Oral l-arginine supplementation impacts several reproductive parameters during the postpartum period in mares

l-Arginine is an amino acid which can alter pituitary function and increase blood flow to the reproductive tract. The objective was to determine the effect of supplementing 100 g of l-arginine on plasma arginine concentrations, follicular dynamics and ovarian and uterine artery blood flow during the estrus that occurs subsequent to foaling. In Experiment 1, mares were fed 100 g l-arginine for 1 day during the last 3 weeks of pregnancy and plasma samples taken for every hour for the first 4 h and every other hour until 12 h. l-Arginine supplementation elevated plasma arginine concentrations from 1 to 8 h post feeding; arginine peaked at 6 h (arginine: 515 ± 33 μmol/L; control: 80 ± 33 μmol/L). In Experiment 2, mares received either 100 g l-arginine or control diets beginning 21 d before the expected foaling date and continued for 30 d postpartum. The reproductive tract was evaluated by transrectal Doppler ultrasonography from Day 1 postpartum through Day 30. There were no differences in ovarian follicular dynamics, ovarian or uterine resistance indices between groups. Vascular perfusion of the F1 follicular wall was greater in l-arginine supplemented mares (37.3 ± 2.6%) than controls (25.4 ± 2.7%; P < 0.05). l-Arginine supplemented mares had a smaller uterine body and horns and accumulated less uterine fluid than controls (P < 0.05). The combination of reducing uterine fluid accumulation, while not altering follicular development, raises the possible use of l-arginine supplementation as a breeding management tool during the postpartum period to increase reproductive success.