Managing Milk Fat Depression: Interactions of Ionophores, Fat Supplements, and other Risk Factors

The biohydrogenation (BH) theory represents a unifying concept to explain the basis for diet-induced milk fat depression (MFD) where unusual intermediates of ruminal fatty acid BH accumulate in the rumen and eventually reduce milk fat synthesis in the mammary gland. Under certain dietary situations the rumen environment is altered and a portion of BH occurs via a pathway that produces trans-10, cis-12 CLA and trans-10 18:1 (Figure 1). Bifidobacterium, Propionibacterium, Lactococcus, Streptococcus, and Lactobacillus isolates from other habitats have been reported to produce trans-10, cis12-CLA. As these genera occur in the rumen, although generally at rather low numbers, they may contribute to BH and specifically to trans-10, cis-12-CLA formation in the rumen. Propionibacterium, Streptococcus, and Lactobacillus are also more numerous in the rumen with concentrate diets (Jenkins et al., 2008), which would again be consistent with greater trans-10, cis-12 CLA production with concentrate diets. Therefore, dietary situations causing MFD alter the pathways of rumen BH resulting in changes in the specific trans-18:1 and CLA isomers available for uptake by the mammary gland and incorporation into milk fat.