Meat grading in sheep: is muscle area more reliable than depth?
Fat and muscle depth have been measured using ultrasound in cattle, pig and sheep meat grading work for well over two decades. In sheep, measurements are most commonly taken at the 12th-13th ribs, although using the third to forth lumbar vertebrae has been shown by some to give greater correlation with actual carcass measurements (Thériault et al., 2009). Signet Breeding Services are UK leaders in such work, and using tailor-made ultrasound equipment from Bromley-based Vet Image Solutions, they also favour this site.
Obtaining these measurements involves a steep learning curve, but once an operator has been properly trained and has scanned a sufficient number of animals, repeating these measurements is relatively straightforward and excellent intra- and interoperator repeatibility can be achieved.
So is muscle area better?
On the face of it, tracing of any area on ultrasound is far more susceptible to error than a simple linear measurement. The reasons for this include ultrasound physics (muscle depth measurements take full advantage of the machine’s superior axial resolution, whereas any area trace will also have to rely on the system’s lateral resolution) and also intra- and interoperator variability. Yet, muscle area has been shown to outperform muscle depth in some studies (see Leeds et al., 2008). Why?
According to Thériault et al. (2009), the main reason is that meat grading relies heavily upon the comparison of one animal with another, but in sheep, there is relatively little variability between animals – or at least, the variability is so small that measurement technique would have to be perfect to ensure that any differences observed were truly down to the differences between animals, and not operators. The most common pitfalls that Thériault et al. highlight in their paper include:
– Probe pressure: Pressure from the transducer deforms fat uniformly, but the same is not true of muscle due to the underlying ribs (Young et al., 1992). In my experience, however, teaching technicians to use the correct probe pressure is not difficult, and ribs do not present a problem.
– Third fat layer: Thériault et al. hypothesise that the third fat layer may often be misinterpreted as the start of the muscle layer, leading to its inclusion in measurements of muscle depth. This layer can definitely be a source of confusion, but I have found that this third fat layer (which tends to be found on fattier sheep and is absent in leaner ones) does not coincide with the deepest area of muscle, which is the area from which muscle depth is always measured.
– Frequency: The muscle depth in sheep and lambs is small compared with pigs and cattle. Special low frequency linear probes are made for beef work, but higher frequencies (the authors recommend higher than 3.5MHz) should be used for sheep scanning, to improve resolution and measurement accuracy. I agree, and would add to this point that a full knowledge of basic ultrasound physics and equipment controls is essential, including the role of the focal zone, a misunderstanding of which can actually be detrimental to image quality.
Above: The third fat layer is seen to the right of the image here, taken on the Vet Image Solutions customised sheep meat grading system.
The above are certainly important points to bear in mind for any organisation starting a new meat grading programme, but cannot account for the poorer performance of lean muscle depth versus area in predicting lamb carcass quality; for any experienced technician working to a well validated protocol, none of these points would present a challenge.
The only way to truly resolve this debate will be for the current leaders in the field of sheep meat grading to introduce area tracing into their standard protocols. If the same organisations which are already achieving excellent results with fat and muscle depth find improvements with area tracing, then the case will be settled.
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