Beef cattle muscle and marbling development
April 22, 2013
The past summer's dry conditions and high prices for feed this winter, not only impacts our pocket books, but it can potentially affect the calf crop, as well as bull and replacement heifer development for next year's sales. Nutrition and management of the cow herd during pregnancy can potentially influence muscle development as well as marbling in the yearling animal, whether it's a yearling bull, replacement heifer or feedlot steer
Research conducted during the last 2 to 3 years has improved our understanding on how muscle and adipose (fat) is developed in beef cattle. One of the leading muscle biologists in this field is Dr. Min Du – a professor that spent several years in the Animal Science Department at the University of Wyoming. One of the unique aspects of his research is that we can directly apply much of this information to both managing our beef herds, as well as using the research results to help explain some of the year to year variation in carcass characteristics that challenge our industry.
In the beginning …
One of the most important concepts to consider is that a large percentage of both muscle as well as marbling development takes place during gestation. Certainly there are genetic influences to consider, and we have all seen presentations and data that indicate that early weaning and creep feeding can enhance quality grade or marbling. Much of this potential for either muscling or marbling is established during pregnancy. Both under-nutrition, as well as over-nutrition, can impact the subsequent expression of muscling and marbling in the offspring produced. During the embryonic stage of early pregnancy, muscle cells, fat cells and fibroblasts all develop from one unique class of cells known as Mesenchymal Stem Cells (MSC). Nutrition and management of the pregnant cow during early, mid and late gestation will influence how these MSC cells develop and specialize, affecting the total number of muscle cells and fat cells that develop in the calf.
“Newer research findings continue to suggest that fetal development, and management of the beef herd during pregnancy, may have larger impacts than we once realized.”
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Skeletal muscle development is initiated during early and mid gestation. Primary myofibers, or muscle cells, are formed during an initial stage of myogenesis during the first 2 months of pregnancy. Secondary muscle cells develop during a second wave of development, months 2-8 of gestation. This second wave of development generates a majority of the skeletal muscle fibers in the animal. There is a smaller contribution by additional cells known as "satellite cells," but for the most part, a large majority of skeletal muscle cells are developed during mid-pregnancy. Any additional muscle growth that occurs after birth, as the calf develops, is due mainly to increases in muscle SIZE. The total number of muscle cells has already been determined during fetal development. Because a majority of muscle fiber development occurs during mid-gestation, it is an important stage to consider, as poor nutrition during early and mid gestation will affect muscle expression in the calf. Ultimately, animals can somewhat compensate by increasing muscle fiber size, but the total number of skeletal muscle fibers is essentially set by the 8th month of pregnancy.
I look at this stage somewhat as an adaptive response by cattle. Range-based herds that are often faced with lack of forage or poor range conditions may ultimately produce calves with fewer skeletal muscle cells, and potentially less overall muscling. This may be a very effective response, as the maintenance of skeletal muscle requires energy, and more lightly muscled animals may have a competitive advantage in poor range conditions because their energy requirements are slightly lower. Unfortunately, light muscling can have a negative impact on both feedlot performance and carcass merit, with lightly muscled animals having a greater chance of grading out as yield grade 4 and 5 carcasses and receiving a discounted price.
Development of marbling
As mentioned above, fat cells known as adipocytes are developed from these same Mesenchymal Stem Cells in the young developing fetus. Where muscle cells develop in early and mid gestation, fat cells develop during late gestation and to a lesser extent during the early life of the calf. Later maturing cells, known as satellite cells, can also develop into fat cells after birth. Several management programs suggest that we can maximize the animal's potential for marbling by weaning at an earlier age (150 days of age), or by providing supplemental "creep" feed. While this is true, the largest influence on fat cells, and potential marbling, is accomplished during late pregnancy.
If you think back to last winter, much of Northeastern Wyoming and the Dakotas were hit by large storms in late October and November. Stresses associated with these storms had the potential to impact the IMF scores of yearling bulls being marketed right now, as well as the quality grade of steers that are approaching finishing weights right now.
Continued research continues to sharpen our understanding of both muscle development and marbling in beef cattle. We know from previous research that nutrition, health, and feedlot management all impact quality grades, feedlot performance and carcass merit. Newer research findings continue to suggest that fetal development, and management of the beef herd during pregnancy, may have larger impacts than we once realized. This information can be used to not only improve the consistency of the product produced, but to also help us understand some of the yearly variation that we see in feedlot pen closeouts.