Abstract

Ammonia kinetics in the rumen, and the rates of urea synthesis, excretion, and recycling to the rumen and post-ruminal digestive tract were estimated by means of tracer dilution methods in eight 12-monthold Hereford steers, all given free access to a mature, subtropical grass (Axonopus spp.) hay (7.8 g N and 5.8 MJ of metabolizable energy (ME)/kg dry matter). These estimates were made towards the end of a 45-day study, including a 12-day adjustment period, and 33 days in which four steers were supplemented with pelleted protein meal and minerals; the other four were supplemented only with minerals. After 10-20 days of supplementation (days 23-32 of the experiment), the voluntary intake of hay was 19% higher (P < 0.05) in supplemented compared with non-supplemented steers, and from days 33 to 42 was 23% (P < 0.01) higher. Therefore, during the period between days 22 and 42 of the experiment when ammonia and urea kinetics were estimated, total ME and N intakes were higher (30 v. 22 MJ/day and 71 v. 29.5 g N/day), and liveweight gain was also higher (P < 0.01) in supplemented steers (800 v. 200 ¦ s.e.d. 88 g/day). Ammonia and volatile fatty acid concentrations in rumen fluid were higher (P < 0.05) in supplemented steers (55 v. 7 mg N/l and 93 v. 77 mmol/l respectively). The rate of synthesis of urea in the body, and the concentrations of urea in plasma and saliva, predominantly of parotid origin, were also higher (P < 0.01) in supplemented steers. Non-supplemented steers appeared to conserve nitrogen, excreting only 0.41 g urea N/day in urine, which was less than 3% of their daily urea synthesis compared with 9 g N/day or 21% of the daily urea synthesis in supplemented steers. More urea N (P < 0.01) was recycled to the digestive tract in supplemented than in non-supplemented steers, but in either case c. 60% of the total amount recycled was transferred to the rumen. A model summarizing N transactions in the body is presented for steers on the basal hay diet and when supplemented with the pelleted meal.