Factors affecting the voluntary intake of some Tasmanian forages

Measurements of voluntary intake, digestibility and chemical composition were made on a range of forage species and varieties in three series of indoor feeding trials. The aim was to determine the intake and digestibility levels of these forages, and the reasons for variations between forages in the voluntary intake - digestibility relations. In the first trial, species were white clover, perennial ryegrass, short rotation ryegrass, Ariki ryegrass, currie cocksfoot and Apanui cocksfoot. These were harvested eight times between May 1969 and August 1970. Digestibility and voluntary intake of white clover was higher than that of the grasses, digestibility of Apanui cocksfoot was lower than that of the other grasses. Within seasons, all species showed a similar intake - digestibility relation, however, at any digestibility level, voluntary intake of winter and autumn cut herbage was less than that of spring and summer cut herbage. Information from this trial did not exclude the possibility that the low winter intakes were due to environmental factors affecting animal appetite. In the second trial, four species, short rotation ryegrass, Italian ryegrass, Tama ryegrass and oats were harvested three times between April and October 1973. There were no differences between species or harvests in digestibility. However, voluntary intake in the spring harvest was higher than in the autumn and winter harvests. A difference in intake between early and late cuts occurred in the autumn - winter harvest and as these cuts were fed in the same feeding trial, this indicated that the differences in intake were due to plant rather than animal factors. In the third trial, perennial ryegrass and demeter fescue were harvested four times between October 1974 and October 1975, and lucerne was harvested seven times between January 1976 and March 1977. Digestibilities of ryegrass and fescue were, similar. No species differences in intake occurred but voluntary intake of winter harvests was lower than that of spring harvests. Digestibility of lucerne was lower than normally found with the grass species and peaked at about 70%. Voluntary intake tended to be high despite the moderate digestibility. Within spring and summer cuts, voluntary intake of all species was closely related to digestibility but, at a given digestibility level, voluntary intake of lucerne but not clover was significantly higher than that of grass. This indicated that the higher intake of legume compared with grass mainly occurred with lower digestibility forages. Differences in intake between grass and legume could be explained by the lower level of neutral detergent fibre in legume. In all trials, voluntary intake of winter and autumn cuts was lower than that of spring and summer cuts. These low intakes could not be explained on the basis of the chemical or in vitro measurements carried out. However, even within winter cuts, there was a positive relation between intake and digestibility and this indicated that the causal mechanism involved gastro-intestinal physical factors. The best predictor of voluntary intake of combined season cuts was a multiple regression including digestibility and a measure of forage density. This indicated that a reduced density of packing of material in the rumen may have been the factor musing the reduced intakes on winter pasture.