EFFECT OF ALFALFA GRASS-SILAGE DRY-MATTER CONTENT ON RUMINAL DIGESTION AND MILK-PRODUCTION IN LACTATING DAIRY-COWS

In trial one, seven lactating Holstein cows fitted with rumen cannulae were used in a replicated 3 x 3 Latin square design to investigate the effect of feeding alfalfa/grass ensiled at three dry matter (DM) levels (27.5, 46.1 and 56.4%) on the rate and extent of protein degradation. Cows were fed a total mixed ration of 70% alfalfa/grass silage, at one of three DM levels, and 30% concentrate (high moisture corn and mineral) (DM basis). Silage samples were incubated in sacco in the rumen for up to 48 h to model disappearance of DM, neutral detergent fibre (NDF) and nitrogen (N). Passage rates of silage, using Cr-mordanted NDF, were determined for small and total particulate fractions. Rates of DM, NDF and N disappearance, as well as rates of passage were unaffected by silage DM content. The 27% DM silage contained a larger soluble N fraction, but a smaller fraction of potentially digestible N than either the 46 or 56% DM silages. Estimates of effective protein degradability corrected for microbial contamination of digesta residues, for the 27, 46 and 56% DM silages were 75.8, 71.2 and 65.0%, respectively. In a second trial, 30 Holstein cows in mid-lactation were fed the same silages as in trial 1 using a completely randomized design. Milk protein percent and yield were higher (P < 0.05) for cows fed the 56% DM silage than those fed the 27 or 46% DM silages. Wilting alfalfa/grass silages prior to ensiling decreased estimated net ruminal silage protein degradation, which was associated with improved milk protein yield.