Effects of short-term rumen-protected folic acid supplementation on pregnancy rate, one-carbon metabolism, and steroid hormone in Holstein dairy cows

This study investigated the effects of rumen-protected folic acid (RPFA) supplementation on pregnancy rate at first artificial insemination (AI) and metabolic parameters in dairy cows. A total of 256 multiparous Holstein dairy cows were blocked by parity, average daily milk yield, days in milk, and body condition scores, and then randomly assigned to 1 of 4 treatments: cows were orally given 0, 3, 6, or 9 g RPFA per day. RPFA feeding was started from 50 ± 3 days in milk through 36 days post-timed AI. Pregnancy rate at first-AI was not significantly affected by RPFA supplementation. Comparisons of folic acid metabolism, one-carbon metabolism, and steroid hormone concentration in both blood plasma and follicular fluids were carried out between 3 g RPFA group and control. Cows supplemented with 3 g RPFA had greater plasma concentrations of folic acid, 5-methyltetrahydrofolate (5-MTHF), S-adenosylmethionine, and leucovorin, while reduced homocysteine and p-aminobenzoylglutamate. One-carbon metabolic analysis showed 3 g RPFA supply increased folinic acid and 5-MTHF concentrations in follicular fluids. Meanwhile, cows in 3 g RPFA group had elevated plasma estradiol concentration, while decreased androstenedione in both plasma and follicular fluids. Enhanced CYP19A1 gene expression in granulosa cells was examined when supplied with 50 μM folic acid in vitro. These findings indicated that the pregnancy rate at first-AI were not affected by short-term RPFA supplementation during the estrus synchronization period, it reduced plasma homocysteine levels without altering follicular fluid homocysteine concentrations. Additionally, RPFA supplementation was associated with changes in steroid hormone regulation, although further studies are needed to investigate the underlying mechanisms.