Fresh and wilted silages (dry matter 169 and 359 g/kg; pH 4·00 and 5·09; lactic acid 165 and 34 g/kg D.M., respectively) were made from ryegrass and used in intake and metabolism experiments with cattle and sheep. Complete ruminant diet (CRD) was used as a control.
Voluntary dry-matter intake of the silages by wether sheep was not significantly different (70·7 and 69·1 g/kg W0·75 for fresh and wilted material, respectively) but was significantly higher (P < 0·001) for CRD (97·9 g/kg W0·76). With rumen-fistulated steers the voluntary intakes of dry matter for the fresh and wilted silages and CRD were 117·1, 105·7 and 130·4 g/kg W0·76, respectively, and were not significantly different. The digestibility of dry matter was significantly higher (P < 0·01) for the fresh and wilted silages (0·751 and 0·698, respectively) than for the CRD (0·608). When incubated in nylon bags in the rumen the silages rapidly lost nitrogen and only 0·2 and 0·3 of nitrogen of the fresh and wilted materials, respectively, remained after 1 h. Between 0·4 and 0·5 of the residual nitrogen in the silages was subsequently lost over a period of 24 h. The loss of the nitrogen of the CRD was much slower and 0·3 of the nitrogen remained after 24 h. The rate of loss of digestible cell wall constituents was similar for the fresh and wilted silages but was slower for the CRD. The silages gave similar rumen fermentation patterns (pH, total volatile fatty acids, acetic, propionic, butyric, valeric and hexanoic acids and ammonia concentrations) during the period following their consumption.
The palatability of the fresh and wilted silages, measured with rumen-fistulated sheep, was less than that of the CRD and a ruminal effect on intake was observed in the case of the silages.
The high intake of the fresh silage is noteworthy, as it casts grave doubt on the widely held view that fermentation acids, particularly lactate, are pre-eminent in governing the voluntary intakes of silages.
From the rapid loss of nitrogen from the silages in nylon bags in the rumen it is concluded that, to increase the utilization of nitrogen, it would be advantageous to reduce protein breakdown in the silo and to supplement silage with a readily available energy source.