Efficiency of D- vs L-methionine utilization by growing steers

Five ruminally cannulated Holstein steers (180 kg) were used in a 5 x 5 Latin square design experiment to evaluate the efficiency of D- vs L-methionine (Met) utilization in growing steers. Treatments were continuous abomasal infusions of water or water with 2 or 4 g.steer-1.d-1 L-Met or 2 or 4 g.steer-1.d-1 D-Met. All steers were fed 2.7 kg/d (DM basis) of a soyhull-based diet to minimize basal Met flow to the small intestine to create a paradigm by which responses to Met supplementation could be measured easily. To ensure that amino acids other than Met did not limit animal performance, an essential amino acid mixture simulating the nonsulfur amino acid profile of ruminal bacteria was infused abomasally. Energy supply to the steers was increased by continuously infusing acetate, propionate, and butyrate into the rumen at rates of 150, 150, and 37.5 g.steer-1.d-1, respectively. Abomasal infusions of dextrose (300 g/d) also provided an additional energy source. Nitrogen retention increased linearly (P < .05) in response to both L-Met and D-Met infusions, with responses being similar between the two Met isomers. Regression of N retention against Met supplementation yielded: Nitrogen retention (g/d) = 19.2 + 1.81 D-Met (g/d) + 1.90 L-Met (g/d). By slope ratio analysis, the efficiency of D-Met utilization relative to L-Met was 95.5%. In a concurrent study using intestinally cannulated steers, both D- and L-Met were absorbed essentially completely from the small intestine. These results show that D-Met can be utilized efficiently as a Met source by cattle.