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Kevin Spurlin, ANR Extension Agent, Grayson County VCE, spurlink@vt.edu

Milk fat depression is one of the more complex nutritionrelated issues dairymen and their nutritionists face. Dr. Tom Jenkins of Clemson University is one of the foremost experts on this subject. He suggests that five main nutritional factors impact fat test results including: dietary fat amount and source; dietary starch level; amount of fiber, particularly from forages; yeast and mold contamination; and diet management.

This subject is too extensive to cover all the dietary and management interactions related to milk fat depression, so the focus here is on research related to “bad fat”. Zhang et al. (2008) shows that rumen microflora and the products of rumen fermentation were altered due to the addition of unsaturated fatty acids into the rumen. However, not all dietary fat impacts the rumen in the same way.

Researchers have characterized fats fed to cows which are most likely to contribute to milk fat depression as “high risk”, or those which contribute to a high rumen unsaturated fatty acid load (RUFAL). Those include oleic (18:1), linoleic (18:2), and linolenic acids (18:3). The presence of these three in the diet at elevated levels may predispose cows to low milk fat if other conditions also exist.

Rumen microbes don’t like unsaturated fats found in many plant oils. The microbes try to saturate them, and that process— called biohydrogenation—is influenced by other dietary characteristics. Low rumen pH, high starch, and low forage content can exacerbate a high RUFAL and cause milk fat depression. Conversely, a diet with a high RUFAL will not automatically result in low milk fat, especially if some precautions are taken. Those precautions include maintaining at least 0.85% body weight as forage NDF and 1.1—1.2% BW as total NDF, keeping dietary starch levels under 30%, adding buffers up to 0.8% of total diet dry matter, and keeping yeast and mold counts under 1 million cfu/ gram. Manage particle size so that not over 47% diet is in the bottom of Penn State Shaker Box, and 49% or more is in the middle pan.

One important point to consider is the total unsaturated fatty acid content of the diet. It is easy to discount “bad fat” of base ingredients such as forages, protein and starch sources, but they all contribute to the RUFAL. Dr. Jenkins illustrates this point in that a corn silagebased diet can be either low risk or high risk simply because of the fatty acid content of the silage, since it is the largest component of the diet. Forages are not often tested for fatty acid content, and book values may not represent true conditions for a particular sample. It may not pay to routinely test forages for fatty acid profile, but this test may be considered if the fat test dilemma on a farm cannot be explained by other means.

A herd that is experiencing milk fat depression should work with a nutritionist to evaluate indicators of rumen health such as observing rumination behavior and manure consistency. That should be done along with a review of feeding management to ensure the delivery of the diet matches what has been formulated. If the answer to low milk fat is not found in those areas, it may be connected to RUFAL, or too much bad fat.


Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

Publication Date

September 30, 2015

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