The utilisation of salts of volatile fatty acids by growing lambs

• Main Author: Hovell, Francis Dickon de Berdt
• Published: University of Aberdeen 1972
• Subjects: 636.089
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.592591

The literature relating to the production, absorption and utilisation of volatile fatty acids by growing and fattening ruminants was reviewed. It was concluded that the hypothesis which moat satisfactorily explained the poor utilisation of high fibre diets by fattening ruminants was that of Armstrong, Blaxter, Wainman and Graham (1958) which attributed the poor utilisation of such diets to an inefficient utilisation of acetate. However, subsequent work on the utilisation of the VP A, and of acetate in particular did not confirm this hypothesis, and it was suggested that apparently conflicting results reported in the literature might be reconciled if the efficiency with which acetate was utilised declined as its contribution to the energy available for lipogenesis increased. 2) In the work reported here the utilisation of acetate by growing lambs was investigated in two cooperative slaughter experiments and one calorimetry experiment. The utilization of propionate and butyrate was also investigated in the second of the comparative slaughter experiments. In all experiments the VFA were given as a mixture of sodium and calcium salts which were substituted for the ME provided by the carbohydrate of a barley based control diet. The ME value of the salts was assumed to be equal to the heat of combustion of their acid equivalents. The rumen pH, VFA and cation levels produced by some of the diets used were measured in too further experiments. 3) It was concluded that differences in the efficiency with which the diets were utilised for energy retention was mainly due to the utilisation of the VPA per sc. And was not due to differences in the metabolisability of the basal constituents of the diets, or to a cation effect. 4) Differences between diets in the efficiency with which they were utilised were caused by differences in fat deposition. There were no differences between VPA in their ability to promote protein deposition. 5) In the second comparative slaughter experiment, acetate provided 0, 7, 15 or 22% of ME and was utilised with a partial efficiency for the three acetate diets respectively. The difference between the Acetate-7 % diet and the Acetate-22 % diet was significant. When the overall efficiency with which the complete diet was utilised for energy retention (kf) was plotted against the proportion of ME provided by the acetate e. its there was a significant quadratic relationship. In the first comparative slaughter experiment (Experiment one), acetate provided 0, 14 or 19% of ME and was utilised with a partial efficiency of 2 or 10--13% for the two acetate diets respectively. The overall efficiency with which the two acetate diets were utilised to promote energy retention was significantly lower than that of the Control. In the calorimetry experiment (Experiment Five) acetate provided 4 or 16% of ME, end the partial efficiency with which the additional acetate of the Acetate-16% diet was utilised was 72%. The overall efficiency of the complete diets was practically identical. 6) The second comparative slaughter experiment (Experiment Three) diets which contained 22% of MS as propionate or butyrate were utilised less efficiently than the Control, but not significantly differently from the Acetate-22% diet described above. The partial efficiency with which propionate and butyrate were utilised was 35 and 17--15% respectively. 7) It was concluded that the efficiency with which acetate was utilised for energy retention was not constant but decreased as its contribution to the MS available for lipogenesis was increased, which is consistant with the concept that efficient utilisation of acetate for lipogenesis is dependent upon a sufficient supply of glucose or glucone precursor. The poor utilisation of butyrete sea consistent with this model if butyrate is utilised for lipo-genesis as a source of two-carbon units. The poor utilisation of propionate may have been due to the ability of the animals to synthetise or utilise glucoses being limited, and the fact that when acetate was substituted for 7% of barley ME, the diet was utilised alightly more efficiently than the control may imply that there is an optimum relationship for the efficient utilisation of both these metabolites. 8) The depot fat of lambs given propionate contained enhanded proportions of odd-chain and branched-chain fatty acids 9) The rumen VFA of cheap given diets containing VFA salts showed considerable diurnal variation which reflected the composition of the diet given. The rumen pH of sheep given diets containing VFA salts fluctuated less than when given the barley based control. The concentration of calcium in the digests of sheep given diets containing VFA salts was significantly increased whereas the concentration of sodium and potassium was similar to that from sheep when given the control diet.