Fat lamb studies in Victoria. I. An assessment of the relative value of various external measurements for differentiating between various grades of export lamb carcasses

Abstract

Sixteen measurements were made on 240 intact export lamb carcasses, representing 12 weight-grade classifications of 20 each, with the object of determining measurements likely to be of value for the assessment of conformation. Six were immediately discarded as being of little value. The remaining 10, which included three probe measurements, were subjected to statistical analysis to determine linear combinations of measurements most convenient for discriminating between grades in each of three weight categories, namely, light (27-28 lb), medium (35-36 lb), and heavy (43-44 lb). The 10 measurements were length of leg ( F ) , depth of thorax (Th), thickness of loin (BC), thickness of flank (P), length of carcass (K), thickness of shoulder (Sh), width of shoulder ( W) , width of flank (E) , width of gigots (G), and twist (Z), with discriminating capacity roughly in that order. BC, P, and Sh were all estimates of muscle and fat obtained by using a probe, and results obtained compared favourably with overseas measurements made on the cut carcass. E, G, and Z had little or no discriminating capacity. Optimum discriminations between grades were given within each weight category by the following combinations of measurements: light — F, K, BC, Sh, P; medium — F, Th; heavy — F, K, W, P; overall — F, Th, BC, P. The groups of carcasses separated into clear weight and grade zones, and several trends were noticeable. The most important of these were: (i) grading of light-weight carcasses was highly subjective between all grades, as both conformation and fat cover were considered; (ii) grading of the top grades in heavier carcasses was more clear-cut and depended primarily on conformation; (iii) there was a tendency for heavier carcasses to be up-graded relative to light carcasses, owing to their heavier fat cover. In view of the fairly clear pattern that emerged from the statistical model used, it is suggested that a rounded form of a discriminant function, namely, Y' = 0.057F + 0.050Th – 0.097BC – 0.17P, might be a useful index of conformation for fat lamb carcasses of all weights particularly if used in the form: Conformation score = 250 – 10Y'.