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Influence of Diet on Growth Rate and Carcass Composition of Steers Differing in Frame Size and Muscle Thickness^1,2,

Colorado State University, Fort Collins 80523

Abstract

One hundred seventy-three steer calves, selected at weaning to represent USDA classifications for frame size (Large, L; Medium, M; Small, S) and muscle thickness (No. 1, No. 2, No. 3), were assigned to three finishing diets (grain, 3.03 Mcal ME/kg DM; silage, 2.46 Mcal ME/kg DM: forage, 2.06 Mcal ME/kg DM), forming a 3 x 3 x 3 factorial with unequal subclass numbers. The steers were slaughtered at constant weights corresponding to their respective frame size classifications (L = 590 kg, M = 499 kg; S = 408 kg). Individual live weights were recorded every 28 d during finishing and, following slaughter, one side of each carcass was dissected into muscle, bone and fat. Diet interacted with frame size to affect growth rate. Frame-related differences in growth rate were very pronounced among grain-fed steers (L > M > S) but became smaller in magnitude as feeding intensity was reduced. Diet also had a significant effect on percentage separable carcass fat, despite the fact that the steers were slaughtered at a similar percentage (approximately 60%) of their estimated potential mature weights. Carcasses produced by grain-finished steers were fattest, and those produced by forage-finished cattle were leanest. The comparative fatness of steers finished on silage was dependent on frame size. Within the L class, carcasses produced by silage-finished steers were comparatively lean and were similar in fatness to carcasses produced by forage-finished cattle. Conversely, S steers finished on silage produced carcasses that were comparable in fatness to carcasses from grain-finished cattle. Frame-related differences in percentage carcass fat were significant only within the silage treatment group (S > L). Differences in carcass muscle-to-bone ratio were directionally consistent with visually discernible differences in feeder cattle muscling (No. 1 > No. 2 > No. 3). The effects of muscle thickness on muscle-to-bone ratio were most pronounced within the L class. Diet was a significant source of variation in carcass muscle-to-bone ratio (grain > silage = forage) but did not alter the relationship between muscle thickness classification and muscle-to-bone ratio.

¹ This research was supported by the USDA-Agric. Marketing Serv. and the Colorado Dept. of Agric. under the Federal-State Marketing Improvement Program and by the Colorado Agric. Exp. Sta.

² The authors express appreciation to W. E. (Ned) Tyler and R. E. Taylor for their contributions in design and direction of the experiment and to R. P. Clayton, D. A. Daley, H. G. Dolezal, R. K. Miller, C. W. Moran, M. D. Self, T. D. Stromberg, M. L. Wittler and D. W. Woodburn for their assistance in data collection.

³ Present address: Texas Meat Purveyors, Inc., Dallas.

⁴ Livestock Div., Agric. Marketing Serv., USDA.

⁵ Monfort of Colorado, Inc., Greeley.