U.S. consumers prefer beef with white fat, as compared to yellow fat, so that is the major reason the packing plants purchase grain-finished beef. The U.S. consumer viewing a meat display of beef products in the supermarket is not accustomed to selecting and purchasing meat that does not have white fat and has difficulty selecting meat that is off-color or contains yellow fat.
The major cause of yellow fat is the intake of the yellow carotenoid pigments, especially b-carotene, which can be metabolized to vitamin A, a vitamin essential to many body processes. Excess b-carotene is stored in fat, giving rise to a yellow-colored fat. Grass/forages are the major source of carotenoid pigments. Finishing diets for beef cattle contain mostly grain and very little forage.
Iowa State has conducted studies on feeding and marketing culls cows. Their data suggest that it takes between 70 and 90 days to change yellow fat to white fat. Most feedlot cattle would be on a high-grain finishing diet for at least 90 days.