GROWTH AND CARCASS COMPOSITION IN SHEEP OFFERED ISOENERGETIC RATIONS WHICH RESULTED IN DIFFERENT CONCENTRATIONS OF RUMINAL METABOLITES

The objective of this study was to determine if isoenergetic rations, designed to result in different ruminal fermentation patterns, altered growth and carcass composition in wethers. Two basal rations, based e ither on starch (S) or a digestible fibre/sugar mixture (F) were formu lated. Sodium propionate was included at 0 (C) or 40 g/kg (P) to give four isoenergetic (12 MJ metabolisable energy/kg dry matter) and isoni trogenous (165 g crude protein/kg dry matter) rations. When fed at 22 g/kg bodyweight (BW) + 75 g hay daily to four ruminally fistulated she ep in a 4 (ration) by 4 (period) Latin square experiment, there was no difference between diets in mean volatile fatty acid concentrations i n ruminal fluid (92, 90, 86 and 105 (S.E. 7.5) mmol/l for SC, SP, FC a nd FP, respectively). The acetate to propionate ratio was lower (P < 0 .001) for S-based diets and decreased (P < 0.01) by the inclusion of P (1.28, 0.93, 1.98 and 1.70 (S.E. 0.035), for SC, SP, FC and FP, respe ctively). When fed to wethers (BW of 46.2 kg; n = 6/treatment) for 64 days before slaughter, the digestible organic matter concentration (g/ kg dry matter) measured in vivo was lower (P <0.01) for S-based diets but unaffected by the inclusion of P (803, 812, 831 and 841 (S.E. 7.0) for SC, SP, FC and FP, respectively). Bodyweight gain and carcass wei ght were similar for all treatments (123, 123, 136 and 120 (S.E. 26.0) g/day and 26.2, 26.9, 27.7 and 26.6 (S.E. 0.87) kg for SC, SP, FC and FP, respectively). Inclusion of P decreased (P < 0.05) the proportion of dissectible fat in the carcass (267, 233, 250 and 223 (S.E. 10.3) g/kg for SC, SP, FC and FP, respectively). Inclusion of P increased (P < 0.05) estimated skeletal muscle growth when included in the S-based diet only (38, 64, 63 and 62 (S.E. 8.0) g/day for SC, SP, FC and FP, respectively). It is concluded that a decrease in the acetate to propi onate ratio in ruminal fluid, by addition of sodium propionate, result ed in a decrease in fat deposition in both S- and F-based rations and increased skeletal muscle growth in the S-based ration. (C) 1998 Elsev ier Science B.V. All rights reserved.