Troubleshooting milk fat depression on dairy farms (Proceedings)


Troubleshooting milk fat depression on dairy farms (Proceedings)

Aug 01, 2011

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.

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