Animal and sward behavior as affected by levels of forage on-offer

Typescript (photocopy).A trial was conducted on pastures of Brazos bermudagrass in 1981 and 1982 to study both animal and pasture responses to grazing. Each pasture was divided into four paddocks to permit rotational grazing. Measurements were made of the standing forage (SF) and residual forage (RF) mass in a paddock at the beginning and end of each seven-day grazing period, respectively. Three yearling steers remained permanently on each pasture throughout the season each year. However, extra animals were added or removed to maintain four levels of forage on-offer (FOF); 25, 50, 75, and 100 g DM/kg animal/day. Growth rate of the swards was determined from the dry matter accumulation rate during each paddock's rest period, while FOF was calculated from SF, growth rate during grazing as extrapolated from dry matter accumulation rate during the rest period, and animal weights on pasture. Apparent dry matter intakes were estimated from the dry matter disappearance as a paddock was grazed. Digestibilities of the RF swards and one-third horizontal layers of SF swards were used to calculate the digestibility of material estimated to be consumed by the animals. A curvilinear relationship was shown between FOF and ADG. As levels of FOF increased, ADG increased, too, eventually reaching a maximum of 0.37 kg at an FOF of 99 g/kg animal/day. Almost the same maximum ADG (0.38 kg) was achieved from a statistically significant (P (LESSTHEQ) 0.05) curvilinear relationship using RF as the independent variable. SF showed a highly significant linear relationship to ADG. The coefficients of determination of the models using RF or FOF as independent variables, while equal to each other, were higher than that of the ADG-SF model. There was little evidence that pasture growth was affected by grazing intensity. Estimated dry matter intakes were shown to increase linearly as levels of FOF increased. In turn, ADG increased linearly as estimated intakes increased. Digestibility of both SF and RF swards declined as levels of FOF increased. At a particular level of FOF, the digestibility of the RF sward was lower than that of the SF sward, indicating that the highest quality material disappeared during grazing. The digestibility of material estimated to be consumed declined as FOF increased. ADG increased in a linear fashion concomitantly with estimated digestible organic matter intake. The coefficients of determination were lower than those for models relating ADG to FOF and RF. This indicates the difficulty encountered when estimating the quantity and quality of ingested material