Influence of progressive defoliation of a grass sward on the eating behaviour of cattle

Abstract

Changes in sward structure, diet selection and eating behaviour of cattle (grazing time, biting, bite size and estimated intake) were studied as contrasting swards of Setaria anceps cv. Kazungula were progressively defoliated. In one experiment three replicates in time of 9-week autumn regrowths were grazed down over 14-day periods, and a further two time replicates of 5-week regrowths were grazed down over 10-day periods in spring. A second experiment investigated the causes of changes in grazing behaviour. In the early stages of defoliation cows selected mainly leaf (> 80%) from the uppermost layers of the sward. As the quantity of leaf on offer became less, the animals took smaller bites, increased the time spent grazing (up to a mean of 646 min in autumn and 593 min in spring), and increased the rate of biting (up to 62.4 and 59.4 eating bites min-1 in autumn and spring respectively) and the total number of eating bites (up to 39,500 and 34,000 day-1 in autumn and spring). At this stage bulk in the rumen appeared to limit intake of herbage. Peak grazing time and highest number of bites occurred when leaf yields averaged 1000 kg dry matter ha-1, although total yield and dry green yield varied between swards. In the later stages of defoliation, grazing time and eating bites declined and, together with a reduced bite size (as low as 66 and 79 mg organic matter per bite eaten in experiments 1 and 2 respectively), resulted in a low intake of herbage. Stem and senescent material in the diet increased up to 77%, but bulk was not the major cause of reduced intake. Nitrogen and mineral levels appeared sufficient for maximum intake. Low leaf density prevented animals harvesting large quantities of herbage from the swards. It was concluded that leaf is the most important component of the sward and that leaf yield, percentage leaf and bulk density of green material (leaf and stem) are the major sward factors influencing intake by grazing animals.