Here is an excerpt from an older article in Drovers:
Johnson said several cattle breeds in Japan, Korea and China are bred to have very high marbling content. To compare, USDA choice beef usually is about 6 percent intramuscular fat, where Japanese Wagyu cattle produce about 30-35 percent intramuscular fat. He added high-end steakhouses in the U.S. have a huge demand for prime steaks with marbling, and those usually have about 6-8 percent marbling.
Asian cattle, however, are fed for much longer periods of time, close to three years in fact, where U.S. beef breeds are usually raised and harvested for meat between 18 and 22 months of age.
Johnson said the longer cattle feed, the more they are able to replace saturated fats with unsaturated fats, such as marbling. Cattle release an enzyme called stearoyl-CoA desaturase, which takes steric acid, a saturated acid, and saturates it to form double bonds to convert it into oleic acid.
"As the animal matures, it gets fatter, has more days on feed, it increases the activity or this enzyme," Johnson said. "So, in fact, we are creating more monounsaturated fat. The best way to get more oleic acid is to feed cattle longer."
Johnson said the genetics of cattle breeds used in the U.S. simply won't allow for marbling comparable to Asian breeds, although dairy breeds such as Holsteins, which make up about 20 percent of the beef cattle industry, do marble very well and without much backfat. U.S. beef producers would have to introduce Asian breeds to have cattle with high marbling, but because they feed so long it would not be economically feasible.
If you want to see the whole article:
https://www.drovers.com/article/mysteries-marbling
