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Melatonin alters bovine uterine artery hemodynamics, vaginal temperatures and fetal morphometrics during late gestational nutrient restriction in a season-dependent manner.

The objectives were to examine melatonin mediated changes in temporal uterine blood flow (UBF), vaginal temperatures (VT), and fetal morphometrics in 54 commercial Brangus heifers (Fall, n = 29; Summer, n = 25) during compromised pregnancy. At d160 of gestation, heifers were assigned to 1 of 4 treatments consisting of adequately fed (ADQ-CON; 100% NRC; n = 13), global nutrient restricted (RES-CON; 60% NRC; n =13), and ADQ or RES supplemented with 20 mg/d of melatonin (ADQ-MEL, n = 13; RES-MEL, n = 15). In the morning (0500 h; AM) and afternoon (1300 h; PM) of d220 of gestation, UBF was determined via Doppler ultrasonography while temperature data loggers attached to progesterone-free CIDRs were used to record VT. At d240 of gestation, heifers underwent Cesarean sections for fetal removal and morphometrics determination. The UBF and VT data were analyzed using repeated measures of ANOVA; while the morphometrics were analyzed by the MIXED procedure of SAS. Seasons were analyzed separately. In Fall, a nutrition by treatment interaction was observed, where the RES-CON heifers exhibited reduced total UBF compared to ADQ-CON (5.67±0.68 vs. 7.97±0.54 L/min; P = 0.039). In Summer, MEL heifers exhibited increased total UBF compared to the CON counterparts (8.16±0.73 vs. 6.00±0.70 L/min; P = 0.048). Moreover, there was a nutrition by treatment by time interaction in VT for Fall and Summer heifers (P ≤ 0.005). In Fall, all groups had decreased VT in the morning compared to the afternoon (P < 0.05). Whereas, in Summer, VT increased for ADQ-CON and RES-CON (P < 0.0001) from morning to afternoon, while the ADQ-MEL and RES-MEL remained constant throughout the day (P = 0.648). Furthermore, the RES-MEL-PM exhibited decreased VT compared to ADQ-CON-PM (38.91±0.09 vs. 39.26±0.09°C; P = 0.018). Lastly, in Fall, a main effect of nutrition was observed on fetal weights, where the RES dams had fetuses with decreased body weight when compared to ADQ (24.08±0.62 vs. 26.57±0.64 kg; P = 0.0087). In Summer, a nutrition by treatment interaction was observed on fetal weights where the RES-CON dams had fetuses with reduced weight when compared to ADQ-CON and RES-MEL (P < 0.05). In summary, nutrient restriction decreased UBF and melatonin supplementation increased UBF depending on the season. Additionally, melatonin appeared to decrease VT and rescue fetal weights when supplemented in the Summer.