Influence of Rumen-Protected Methionine and Lysine on Growing Cattle Fed Forage and Corn Based Diets

Abstract

Microbial protein in ruminants may be inadequate to support desirable levels of performance if the diet does not supply sufficient amounts of intestinally absorbable amino acids. Three experiments were conducted to determine the effects of RPM and/or a combination of RPM and RPL on ADG, serum metabolites, and DMI of calves fed diets based on forage or corn (Zea mays, L.). In Exp. 1, 36 steers were fed alfalfa/fescue (Medicago sativa [L.], Festuca spp. [L.]) hay plus one of three supplemental treatments for 28 d: (i) wheat (Triticum aestivum [L.] em. Thell.) middling basal supplement (WM); (ii), WM + 0.07 oz of RPM (RPM2); or (iii) WM + 0.14 oz of RPM (RPM4). Although there were no overall differences (P > 0.05) in ADG among treatments, there was a numerical trend (P 0.05) among treatments, ADG of RPML1 and RPML2 treatments was increased (P < 0.05) by 9 and 8%, respectively, over the control, during the 58-d period. In Exp. 3, 24 Holstein heifers grazed Kentucky 31 tall fescue (Festuca arundinacea Schreb.) pasture and were fed one of four supplements: (i) soyhulls (Glycine max (L.) Merr.) and cracked corn (control); (ii) control + 0.07 oz of RPM (RPMI); (iii) control + 0.07 oz and 0.25 oz of RPM and RPL (RPML); or (iv) control + 3.5 oz of blood meal (BM). Heifers had greater (P = 0.005) ADG and DMI (P = 0.03) for the first 30 d as compared with the second 30 d. Serum methionine was greater (P = 0.01) for RPML than control and BM treatments. These data suggest that lysine and methionine were the first limiting amino acids in corn/corn silage-based diets but not for heifers grazing tall fescue.