The effect of supplementary protein on in vivo metabolism of the mammary gland in lactating dairy cows.

Four lactating cows equipped with rumen and duodenal cannulas were fed a diet of grass silage and concentrates containing either 12.4 or 17.2% CP (DM basis) in a change-over design. Additional protein was supplied as white fish meal. Fish meal did not affect molar proportions of VFA in the rumen, but duodenal NAN supply was increased .69 g/g of N in supplementary feed. In Experiment 2, three lactating dairy cows that had been prepared with catheters across the mammary gland were fed the same diets using a switchback design. Blood samples were taken to determine changes in metabolite flux to the mammary gland. In both experiments, milk production and protein yield were nonsignificantly increased by addition of fish meal. Milk urea output was increased from 3.18 to 4.74 g/d by fish meal supplementation, reflecting increased arterial concentrations of urea. Concentrations of glucose, VFA, and BHBA in blood showed no substantial changes because of dietary supplementation of fish meal. Supply of essential AA increased 26% with fish meal supplementation, mammary uptake increased 34%, but milk protein output only increased 5%. The low efficiency of conversion of supplementary protein to milk protein appears to be related to the inability of the gland to utilize the additional AA.