Supplemental protein degradation, bacterial protein synthesis and nitrogen retention in sheep eating sodium hydroxide-treated straw.

1. Alkali (sodium hydroxide)-treated wheat straw was given to six rumen- and abomasal-cannulated sheep to study the rumen degradation of cotton-seed meal (CSM) and barley (B), and the effects of these supplements on nitrogen retention and efficiency of bacterial protein synthesis were measured. 2. N degradation, using porous synthetic (nylon) bags incubated within the rumen (P), and in vivo measurement determined from the abomasal flow of N (V), distinguished quantitatively between the two supplements. Estimates of P, corrected for fractional outflow rates/h (FOR), underestimated estimates of V when FOR of undegraded protein from the rumen (k) of 0.05 and 0.08 were used. Estimates of V for CSM and B were 70.9 and 80.8% respectively. 3. Intakes of alkali-treated straw were not affected by the supplements. Intakes of digestible organic matter (DOM) for the diets comprising alkali-treated straw alone (W), straw plus CSM (WC) and straw plus barley (WB) were 477, 575 and 590 g/d respectively (P less than 0.05) and organic matter (OM) apparently digested in the rumen (OMADR) was 339, 399 and 435 g/d respectively (P less than 0.05). 4. On W, WC and WB respectively, flows at the abomasum were 11.0, 14.0 and 13.3 g/d for bacterial N (P less than 0.05) and 0, 2.8 and 0.5 g/d for dietary supplemental N; g bacterial N/kg OMADR were 32.4, 35.6 and 30.9 (P greater than 0.05) and N balances were 2.37, 4.27 and 3.29 g/d (P less than 0.05) on the respective treatments. It was suggested that supplements increased total OM intake as a result of increased OM digested in the rumen rather than OM flow from the rumen.