Effects of bacterial inoculation of unwilted and wilted grass silages. 1. Rumen microbial activity, silage nutrient degradability and digestibility

Abstract

A set of four silages, comprising unwilted and wilted silages, both with and without bacterial inoculation, was prepared from perennial ryegrass swards on each of eight harvesting occasions during the 1994 growing season. The four silages, within a single harvest, were offered as the total diet at the maintenance level of feeding to 16 wether sheep in an 8-period change-over study with experimental periods of 3 weeks duration, in order to determine whole tract digestibility and urinary excretion of purine derivatives (PD). Silage dry matter (DM) and nitrogen (N) degradabilities were also examined in the rumen of four rumen-fistulated steers offered a medium quality silage and a concentrate supplement at a ratio of 60[ratio ]40 (silage[ratio ]concentrate) on a DM basis. The data presented are the mean results obtained across the eight harvests.

There were no significant effects of inoculation on silage pH and ammonia-N/total-N across the unwilted and wilted materials. Inoculation of either the unwilted or wilted silages had no significant effect on microbial activity in the rumen of sheep, as indicated from excretion of urinary PD, or on silage DM or N degradability assessed with rumen-fistulated steers. However, inoculation significantly increased the digestibility of organic matter (1·7%, P<0·01), N (4·6%, P<0·001), energy (2·3%, P<0·01), neutral detergent fibre (1·5%, P<0·05) and acid detergent fibre (2·8%, P<0·001) in the silage. These increases in digestibility following inoculation, in general derived equally from both the unwilted and wilted silages. The results indicate that the increase in silage nutrient digestibility following inoculation probably reflects a more extensive digestion in the abomasum and intestine, rather than in the rumen.

Wilting of grass prior to ensiling resulted in silages with significantly lower pH (5·4%, P<0·05) and ammonia-N/total-N (42·7%, P<0·001) across the untreated and inoculant-treated materials. Wilting also significantly increased (P<0·05) urinary PD-N output of sheep by 6·9% and silage DM degradability by steers by 1·6–2·4% across the untreated and inoculant-treated silages. However, there were no significant effects of wilting on silage N degradability and silage nutrient digestibility.