Nitrogen kinetics in cattle fed a mature subtropical grass hay with and without protein meal supplementation

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.