Changes in body composition associated with weight loss in early- and later-maturing types of steers

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Abstract
A trial is reported with eight steers in a 2 x 2 factorial design to compare the effects of weight loss in two maturity types of steers, Charolais x Hereford (C x H) and purebred Hereford /H x H), having similar estimated initial carcass fat contents (30% and 32% respectively) but different liveweights (402 kg v. 332 kg). The steers were aged about 61 weeks at the beginning of the trial, when a pair of C x H and of H x H steers (group 1) were slaughtered. The initial body composition was established by sample cut dissection and half-carcass chemical analysis. The remaining two pairs of steers (group 2) here slaughtered and analysed following 234 days of grossly submaintekmce feeding. During the submaintenance period the C x H steers lost considerably more liveweight (134 kg v. 107 kg) than the H x H steers; however, the percentage loss was similar in all four steers. All cattle consumed identical weights of feed. It is estimated that in losing about 30% of their initial liveweight, the four steers lost about 40% of their carcass weight and 20% of their offal weight, regardless of initial liveweight differences. The carcass loss in both breeds consisted of about 70% of the initial fat weight, 40% of the muscle weight, and less than 10% of the bone weight. Breed differences in the distribution of carcass tissues and retail cuts were small and generally not significant. Three classes of body components are defined and listed: class 1, those which lost weight at a relatively greater rate than the carcass; class 2, at a similar rate; class 3, at a slower rate. Estimated 'negative growth coefficients' (k in the allometric equation y = bxk) for carcass, offal, bone, muscle and fat weights, relative to liveweight, were about 1.4, 0.5, < 0.25, 1.3 and > 3.0 respectively.