Effect of silage dry matter content and rapeseed meal supplementation on dairy cows: 2. Rumen fermentation and digesta passage rate

The effects of direct-cut and wilted silage with or without rapeseed meal (RSM) supplementation on rumen fermentation and digesta passage rates were studied in a 4×4 balanced Latin square experiment with four multiparous cows. The dry matter (DM) and crude protein (CP) degradabilities of silages were determined in sacco. RSM supplementation replaced 16% of the oat–barley concentrate mixture. The daily concentrate amount was 10 kg. Silages were given ad libitum. The effective degradabilities of dry matter and protein (EPD) were higher (p<0.001) in direct-cut silage but wilted silage had higher (p<0.001) content of rapidly degradable protein. Rumen ammonia concentration was higher with direct-cut silage (10.16 versus 8.47 mmol/l, p<0.01), reflecting higher CP content and EPD. Rumen pH and total volatile fatty acid (VFA) concentration in rumen were not affected by silage type, and the differences in molar proportions of rumen VFA’s were small. The proportion of propionate tended to be higher (p<0.10) with wilted silage, whereas the proportion of butyrate was higher (133 versus 128 mmol/mol, p<0.01) with direct-cut silage. The retention time of the digesta particle phase in alimentary tract was shorter (p<0.05 or better) with wilted silage. Pool mean retention time (PMRT), which illustrates the retention time of particles in rumen, was shorter (34.0 versus 28.3 h, p<0.05) with wilted silage. Similarly, retention time of particles in the rumen pool of particles small enough to leave the rumen tended to be shorter (p<0.10) with wilted silage. RSM supplementation tended to increase (p<0.10) rumen ammonia concentration and decreased the molar proportion of rumen butyrate (132 versus 127 mmol/l, p<0.05). The retention time of particles in pool, which illustrates the rate of particle break down in the rumen, was shorter (p<0.10) with RSM. In conclusion, the ruminal retention time of particles was shorter with wilted silage. This may affect silage digestibility negatively.