"Yield of milk components continues to be the principal driver of variation in producer milk payments. Therefore, diet-induced milk fat depression negatively impacts financial income of dairy farmers. Our overall objective is to determine the effects of dietary factors and rumen pH and their interactions on biohydrogenation pathways and the formation of biohydrogenation intermediates (e.g. t10, c12 conjugated linoleic acid, [CLA]) that limit fat synthesis in the mammary gland. By using an in vitro batch culture system in the first three experiments, we determined the effects of common dietary factors (dietary unsaturated fatty acid concentration, starch content and starch fermentability, and Saccharomyces cerevisiae fermentation product) and culture pH on biohydrogenation pathways, as well as their interactions. In all three experiments, culture pH had the greatest influence on biohydrogenation pathways, with low culture pH increasing the formation of t10, c12 CLA in vitro. In the first experiment, low culture pH and increasing concentration of corn oil increased the formation of t10, c12 CLA. Increasing corn oil concentration at low culture pH increased t10, c12 CLA concentration. In the second experiment, low culture pH, combined with highly fermentable starch (high moisture corn), increased t10, c12 CLA concentration. Although starch fermentability did not affect t10, c12 CLA overall, high starch content provided by high moisture corn increased t10, c12 CLA concentration at low culture pH. In the third experiment, highly fermentable starch (high moisture corn) at low culture pH increased t10, c12 CLA concentration. Rumen fluid collected from cows supplemented with Saccharomyces cerevisiae fermentation product decreased t10, c12 CLA concentration, especially when combined with high moisture corn at low culture pH. The fourth experiment was an in vivo study, which determined the effect of production level on severity of diet-induced milk fat depression and biohydrogenation pathways for mid- and late lactation cows. A milk fat depression-inducing diet decreased milk fat content and fat yield, and increased t10, c12 CLA concentration in milk. 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 t10, c12 CLA concentration than lower producing cows. Cows fed a milk fat depression-inducing diet had a lower mean rumen pH and greater rumen pool of t10, c12 CLA than cows fed a control diet. Dietary factors interacted with rumen pH to influence biohydrogenation pathways and t10, c12 CLA concentration, and production level also impacted cow response to diet-induced milk fat depression. Further work is required to clarify interactions between dietary factors and rumen pH and their effects on rumen bacterial populations. Mechanisms behind the interaction between production level and diet induced-milk fat depression are still unclear and should be examined."--Pages ii-iii.
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