Selection for increased visual muscling increases carcass leanness without compromising predicted Meat Standards Australia eating-quality index

Context Selection using visual muscle score (MS) has been proposed to increase carcass leanness (i.e. meat yield), without compromising eating quality. Aims The aim of the present study was to examine the impact that selection for divergent MS has on live animal, commercial carcass and carcass tissue weights by using computed tomography (CT) including Meat Standards Australia (MSA) index-predicted eating quality. Methods Data from 67 steers originating from three muscling lines, namely, low, high and heterozygous high (HighHet – heterozygous for the 821 del11 myostatin mutation), were used. Visual MS was assessed on all steers. All steers were slaughtered and the left-hand side of each carcass was processed with fat trimming limited to only that required for hygiene purposes and kidney fat was not removed. All carcasses were MSA graded and then boned-out into untrimmed boneless primals (e.g. rump, cube roll). A CT scan of each beef primal was processed with image analysis software to estimate lean and fat tissue weights. The following traits were analysed: MS, weaning and slaughter weights; commercial carcass traits, including cold carcass weight, rump fat, MSA rib fat, MSA eye-muscle area, MSA marble score and MSA index; and CT-scanned compositional carcass traits, including lean, fat and bone tissues (%) and lean:bone ratio. All data were analysed with a linear mixed-effects model using REML. Least-squares means for the three muscling lines are reported. Linear trends between MS and seven carcass traits, with and without the myostatin mutation, are presented graphically. Key results Muscling line effects (P < 0.05) were found for visual MS and carcass traits. Linear trends between MS and carcass traits with and without the myostatin mutation demonstrate that increases in MS (P = 0.24) did not compromise predictions of MSA index even though MSA marble score decreased (P = 0.026), but myostatin decreased MSA marble score and tended to decrease MSA index (P = 0.097). Increases in the MSA eye-muscle area were associated with increases in MS (P < 0.01), with little effect of myostatin. Increases in MS and the myostatin mutation were both associated with increases (P < 0.01) in lean tissue (%) and the lean:bone ratio, and decreases (P = 0.02) in fat tissue (%). Conclusions The results indicate selection for high MS can be used to increase carcass yield, without negatively affecting MSA index predictions of eating quality. Implications Producers can use MS to identify animals with higher yields to increase carcass leanness and decrease carcass waste fat, without compromising MSA index predictions of eating quality, but should do so while considering all traits that affect profitability, in particular marble score and its association with eating quality.