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

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<0.20) for ADG to increase (1.50,1.69, and 2.09 lb) with increasing level of RPM during the second 14 d of the experiment. Serum methionine concentration on Day 28 was greater (P<0.01) for calves fed RPM. In Exp. 2, 48 steers were fed a corn/corn silage-based diet plus one of three treatments: (i) basal supplement (control); (ii) control + 0.03 oz and 0.09 oz of RPM and RPL (RPML1); or (iii) control + 0.054 oz and 0.18 oz of RPM and RPL (RPML2). Although DMI did not differ (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)soy-hulls (Glycine max (L.) Merr.) and cracked corn (control); (ii) control + 0.07 oz of RPM (RPM1); (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. Research QuestionCertain rumen-protected amino acids may be limiting for growth of cattle fed forage or concentrate based diets. Supplementing rumen-protected amino acids to these diets offers potential for improving growth and/or gain efficiency of steers. The growth response from supplemental rumen-protected amino acids may not be the same under different dietary conditions. The primary objective of this study was to determine the dietary situations in which supplemental rumen-protected amino acids enhanced growth of cattle. Literature SummaryPast research has revealed that lysine and methionine are likely amino acids that may be limited for growth in certain diets fed to cattle. Most studies have revealed inconsistent responses in growth to increasing levels of supplemental rumen-protected amino acids in cattle diets. Feeding graded levels of rumen- protected amino acids to growing cattle in forage vs. concentrate diets may determine their usefulness as feed additives. Study DescriptionThree experiments were conducted to evaluate effects of supplemental rumen- protected methionine and methionine plus lysine on daily gain, feed intake, and serum metabolites. In Experiment 1, 36 Angus crossbreed steers (avg weight 418 lb) were fed free- choice alfalfa/fescue hay and 4 lb of a wheat middling based supplement to which either 0,0.07, or 0.14 oz of rumen-protected methionine was added as the dietary treatment. In Experiment 2,48 crossbreed steers (avg weight 593 lb) were fed a whole- shelled corn/corn silage diet. The three dietary treatments included basal diet, basal diet + 0.03 oz rumen-protected methionine and 0.09 oz rumen-protected lysine, or basal diet + 0.053 oz rumen-protected methionine and 0.18 oz rumen- protected lysine. In Experiment 3, 24 Holstein heifers (avg. wt. 566 lbs) grazed low-endophyte tall fescue pasture and provided daily a supplement containing no rumen-protected amino acids, 0.07 oz rumen-protected methionine, 0.07 oz rumen-protected methionine and 0.25 oz rumen-protected lysine, or 2.5 oz bloodmeal as the four dietary treatments. The daily gain of animals was determined in all experiments while feed intake was monitored in Experiments 2 and 3, and gain efficiency calculated in Experiment 2. The serum amino acid concentrations were measured in Experiments 1 and 3. Applied QuestionsWhich feeding situations with rumen-protected amino acids increased gain of steers? Feeding steers the whole-shelled corn/corn silage diet with either rumen-protected methionine and rumen-protected lysine fed as smartamine ML increased daily gain. The fact that the higher level of smartamine gave no further increase in daily gain suggests that 0.03 oz of rumen- protected methionine and 0.09 oz Full scientific article from which this summary was written begins on page 696 of this issue. of rumen-protected lysine was sufficient to maximize growth of growing steers fed the whole-shelled corn/corn silage diet. Why is growth response in steers fed forage-based diets supplemented with rumen- protected amino acids inconsistent? With lower daily gains (1–1.5 lb/d), forage-based diets may support amino acid requirements of steers for growth, and additional rumen-protected amino acids yield no further growth response. Steers, on the other hand, consume forage-based diets that achieve rates of gain in excess of 2 lb/d, may show a growth response to supplemental rumen-protected amino acids. Supply of rumen-protected amino acids to supplements for growing calves grazing tall fescue in hot and humid conditions may actually depress daily gains of steers.