One-carbon Metabolite Supplementation to Heifers for the First 14 D of the Estrous Cycle Alters the Plasma and Hepatic One-Carbon Metabolite Pool and Methionine-Folate Cycle Enzyme Transcript Abundance in a Dose-Dependent Manner

The objective of this study was to determine the dose of folate and vitamin B-12 in beef heifers fed rumen protected methionine and choline required to maintain increased B-12 levels and intermediates of the methionine-folate cycle in circulation. Angus heifers (n = 30; BW = 392.6 +/- 12.6 kg) were individually fed and assigned to one of five treatments: 0XNEG: Total mixed ration (TMR) and saline injections at day 0 and 7 of the estrous cycle, 0XPOS: TMR, rumen protected methionine (MET) fed at 0.08% of the diet DM, rumen protected choline (CHOL) fed at 60 g/d, and saline injections at day 0 and 7, 0.5X: TMR, MET, CHOL, 5 mg B-12, and 80 mg folate at day 0 and 7, 1X: TMR, MET CHOL, 10 mg vitamin B-12, and 160 mg folate at day 0 and 7, and 2X: TMR, MET, CHOL, 20 mg B-12, and 320 mg folate at day 0 and 7. All heifers were estrus synchronized but not bred, and blood was collected on day 0, 2, 5, 7, 9, 12, and 14 of a synchronized estrous cycle. Heifers were slaughtered on day 14 of the estrous cycle for liver collection. Serum B-12 concentrations were greater in the 0.5X, 1X, and 2X, compared with 0XNEG and 0XPOS on all days after treatment initiation (P < 0.0001). Serum folate concentrations were greater for the 2X treatment at day 5, 7, and 9 of the cycle compared with all other treatments (P <= 0.05). There were no differences (P >= 0.19) in hepatic methionine-cycle or choline analyte concentrations by treatment. Concentrations of hepatic folate cycle intermediates were always greater (P <= 0.04) in the 2X treatment compared with the 0XNEG and 0XPOS heifers. Serum methionine was greater (P = 0.04) in the 0.5X and 2X heifers compared with 0XNEG, and S-adenosylhomocysteine (SAH) tended (P = 0.06) to be greater in the 0.5X heifers and the S-adenosylmethionine (SAM):SAH ratio was decreased (P = 0.05) in the 0.5X treatment compared with the 0XNEG, 0XPOS, and 2X heifers. The hepatic transcript abundance of MAT2A and MAT2B were decreased (P <= 0.02) in the 0.5X heifers compared with the 0XNEG, 0XPOS, and 2X heifers. These data support that beef heifers fed rumen protected methionine and choline require 20 mg B-12 and 320 mg folate once weekly to maintain increased concentrations of B-12 and folate in serum. Furthermore, these data demonstrate that not all supplementation levels are equal in providing positive responses, and that some levels, such as the 0.5X, may result in a stoichiometric imbalance in the one-carbon metabolism pathway that results in a decreased SAM:SAH ratio. This study determined the optimal doses for vitamin B-12 (20 mg injected weekly) and folate (320 mg injected weekly) injection to beef heifers during the periconceptual period when rumen protected methionine and choline were fed at 0.08% of dry matter and 60 g/d, respectively. These doses set the basis upon which future studies can be based when feeding and injecting one-carbon metabolites. Lay Summary The strategic inclusion of one-carbon metabolites, which include vitamins and minerals that are found in human prenatal vitamins, to beef cattle feeding and management protocols during the periconceptual period (the time around breeding) is a novel concept. Therefore, this study aimed to identify the feeding and injection doses of one-carbon metabolites in beef heifers to maintain increased circulating concentrations of one-carbon metabolites for use as a model from which other studies could base their treatments on. We determined that daily feeding of methionine and choline at 0.08% of dry matter and 60 g/d, respectively, and administration of vitamin B-12 and folate at 20 mg and 320 mg once per week, respectively resulted in sustained elevated concentrations of one-carbon metabolites.