Effects of supplementation of a grass silage and barley diet with urea, rapeseed meal and heat-moisture-treated rapeseed cake on omasal digesta flow and milk production in lactating dairy cows

Four multiparous dairy cows, fitted with rumen and duodenal cannulas, were fed grass silage and one of four concentrates of barley (B), barley + urea (BU), barley + solvent-extracted rapeseed meal (BRM) or barley + heat-moisture-treated rapeseed cake (BRC). Despite marginal N deficiencies for the control diet (B), BU did not affect milk yield, ruminal fibre digestion or omasal total non-ammonia-N (NAN) or microbial NAN flow. In contrast, BRM and BRC diets significantly increased milk and milk protein yields and omasal dietary NAN flows. Effects of rapeseed meal (RSM) and rapeseed cake (RSC) on milk yield and omasal NAN flow were similar. The diets had no effect on rumen microbial protein synthesis. In general, nonprotein N and soluble true protein comprised 0.20 of omasal total NAN flow, for all diets. Supplementary AAT from rapeseed feeds appeared to escape from the rumen as insoluble feed particles.