l-Arginine supplementation 0.5% of diet during the last 90 days of gestation and 14 days postpartum reduced uterine fluid accumulation in the broodmare

l-Arginine is an essential amino acid in many species that has been shown to influence reproduction. However, in horses a dose of 1% l-arginine of total dietary intake impaired absorption of other amino acids, whereas a dose of 0.5% did not. The objectives of this experiment were to evaluate postpartum parameters on mares supplemented with 0.5% l-arginine through the last 90 d of gestation and 14 d postpartum. Sixteen light-horse mares were randomly divided in two groups: 8 mares supplemented with 0.5% l-arginine and 8 mares fed an isonitrogenous equivalent. Gestation length, days to uterine clearance and days to first ovulation were compared. Uterine body depth, diameter of uterine horns, and length of largest pocket of uterine fluid were recorded daily via transrectal ultrasound. Measurements of foal weight, height, and cannon bone circumference were recorded for 9 weeks. Arginine treatment had no effect on gestation length (P = 0.58). Supplemented mares cleared fluid quicker postpartum (6.8 ± 0.53 d; P = 0.026) compared to control (9.0 ± 0.38 d). Mares supplemented with l-arginine had smaller diameter of fluid present in the postpartum uterus (P ≤ 0.05). Days to first postpartum ovulation were not affected by treatment nor any influence on uterine involution. Finally, treatment had no effect on any foal's measured parameters. l-Arginine supplementation fed at 0.5% of daily intake during the last 90 d of gestation and early postpartum in mares decreased uterine fluid accumulation, yet did not appear to have any effect on any other parameters measured.