Amino acid and energy interrelationships in growing beef steers: II. Effects of energy intake and metabolizable lysine supply on growth

We conducted three experiments to determine the optimal metabolizable Lys:n et energy ratio for growth of beef calves. The single basal diet fed contai ned corn (56.1%), soybean hulls (18%), cottonseed hulls (15%), animal fat ( 4.25%), and corn gluten meal (5.6%). In Exp. 1, 54 steers were individually fed the basal diet at 1.5, 2.25, and 3.0 times NE, requirement; rations we re top-dressed with 3.4 g of rumen-stable (RS) Met and either 0, 2, 4, 6, 8 , or 12 g of RS-Lys daily. An additional 18 steers were fed the same three levels of energy and supplemented with 125 g of blood meal per steer. In Ex p. 2, 68 crossbred steers were subjected to the same experimental protocol, with the exception that only the two highest levels of energy were used. O f these steers, 48 were fed individually and received the RS-Lys treatments ; the remaining 20 steers received 125 g of blood meal per steer. No intera ction ( P > .10) was detected between level of supplemental Lys and energy intake in Exp. I or 2. Supplementation with RS-Lys improved(P < .01) ADG in Exp. I, but it had no effect (P > .10) on growth in Exp. 2. The Lys requir ement estimates were 44.3 and 51.3 g/d, corresponding to maximal growth rat es of 1.21 and 1.64 kg/d for the 2.25 and 3.0 times maintenance treatments, respectively. Comparing the growth rates of steers fed supplemental Lys wi th those of steers fed blood meal in Exp. 1 and 2 revealed an ADG advantage (P < .03) with blood meal supplementation. To confirm the blood meal respo nse, Exp. 3 used 75 crossbred steers fed the basal diet at 3.0 times NE, re quirement plus either 3.4 g RS-Met, 3.4 g RS-Met and 12 g RS-Lys, or 125 g of blood meal per steer. Blood meal supplementation improved (P < .01) grow th of steers over those fed supplemental Met or Met plus Lys. Although a di stinct relationship between amino acid requirements and energy supply may e xist, Lys and Met were not first-limiting in these experiments, or selectiv e supplementation with undegradable protein may have provided some factor t hat enhanced performance beyond that detected with Lys and Met alone.