Live animal predictions of carcass components and marble score in beef cattle: model development and evaluation.

Until recently, beef carcass payment grids were predominantly based on weight and fatness categories with some adjustment for age, defined as number of adult teeth, to determine the price received by Australian beef producers for slaughter cattle. With the introduction of the Meat Standards Australia (MSA) grading system, the beef industry has moved towards payments that account for intramuscular fat (IMF) content (marble score (MarbSc)) and MSA grades. The possibility of a payment system based on lean meat yield (LMY, %) has also been raised. The BeefSpecs suite of tools has been developed to assist producers to meet current market specifications, specifically P8-rump fat and hot standard carcass weight (HCW). A series of equations have now been developed to partition empty body fat and fat-free weight into carcass fat-free mass (FFM) and fat mass (FM) and then into flesh FFM (FleshFFM) and flesh FM (FleshFM) to predict carcass components from live cattle assessments. These components then predict denuded lean (kg) and finally LMY (%) that contribute to emerging market specifications. The equations, along with the MarbSc equation, are described and then evaluated using two independent datasets. The decomposition of evaluation datasets demonstrates that error in prediction of HCW (kg), bone weight (BoneWt, kg), FleshFFM (kg), FleshFM (kg), MarbSc and chemical IMF percentage (ChemIMF%) is shown to be largely random error (%) in evaluation dataset 1, though error for ChemIMF% was primarily slope bias (%) in evaluation dataset 1, and BoneWt had substantial mean bias (%) in evaluation dataset 2. High modelling efficiencies of 0.97 and 0.95 for predicting HCW for evaluation datasets 1 and 2, respectively, suggest a high level of accuracy and precision in the prediction of HCW. The new outputs of the model are then described as to their role in estimating MSA index scores. The modelling system to partition chemical components of the empty body into carcass components is not dependent on the base modelling system used to derive empty body FFM and FM. This can be considered a general process that could be used with any appropriate model of body composition.

Breed variation and genetic parameters for growth and body development in diverse beef cattle genotypes.

Conformation scores can account for more than 20% of cattle price variation at Australian livestock sales. However, there are limited available references which define genetic factors relating objective live developmental traits to carcass composition. Weaning and post-weaning weight, height, length, girth, muscle (ratio of stifle to hip width) and fat depth of 1202 progeny from mature Hereford cows (637) mated to seven sire breeds (Jersey, Wagyu, Angus, Hereford, South Devon, Limousin and Belgian Blue) were examined for growth and development across ages. Crossbred Wagyu and Jersey were both lighter in weight and smaller in size (height, length and girth) than purebred Hereford and crossbred Angus, South Devon, Limousin and Belgian Blue. Within the five larger crossbreds, there were significant changes in relative weight from weaning to 600 days. Sire breeds differed in fat depth, with Angus being the fattest (9% on average fatter than Hereford and Wagyu), and Jersey 5% less fat than Hereford, followed by South Devon and Limousin (19% lower than Hereford) and Belgian Blue (39% lower than Hereford). Direct heritability ranged from 19 to 42% and was higher than the proportion of total phenotypic variance accounted for by maternal effects (which ranged from 0 to 17%) for most body measurement traits except for weight (38 v. 18%) and girth (36 v. 9%) traits at weaning, an indication of maternal effect on some body conformation traits at early ages. Muscularity (19 to 44%) and fat depth (26 to 43%) were moderately to highly heritable across ages. There were large differences for growth and the objective measures of body development between crossbreds with a degree of overlap among the progeny of the seven sire breeds. The variation between genetic (positive) and environmental (negative) correlations for dry versus wet season average daily gains in weight and fat, suggested the potential use of live-animal conformation traits for within breed selection of genetically superior animal in these traits across seasons.