Response of beef cattle to infused supplements of urea and of urea-molasses when offered a low quality grass hay

Eight Hereford steers, fistulated at the rumen and offered a low quality grass hay (Axonopus affinus comprised 75 per cent of the pasture) were studied in pens for 100 days to evaluate urea and urea-molasses supplements in terms of their effect on hay intake, rumen fermentation and liveweight changes. The hay contained 0.58 per cent nitrogen and had an apparent digestibility of 49.8 per cent. Supplements were made up in a liquid form and infused directly and continuously into the rumen so that on a daily basis 56 g urea with and without 500 g molasses, or 112 g urea with 500 g or 1000 g molasses was delivered. When urea alone was infused, rumen ammonia and plasma urea concentrations were increased (P < 0.05) but the urea had no significant effect on liveweight change, on digestible organic matter intake (DOMI) or on the pattern of rumen fermentation. When infusions of urea and molasses were given, total DOMI was increased (P < 0.08) and the pattern of rumen fermentation changed: total rumen volatile fatty acid (VFA) concentrations were increased (P < 0.05), with an increased (P < 0.05) molar proportion of propionate in one of the treatments. Neither urea nor urea and molasses infusions significantly increased the animal's intake of hay. Liveweight change was related (r= 0.60) to DOMI and it was estimated that steers in the experiment required 41 g DOM kg.-0.75 day-1 for maintenance of liveweight. This level of intake occurred only when supplements containing molasses were given. Overall, the results indicated that little improvement in growth of cattle or of their utilization of low quality pastures could be obtained from a urea supplement. However, there may be potential for molasses as a concentrate supplement for cattle grazing native pastures.