ANIMAL NUTRITION: NEW STRATEGIES FOR NUTRITIONAL OPTIMIZATION

This dissertation focuses on the impact of nutrition to modulate and optimize milk production, blood metabolites and liver metabolism. Specifically, the main aim was to elucidate the effects of rumen protected choline (RPC) supplementation to lactating dairy cows on production, metabolic health and hepatic gene expression, for which two different studies were performed. The first study, a meta-analysis of the effect of RPC supplementation on milk yield (MY), nonesterified fatty acids (NEFA) and β-hydroxybutyrate (BHBA) in lactating dairy cows was performed in order to obtain an overall view of the effect of rumen protected choline. Rumen protected choline supplementation has been reported to have a positive effect on milk yield and metabolic health in lactating dairy cows. In light of this, a meta-analysis has been performed in order to elucidate the effect of choline chloride supplemented as RPC on both milk yield and selected blood metabolites. For this purpose, 21 peer-reviewed articles published from 1985 to 2016 were selected. This systematic review was carried out to evaluate the effects of RPC supplementation on MY, NEFA, and BHBA. Results obtained showed positive effects of RPC supplementation on MY in lactating dairy cows. The studies selected for this meta- analysis supplemented choline chloride in a range from 6.25 to 50g/d and milk production increase averaged 2.14±1.86 kg/d. Meta-regression on the dose- response relationship between dietary RPC and MY was significant. When NEFA and BHBA were evaluated, no overall effect was detected. Moderator analysis revealed that all outcomes, MY, NEFA and BHBA, were not significantly affected by the mode of choline supplementation (blended vs. topdressed). In the second study, the mechanism beyond the metabolic changes due to RPC was investigated, with emphasis on hepatic gluconeogenesis, lipid oxidation and transport that occur during the transition period; particularly, the interaction of RPC and dietary energy concentration was tested and the expression of selected hepatic genes was analyzed. Hepatic gluconeogenic and oxidation genes were studied during the transition from late pregnancy to early lactation dependent upon RPC supplementation during the periparturient period, and prepartum energy intake. Controlling prepartum energy intake or supplementing RPC during the periparturient period, are two strategies to optimize hepatic metabolic function. At -48 days relative to calving (DRTC), multiparous Holstein cows were assigned to either a controlled (1.40 Mcal of NEL/kg DM; CE) or high (1.63 Mcal NEL/kg DM; HE) energy prepartum diet with or without RPC (top-dressed daily from -21 to +21 DRTC). Postpartum diets only differed in addition vs absence of RPC. Liver tissue biopsy samples were collected at -14, +7, +14, and +21 DRTC for RNA isolation and cDNA generation (n=16/treatment). Six genes involved in gluconeogenesis, lipid oxidation and lipid transportation were selected. Results obtained indicate that an increase in the expression of pyruvate carboxylase mRNA was reduced in cows receiving RPC after calving, which suggests that RPC had improved energy status and carbohydrate metabolism in the liver and reduced the need for pyruvate carboxylase. RPC supplementation decreased PCK1 in HE+RPC probably due to higher oxidation of increased circulating NEFA that translated to increased oxidative capacity of the TCA cycle. Moreover, this change can help to maintain the oxaloacetate pool. No effect of RPC supplementation was observed with regard to CPT1A, which is involved in fatty acid transportation. On the other hand, PPARA and MTTP were affected by RPC treatment, indicating that RPC supplementation may have modulated FA transport and oxidation. In light of the two studies performed, RPC can improve milk production through lactation. Additionally, RPC supplementation may support and increase hepatic oxidative capacity.