Responses to postruminal infusions of glucose and casein in lactating goats

Abstract

1. A study was made in goats of the response in terms of milk production, nitrogen utilization, plasma amino acids and glucose kinetics to postruminal infusions of glucose and casein. Goats in mid-lactation, housed in metabolism cages, were fed on 2 kg basal ration (containing 112 g crude protein (N × 6.25)/kg)/d alone (control) and with daily infusions into the abomasum of 45 g casein or 45 g glucose.2. Milk production increased with casein infusion (P < 0.01), but not with glucose infusion, being 1.59, 1.86 and 1.62 kg/d with the control, casein and glucose infusions respectively. Milk composition was unaffected except for milk fat, which was decreased by the glucose infusion.3. The goats were in positive N balance on the basal ration. Milk N output increased with casein infusion, by an amount equivalent to 27% of the infused N.4. The concentration of dispensable amino acids in arterial plasma was decreased with casein infusion and increased with glucose infusion.5. Glucose kinetic measurements with control, casein and glucose infusions indicated that pool sizes were 247, 279 and 302 mg/kg live weight^0.75, and irreversible losses were 5.5, 6.7 and 7.0 mg/kg live weight^0.75 per min respectively. The differences between the results obtained from the glucose and casein infusions were not significant (P < 0.05).6. The results of the experiment indicate that the increased milk production obtained when casein was infused was not due to enhanced gluconeogenesis.