Evaluate Fat Sources Carefully

July 26, 2013 11:16 AM

Your cows need it. But what type of fat – and how much – should your herd’s rations include?

Essie Evans 7 13By Dr. Essi Evans, Essi Evans Technical Advisory Services, Inc.

Fat is a key source of storage for energy since tissues use stored fat when cows are in negative energy balance. Cows have the ability to synthesize fat for tissue storage and milk fat, but it is more efficient to use fat obtained from the diet for these purposes.

The big questions to answer when formulating diets are: What type of fat and how much of these fat sources should rations include?

How Much Fat?

A rule of thumb is to formulate rations with adequate levels of fat in the diet to roughly equal the amount of fat cows produce in milk.

Therefore, if a group of cows produces 100 pounds of milk with a 3.5% fat test, daily intake should not be more than 3.5 pounds of dietary fat in the ration.

What Kind of Fat?

The question of what kind of fat to include in rations is a bit more difficult to answer.

Fat is made up of fatty acids, much in the same way proteins are made up of amino acids. Fatty acids can be:
• Saturated (usually limited to tallow in dairy diets)
• Monounsaturated
• Polyunsaturated

How Do Fats Work?

Saturated fats do not have a significant impact on rumen fermentation, and pass through the rumen into the small intestine for digestion. However, saturated fats are not digested as well as unsaturated fats.

In general, rumen microbes try to saturate the mono- and polyunsaturated fatty acids. High dietary levels of unsaturated fatty acids can disrupt rumen fermentation leading to a reduction in the amount of valuable microbial protein that is produced.

In addition, high dietary levels of unsaturated fatty acids will reduce the amount of fiber that is digested in the rumen. Furthermore, partial saturation of the polyunsaturated fatty acids can interfere with milk fat synthesis. For example, polyunsaturated fatty acids are a key contributor to milk fat depression.

But polyunsaturated fatty acids are not entirely bad. Lactating dairy cows actually require two polyunsaturated fatty acids: linoleic acid and linolenic acid. Saturation (or hydrogenation) of all fatty acids is not the answer.

It is important that enough of these fatty acids escape the rumen biohydrogenation process to support cows’ needs.

Fat Sources

Fats are derived from a variety of dietary ingredients, as well as supplemental sources. Fats can be considered to be rumen active or rumen inert.
• Rumen active fats contribute a significant amount of fatty acids into the rumen. Examples include forages, byproducts like distillers grains and ground oilseeds.
• Inert fats include whole oilseeds, calcium salts of fatty acids and hydrogenated fats.

Lately, a number of new products have entered the supplemental fat market containing almost exclusively one fatty acid: palmitic acid (C 16:0). Many producers want to know if this rumen inert product is superior to others on the market. To help answer this question, following is a look at published research results.

Research Results

A few short-term studies have been conducted with these products. Most of the studies compared added palmitic acid to no added fat. The inclusion of any fat supplement tends to increase energy density and therefore improve feed efficiency.

In short-term research studies, fat yields have decreased, increased or stayed the same.

For example, a University of California study showed that milk fat percentage, but not yield, was reduced when palmitic acid was added. In contrast, a study at Michigan State University reported in 2011 showed that milk fat yield increased when palmitic acid was provided during a 25-day feeding period compared to no supplemental fat addition.

Only two reports were found that compared differences between palmitic acid and other fat sources.
• Two short-term trials were conducted at Penn State University.

  1. In the first study, short term differences between MEGALAC® Rumen Bypass Fat and palmitic acid on a percentage basis were recorded, but not on a yield basis.
  2. In the second study fat yield was higher with the palmitic acid supplement. However, the feeding periods were only 14 days in length.
Only one long-term study has been conducted to date. Results show that palmitic acid appeared to be less promising after four weeks into the feeding period.
• In a University of Delaware 12-week study where production persistency could be evaluated, results showed that milk production and milk component production were higher with MEGALAC when compared with the palmitic acid supplement.

What Does This Mean?

In conclusion, rumen inert fatty acid supplements provide a concentrated source of energy to cows. Cows respond to added fatty acid supplements, but time is needed to see the real response.

When evaluating diets on-farm, it is important to remember that pounds of milk fat, and not just changes in milk fat percentage, are the defining standard of ration success.

Contact Dr. Evans at essievans@sympatico.ca.

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