Relationship of feeder lamb frame size to feedlot gain and carcass yield and quality grades

Small-(S), medium-(M), and large-(L) framed feeder lambs (n = 243 S, 247 M, and 245 L) were finished on five diets differing in amount of concentrate (C) and crude protein (CP): 1) 30% C, 12.5% CP; 2) 55% C, 12.5% CP; 3) 55% C, 14.5% CP; 4) 80% C, 12.5% CP; and 5) 80% C, 14.5% CP. Lambs were sampled over a wide weight range to establish relationships of frame size to daily gain and live weight at specific carcass grade end points. Increased frame size (F) was associated with more rapid gains during finishing, although differences in daily gain among L, M, and S lambs were not expressed consistently across all dietary treatments (D), as reflected by a significant F x D interaction for ADG. The rate at which external fat thickness increased as the lambs became heavier was not affected by frame size, sex, or diet. However, increased frame size was associated with lower (P < .05) values for fat thickness (FT), body wall thickness (BWT), yield grade (YG), and quality grade (QG) when comparisons were made at a constant slaughter weight. Similarly, later-maturing wether lambs were leaner and had lower YG and QG than ewes of the same weight. Dietary treatment had no effect on FT, BWT, and YG when treatment groups were compared at a constant live weight. Subclass regressions of YG on live weight were used to project the weights at which lambs of various frame sizes would be expected to produce YG-2 carcasses. Projected final weights for wethers were less than 50 kg for S, 50 to 55 kg for M, and greater than 55 kg for L. Projected final weights for ewe lambs were approximately 2.5 kg lower. Dietary treatment did not affect relationships among frame size, live weight, and YG. A grading/classification system for feeder lambs based on frame size could be developed to predict carcass grade end point.