Behavioural mechanisms of intake rate by heifers grazing swards of contrasting structures

The relationship between herbage intake by herbivores and herbage available depends on the interaction between sward structure and animal behaviour. This relationship is a crucial component determining the stability, function and productivity of grazing systems. The present study aims at quantifying the mechanisms by which intake rate is determined in swards of contrasting structure and forage species, and it reveals the importance of animal choice in the interaction between animal and sward. We hypothesised that as sward height and herbage mass increased, bite mass and intake rate would increase. The relationship between intake rate and bite mass should exhibit a higher asymptote for the more succulent, less fibrous Avena than for Cynodon due to differences in chewing requirements per unit dry mass of intake. In four different experiments, Cynodon sp. cv. Tifton 85 and Avena strigosa cv. Iapar 61 swards of different heights were obtained by various durations of short time high-intensity grazing or by different growth periods. Bite mass and intake rate were estimated by the double-weighing technique with correction for insensible losses. Number and timing of jaw movements and bites were measured with behaviour recorders. Bite mass was the main variable determining intake rate. Contrary to expectations, bite mass first increased and then decreased with increasing sward height, a result that seems due to animal choice and not from restrictions imposed by the swards. As expected, Cynodon sp. swards required more chewing per unit DM intake due to their higher DM content (39.5 vs. 22.6, P < 0.0001) and higher fibre (67.8 vs. 51.3, P < 0.0001) content than A. strigosa. Harvesting and chewing jaw movements, the components of total jaw movements and time per bite, had different time costs, and values were remarkably similar to previously published values. On average, each harvesting jaw movement took 1 s and a chewing jaw movement took 0.68 s. The reduction in bite mass with increasing sward height in tall swards is discussed as a destabilising mechanism that generates spatial heterogeneity in sward height and mass.