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.