Young Scholar Presentation: Effects of Dietary Factors and Rumen pH on Rumen Biohydrogenation Pathways and Risk of Milk Fat Depression

Our overall objective was to determine the effects of dietary factors and rumen pH and their interactions on biohydrogenation pathways and the formation of biohydrogenation intermediates (e.g. trans-10, cis-12 18:2) that reduce fat synthesis in the mammary gland. By using an in vitro batch culture system in the first three experiments, we determined the effects of culture pH, common dietary factors (unsaturated fatty acid concentration, starch content and starch fermentability), as well as Saccharomyces cerevisiae fermentation product on biohydrogenation pathways. In all three experiments, culture pH had the greatest influence on biohydrogenation pathways, with low culture pH increasing the formation of trans-10, cis-12 18:2 (all P < 0.01). In the first experiment, increasing unsaturated fatty acid concentration, provided by corn oil, at low culture pH resulted in a greater increase in trans-10, cis-12 18:2 concentration than at high culture pH (interaction P < 0.05). In the second experiment, starch content and fermentability interacted with pH to affect trans-10, cis-12 18:2 concentration; high moisture corn at high starch content and low culture pH increased trans-10, cis-12 18:2 concentration (interaction P < 0.01). In the third experiment, rumen fluid collected from cows supplemented with Saccharomyces cerevisiae fermentation product decreased trans-10, cis-12 18:2 concentration, especially in cultures containing high moisture corn at low culture pH (interaction P < 0.01). The fourth experiment was an in vivo study, which determined the effect of production level on rumen biohydrogenation and risk of diet-induced milk fat depression. A milk fat depression-inducing diet decreased milk fat content (P < 0.01) and milk fat yield (P = 0.05), and increased trans-10, cis-12 18:2 concentration in milk (P < 0.01) and rumen pH range (P = 0.01). Rumen pH range was negatively correlated with milk fat concentration and positively correlated with trans-10, cis-12 18:2 concentration in milk (both P < 0.01). Higher producing cows were at higher risk for diet-induced milk fat depression, exhibiting greater reductions in milk fat content and yield and a greater increase in milk trans-10, cis-12 18:2 concentration than lower producing cows (all interaction P < 0.10). Our results indicate that dietary factors interacted with rumen pH to influence biohydrogenation pathways and trans-10, cis-12 18:2 concentration, rumen pH was the main driver of changes in biohydrogenation pathways, and production level also impacted cow response to diet-induced milk fat depression.