The Milk Fat Depression Story
Milk fat depression is a problem that has been a concern of dairy producers for over 150 years. During this time, a number of theories have been postulated in an attempt to explain the occurrence of milk fat depression, however most have been shown to be inadequate. Research conducted within the last decade has substantially increased our understanding of the mechanisms underpinning milk fat depression. Based on these discoveries, Bauman and Griinari (2001; 2003) proposed the biohydrogenation theory of milk fat depression. This theory proposes that "under certain dietary situations the rumen environment is altered and this results in changes in the biohydrogenation of unsaturated fatty acids resulting in the formation of unique fatty acids that directly inhibit milk fat synthesis in the mammary gland."
One of the first studies to look at the effects of CLA on milk fat synthesis was performed by Chouinard et al. (1999). This study used abomasal infusion to avoid alteration of the CLA supplement by the rumen bacteria. We provided increasing amounts of a CLA mixture which predominantly contained four isomers (trans-8, cis-10; cis-9, trans-11; trans-10, cis-12; and cis-11, trans-13). Infusion of the supplement caused a dramatic reduction in milk fat synthesis that was reversed when supplementation was terminated. Thus, these results indicated that at least one inhibitor of milk fat synthesis was present in the CLA mixture.
The next step was to identify which CLA isomer or isomers were causing the milk fat depression to occur. We first compared the effects of cis-9, trans-11 CLA, the predominant CLA isomer found in milk fat, to those of trans-10, cis-12 CLA Baumgard et al. (2000). As shown in the graph at the left, trans-10, cis-12 CLA was found to be a potent inhibitor of milk fat synthesis. Subsequently, we investigated the effects of the other two major CLA isomers that were present in the supplement used by Chouinard et al. and found that neither effected milk fat synthesis Perfield et al (2004).
A combination of studies by Baumgard et al. (2001) and Peterson et al. (2002) demonstrated the ability of small doses of trans-10, cis-12 CLA to greatly decrease milk fat production. The curve to the left summarizes data from seven studies and demonstrates the curvilinear decrease in milk fat yield that occurs as increasing amounts of trans-10, cis-12 CLA are abomasally infused (de Veth et al. 2004).
The above effects of trans-10, cis-12 CLA are specific for milk fat and there is no effect on yield of milk or other milk components. The implications of the above research are multifaceted. Research with trans-10, cis-12 CLA has branched out to other animal models looking at effects milk fat synthesis as well as on body fat accretion. In regards to milk fat depression, the trans-10, cis-12 CLA isomer can now be used for more specific investigations into the mechanism by which this situation occurs. The ability to reduce milk fat synthesis in the dairy cow without affecting other production variables may also have potential as a management tool under certain production systems. Our group has investigated some of these management possibilities in more detail.
While the effects of trans-10, cis-12 CLA on milk fat synthesis are clear, they do not completely explain all situations of milk fat depression. Another active area of research in our lab is the identification of other inhibitors of milk fat synthesis. We hope that through identification of other inhibitors we can gain more insight into the reason(s) cases of milk fat depression occur and develop approaches for prevention. At the same time we are building a better understanding of the mechanism by which lipid metabolism is altered by these compounds, and may be able to apply this knowledge to areas other than milk fat synthesis in the lactating dairy cow.