The rate and pattern of urea infusion into the rumen of wethers alters nitrogen balance and plasma ammonia

Summary Changes in N balance, urinary excretion of purine derivative ( PD ), urea, creatinine and ammonia and plasma ammonia, glucose, urea, insulin and IGF ‐1 were examined in four wethers (37 ± 2.6 kg BW ). The animals were fitted with permanent ruminal catheters, fed lucerne hay (9.4 MJ /day; 23 g N/day; 7 g soluble N/day, 6 equal meals/day) and treated with contrasting rates of urea infusion into the rumen: first, a continuous infusion ( CT ), at 3.2 mg urea‐N/min for 10 days and then a discontinuous infusion ( DT ) at 156 mg urea‐N/min for 4 min; in 6 daily doses with the meals for 7 days. N balance was calculated from pooled samples of faeces and urine. Jugular blood samples were collected before and 1.5 h after the morning meal (M1) on days CT 10, DT 2, DT 4 and DT 6. N retention decreased during DT (p = 0.01) due to a significant increase of N excretion in urine (4 g/day; p = 0.009) and faeces (1 g/day; p = 0.02). Dry matter (p < 0.001) and N digestibility in vivo (p = 0.01) decreased significantly during DT . Urinary urea and PD excretion were not altered by treatment. Significant linear (p = 0.004) and quadratic (p = 0.001) effects were observed for plasma ammonia in M1 (from 170 CT 10 to 235  μ m DT 2 and returned to 120  μ m DT 6). No changes were observed in plasma glucose, urea, insulin and IGF ‐1. Results indicate that changes from CT to DT reduced N retention in sheep due to enhanced urinary N excretion, but it was not associated with changes in urinary urea or PD excretion; or plasma concentrations of insulin and IGF ‐1. As the dry matter ( DM ) an N digestibility could account a 0.23 of the decrease in N retention; the largest fraction of the reduction in N retention remained unexplained by the results.