ALLOWANCE-INTAKE RELATIONS OF CATTLE GRAZING VEGETATIVE TALL FESCUE

Herbage allowance is one of the important pasture factors in the determination of intake by grazing livestock. Ingestive behaviour of 12 adult Angus cows (Bos taurus) was measured over a range of allowances (0.25 to 0.72 kg dry matter (DM) per 100 kg live weight (LW) for a 1-h period) of vegetative tall fescue (Festuca arundinacea Schreb.). A balanced change-over design was used to estimate direct, residual and permanent effects of herbage allowance on rate of DM intake, rate of biting and herbage DM intake per bite. In Experiment 1, herbage DM intake per meal increased linearly from 0.68 to 1.72 kg (100 kg LW)-1 as DM allowance increased from 0.25 to 0.72 kg (100 kg LW)-1 h-1. Cows grazed at 0.30 kg (100 kg LW)-1 h-1 and stopped grazing when the sward was reduced to a height about 10 to 12 cm above the soil surface, approximately defined by the tops of pseudostems. In Experiment 2, herbage DM intake rates of 0.29, 0.47 and 0.42 kg (100 kg LW)-1 h-1 were recorded as cows grazed allowances of 0.43, 0.70 and 0.90 kg (100 kg LW)-1 h-1 for most of the 1-h grazing period. Limiting herbage DM allowances in Experiment 2 were associated with small reductions in rate of biting and herbage DM intake per bite as allowance declined. Sward DM density (> 5 cm) was an important variable in the determination of herbage DM intake rates at lower herbage allowances.