Supplemental fat generally does result in a slight decrease in milk protein percent. Fatty acid composition of milk fat is altered by supplementation of the diets with fat. Changes in milk fat fatty acid composition reflect the fatty acids supplemented to the diet. Most fat supplements contain long chain fatty acids. Several other nutrient changes need to be considered when supplementing diets with fat.
Adequate fiber needs to be available in the diet to stimulate ruminal fermentation.
Calcium complexes with the fatty acids to form soaps, resulting in lowered availability of Ca for absorption. Calcium level in the diet should be adjusted when fats are added. And, the decrease in proportion of grain in the diet that occurs when adding fats results in lowered available energy to the rumen microorganisms, and therefore less microbial protein synthesized and made available to the cow. A good source of dietary protein that is of low rumen degradability will make up for this deficit. The type of fat used in the diet can substantially affect the result.