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 (High^Het – 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.

背景：已经提出使用视觉肌肉评分（MS）进行选择，以增加car体的瘦度（即肉的产量），而不影响进食质量。

目的：本研究的目的是通过使用计算机断层扫描（CT），包括澳大利亚肉类标准（MSA）指数预测的进食质量，检查选择不同的MS对活体动物，商业car体和car体重量的影响。

方法：来自低，高和杂合高（高^Het）三种肌肉系的67个ers牛的数据–使用了821 del11 myostatin突变的杂合子）。对所有ste牛进行视觉MS评估。屠宰所有ste牛，对每个car体的左侧进行除脂处理，仅限于出于卫生目的使用，并且不去除肾脏脂肪。所有car体均经过MSA分级，然后剔除成未修剪的无骨原基（例如，臀部，方块状卷皮）。用图像分析软件对每张牛肉原肉进行CT扫描，以估计瘦肉和脂肪组织的重量。分析以下特征：MS，断奶和屠宰体重；商业car体特征，包括冷cold体重量，臀部脂肪，MSA肋骨脂肪，MSA眼肌面积，MSA大理石评分和MSA指数；以及CT扫描的composition体成分特征，包括瘦肉，脂肪和骨组织（％）以及瘦肉：骨头的比例。使用REML，使用线性混合效应模型分析所有数据。报告了三个肌肉线的最小二乘均值。MS与7个car体性状之间的线性趋势，有无肌生长抑制素突变，以图形方式显示。

关键结果：发现视觉MS和car体性状的肌肉线条效应（P <0.05）。MS和具有和没有肌肉生长抑制素突变的car体性状之间的线性趋势表明，即使MSA大理石得分降低（P = 0.026），MS的升高（P = 0.24）也不会损害MSA指数的预测，但是肌肉生长抑制素降低了MSA大理石得分并趋于降低MSA指数（P = 0.097）。MSA眼肌面积的增加与MS的增加有关（P <0.01），而肌肉生长抑制素的作用很小。MS的增加和肌生长抑制素突变都与增加有关（P瘦组织（％）和瘦骨：骨骼的比率<0.01），脂肪组织（％）下降（P = 0.02）。

结论：结果表明，选择高MS可以提高car体产量，而不会对MSA饮食质量指数的预测产生负面影响。

含义：生产者可以使用MS来鉴定具有更高产量的动物，以增加体瘦肉率并减少car体废脂肪，而不会影响MSA食用质量指数的预测，但在考虑影响盈利能力的所有特征（尤其是大理石评分及其关联性）时应这样做与饮食质量。