Troubleshooting milk fat depression on dairy farms (Proceedings)

Milk fat depression is a common finding of veterinarians and nutritionists on dairy farms, especially during the warmer summer months. Troubleshooting cases of milk fat depression can be challenging as often there are a number of causes that are contributing to the problem. While veterinarians seem to reflexively be concerned about rumen acidosis as the cause of milk fat depression, we cannot fall into the trap of just recommending feeding more long-stemmed forage and thinking the problem will be solved. The following discussion will dig a little deeper into current thoughts on the causes behind milk fat depression and how to utilize that information as you troubleshoot milk fat depression issues on your clients' dairy farms.

Theories for the cause of milk fat depression have included decreased rumen production of acetate and butyrate, the glucogenic-insulin theory of competition for nutrients between the mammary tissue and other tissues, and the trans fatty acid theory (Bauman and Griinari, 2003). The most recent theory is known as the biohydrogenation theory and it was proposed by Bauman and Griinari in 2001 (Bauman and Griinari, 2001). This theory has been further over the past 10 years. At the heart of the theory is the concept that under the right nutritional and ruminal conditions, rumen biohydrogenation is altered and specific fatty acid intermediates are produced that, when absorbed, decrease milk fat synthesis in mammary tissue by causing a decreased expression of enzymes. While a number of specific intermediates in biohydrogenation have been identified, trans-10, cis-12 conjugated lineoleic acid (CLA) was the first and seemingly most important intermediate that was linked to milk fat depression (Baumgard, et al, 2000). Potentially, the production of only 1-2 g/d of trans-10, cis-12 CLA can result in significant milk fat depression (Lock and Bauman, 2007). With that small an amount of intermediate potentially causing milk fat depression and possibly multiple factors leading to the development of that intermediate, it is easy to see why troubleshooting milk fat depression cases can quicly become very complicated.

As we think about troubleshooting milk fat depression it is once again essential to remember that under normal circumstances, C-18 polyunsaturated fatty acids (PUFA) are biohydrogenated to stearic acid (18:0) by way of rumenic and vaccenic acid as shown in Figure 1. Under select conditions, the polyunsaturated fatty acids take a different pathway to get to the final end product of stearic acid. The intermediates in this altered pathway, namely trans-10, cis-12 CLA, are the intermediates identified by the biohydrogenation thory as potent stimulators of milk fat depression. So what leads to the development of the "altered pathway"?

Figure 1. Rumen biohydrogenation pathways. Adapted from Bauman and Griinari, 2003

The causes of the development of the altered pathway can be generally lumped into three separate groups: increased polyunsaturated fatty acid stearic acid precursors (increased PUFA load), altered rates of biohydrogenation, and altered rumen fermentation (Mechor et al, 2009). While this list is by no means meant to be exhaustive, it does provide a framework for performing a systematic investigation of a case milk fat depression

1. Factors affecting PUFA load include the following

     a. Total unsaturated fatty load

     b. Fats and oils in basal ingredients such as corn and corn silage

     c. Amount of fat from coproducts such as DDGS and cottonseed

     d. Fat from animal fats such as tallow or choice white grease or from ruminally inert fat sources.

2. Factors affecting rate of biohydrogenation include the following;

     a. Increased DMI

     b. Increased processing causing finer ration particles

     c. Low effective fiber levels due to ration construction or harvesting at finer or shorter chop lengths

     d. Sorting

     e. Presence of PUFAs

3. Factors leading to altered rumen fermentation include the following:

     a. Ration factors

          i. Decreased ration moisture content

          ii. Improperly mixed tmr

          iii. Short particle length

          iv. Frequency of tmr delivery

          v. Improper diet formulation to produce low effective fiber levels

     b. Ingredient factors:

          i. Condition of fermented feeds

               1) Moisture %,

               2) Presence of yeasts or molds

               3) Processing

               4) Fermentation

          ii. Source and forms of unsaturated fats in the ration

          iii. Source and form of carbohydrates in the ration

          iv. Water

          v. Forage types

          vi. Additives

     c. Cow factors

          i. Eating behavior

          ii. Sorting of tmr

          iii. Heat stress

     d. Facility factors

          i. Overcrowding

          ii. Water availability

          iii. Cooling capability

          iv. Cow comfort

Obviously, there are a great number of factors to investigate when one becomes involved in a milk fat depression situation. Having been involved in a few milk fat depression investigations, it is important to recognize that for the most part, these investigations will take time and require some sampling of feed ingredients. Usually the causes are not simple and easy to find. Often there are multiple causes that together contribute to the development of the problem. With that said, focusing on effective fiber availability, starch level and degradability, and PUFA load oftentimes will lead to a solution to the problem. These situations can be challenging but they are also rewarding as they allow us, as dairy veterinarians, to use our skills and training to solve problems for our clients

References

Bauman, D.E, and J.M. Griinari. 2003. Nutritonal regulation of milk fat synthesis. Annu. Rev.Nutr.23:203-227.

Lock, A.L. and D.E. Bauman. 2007. Milk fat depression: what do we know and what can we do about it? Proceedings of 2007 Penn State Dairy Cattle Nutrition Workshop. pp 9-18.