The effect of monensin supplementation of dried grass or barley diets on aspects of propionate metabolism,insulin secretion and lipogenesis in the sheep

The effects of 33 mg kg ^?1 monensin supplementation (+M) of barley (HC) or dried grass (HF) diets on aspects of propionate metabolism, insulin secretion, rumen vitamin B₁₂ synthesis and lipogenesis in sheep were investigated. Monensin had no significant effect on the estimated metabolisable energy intake, or the liveweight gain, but increased the proportion of propionate in rumen volatile fatty acids (VFA), P<0.01 and P<0.05 for HC and HF diets respectively, and increased rumen ammonia concentration, P<0.01 for both diets. Plasma propionate concentrations were increased (P<0.01) with the HC+M diet. Significant increases in adipose tissue and liver-propionyl-CoA carboxylase (P<0.001), a non-significant trend toward increases in fatty acid synthetase and significant increases in malic enzyme (P<0.01), indicated an induction of the pathways of propionate metabolism and lipogenesis; this was also supported by a trend toward increases, or significant increases, in total propionate derived fatty acids (PDFA), branchedchain fatty acids (BCFA) (P<0.01) and 18:1/18:0 ratio (P<0.02) in adipose tissue triacylglycerols of sheep fed the HC+M diet. Dietary monensin did not influence plasma glucose but reduced plasma insulin concentrations with both diets, P<0.001 and P<0.05 for HC and HF diets respectively, possibly due to a direct effect of monensin on insulin secretion. Monensin supplementation led to significant increases (P<0.02) or a trend toward increases in serum vitamin B₁₂, for HC and HF diets respectively, and to significant increases in liver vitamin B₁₂ (P<0.02) for the HF diet.