Relationship between oxygenated fatty acid and milk fat concentration during diet-induced milk fat depression in dairy cows.

Altered rumen biohydrogenation of UFA is central to the current understanding of diet-induced milk fat depression (MFD) in dairy cows, and associations with numerous trans fatty acids (FA) have been characterized. Although some specific CLA isomers have been demonstrated to be antilipogenic, they fail to account for the full decrease in milk fat synthesis during diet-induced MFD, indicating that other undiscovered bioactive causative actors likely exist. Microbial FA metabolism also results in synthesis of oxy and hydroxy FA and the objective of the current study was to quantify changes in these during diet-induced MFD. We hypothesized that 10-oxo-stearic and 10-hydroxy-stearic acid (10-O-18:0 and 10-OH-18:0) would be increased in milk fat during diet-induced MFD. Milk fat samples from 2 experiments that fed diets resulting in MFD were analyzed. In the first experiment (n = 24), diet-induced MFD increased milk fat concentration of 10-O-18:0 from 0.04% to 0.14% of FA and 10-OH-18:0 from 0.07% to 0.21% of FA, whereas milk fat concentration was decreased 43% and milk fat trans-10 18:1 increased from 0.04% to 12.0% of FA. A second experiment that resulted in varying degrees of MFD between the cows was combined to create a dataset for regression analysis (n = 96; trans-10 18:1 ranged from 0.27% to 6.69% of FA). In this dataset, 10-O-18:0 and 10-OH-18:0 were quadratically related to trans-10 18:1 and ratio of trans-10 18:1 to trans-11 18:1 in milk fat but not trans-11 18:1. These results indicate an association of the oxygenated FA 10-O-18:0 and 10-OH-18:0 with diet-induced MFD. Their causative roles in the physiology of MFD warrant further investigation.