Effects of energy supply on leucine utilization by growing steers at two body weights

¹ Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600
² Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600
³ Department of Clinical Sciences, Kansas State University, Manhattan 66506-1600

^* To whom correspondence should be addressed. E-mail: etitgeme{at}ksu.edu.

	Abstract

The effects of energy supplementation on Leu utilization in growing steers were evaluated in 2 experiments using 6 ruminally cannulated Holstein steers. In Exp. 1, steers (initial BW = 150 ± 7 kg) were limit-fed (2.3 kg of DM/d) a diet based on soybean hulls, and received a basal ruminal infusion of 100 g of acetate/d, 75 g of propionate/d, and 75 g of butyrate/d, as well as abomasal infusions of 200 g of glucose/d and a mixture (215 g/d) containing all essential AA except Leu. Treatments were arranged as a 3 x 2 factorial, with 3 amounts of Leu infused abomasally (0, 4, and 8 g/d) and supplementation with 2 amounts of energy (0 and 1.9 Mcal/d of GE). Supplemental energy was supplied by ruminal infusion of 100 g of acetate/d, 75 g of propionate/d, and 75 g of butyrate/d, as well as abomasal infusion of 200 g of glucose/d to provide energy to the animal without affecting microbial protein supply. When no supplemental energy was provided, Leu supplementation increased N balance without a difference between 4 and 8 g/d of Leu (24.5, 27.0, and 27.3 g/d for 0, 4, and 8 g/d of Leu), but when additional energy was supplied, N retention increased linearly in response to Leu (25.6, 28.5, and 31.6 g/d for 0, 4, and 8 g/d of Leu; Leu x energy interaction, P = 0.06). The changes in N balance were results of changes in urinary N excretion. The greater Leu retentions in response to energy supplementation when Leu was the most limiting nutrient indicate that energy supplementation improved the true efficiency of Leu utilization. In addition, supplemental energy increased the gross efficiency of Leu utilization when Leu supply was not limiting by increasing maximal rates of protein deposition. Experiment 2 was similar to Exp. 1, but steers had an initial BW of 275 ± 12 kg and were limit-fed at 3.6 kg of DM/d. Retention of N was not affected (P = 0.22) by Leu supplementation, indicating that Leu did not limit protein deposition. Energy supply increased N retention (P < 0.01) independent of Leu supplementation (33.0 vs. 27.8 g/d). Overall, energy supplementation improved Leu utilization by modestly increasing N retention when Leu was limiting and by increasing the ability of steers to respond to the greatest amount of supplemental Leu. We conclude from these results that the assumption of a constant efficiency of AA utilization is unlikely to be appropriate in growing steers.

Key Words: cattle, energy, growth , leucine , utilization