Effect of Melatonin Supplementation during Mid- to Late- Gestation on Maternal Uterine Blood Flow and Calf Size at Birth

Melatonin is a strong antioxidant and causes an increase in uteroplacental blood flow when supplemented during gestation. The objective of the current study was to examine the effects of supplemental melatonin on uterine blood flow from mid- to late- gestation in cattle. Commercial beef heifers (n = 32) and cows (n = 25) were artificially inseminated and assigned to one of two treatment groups supplemented with (MEL) or without (CON) melatonin delivered as two- 24 mg implants or placebo at d 180, 210, and 240 of gestation. Uterine artery blood flow was determined using color Doppler ultrasonography. At birth, calf body weight, thoracic girth, abdominal girth, curved crown rump (CCR) length, and ponderal index (BW [kg]/CCR [m]³) were determined. Total uterine artery blood flow (summation of left and right arteries) were analyzed using repeated measures of the MIXED procedure of SAS and the model statement included day, treatment, breed, fetal sex and the respective interaction. Calf morphometric measurements were analyzed using the MIXED procedure of SAS and the model statement included treatment, calf sex and the respective interaction with gestation length as a covariant. Heifers and cows were analyzed separately due to a difference in breed between the two groups. In heifers, a treatment by day interaction (P = 0.01) was observed for total uterine artery blood flow, whereby total uterine artery blood flow was increased in MEL vs. CON on d 240 of gestation. In cows, total uterine artery blood flow increased (P < 0.05) in MEL vs. CON. Gestation length was increased (P < 0.05) in MEL heifers gestating male calves vs. female calves, while CON heifers gestating male and female calves were intermediate. In cows, gestation length was not different (P = 0.15) between treatments. At birth, calf weight, thoracic girth, abdominal girth, CCR, and ponderal index were not different (P > 0.05) between treatments in heifers. In cows, calf weight, thoracic girth, and abdominal girth were not different (P > 0.05) between treatments. However, CCR was increased (P = 0.03) and ponderal index was decreased (P = 0.01) in calves born to MEL treated cows vs. CON. In summary, melatonin supplementation increased uterine blood flow in mid- to late- gestating cattle but this was not accompanied by an increase in calf birth weight. Additional studies regarding the physiological impact of these hemodynamic changes on nutrient transport and offspring development are warranted.