Effect of casein and propionate supply on whole body protein metabolism in lactating dairy cows

The effects of Casein (Cas) and propionate (C3) on whole body (WB) leucine (Leu) metabolism were determined in three multiparous Holstein cows, fitted with both duodenum and rumen cannulas, used in a Youden replicated square with 14-d periods. Cows were fed a grass silage-based diet estimated to supply 124.1 MJ d-1 of NEL and 1593 g d-1 of protein digested in the intestine. Cas (743 g d-1 in the duodenum) and C3 (1041 g d-1 in the rumen) infusions were tested in a factorial arrangement. For each period, on day 11, L[1–13C] leucine (4.3 mmol h-1) and on day 13, [13C] sodium bicarbonate (4.2 mmol h-1) were infused into a jugular vein. Blood samples were taken from a carotid artery to measure enrichments of 13CO2 (days 11 and 13) and of 13C[4-methyl 2-oxopentanoate] (MOP, day 11). Both Cas and C3 treatments separately increased milk protein concentration and yield, but only Cas treatments increased milk yield (18%). The overall increments in Leu irreversible loss rate (ILR) (26%), oxidation (146%), protein s...