Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    SUPPLEMENTAL CHOLINE AND METHIONINE FOR DAIRY CATTLE: EFFECTS ON PERFORMANCE, LIVER FAT CONTENT AND GENE EXPRESSION, AND PLASMA AMINO ACID AND CHOLINE METABOLITE CONCENTRATIONS
    (2019) Potts, Sarah B; Erdman, Richard A; Animal Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The objectives of this dissertation were to 1) determine markers of post-ruminal choline supply so that bioavailability of rumen-protected choline (RPC) sources can be calculated in lactating cows and 2) investigate the production performance and metabolic effects of supplemental choline, methionine, or both on periparturient dairy cows. Observations from Experiment 1 indicated that of the 26 choline metabolites investigated, including 16 species of phosphatidylcholine (PC) and 4 species of lysophosphatidylcholine, free choline and betaine in blood and milk were most responsive to post-ruminal choline supplied via abomasal infusion. However, RPC did not elicit changes in blood or milk choline metabolites, even at the very high doses tested in Experiment 2. These results suggest that choline supplied as RPC is absorbed differently than choline supplied via abomasal infusion, that RPC is over-protected such that choline supplied in this form is not available to the cow, or that responses to RPC vary depending physiological state of the cow. Results from Experiment 2 indicated that primi- and multiparous cows respond differently to supplemental choline and methionine fed during the periparturient period. Feeding RPC to primiparous cows increased milk yield, while feeding rumen-protected methionine (RPM) had minimal effects on production. In contrast, RPM improved milk components and fat-corrected milk yield for multiparous cows. These observations suggest that primi- and multiparous cows have different methionine and choline requirements in the periparturient period. Investigation into the specific metabolic effects of choline and methionine fed to periparturient cows in Experiment 3 suggested that both RPC and RPM modify choline metabolism. The milk and blood PC profile was altered by both RPC and RPM. In line with this observation, RPC increased hepatic expression of the gene that encodes the enzyme responsible for catalyzing the rate-limiting step of PC synthesis via the CDP-choline pathway. The RPC-induced increase in hepatic betaine-homocysteine methyltransferase expression provided additional support for the connection between choline and methionine metabolism via one-carbon metabolism. Modification of postpartum plasma lactate concentrations by RPC for both primi- and multiparous cows, in conjunction with alterations in pre- or postpartum body condition, also suggest a choline-induced modification of tissue mobilization.
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    CAN CHOLINE SPARE METHIOININE FROM CATABOLISM IN LACTATING MICE AND DAIRY COWS?
    (2009) Benoit, Sarah Lee Ann; Erdman, Richard A; Animal Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Several studies have demonstrated that supplementation of rumen-protected choline (RPC) improves milk production in the lactating dairy cow; however, there are an equal number of studies failing to observe production responses. To date, there are only three studies that provide quantitative information in ruminants on the metabolic fates of methyl groups derived from choline and Methionine (Met). This has limited the ability to predict when, and under what conditions, RPC supplementation will be beneficial. The objectives of this thesis were to determine the interaction of choline and Met methyl group metabolism and the extent of methyl group transfer during lactation, and define what role, if any, is there for RPC in remethylation of homocysteine and in the sparing of Met in lactating animals. A preliminary study with lactating mice consuming a low protein basal diet (10%) was conducted. From 11 to 15 d postpartum, stable isotopes of [methyl,2H3] choline and [methyl,2H3] Met replaced the unlabeled choline and Met in the basal diet to measure the metabolic fates of choline and Met including Met remethylation and sources of Met methyl in the mammary gland. Isotopic analysis revealed that the liver is a major site of Met remethylation from dietary choline with a minimum choline methyl group contribution to Met remethylation of 35%. Mammary tissue was not a major site of Met remethylation from dietary choline (< 10% of Met methyl) as measure by Met methyl in mammary tissue and milk casein. However, there was a significant unlabeled source of methyl groups contributing at a minimum of 45% Met remethylation in the mammary tissue, presumably by de novo synthesis. This suggested that in addition to the liver, the mammary gland is an active site of methyl group transactions. In a subsequent study, lactating dairy cows were fed a total mixed ration formulated to meet the nutrient requirements with exception of metabolizable Met that was restricted to 1.49 % of metabolizable protein. Treatments included a Control (basal diet) and RPC supplemented diet where the basal diet was top dressed with 15g/d RPC, diets were fed in a single reversal design with 2 week experimental periods. Stable isotopes of Met, [1-13C] Met, [13CH3] Met, and [methyl-2CH3] choline were continuously infused on d 14 of each period to determine the metabolic fate and methyl transactions of Met methyl as measured in blood and milk casein. Treatment had no effect on milk production or composition. However, plasma free Met from choline transmethylation was shown to act as a significant contributor to casein synthesis. Fractional Met remethylation rates in the control and RPC treatments were 26 and 23%, respectively. Methionine Met methyl loss within the mammary tissue appears to be minimal. Based on casein Met enrichment, upwards of 40% of Met present in casein had undergone transmethylation with choline serving as the ultimate methyl donor. Furthermore, plasma versus casein Met methyl enrichment data suggested that a significant amount of de novo methyl group synthesis occurs in the dairy cow's mammary gland with choline serving as a major methyl donor.