Effects of energy source on methionine utilization by growing steers.

We evaluated the effects of different supplemental energy sources on Met use in growing steers. Ruminally cannulated Holstein steers were used in two 6 x 6 Latin squares, and data were pooled for analyses. In Exp. 1, steers (148 kg) were fed 2.3 kg of DM/d of a diet based on soybean hulls. Treatments (2 x 3 factorial) were abomasal infusion of 0 or 3 g of l-Met/d, and supplementation with no energy or with glucose (360 g/d) or fat (150 g/d) continuously infused into the abomasum. In Exp. 2, steers (190 kg) received 2.6 kg of dietary DM/d and were provided (2 x 3 factorial) with 0 or 3 g of l-Met/d, and with no supplemental energy or with acetate (385 g/d) or propionate (270 g/ d) continuously infused into the rumen. In both experiments, the energy sources supplied 1.3 Mcal of GE/d, and all steers received basal infusions of 400 g of acetate/d into the rumen and a mixture (125 g/d) of all essential AA except Met into the abomasum. Nitrogen balance (18.8 vs. 23.5 g/d; P < 0.01) and whole-body protein synthesis (2.1 vs. 2.3 kg/d; P < 0.07) were increased by Met supplementation, indicating that protein deposition was limited by Met. Supplemental energy reduced (P < 0.01) urinary N excretion and increased (P < 0.01) N retention without differences among energy sources. Increases in N retention in response to Met were numerically greater when energy was supplemented. Efficiency of supplemental Met use was 11% when no energy was supplemented but averaged 21% when 1.3 Mcal of GE/d was provided. Whole-body protein synthesis and degradation were not affected by energy supplementation. Serum insulin concentrations were increased by glucose and propionate supplementation. Serum IGF-I concentrations were increased by supplementation with Met or glucogenic sources of energy. In growing steers, N retention was increased by energy supplementation even though protein deposition was limited by Met, suggesting that energy supplementation improves the efficiency of AA use. These responses were independent of the source of energy.