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Marbling, Genetic Data Lacking

Marbling, Genetic Data Lacking

A new study shows there's a lot more mystery than information when selecting for carcass traits.

If you select for carcass traits, the effects on your cow herd remains too much a mystery but a recent study provides some insights.

Looking at the upward trend in marbling expected progeny differences (EPDs), it's clear genetic selection is effective. However one advisor will tell you to back away from marbling selection because you have plenty, while another will say you should maximize it while avoiding single-trait selection.

Texas A&M geneticist Andy Herring recently authored a paper which sheds some light on this dilemma. Called "Genetic aspects of marbling in beef carcasses," his literature review encompasses 52 studies spanning several decades.

MAKING DATA PAY: More work needs to be done to get more consistent, well-marbled cattle.

One of the things he found was a lack of correlation between marbling and backfat thickness. This was shown in comparisons of high- and low-marbling EPD, registered Angus bulls bred to composite cows at the USDA Meat Animal Research Center. The top bull EPDs were +.33 and the low bulls were -.35 in the 1995 Angus Sire Summary, but fat thickness EPD was similar for all.

Progeny were fed and harvested in two groups 60 days apart. Calves from high-marbling bulls averaged 52% and 96% Choice, compared to 17% and 78% Choice for the progeny of low-marbling EPD bulls, while Yield Grades did not vary significantly. Further analysis suggested the higher-marbling progeny also may have a faster rate of marbling deposition.

Marbling, not backfat

Herring says this research demonstrated selection can increase marbling ability without increasing external fat or causing detrimental effects on other feedlot or ranch traits.

Although the marbling heritability average estimate is .45, the reports range from .12 (barely worth the selection effort) to .88, or 88% effective. Yet the amount of genetic variation itself varies and the relationship of marbling to other traits is probably not constant across all breeds, Herring says.

The variability in external fat appears to be larger on average than that for marbling, with heritability estimates from .02 to .86 across several studies.

"Fat thickness is thought of as the result of feeding management but there are significant genetic differences when cattle are subjected to the same environment," Herring adds.

Phenotypic correlation estimates between marbling and fat thickness have ranged from slightly negative to moderately positive.

"That means fat thickness phenotype alone may only describe 0.64% to 9% of the variation in marbling," he explains. Ironically, most cattle are marketed on quality grids based on estimated backfat thickness.

End-point inconsistencies

Marbling-related research varies in methodology of carcass end-point constant, from age to weight to fat-thickness basis. Most genetic research uses an age-constant basis, while nutritional studies favor a fat-constant one.

Herring calls for more research looking at both in the same trial, "especially as age verification programs become more popular."

He says the industry needs to find better ways to evaluate and incorporate herd and calf genetic and management factors when evaluating marbling ability and other carcass traits.

"The main point about cattle that grade Prime is that they have the genetic ability to marble, and it is not because they are fat," he says, noting results of three National Beef Quality Audits.

Other effects

As producers apply selection pressure to get a few more Prime grades and pounds, cows change. Herring says there's a shortage of research, but mature cow weight and height may be lowly, negatively correlated with marbling score.

Cow body condition score seems not correlated with marbling, slightly with carcass weight and moderately with fat thickness of steers.

"Within beef production systems, we must always consider the relationships between cowherd and end-product traits," he concludes.

Source: Certified Angus Beef

Make the system work and pay better

Texas A&M Geneticist Andy Herring makes several observations about carcass quality and how it could be better quantified and managed. Here are a few of his suggestions.

  • The industry needs to better evaluate and incorporate calf and/or herd background information when evaluating marbling ability, as well as all carcass traits. Several reports in the literature document the influence of things such as animal age at harvest, age of dam, effects of creep feeding, individual year effects. Such things may be viewed as "nuisance" variables, and are generally not known on most feedlot cattle. They should receive more attention as source- and age-verified programs become more important. Variation in these types of effects could mask genetic differences if not documented.
  • A large inefficiency in the beef industry comes from managing cattle of different genetic potentials in the same manner because their ultimate potential is not known, or is ineffectively projected based on appearances or stereotypes. Therefore, premiums and discounts that are reliably related to end-product differences are needed earlier in the U.S. beef production system. Simply relying on external fat thickness to predict marbling ability is ineffective and inefficient.
  • Ultrasound evaluation of body composition provides important information to predict marbling of carcasses from feeder calves and feedlot cattle, as well as to identify genetic potential for marbling among breeding animals.
  • Multiple sources of information should be jointly utilized to genetically change marbling. A 2004 study found combining live animal ultrasound and carcass data gave a larger range and more accurate EPD estimation than either source did individually.
  • There needs to be more focus on evaluation of beef females in regard to improving all carcass traits, although many seedstock producers have been more concerned with obtaining ultrasound information on yearling bulls than heifers. Yet a study in 2000 found higher genetic correlations between yearling heifer ultrasound intramuscular fat and carcass intramuscular fat in Australian Angus and Hereford cattle, as compared to yearling bull ultrasound intramuscular fat. A 2001 study found much higher genetic correlations between ultrasound fat thickness in yearling heifers and carcass fat in steers than between yearling bulls and steers.


TAGS: Regulatory
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