The effects of intraruminal infusions of urea, casein, glucose syrup and a mixture of casein and glucose syrup on nitrogen digestion in the rumen of cattle receiving grass-silage diets

In an incomplete 5 X 5 Latin square experiment, four cattle were given grass silage in two meals per d to satisfy 1.15 maintenance energy requirements. In addition, water or casein (21 g nitrogen and 0.17 kg organic matter (OM)/d) or urea (U; 28 g N/d) or a glucose syrup (G; 0.87 kg OM/d) or casein and glucose syrup (CG; 17 g N and 0.93 kg OM/d) were infused intraruminally at a constant rate. A 24 h collection of duodenal digesta was made using chromic oxide for flow estimation and 35S as a marker of microbial N entering the small intestine. Samples of rumen fluid were also taken for estimation of rumen pH, and concentrations of ammonia-N and volatile fatty acids. The intraruminal infusions had no significant effects on rumen pH, concentrations of volatile fatty acids or their molar proportions. Infusion of either C or U significantly (P less than 0.05) increased rumen NH3-N concentrations whereas infusions of either G or CG lowered rumen NH3-N concentrations. Infusions of C or U had no significant effect on the quantities of OM, acid-detergent fibre (ADF) or N constituents which entered the small intestine. Infusions of G or CG increased the quantities of OM (G P less than 0.05, CG P less than 0.01), ADF (CG P less than 0.05), non-NH3-N (G P less than 0.05, CG P less than 0.01), amino acid N (G P less than 0.05, CG P less than 0.01) and microbial N (G P less than 0.05, CG P less than 0.01) which entered the small intestine. The efficiency of rumen microbial N synthesis was unchanged by the infusion of C, U or G (P greater than 0.05) but increased significantly (P less than 0.05) when CG were infused.