Bite dimensions for cattle grazing herbage at low levels of depletion

Abstract

The processes that occur during the depletion of a single feeding station by grazing cattle are important to our understanding of intake at larger spatial scales. Factorial experiments were conducted in which feeding stations of different sizes were grazed individually by cattle to various levels of depletion, defined as the number of bites removed. Feeding stations in Experiment 1 (alfalfa) measured 0·11, 0·24 and 0·45 m2, and the numbers of bites removed for depletion levels 1–3 were 35, 70 and 104 m−2 respectively. Feeding stations in Experiment 2 (oats) measured 0·11 and 0·24 m2, and the numbers of bites were 45, 95 and 140 m−2 offered. In both experiments a fourth depletion level (not included in the analysis of variance) determined the maximum voluntary depletion, and exceeded 250 bites m−2 offered. Initial sward height was 20 cm. Bite dimensions were derived from the frequency distribution of residual herbage heights. Treatments were replicated over six and four animals of approximately 500 kg live weight in Experiments 1 and 2 respectively.The mean residual herbage height and the frequency distribution of residual heights indicated that bites were removed predominantly from the top grazing horizon at depletion levels 1–3, with a mean effective bite depth of 8·6 cm. Nevertheless, at the same depletion levels, the mean effective bite area declined from 148 to 87 cm2 in Experiment 1 (alfalfa, Medicago sativa L.) and from 86 to 58 cm2 in Experiment 2 (oats, Avena sativa L.). Feeding station size did not significantly affect mean effective bite area in either experiment.Simulation was used to examine the implications of various overlap rules (from completely random to highly systematic) for within‐grazing‐horizon placement of a circular bite of constant potential area. These rules shaped the relationship between mean effective bite area and number of bites removed per unit sward area offered. Bite placement in which permissible overlap became progressively more lenient as the grazing horizon was depleted, and, with acceptable estimates of potential bite area, yielded results that were similar, though not identical, to those measured. These results can help understanding of the factors that determine the intake gain function at a single feeding station.