Sources of variation in mechanical shear force measures of tenderness in beef from tropically adapted genotypes, effects of data editing and their implications for genetic parameter estimation

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 ed iting the raw data and to partition sources of variation for meat tendernes s measured in 2 different muscles. The effect of different methods of elect rical stimulation was examined: non-stimulated, extra low voltage or high v oltage. The results showed stimulation method had a large effect on the mea n and variance of the shear force. Non-stimulated slaughter groups were mor e variable than high voltage treated groups, which were more variable than low voltage treated groups. The effect of stimulation method was greater fo r shear force measured in M. longissimus dorsi than in M. semitendinosus. T he variability in tenderness associated with different methods of electrica l stimulation and the larger effect seen for the M. longissimus muscle sugg est 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 amon g the independent variables of tenderness were examined. Removal of non-sti mulated slaughter groups and other outliers resulted in a large reduction i n slaughter group variance and residual variance, with the estimate of heri tability for shear force of the M. longissimus increasing from 19 to 39%. B eef tenderness, as measured mechanically, could be improved by selection. H owever, the fluctuating heritability estimates reflect differences in handl ing pre- and post-slaughter and thus highlight difficulties in measuring te nderness consistently. The low genetic correlation between the 2 muscles (r (g) = 0.34) suggests improving overall tenderness of the carcass may be dif ficult. The large variance of the slaughter date effect within an abattoir (15% of the total variance) presents a problem in achieving consistently te nder meat.