Abstract

Warner-Bratzler shear force measures of tenderness were taken on 2 muscles from 2661 carcasses from 3 tropically adapted breeds: Belmont Red, Brahman and Santa Gertrudis. The data were used to determine suitable methods of editing the raw data and to partition sources of variation for meat tenderness measured in 2 different muscles. The effect of different methods of electrical stimulation was examined: non-stimulated, extra low voltage or high voltage. The results showed stimulation method had a large effect on the mean and variance of the shear force. Non-stimulated slaughter groups were more variable than high voltage treated groups, which were more variable than low voltage treated groups. The effect of stimulation method was greater for shear force measured in M. longissimus dorsi than in M. semitendinosus. The variability in tenderness associated with different methods of electrical stimulation and the larger effect seen for the M. longissimus muscle suggest cold shortening, a processing effect, may have occurred in some of the slaughter groups. Several methods of editing outlier records were used and the effects of removing these records on the partitioning of variances among the independent variables of tenderness were examined. Removal of non-stimulated slaughter groups and other outliers resulted in a large reduction in slaughter group variance and residual variance, with the estimate of heritability for shear force of the M. longissimus increasing from 19 to 39%. Beef tenderness, as measured mechanically, could be improved by selection. However, the fluctuating heritability estimates reflect differences in handling pre- and post-slaughter and thus highlight difficulties in measuring tenderness consistently. The low genetic correlation between the 2 muscles (r _g = 0.34) suggests improving overall tenderness of the carcass may be difficult. The large variance of the slaughter date effect within an abattoir (15% of the total variance) presents a problem in achieving consistently tender meat.