The effect of the biohydrogenation-induced milk fat depression in dairy cows on milk odd- and branched-chain fatty acids across multiple studies

Diet-induced milk fat depression (MFD) caused by UFA and low-fiber diets results in an increase in alternate rumen biohydrogenation intermediates. The effect of these MFD-inducing diets on milk odd- and branched-chain fatty acids (OBCFA) is not well known. The first objective of this study was to characterize the time course of changes in OBCFA in milk fat during induction and recovery of MFD induced with a high-UFA and low-fiber diet in 3 separate experiments. In the first experiment, milk OBCFA were quantified in a previous experiment that sampled milk fat every other day during induction and recovery of MFD. Induction of MFD decreased all OBCFA in milk fat except iso 17:0, which was increased by 15%. The temporal response varied among OBCFA, with iso 15:0, anteiso 15:0, 17:0, anteiso 17:0, and total anteiso FA and OBCFA progressively decreasing after the first day, while iso 14:0, iso 16:0, and total iso FA decreased after d 5. The OBCFA progressively increased during recovery, and most were similar to the control by d 3. Two additional experiments investigated the change in milk OBCFA during recovery from MFD with diets that differed in UFA, fiber, or monensin supplementation. The effect of MFD on milk OBCFA was similar to the results from experiment 1 and the temporal response during recovery was similar to recovery of normal rumen fermentation. The second objective was to characterize the response across MFD experiments conducted in our research group. The database included 7 experiments that fed either a control diet or a diet that caused MFD through either a change in fiber, UFA, or both. The MFD diet decreased most OBCFA in milk fat, including iso 14:0, 15:0, iso 15:0, anteiso 15:0, iso 16:0, 17:0, anteiso 17:0, and the sum totals of iso FA, anteiso FA, and OBCFA. However, iso 17:0 and cis-9 17:1 were not different between the control and MFD diets. In conclusion, milk OBCFA other than iso 17:0 and cis-9 17:1 are reduced when feeding a diet that causes biohydrogenation-induced MFD and the temporal response was similar to the alternate rumen biohydrogenation intermediates and likely indicates the time course of adaptation of rumen microbial populations and fermentation during these diet alterations.