The contribution of recycled urea to the synthesis of microbial protein in the rumen of dairy cows fed high grain and forage diets using nitrogen-15-labelled urea.

Abstract

Lactating Holstein cows were used to evaluate the contribution of recycled urea nitrogen to bacterial protein exiting the rumen, and appearing in milk and feces during continuous jugular infusion of ¹⁵N-labelled urea in long-term infusion (72 h) and short term infusion (3, 12, 20, and 24 h) studies. Two cows in the first trial, and four cows in the second trial fitted with T-type duodenal cannulae were fed either a high grain or high forage diet. Urea labeled with ¹⁵N was infused continuously into the jugular vein for 72 h in trial 1 and for 3, 12, 20, and 24 h in trial 2. Cr₂O₃ was used as a digestibility marker. In trial 1, duodenal digesta, feces and urine were sampled every 4 h for 5 d (during 72 h of infusion and 48 h thereafter). Milk and blood were sampled every 12 h for 5 d, and total urine was collected for 5 d. In trial 2, duodenal digesta and urine were sampled every 6 h for 3 d. Feces, milk and blood were sampled every 12 h for 3 d, and total urine was collected for 3 d. Percent of total N in digesta and bacteria entering the duodenum which originated from recycled N was greater in cows fed high grain than high forage, 20 vs 10%, P < .23 and 28 vs 15%, P < .18 respectively, for trial 1. In both trials, ¹⁵N enrichment of digesta and bacteria entering the duodenum was higher for high grain from shortly after the beginning until the end of infusion periods of more than 20 h. Enrichment patterns of urine (and blood) were inverse of those in digesta and bacteria, reflecting a greater recycling of urea on the high grain diet. Flow of N from the rumen to the small intestine was higher on high grain than on high forage in both trials, (128 vs 107% of N intake for trial 1, and 123 vs 98% of N intake for trial 2), which was consistent with the larger amount of recycled N on high grain. Urinary excretion of ¹⁵N accounted for about 84% of that excreted in trial 1, and 76% in trial 2; whereas, feces and milk accounted for a total of 16% in trial 1, and 24% in trial 2. Percent of ¹⁵N infused which was retained in the body 2 d after infusion ceased was 23% in tiral 1, and 17% at 72 h in trial 2. It was estimated by the ¹⁵N ratio techniques that 64 to 74% of the total N passing into the duodenum was bacterial N in trial 1. In conclusion, these data demonstrate that about twice as much endogenous N is recycled and incorporated into bacterial N on a high grain than a high forage diet fed to lactating cows. The higher N passage into the duodenum than N intake on a high grain diet might be explained by more N being recycled into the rumen.