The influence of higher volatile fattty acids on the intake of urea-supplemented low quality cereal hay by sheep

Abstract

The intake of milled oaten hay (0.7% nitrogen) by sheep was markedly increased by the addition of urea. Where molasses or an equivalent level of sucrose was fed in addition to urea no further significant increase in hay intake occurred, although there was an increase in the concentration of microbial protein (trichloroacetic acid-precipitated nitrogen). The addition of 0.56% of a mixture of isobutyric, n-valeric, and isovaleric acids with urea increased the microbial protein concentration to a similar extent but, in addition, significantly increased the intake of hay. All urea treatments resulted in significant increases in the rate of cotton thread digestion, in the rate of passage of food through the animal, and in the concentrations of total volatile fatty acid (V.F.A.) and ammonia nitrogen in the rumen, and caused an improvement in nitrogen balance from negative to positive values. Since the amount of energy added in the V.F.A. mixture was negligible compared with the molasses or sucrose supplements, it is suggested that the V.F.A. directly stimulated microbial growth in the rumen. The implications of these findings are discussed in relation to the supplementation of low-quality roughage and the definition of 'quality' of ruminant feedstuffs.