NITROGEN KINETICS IN THE LARGE INTESTINE OF SHEEP GIVEN BROMEGRASS PELLETS

Abstract

Six sheep fed 606 g of pelleted bromegrass (Bromus inermis) hay per day were used in acute experiments to study nitrogen (N) metabolism. With three sheep (15NH4)2SO4 was infused into the caecum and with another three sheep a mixture of 14C- and 15N-urea was infused into the blood. Samples were obtained before infusions commenced and on tracer plateau before slaughter for determination of blood urea N and rumen ammonia N concentrations, enrichments and specific radioactivities. Digesta were also obtained at slaughter from the rumen, abomasum, ileum and five segments of the large intestine for determination of concentrations and enrichments of ammonia N, microbial N and nonurea nonammonia N (NU-NAN). Urine was obtained from the bladder at slaughter. Flows of N calculated from these data were represented by an eight-pool model. There was negligible transfer of endogenous urea into the caecum and proximal colon either via ileal digesta or by direct transfer across the gut wall. Approximately 9% of caecal ammonia N was derived from blood urea. Approximately 0.61 g endogenous non-urea N/day was secreted into the caecum and proximal colon. Proteolysis and deamination produced 1.21 g ammonia N/day in the caecum and proximal colon. Absorption of ammonia from the caecum and proximal colon was 0.80 g N/day, while at least 0.80 g ammonia N was absorbed from the spiral colon descending colon and rectum. Ammonia N (0.40 g N/day) was incorporated into microbial N in the caecum. Microbial N constituted 49% of the NU-NAN in digesta flowing from the caecum; some 72% of this microbial N was excreted in the faeces. The rate of irreversible loss of the blood urea pool measured with 14C-urea (6.52 ± 0.76 g N/day) was greater (P < 0.05) than that measured with 15N-urea (5.20 ± 0.32 g N/day). Caecal ammonia contributed 10% of the N entering the blood urea pool and 2% of that entering the rumen ammonia pool. Key words: Sheep, large intestine, nitrogen kinetics, models