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

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 +/- 2.6kg BW). The animals were fitted with permanent ruminal catheters, fed lucerne hay (9.4 MJ/day; 23gN/day; 7g soluble N/day, 6 equal meals/day) and treated with contrasting rates of urea infusion into the rumen: first, a continuous infusion (CT), at 3.2mg urea-N/min for 10 days and then a discontinuous infusion (DT) at 156mg urea-N/min for 4min; in 6 daily doses with the meals for 7days. N balance was calculated from pooled samples of faeces and urine. Jugular blood samples were collected before and 1.5h after the morning meal (M1) on days CT10, DT2, DT4 and DT6. N retention decreased during DT (p=0.01) due to a significant increase of N excretion in urine (4g/day; p=0.009) and faeces (1g/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 CT10 to 235m DT2 and returned to 120m DT6). 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.