| Abstract: | Development of simulation models of grazing beef cattle requires measurement of the components of the ingestive process and the establishment of relationships between these components and the structure of the sward. The ingestive behaviour of eight half-sib Angus steers (live weight (LW), x?= 270 kg) grazing alfalfa (Medicago sativa L.) was studied at three stages of maturity (26, 40 and 47 days of regrowth) and at four allowances of herbage dry matter (DM) (1·0, 1·5, 20 and 2·5 kg per 100 kg LW) at each of two daily grazing sessions. A tethering system of grazing was used in which the experimental unit was a tethered steer and its plot for one grazing session. Grazing sessions commenced at 08.00 and 14.00 h EDT. Intake (DM) increased linearly from 1·98 kg per steer session at a DM allowance of 1 kg (100 kg LW)?1 to 2 89 kg steer session at an allowance of 2·5 kg (100 kg LW) ?1 as utilization of herbage declined linearly from 0·69 to 0·43. Herbage DM in take per bite increased from 1 0 g at 1 kg (100 kg LW) ?1 allowance to 1·5 g at 2·5 kg (100 kg LW) ?1 allowance. Rates of biting were not affected by herbage allowance and averaged 21 bites min?1. Dry matter intake increased from 1·77 to 3 41 kg per steer session as the alfalfa matured and herbage mass changed from 1500 to 4656 kg ha?1. Mean rates of biting were 24 bites min?1 for steers grazing the youngest alfalfa and 16 bites min?1 for steers on the oldest forage. Herbage DM intakes per bite were 1·1 g and 1·7 g at the same stages. Rates of DM intake approached 2 kg h?1 and maximum daily DM intake was estimated at 2 75 kg (100 kg LW) ?1. Intake of alfalfa was limited by allowance and mass of herbage above a canopy horizon of 20 cm and, to a lesser extent, by the length of fast. |
