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Effects of Feeder-Cattle Frame Size and Muscle Thickness on Subsequent Growth and Carcass Development. III. Partitioning of Separable Carcass Fat^1,2,

Colorado State University^,5, Fort Collins 80523

Abstract

Yearling feeder steers (n = 324) of diverse genotypes, representing three frame sizes (Large, L, Medium, M; Small, S) and tliree muscle-thickness classes (No. 1, No. 2, No. 3), were backgrounded for 28 d and slaughtered after O, 28, 56, 84, 112 or 140 d on a high-energy, finishing diet. The left side of each carcass was separated into primal and subprimal cuts and each cut was dissected. Weights of intermuscular (IM), subcutaneous (SC) and intemal (INT) fat components were recorded and analyzed to determine the effects of feeder-cattle frame size (F) and muscle thickness (M) on carcass-fat partitioning. Allometric coefflcients for the three fat depots, calculated using total separable fat (TSF) as the independent variate, were similar for steers in all nine F x M subclasses. However, there were among-class differences in relative proportions of SC and INT (expressed as percentages of TSF) when the subclasses were compared at a constant TSF percentime or at a common USDA marbling score. Increased frame size was associated with lower (P<.05) proportions of SC (S>M>L) and higher (P<.05) proportions of INT (L>M>S). Additionally, L steers attained a "small" degree of marbling at a lower TSF percentage than M and S steers. Among muscle-thickness groups, No. 1 and No. 2 steers partitioned similar proportions of TSF into the three depots; however, No. 3 steers had lower (P>.05) levels of SC and higher (P<.05) levels of INT. Moreover, No. 3 steers had lower percentages of TSF than did No. 1 and No. 2 steers, when the groups were compared at a common marbling score. Differences in fat partitioning observed in the present study were attributed to variadon in selection history and maturity-type.

¹ Sci. Ser. Paper No. 3003 published with the approval of the Colorado State Univ. Agr. Exp. Sta. This research was supported by the USDA Agr. Marketing Serv. and the Colorado Dept. of Agr. under the Federal-State Marketing Improvement Program.

² The authors express appreciation to Mr. W. E. (Ned) Tyler and Dr. R. E. Taylor for their contributions in design and direction of the experiment and to R. P. Clayton, D. A. Daley, H. G. Dolezal, B. K. Klein, W. R. Lloyd, 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 Livestock Division, Agr. Marketing Serv., USDA.

⁴ Monfort of Colorado, Inc. Greeley, CO.

⁵ Dept. of Anim. Sci.