Efficiency of utilization of volatile fatty acids for maintenance and energy retention by sheep

1. Two experiments were conducted with lambs sustained entirely by intragastric infusion of volatile fatty acids (VFA), protein, minerals and vitamins. 2. In the first experiment to determine the effects of VFA on nitrogen retention four mixtures of VFA (B, C, D and E) were used containing acetic, propionic and butyric acid in the following molar proportions respectively: 45, 45 and 10; 55, 35 and 10; 65, 25 and 10; 75, 15 and 10. The level of infusion was 836 kJ/live weight0.75 per d and the design was a 4 X 4 Latin square with 14 d periods. There were no significant differences in the N balance between the different mixtures of VFA though mixture B tended to give the highest N retention. 3. Thirty-two lambs were used in the second experiment for measurements of heat production in closed-circuit respiration chambers. Six mixtures of VFA were used. These included mixtures B-E from Expt 1 and in addition two mixtures (A and F) containing acetic, propionic and butyric acid in the following molar proportions respectively: 35, 55 and 10; 85, 5 and 10. The heat production was measured both at 450 and 900 kJ/W0.75 per d, except for mixture F, where it was not possible to achieve a rate of infusion in excess of 675 kJ/W0.75 per d. 4. The energy required for maintenance was determined to be 0.45 +/- 0.02 MJ/kg live weight0.75 per d regardless of the mixture used. 5. The efficiency of utilization for fattening (kf) values for the six mixtures were 0.78, 0.64, 0.57, 0.61, 0.61 and 0.59 for mixtures A, B, C, D, E and F respectively. Only mixture A was significantly better utilized than the other mixtures. This mixture also gave the most efficient N utilization. 6. It is concluded from this evidence that differences in kf for diets normally given to ruminants cannot be attributed to differences in utilization of volatile fatty acids.