Quality of fat differs among sources of supply. Specialty fats are treated to make them more inert in the rumen. These are sometimes called by-pass fats. Fats may be made more inert with a physical form such as flaked tallow or by chemical means such as forming a calcium salt. The increase in milk production due to added fat varies with a number of factors.
Older cows seem to respond better than first lactation cows.
Thin cows tend to respond more than fat cows. The milk production response increases with better genetic merit. Improved milk production from feeding fat is rare during the first few weeks of lactation. The reason for lack of response during early lactation is not known, but it could be that cows are overloaded with fat mobilized from their body stores at this time. The best response seems to occur when the cow approaches a positive energy balance about a month after parturition.