428 - Effects of Melatonin Supplementation During Mid- to Late-gestation Nutrient Restriction on Maternal and Fetal Amino Acid Concentrations

Nutrient restriction during mid- to late-gestation compromises pregnancy, alters maternal metabolism, and afflicts fetal growth and development. The objective of this study was to determine if dietary melatonin supplementation in global nutrient-restricted cows alters fetal-maternal circulating amino acids. In a 2x2 factorial, Brangus heifers of two calving seasons, spring (n = 29) and fall (n = 25), were fed a total mixed ration at either 100% (adequate; ADQ) or 60% (nutrient restricted; RES) of nutritional requirements based on NRC guidelines and were either supplemented with dietary melatonin (MEL; 20 mg/d) or no supplemented control (CON) from d 160 to d 240 of gestation. Heifers were supplemented and fed at 0900 h daily. Maternal blood, fetal blood, and amniotic fluid were collected at Cesarean section on d 240 of gestation in either the morning (AM; 0500 h) or afternoon (PM; 1330 h). Plasma and amniotic fluid AA were determined by a gas chromatography - mass spectrometry method with internal standard calibration using authentic amino acid standards. Total AA concentrations (mM) were calculated and the total fetal AA concentration was subtracted from the total maternal AA concentration to calculate maternal-fetal differences. Data were analyzed separately by calving season, using the MIXED procedure of SAS 9.4 to determine the effects of nutritional plane, melatonin supplementation, time of day, and their interaction on AA concentrations. Spring fetal-maternal AA differences were less (321±194 vs. 935±190 vs. 1,135±194 vs. 1,352±212 mM, respectively; P < 0.05) in RES-CON than in RES-MEL, ADQ-MEL, and ADQ-CON groups. Fall fetal-maternal AA differences did not differ (P > 0.05). Spring amniotic fluid AA concentrations were less (919±208 vs. 1,416±212 vs. 1,174±213 mM, respectively; P < 0.01) in RES-MEL heifers than in ADQ-MEL and RES-CON groups. Fall amniotic fluid AA concentrations were less (1,132±258 vs. 2,877±308 mM, respectively; P < 0.05) in all groups compared with ADQ-CON-AM. These findings demonstrate nutrient restriction during mid- to late-gestation alters circulating maternal AA concentrations, affecting fetal AA concentrations.