Abstract:
Over 3 dry seasons (1984-86), delivery systems of urea supplements were evaluated on weaner heifers grazing native pasture. Daily treatments per heifer were urea-ammonium sulfate (US) (28g + 6g), via the drinking water; urea-molasses (UM) (30g + 250g), via roller drum lick feeders; UM2 (60g + 1000g), via open troughs. Rainfall recordings were well above average in all dry seasons. The only liveweight response to supplementation occurred in 1986, during a dry period from June to October. During this period the liveweight performance of the US group was improved relative to the control group (+7 v. -57 g/heifer.day; P<0.05). The response of the UM2 group (+82 g/heifer.day) exceeded (P<0.05) that of all other groups. There was no response to the UM supplement (-51 g/heifer.day). Over the combined dry and following wet seasons the UM2 group grew faster (P<0.05) than all other groups. Ammonia-nitrogen concentrations in the rumen fluid of heifers, measured in the 1984 and 1986 dry seasons, were very low for the control group (7-54 mg/L rumen fluid) but were much higher in the US group 2 h after drinking (108-152 mg/L). At 18 h after drinking, the ammonia-nitrogen concentration had declined markedly in the US group (30-54 mg/L) but, in 2 of 3 observations, was still higher than that of the controls (significantly in 1 case; P<0.05).