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Milk fat is a valuable part of what goes into the bulk tank

Kevin Harvatine and Daniel Rico Published on 31 October 2011
cows at feed bunk

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 highly fermentable or containing unsaturated plant fat can reduce milk fat yield.


The diets associated with reduced milk fat cause many changes in the rumen environment (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 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 (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.

Recent research at Penn State

Dietary factors that cause milk fat depression have almost exclusively been studied through induction of milk fat depression. This is useful because it tells us what dietary factors cause milk fat depression, but it does not directly tell how to recover or accelerate recovery once you have milk fat depression.

We recently conducted a time course experiment to characterize the timing of induction and recovery of diet-induced milk fat depression. We induced milk fat depression by feeding a low-fiber and high-soybean oil diet and then recovered by feeding a higher-fiber and low-oil diet. We took milk samples every other day to observe the milk fat yield over time.

Milk fat yield decreased progressively when fed the low-fiber and high-oil diet and was significantly decreased after seven days. When switched to the recovery diet, milk fat yield progressively increased and was not different from control by day 11.

A key insight from the experiment is the expected lag between making diet adjustments and milk fat synthesis. Addition of a risk factor may cause milk fat depression in 7 to 10 days and elimination of a risk factor is expected to take 10 to 14 days to observe a benefit.

Knowing the time course is very important to identify what may have caused milk fat depression and knowing how long to wait to determine if a diet correction has been effective in improving milk fat.

The bottom line

Maintaining stable rumen fermentation is important to the efficiency and health of dairy cows, including optimal milk fat synthesis. Milk fat depression is caused by an inhibition of milk fat synthesis by specific fatty acids produced in the rumen during altered fermentation.

We have a clear understanding of the mechanism of milk fat depression and associated risk factors, but have a poor ability to predict when milk fat depression will occur because of unpredictable feeds and complex interactions that occur in the rumen.

A lag of 7 to 10 days is expected if a diet adjustment induces milk fat depression, and diet corrections will take 10 to 14 days to rescue milk fat synthesis. The question remains if we can accelerate recovery from milk fat depression. PD

Dr. Kevin Harvatine is assistant professor of nutritional physiology, and Daniel Rico is a Ph.D. candidate at Penn State University’s Department of Dairy and Animal Science .

References omitted due to spaceÂbut are available upon request to .

—Excerpts from Penn State Dairy Digest, April 21, 2011

PHOTO: For many years we thought 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. Photo by PD staff.

Kevin Harvatine Ph.D.
  • Kevin Harvatine Ph.D.

  • Assistant Professor of Nutritional Physiology
  • Penn State University
  • Email Kevin Harvatine Ph.D.