Rumen and total diet digestibilities in lactating cows fed diets containing full-fat rapeseed

Effects of full-fat crushed rapeseed (0, 1, or 2 kg/d) on rumen and total digestion, rumen biohydrogenation, and rumen microbial protein synthesis were studied in lactating cows. Rumen digestibilities (%) of DM, NDF, and cellulose were 52.1, 46.1, and 51.8, respectively, for control. Rapeseed decreased rumen and total DM digestibilities and proportion of DM digested in the rumen. Rumen digestibility of cellulose was decreased by rapeseed, but this was apparently compensated by hindgut fermentation. Dry matter, NDF, and hemicellulose digestibilities were compensated at 1 kg but not at 2 kg/d. Biohydrogenation of 18:1 fatty acids increased with increasing dietary fat, whereas that of 18:2 and 18:3 was 85% on all diets. Fatty acid digestibility was not different among diets. Microbial nitrogen in the duodenum increased from 142 g/d for control to 191 g/d for 1 and 2 kg/d. Efficiency of microbial protein synthesis (grams of microbial nitrogen per kilogram organic matter apparently digested in the rumen) was 17.3, 24.8, and 26.6 for 0, 1, and 2 kg/d. Slow release of fat from crushed rapeseed minimized negative effects on rumen metabolism; 1 to 2 kg/d of full-fat crushed rapeseed may be fed to lactating cows without detrimental metabolic effects.