Effect of sodium metabisulphite on proteolysis and rumen degradability of alfalfa silage protein

First cut alfalfa without (16% dry matter (DM)) or after 24 h wilting (28% DM) was ensiled with formic acid (FA) (0.4%) or sodium metabisulphite (SM) (0.5%). The pH tended to be lower for the FA and SM silages compared with the control. At both DMs the control silages had higher ammonia content compared with the experimental silages. At 16% DM the SM treatment resulted in reduction of the ammonia concentration in an extent similar to the FA. The SM silages had higher buffer soluble nitrogen, α-amino nitrogen and lower insoluble trichloroacetic acid nitrogen content than the control and FA silages. The extent of in vitro rumen degradability of the silage protein did not differ significantly between the treatments. A trend for higher degradability of the SM treated silages was observed. In sacco the protein of the SM silages after 48 h rumen incubation was more degradable compared with the control (84.42% and 78.12% at P = 0.005 for 16% DM and 81.82% and 78.72% at P = 0.004 for 28% DM, respectively). The degradability of the FA silage protein was slightly higher compared with the control but the differences were not significant. The effect of the wilting on the silage protein degradability, observed by 24 h tended to disappear by 48 h of rumen incubation. It is concluded that the SM increased alfalfa protein degradability in the rumen.