The trait having the most influence on U.S.D.A. Quality Grade is marbling (the amount and distribution of intramuscular fat within the ribeye) score.  Beef cuts with high levels of marbling are more likely to be tender, juicy and flavorful than cuts with very low levels of marbling.    Ultrasound can be used to estimate the amount of intramuscular fat in the ribeye in live cattle.  It is not an estimate of the marbling score, but a direct estimate of the amount of intramuscular fat present in the ribeye.

The heritability for marbling is in the moderate range.   In addition, the genetic correlation between marbling and fat thickness has been shown to be very low, suggesting that selection for improved marbling can be obtained without an increase in external fat.

The prediction of percentage intramuscular fat is a newer developing aspect of ultrasound and has probably been the source of the most excitement.   Several research groups across the country are working in this area.  Early results are quite encouraging, but it is still unclear how useful this concept will be for measuring genetic merit in breeding animals.

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Gearld Fry
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