1437
Dietary melatonin supplementation during late gestation alters concentrations of progesterone and milk yield in Holstein heifers
The objective was to examine the effects of supplementing dietary melatonin during late gestation Holstein heifers on maternal concentrations of progesterone and estradiol-17β as well as subsequent milk yield during the first 30 days of lactation. On d 190 of gestation, heifers (n = 20) were blocked by body weight and then randomly assigned to one of two dietary treatments: 1). 20 mg of dietary melatonin per day (MEL) or 2). no melatonin supplementation (CON). At 0800 h, MEL heifers received 0.7 kg of grain top dressed with 2 mL of 10 mg/mL melatonin in ethanol while CON heifers received 0.7 kg of grain top dressed with 2 mL of ethanol alone. A TMR was provided after grain consumption. Supplementation ceased on d 262 of gestation for both treatment groups. Blood samples were collected on d 180 (baseline), 210, 240, and 262 of gestation. Serum concentrations of progesterone and estradiol-17β were determined via radioimmunoassay. Milk yield was recorded for the first 30 days of lactation. Dependent variables were analyzed using repeated-measures ANOVA of the MIXED procedure of SAS with the model statement containing dietary treatment, day, and their respective interaction. Main effects of dietary treatment or day are discussed in the absence of significant (P < 0.05) treatment by day interactions. Serum progesterone was decreased (P < 0.05) by 12% in MEL vs. CON heifers. Moreover, concentrations of progesterone were decreased (P < 0.0001) on d 262 of gestation vs. d 180, 210, and 240. Serum estradiol-17β tended to be decreased (P = 0.06) by 19% in MEL vs. CON heifers. Concentrations of estradiol-17β increased (P < 0.0001) as gestation proceeded. Gestation length was not different (P > 0.50) between treatments and averaged 275 ± 2 d. Daily milk yield showed a treatment by day interaction (P< 0.01), whereby milk yield was increased by 41% and 33% on d 2 and 3 of lactation in MEL vs. CON heifers, respectively. Dietary melatonin supplementation during late pregnancy altered steroid synthesis and/or clearance. In addition, the decreased concentrations of steroids during late gestation in MEL heifers had no adverse effects on subsequent milk yield during early lactation.
Keywords: melatonin, pregnancy, progesterone