EFFECT OF LEVEL OF INPUT OF DIFFERENT PROPORTIONS OF VOLATILE FATTY-ACIDS ON ENERGY-UTILIZATION IN GROWING RUMINANTS

Four steers were maintained wholly by intragastric infusion of volatil e fatty acids (VFA) and protein, together with a mineral-vitamin suppl ement. The infusion was given at three levels of energy, namely 450, 6 75 and 900 kJ/kg live weight(0.75), calculated to supply energy at 1.0 , 1.5 or 2.0 times that required for maintenance. The VFA provided 0.8 37 and the protein 0.163 of the energy infused. The molar proportions of individual VFA were varied so that the infusate contained 0.36-0.91 of acetic acid, 0.56-0.01 of propionic acid and a constant 0.08 of bu tyric acid. Heat production was measured in respiration chambers. Urin e was analysed for N, urea, beta-hydroxybutyrate and VFA. Blood plasma was analysed for beta-hydroxybutyrate, free fatty acids, insulin and glucose. As the proportion of acetic acid was increased, and propionic acid reduced, there was no change in blood or urine metabolites or in heat production until acetic acid exceeded a proportion of about 0.75 . At higher proportions beta-hydroxybutyrate increased in plasma and u rine, blood glucose and insulin tended to fall and urinary N excretion rose. At a proportion of acetic acid of > 0.80, acetate appeared in t he urine and at > 0.86 heat production declined. The effect of level o f infusion on the molar proportion at which plasma and urine metabolit es changed was less clear. There was a tendency for the increase in be ta-hydroxybutyrate to occur at a slightly lower proportion of acetic a cid at the highest level of infusion. It is concluded that differences in heat production that are observed between diets are probably not c aused by differences in rumen VFA proportions. The reaction to a highl y elevated proportion of acetic acid is to excrete and beta-hydroxybut yrate and acetate in the urine and so decrease rather than increase he at production. Regardless of level of infusion a metabolic crisis occu rred when the proportion of acetic acid was above the levels found in the rumen content of normally-fed animals.