The effect of protein supplementation on digestion and glucose metabolism in young cattle fed on silage.

1. Ryegrass (Lolium multiflorum ev. RvP) silage was given to twelve Friesian steer calves (initial live weight (LW) 106 kg) at 2.3 kg dry matter (DM)/calf per d, alone, or with a supplement of fish meal at 50 (FM1) or 100 (FM2) g/kg silage DM per d. 2. The rate of flow of nutrients into the duodenum was measured using a dual-marker technique. Glucose irreversible-loss rates and the contribution of propionate to gluconeogenesis were measured using continuous infusions of [U-14C]glucose and [2-14C]propionate. 3. Amino acid flow into the duodenum was significantly (P less than 0.01) increased from 238 g/d on the control treatment to 319 g/d on FM2, but there was no response to FM1 (P greater than 0.05). 4. Glucose irreversible loss rate remained constant with an average value of 1.86 mg/min per kg LW. 5. Neither the percentage of glucose synthesized from propionate (average 30) nor the percentage of propionate converted to glucose (average 45) was significantly altered by treatment (P greater than 0.05).