NET FLUX OF NUTRIENTS ACROSS SPLANCHNIC TISSUES IN WETHERS CONSUMING BERMUDAGRASS OR RYEGRASS-WHEAT HAY SUPPLEMENTED WITH RUMEN UNDEGRADABLE PROTEIN

Crossbred wethers (14, 37 +/- 1.5 kg BW and 10 months old) were used i n a 21-day experiment (2 x 2 factorial) to determine effects of dietar y grass source (tropical vs temperate) on response to rumen undegradab le protein supplementation in oxygen consumption by, and net flux of N fractions across, the portal-drained viscera and liver, Bermudagrass (B; 73.1% NDF and 6.2% CP) or ryegrass-wheat (RW; 65.9% NDF and 8.9% C P) hay was supplemented with 53 g day(-1) (DM) of soybean meal (S) or 53 g day(-1) (DM) of soybean meal plus 70 g day(-1) (DM) of a mixture of feedstuffs high in rumen undegradable protein (SR; 46.6% corn glute n, 26.7% feather and 26.7% blood meals). Digestible energy intake (9.1 , 10.0, 11.3 and 12.1 MJ day(-1)) was greater (P < 0.01) for RW than f or B, and N intake was 12.5, 21.5, 16.5 and 25.3 g day(-1) (SE 1.08) f or B-S, B-SR, RW-S and RW-SR, respectively. Splanchnic energy use as a percentage of DE intake was less (P = 0.03) for RW vs B (21.4 vs 27.8 %), Supplementation with SR increased (P = 0.04) alpha-amino N release by the portal-drained viscera (6.6, 22.0, 9.8 and 20.6 mmol h(-1)) an d hepatic uptake (10.9, 25.6, 11.2 and 14.3 mmol h(-1) for B-S, B-SR, RW-S and RW-SR, respectively; SE 2.29). Supplementation with SR increa sed (P <0.01) hepatic urea N release (26.7, 48.4, 29.0 and 41.8 mmol h (-1)) and ammonia N uptake (17.3, 29.7, 23.3 and 26.6 mmol h(-1) for B -S, B-SR, RW-S and RW-SR, respectively) more (interaction; P = 0.09 an d 0.08, respectively) with B than with RW, In conclusion, these result s indicate that DE intake and splanchnic energy consumption for tropic al and temperate grasses may influence metabolic fate of rumen undegra dable protein N and, thus, impact potential performance benefits of su pplementation. Also, unless forage intake is changed, any improvements in animal performance with rumen undegradable protein supplementation would be through increased N absorption without change in energy avai lable to peripheral tissues, regardless of grass source, (C) 1997 Else vier Science B.V.