Keeping an eye on milk fat production is important because milk fat is a valuable part of milk and is very responsive to herd management. Milk fat concentration is very variable from farm to farm and even between cows on the same farm. Every producer has a different goal for milk fat and has a different definition of “low milk fat.” However, milk fat is a valuable part of what goes into the bulk tank and has a large impact on the efficiency of converting feed to milk, so maximizing milk fat is an important goal. In the research lab, we normally define milk fat depression as milk fat production below the genetic potential of the cow. There are important genetic differences between cows that explain some of the variation in milk fat, but do not explain sudden decreases. Although it is much more difficult to calculate, it is very important to determine if milk fat yield has been reduced or if milk yield has simply increased. Research and on-farm observations clearly show a very large impact of nutrition on milk fat yield
What Causes Milk Fat Depression?
High producing dairy cows have a high-energy requirement, so we commonly increase the fermentability (rumen digestibility) of diets by feeding high starch grains and higher quality forages or feed additional fat. However, it was recognized nearly 150 years ago that diets that are highly fermentable or contain unsaturated plant fat can reduce milk fat yield. The diets that are associated with reduced milk fat cause many changes in the rumen environment (ex. reduced pH), which results in a change in the microbial population because some rumen microorganisms cannot grow well under the new conditions while others grow better. There are many implications for the cow that were thought to cause reduced milk fat, including a change in the ratio of the two main volatile fatty acids produced during fermentation (acetate and propionate) and reduced fiber digestion. For many years we thought that the changes in volatile fatty acids resulted in low milk fat synthesis. However, the altered microbial population also results in a change in how the microbes modify unsaturated fat in the diet. Under unstable rumen fermentation microbes produce different fatty acids, some of which are potent inhibitors of milk fat synthesis in the cow. Research over the past dozen years has described how these bioactive fatty acids reduce milk fat.
The Rumen is Complicated
The good news is that we know what causes milk fat depression, but the bad news is that we cannot accurately predict when alternate rumen fermentation is going to occur. Numerous feed and management factors can destabilize fermentation and result in milk fat depression. Major risk factors include diet fermentability (ex. starch), forage particle size, unsaturated fat concentration, rumensin, slug feeding, and numerous other factors. Almost all diets balanced for modern dairy cows include some risk factors. The extent and interaction of risk factors is very important and makes prediction of rumen fermentation difficult. Importantly, stable rumen fermentation has many additional advantages including increased rumen protein synthesis, high fiber digestibility, and improved cow health.