE CATTLE BITE DIMENSIONS

To identify and quantify the influence of sward structure on the intak e of grazing animals requires an understanding of sward-animal interac tions at the bite level. We tested the hypothesis that bite dimensions on vertically heterogeneous swards are determined by structural chara cteristics of the top leaf stratum, independent of those of a bottom l ayer of pseudostem or stem. Dallisgrass (Paspalum dilatatam Poir.) mic roswards were constructed by hand in a factorial combination of two la mina 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 swa rd height and lamina length. Treatments were replicated over three ste ers (Bos taurus) of 750 kg avg. body weight. Bite area and depth incre ased with sward height in swards with pseudostem but were not affected by lamina length. Pseudostems were not a barrier to defoliation and d id not affect bite dimensions. Steers restricted grazing of tillers wi th stem to the upper lamina horizon. Bite depth was limited by stems o nly when lamina length was less than half of sward height. Bite area w as reduced by the presence of stems because tillers bent at ligule hei ght, restricting the animals to gather fewer tillers in each bite than when tillers bent at the base. Bite weight was reduced severely in sw ards with stem, relative to those with pseudostem. Results confirmed t he importance of stem horizons and their relative positions in the swa rd as determinants of bite weight.