FEED-INTAKE AND LIVEWEIGHT OF CATTLE ON SUBTROPICAL NATIVE PASTURE HAYS .1. THE EFFECT OF UREA

Twenty Hereford weaner steers and heifers, 9-months-old and of mean liveweight (+/- s.e.) 152 +/- 6 kg, were stratified according to sex and liveweight into four groups and assigned to cells in an experimental design which aimed at studying the effects of an increased urea intake on their rumen ammonia concentration, their intake of a low N native pasture hay and on their liveweight change over 49 days. The cattle on the basal hay diet maintained low concentrations of ammonia in rumen fluid (8 mg N/L) during the study and lost weight (at 50 g/day). Urea (as 15, 23, 42 and 53 g head.day) increased significantly (P < 0.01) rumen ammonia concentration (to 152 +/- 10.9 mg N/L), hay intake in the final 7 days (from 2633 to 3738 +/- 72 g organic matter/head.day) and liveweight change (to 273 +/- 33 g/head.day). Hay intake of the cattle increased within 14 days of supplementation with urea which was sprayed onto portions of the hay. Urinary N content, but not faeces, was increased by the urea diets. Urea intakes also increased urea nitrogen concentrations in plasma and saliva which were correlated (r = 0.87). Liveweight change was maximized at daily urea intakes of 23 g/head and these gains are discussed in terms of the suitability of urea for Hereford cattle which are fed only low N native pastures.