Bite mechanics of cattle and sheep grazing grass-dominant swards

Numerous experiments have shown that animals grazing pasture remove only a portion of the sward height offered. Furthermore, bite area increases with sward height but decreases slightly as sward bulk density increases. A number of theories have been proposed to explain these patterns. One suggestion is that bite depth is limited by barrier layers of mature pseudostem or reproductive stem in the canopy. Animals may then adjust their bite area to maintain a constant bite fracture force. However, recent studies show that these hypotheses are unlikely to be true except in limited circumstances. This paper develops a theory of bite mechanics to describe and explain the bite dimensions of sheep and cattle grazing grass swards in terms of physical processes. Bite mechanics are described using simple mathematical functions that assume a non-constant bulk density profile and a non-cylindrical bite volume. Bite `depth' (insertion height) is assumed to be proportional to sward surface height. Bite area responses can then be explained by the extent to which this non-cylindrical bite volume is inserted into the canopy and by the observation that animals decrease potential bite area slightly in response to increasing pasture bulk density. Bite area and bite weight calculations derived from these models are consistent with published data from sheep and cattle grazing artificial swards. Areas for further experimentation are suggested.