Supplemental fat in high-energy rations for lactating cows: effects on intake, digestion, milk yield, and composition.

Supplemental fats (SF) have special value in the diets of dairy cows with superior productive ability, because the high energy density of SF allows greater energy consumption and direct transfer of the fatty acids (FA) of the SF to milk fat; this increases metabolic efficiency. Some SF, especially oils with a high degree of unsaturation, disturb ruminal fermentation, decrease fiber digestibility, and lower milk fat test; however, oilseeds (e.g., whole cottonseed) can be fed without observable ruminal inhibition, probably because of a slow release of the oil into ruminal contents. A number of commercial fat supplements are available that have little effect on ruminal fermentation and are highly digestible postruminally. A product of the calcium salts of palm oil fatty acids was shown to have a NE for lactation of more than three times that of corn. As the emphasis in milk pricing formula changes from milk fat to milk protein, there is greater concern regarding the propensity of fat supplements to decrease milk protein (casein) percentage. In two studies, a supplement of ruminally protected lysine and methionine largely prevented the reduction in casein percentage that occurred with feeding SF, indicating that the amino acid profile of undegraded dietary protein is important with regard to preventing the lower casein percentage. Combining fat supplements that are slowly released in the rumen with those that are inert in the rumen will allow maximal use of SF.