Image analytic characteristics for marbling of rib eye divided into four areas and their genetic parameters in Japanese Black steers

The purposes of this research were to examine the characteristics of marbling in different regions for rib eye of 1,524 Japanese Black steers in progeny testing and to estimate their genetic parameters. Digital images of the rib eye on carcass cross section were taken between the 6th and 7th ribs with photography equipment and the rib eye was divided into four quadrants by inertia main axes. Fat area ratio (FAR), overall coarseness of marbling particles (OCM), coarseness of maximum marbling particle (MMC), fineness index of marbling particles (FIM) and average of luminance in lean (LL) and fat (LF) were calculated for whole and each quadrant of rib eye. Genetic parameters for these traits were estimated with the GIBBS2F90 program using an animal model included in the model were station-year-season and birthplace as fixed effects, age at the beginning of the fattening period as a covariate and additive genetic effect and residual effect as random effects. The FAR showed significantly (P0.05) higher percentage at 3rd and 4th quadrants (34.97, 36.22%) than 1st and 2nd quadrants (33.36, 33.91%). OCM, MMC and FIM of each guardant were significantly different in all combinations (P0.05). Marbling flecks of medial region were larger than lateral region, conversely finer marbling flecks were contained in lateral region than medial region. Average of LL revealed small value in order of 2nd (111.14), 3rd (110.22), 4th (109.63) and 1st (108.66) quadrant, indicating that the lean color of ventral was lighter than that of dorsal. Average of LF of 3rd and 4th quadrants (110.22, 109.63) were significantly higher than 1st and 2nd quadrant (P0.05). Heritability estimates of FAR in 1st and 2nd quadrants (0.68, 0.68) were comparatively higher than 3rd and 4th quadrants (0.56, 0.61). Heritability estimates of OCM (0.31) and MMC (0.21) of 3rd quadrant were lower than other quadrants, and that of FIM (0.19) of 4th quadrant was the lowest among other quadrants. Heritabilities of average of LL and LF were moderate through the quadrants (0.52-0.61). Genetic correlation for MMC between whole and 3rd quadrant was relatively lower value (0.71), than other quadrants (0.85-0.92). These results were indicating that the 3rd quadrant wasn't more efficient for improvement of marbling coarseness of rib eye. However genetic correlations of FIM among whole and all quadrants were generally high. Thus, genetic improvement of FIM for whole of rib eye might increase the fineness of all quadrants unlike MMC. Differences of phenotypic and genetic characteristics of marbling by rib eye regions were clarified from this study and also the possibility of improvement for balance of marbling alignment was suggested.