Utilization of urea and molasses supplements by sheep grazing oat stubble

Abstract

Mature crossbred wethers, grazed for 11 weeks on oat stubble at a stocking rate of 20 sheep ha-1, were unsupplemented or offered supplements of a urea-mineral block, a molasses lick or a urea-molasses lick. Following rain in the fourth week, half of the area was sprayed with desiccant herbicide to establish weed-free stubbles. The stubble initially yielded c. 2600 kg DM ha-1 of dead crop residues plus 900 kg DM ha-1 of fallen grain; after 11 weeks this had fallen to c. 1400 kg DM ha-1 of dead crop residues. Initially the sheep selected diets of 0.03-0.22 grain and 0.78-0.97 dead herbage and, during this time, urea-supplemented sheep selected diets lower in grain and nitrogen (N) content than those selected by unsupplemented sheep. On weedy stubbles, where green herbage yielded more than 100 kg DM ha-1, sheep selected a diet containing c. 0.78 green herbage (N content 2.6%), ate more digestible organic matter (DOM) and gained more weight than those on weed-free stubbles, where the diets consisted of 0.98 dead crop residues (N content 1.1 %). All sheep ate some of the urea-mineral blocks throughout, but 100% acceptance of the licks only occurred by the 6th week. Mean intakes reached desired levels (10 g urea and/or 50 g molasses sheep-1 day-1) thereafter, but individual intakes varied widely on all supplements, with the coefficient of variation generally > 50%. In most periods, supplementation had only small effects on the intake of DOM. During the first 4 weeks, sheep lost weight except those offered the urea-mineral blocks. Over the whole experiment, urea-supplemented sheep lost less weight than unsupplemented sheep, but this was mainly due to higher weight gains on the weedy plots. Mean concentrations of rumen volatile fatty acids (VFA) were < 80 mmol l-1 throughout, and fell to <40 mmol l-1 on the weed-free plots in the latter stages of the experiment. Mean rumen ammonia levels, which were < 100 mg N l-1 in unsupplemented sheep, were generally increased by urea supplements, and by grazing weedy stubbles. Responses to supplementation are discussed in terms of the quality of the available plant material, the diet selected, the levels of rumen metabolites and the intake of supplements.