DIGESTIBILITY, NITROGEN-RETENTION AND PLASMA METABOLITE CONCENTRATIONS IN STEERS OFFERED WHOLE-CROP WHEAT SILAGE-BASED RATIONS

The effects of urea-treatment (56 kg urea / t dry matter (DM)) at ensi ling, and substitution with barley (350 g / kg DM) at feeding, on the nutritive value of whole-crop wheat silage were examined in Friesian s teers. Twelve steers (body weight (BW) = 372 s.d. 26.5 kg) were used i n a 2 (silages) X 2 (levels of barley) factorial experiment with two p eriods each of 34 days duration. The target dietary allowance was 15 g DM/kg BW offered once daily as a complete feed. Untreated (W) and ure a-treated (UW) wheat silage had concentrations (s.d.) of DM (g / kg), crude protein (g / kg DM) and ammonia (g / kg nitrogen) of 394 (28.3) and 390 (29.6), 125 (3.7) and 264 (35.8) and 121 (24.7) and 410 (139.4 ), respectively. Addition of urea at ensiling increased the digestibil ity of DM, organic matter (OM), neutral and acid detergent fibre and i ncreased the digestible OM (DOMD) and estimated metabolisable energy ( ME) concentrations. Substitution with barley increased the digestibili ty of DM and OM, DOMD and estimated ME concentrations but decreased fi bre digestibility. Urea addition at ensiling increased nitrogen retent ion in absolute terms (but decreased nitrogen retention as a proportio n of absorbed nitrogen) and plasma concentrations of urea and non-este rified fatty acids. Nitrogen retention was increased on W diets but de creased on UW diets by substitution with barley. Allantoin excretion d id not differ between treatments. It is concluded that urea-treatment increased whole-crop wheat silage digestibility but that the addition of a readily fermentable carbohydrate to urea-treated wheat silage did not increase microbial protein synthesis in the rumen or whole body n itrogen retention.