| Summary: | Doctor of Philosophy === Department of Animal Sciences and Industry === Barry J. Bradford === The ability of nutritional supplements to generate responses in productive animals at different physiological stages, and their interaction with the particular gastrointestinal tract of ruminants have created the necessity to explore effects beyond productivity. Modulation of immune function and inflammatory processes, modifications of nutrient metabolism, and interactions with the ruminal microbial population are effects attributed to supplements that encouraged the formulation of the set of experiments described in this dissertation. The first experiment was designed to test the effects of arachidonic and docosahexaenoic acids supplemented in milk replacer for Holstein calves during the pre-weaning period on the immune system, lipid and glucose metabolism, and growth performance when animals underwent a vaccination protocol. Neither supplemental fatty acid source affected productivity, cytokine production, antibody production, or CD4 and CD8 cell proliferation. A treatment effect on glucose and NEFA plasma concentration was observed. Polyunsaturated fatty acids, mainly DHA, lowered glucose and NEFA levels compared to control; moreover, a dose effect was observed indicating that increasing amounts of PUFA decreased plasma glucose level. The second experiment tested the effects of Co carbonate (CoCarb) and Co glucoheptonate (CoGH) at different concentrations of Co on in vitro fermentation rate, fermentation end-products, and DM and NDF disappearance. Dry matter and NDF disappearance increased with CoGH relative to CoCarb at 1.0 ppm Co or less. CoCarb at > 3.0 ppm appeared to stimulate the biohydrogenation of long-chain fatty acids whereas CoGH had limited effects on this process. Finally, the third study evaluated responses to chromium propionate during peak lactation and interactions between chromium and rumen-protected lysine and methionine. Chromium propionate increased feed intake and tended to increase energy-corrected milk yield. Primiparous cows showed greater responses in feed intake and milk protein yield than multiparous cows. In this study, feeding chromium propionate near peak lactation increased feed intake and tended to increase productivity but no benefits of supplementing rumen-protected lysine and methionine were observed. Overall, nutritional supplements helped to regulate different biological functions in ruminants; their utilization is not always is feasible, but the results of these experiments provide guidance about effectiveness during different physiological situations.
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