Bite horizons and dimensions for cattle grazing herbage to high levels of depletion [Note 1. Contribution from the Agricultural Research Organization, Institute of Field ...]

A simple model of depletion by a grazing ruminant was tested at high depletion levels and for different sizes of feeding station. The model divides an initially uniform sward profile into grazing horizons, dependent on bite depth, and assumes a constant within‐horizon bite area. Two grazing experiments were conducted using cattle. Uniform areas of oats and alfalfa herbage were grazed individually to a high level of depletion (≈150 bites m –2 ). Feeding station areas were 0·16, 0·30 and 0·53 m 2 . Initial sward heights were 10 and 20 cm in oats and 20 cm in alfalfa. Size of area did not significantly affect the observed number of bites removed per square metre, the mean residual herbage height or mass, or the proportion of each grazing horizon depleted, derived from the frequency distribution of residual heights. The mean residual height for all treatment combinations could be explained by assuming a ‘take half’ rule for mean bite depth, and allowing for the derived proportion of the area of each grazing horizon depleted. A simple bite placement simulator was used to generate, for a single grazing horizon and for a given maximum potential area of a bite, the expected relationship among mean effective area of a bite, the proportion of the area of the horizon grazed and the number of bites removed per unit area. The simulator mimics a loosely systematic grazing style. The observed bite numbers and the derived proportions of grazing horizon depleted can be reconciled if the within‐horizon mean effective bite area is not constant but declines as predicted by the bite placement simulator. The implications for the shape of the gain function within a feeding station are discussed.