Effect of energy status on milk production responses to dietary addition of fat in cows

Adding appropriate fats to diets for dairy cows has the potential to improve the acceptability of dairy products to the health-conscious consumer by increasing the proportion of monounsaturated fatty acids in milk fat. However, concentrations of protein in milk are often concurrently depressed. In these respects responses to fat resemble those seen in animals in negative energy balance, where the contribution of fatty acids mobilized from adipose tissue to milk fat synthesis leads to similar changes in milk fat composition, and milk protein concentrations are relatively low. These similarities give rise to two questions: (i) do the metabolic adaptations during negative energy balance influence the utilization of dietary fat for milk fat synthesis, and (ii) is the mechanism whereby milk protein is depressed the same in both cases? As a preliminary investigation of this, an experiment was conducted to compare the effects of dietary fat supplementation on performance of lactating cows either mobilizing or depositing body tissue.