Effects of Feeder-cattle Frame Size and Muscle Thickness on Subsequent Growth and Carcass Development. I. an Objective Analysis of Frame Size and Muscle Thickness 1'2

A multivariate analysis was performed to provide objective descriptions of visually perceived differences in feeder-cattle frame size and muscle thickness. Three hundred and twenty-four yearling feeder steers were selected to represent USDA classifications for frame size (Large, L; Medium, M; Small, S) and muscle thickness (No. 1, No. 2 and No. 3). The frame-size classification (F) of each steer was determined by a subjective evaluation of skeletal size-height and lengthin relation to maturity; the muscle-thickness classification (M) was based on a visual assessment of thickness of the rear quarter, forearm and gaskln-due to muscle expression and independent of differences in fatness-in relation to skeletal size. Six dimensional measurements (heart girth, hip width, stifle width, forearm circumference, body length and hip height) and a measurement of subcutaneous fat thickness (fat probe) were recorded for each steer to obtain objective assessments of traits that were emphasized during visual classification. A canonical varlate analysis was performed on the system of body measurements to quantify differences in feeder-cattle size and shape corresponding to USDA descriptions of frame size and muscle thickness. Over 95% of the dimensional variation among the nine F X M subclasses (L-No. 1, L-No. 2 . . . S-No. 3) was explained by the first two canonical vatiates. The first canonical variate (Zt) described morphological differences correspondingto variation in frame size and primarily reflected differences in absolute stature (L>M>S). Moreover, Z~ revealed a latent tendency for cattle of different frame sizes to vary in relative body dimensions (length, width and girth in relation to height). These relative dimensional differences were attributed to variation in developmental status. The second canonical variate (Z a ) primarily contrasted variation in rear-quarter thickness (relative to skeletal size) and shape corresponding to visually perceived differences in muscle thickness. Cattle perceived to be thickly muscled (No. I muscle thickness) had disproportionately wide and convexly shaped rear quarters, while steers classified as thinly muscled (No. 3 muscle thickness) were relatively narrow and flat through the rear quarters. Circumference of the forearm received secondary emphasis for discriminating among muscle-thickness groups. Evaluations of muscle thickness did not appear to be influenced by variation in subcutaneous fat thickness. Results of the analysis were used as an objective basis for interpreting the effects of feeder-cattle frame size and muscle thickness on subsequent growth and carcass development. (