Quantitative digestion of fresh herbage by sheep

Summary The utilization by sheep of dietary N provided in high protein, high water content fresh herbages (Ruanui perennial ryegrass, Tama Westerwolds ryegrass, Pitau white clover, and Fakir giant sainfoin at two stages of maturity) was studied at two levels of intake (maintenance and 1·5 maintenance). Feed was provided by constant feeder. Apparent digestibility of N was similar for all herbages ( ca . 85%) except sainfoin which, particularly at a late stage of maturity, was lower (70–80%). A small loss of nitrogen across the stomachs occurred with clover (1–3 g/day) and Tama ryegrass at the higher feeding level (2 g/day), but no loss was observed with the other diets. The apparent digestibility of N and of non-ammonia N (NAN) in the intestines was lower for sainfoin, and estimated true digestibility was also lower. Amino acid N contributed less to the NAN reaching the duodenum on the sainfoin diets than on the grass and clover diets. N retention was negative at the lower level of feeding for ryegrass and clover diets. It was greatest for the sainfoin diets at similar N intakes, so that efficiency of retention of apparently digested N was also greatest for sainfoin. The size of the urea pool, the plasma urea concentration and the urea irreversible loss, using 14 Curea, did not differ significantly between diets at similar N intake. Urea irreversible loss exceeded urinary urea excretion by 35—50% on all but the late-maturity sainfoin diet, where urea irreversible loss was more than double the urinary urea output. These data indicate dietary differences in the extent of degradation of urea on recycling to the gastro-intestinal tract. Urea clearance across the kidney was also lowest for sainfoin. Data are compared in a simple model which illustrates the importance of variable clearance of urea across the kidney and the gut wall and the need for knowledge of factors which control this.