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Managing Milk Fat Depression

Milk fat concentration is variable within and between farms and is modified by genetics, season of the year, and physiological state, but is especially responsive to diet. Synthesis of milk fat is an energy demanding process, but also represents a significant portion of the economic and nutritional value of dairy products. First described over one and a half centuries ago, diet-induced milk fat depression (MFD) is characterized by a decrease in milk fat yield of up to 50% with no change in milk yield or yield of other milk components. Milk fat depression is classically observed in ruminants fed highly fermentable diets or diets high in plant oils. Varying levels of MFD are commonly experienced today in both intensively and extensively managed dairy herds, and this represents a level of milk fat production below the genetic potential of the cow. Milk fat depression is also a useful variable for evaluating herd management. In many cases onset of diet-induced MFD is an indication of modified ruminal fermentation and in more pronounced cases this can be associated with ruminal acidosis and reduced efficiency. Therefore, maintaining optimal milk fat synthesis has value beyond the milk fat sold. Although the past two decades have provided extensive knowledge of the causes and mechanisms of MFD we continue to experience the condition because of the requirement to feed high-energy diets and the desire to maintain optimal milk production. Additionally, numerous dietary factors commonly interact to cause MFD, making prediction difficult. We have investigated the time course of induction and recovery of MFD that provides insight into identifying causative factors and setting expectations for correction of MFD. We have also demonstrated that a rumen available methionine analog reduces the risk of MFD and that feeding management has an important role.