Milk fat depression is a metabolic disorder characterized by a sustained reduction in milk fat and has challenged producers and scientists for over a century. Nutritionists have incorporated supplemental fat in dairy rations for years as a means to increase the energy density of the diet, but often decreased fiber digestibility or milk fat depression was observed. The rumen microbial populations were thought to play a major part in why producers had difficulty using high levels of unsaturated fat. Many different theories evolved throughout the century into the causes of milk fat depression that often appears ex nihilo. Such theories involved concepts centered on substrate limitation, low rumen pH, and specific bioactive fatty acids. The biohydrogenation theory became one of the more widely accepted explanations for milk fat depression. Since the discovery of certain fatty acid isomers that induce milk fat depression more investigations have aimed at how different nutrients interact in the rumen and affect the production of these milk fat inhibiting isomers. Readily available carbohydrates such as sugar, starch, neutral detergent soluble fiber, and rumen availability of these carbohydrate fractions, all effect the microbial populations within the rumen that are involved in the process of biohydrogenation. This review summarizes the knowledge available on milk fat depression, microbial influences, how soluble carbohydrate fractions influence on biohydrogenation.
Koch LE, Lascano GJ