UREA AS A SOURCE OF AMMONIA FOR IMPROVING THE NUTRITIVE-VALUE OF WHEAT STRAW

The effect of straw ammoniation using urea as a source of ammonia on nutritive value of wheat straw was evaluated with male sheep in two experiments of Latin square design. In Experiment 1 treatments were: untreated straw (S1), untreated straw supplemented with 20 g urea kg-1 air-dry straw at feeding time (SU1), straw treated with 30 g anhydrous ammonia kg-1 air-dry straw (AS), straw ensiled with 40 g urea kg-1 air-dry straw (US1) and straw ensiled with 40 g urea plus 10 g sodium hydroxide kg-1 air-dry straw (UCS). In Experiments 2 treatment were: untreated straw (S2), untreated straw supplemented with 20 g urea kg-1 air-dry straw at feeding time (SU2), untreated straw supplemented with 120 g kg-1 air-dry straw of a urea-molasses mixture at feeding time (SUM), straw ensiled with 40 g urea kg-1 air-dry straw (US2) and straw stacked with 40 g urea kg-1 air-dry straw (USS). Straw was chopped before ensiling, while whole rectangular bales were used in treatments AS and USS of Experiments 1 and 2, respectively. Ammonia and urea treatments were carried out at ambient temperature and lasted 60 days. For all the treatments involving the hydrolysis of urea to ammonia (US1, UCS, US2 and USS) the moisture content of the straw was raised to ca. 40% by the addition of water. Straw in the long form was chopped prior to feeding. In all treatments straw was given as the sole feed, with a mineral supplement. In both experiments supplementation with urea or urea-molasses at feeding time significantly increased (P < 0.05) organic matter (OM) digestibility and digestible organic matter (DOM) intake over untreated straw by 47-72 g kg-1 and 4.9-6.2 g day-1 kg W-0.75, respectively. Treating straw by ensiling with urea raised OM digestibility and DOM intake further, the increments being 121-123 g kg-1 and 14.3-15.0 g day-1 kg W-0.75, respectively, over untreated straw. Digestibility of cell wall constituents was particularly enhanced by urea treatment. Urea treatment of stacked straw was less effective than urea treatment of ensiled straw. Anhydrous ammonia and urea treatment by ensiling promoted similar levels of DOM intake. Availability of retained nitrogen by ammoniated straws was high. It is concluded that urea can be an effective source of ammonia for straw treatment by ensiling.