251. Uterine Blood Flow and Placental Size in Beef Cows with Historically Divergent Fetal Growth Fed at a Similar Nutritional Plane During Late Gestation.

Abstract Spring-calving, multiparous MARC II cows managed in multi-sire pastures were bred via natural service. To be selected, females were parity ≥5 and ranked in the top or bottom 25% for previous average calf birth weight within age of dam class. Females with high fetal growth (HIGH; n = 20; historical average calf birth weight = 43.1 kg) or low fetal growth (LOW; n = 20; historical average calf birth weight = 35.0 kg) but similar historical dam body weight (BW), body condition score (BCS), and calf sex ratio were selected. From d -115 ± 10 (SD) relative to calving until the day of calving, all females (BW: 660 ± 46 kg; BCS: 5.6 ± 0.5) were individually-fed a total mixed ration in Calan gates to meet 100% of metabolizable energy and metabolizable protein requirements for maintenance and pregnancy. Transrectal color Doppler ultrasonography of both uterine arteries was conducted on d -119, -73, and -36 ± 8 relative to calving. Expelled placentas were split into ipsilateral and contralateral sides, dissected (cotyledonary vs. intercotyledonary), and dried. Data were analyzed with historical dam fetal growth group, calf sex, and date of calving as fixed effects. Uterine blood flow included day and fetal growth group × day using repeated measures. We have previously reported that calf BW at birth was greater (P < 0.001) for calves born to HIGH dams than LOW dams. Ipsilateral uterine artery mean velocity was greater (P = 0.03) and contralateral mean velocity tended (P = 0.10) to be greater for HIGH dams. Cross-sectional area of the ipsilateral uterine artery was greater (P ≤ 0.04) on d -73 and -36 relative to calving for HIGH dams. Contralateral uterine artery cross-sectional area and blood flow were not affected (P ≥ 0.24) by historical dam fetal growth group. Ipsilateral uterine artery and total blood flow were greater (P ≤ 0.02) at all 3 timepoints for HIGH dams. Ipsilateral dry intercotyledonary weight was greater (P = 0.05) and cotyledonary weight tended to be greater (P = 0.06) for HIGH dams, but contralateral intercotyledonary and cotyledonary weights were not affected (P ≥ 0.27) by fetal growth group. Combined, dry total placental weight tended to be greater (P = 0.07) for HIGH dams. Fetal growth group did not affect (P ≥ 0.36) cotyledonary number, cotyledonary size, or placental efficiency (calf birth BW/total placental weight). Even with similar nutritional management during late gestation, HIGH dams had greater total uterine blood flow and placental size, allowing for increased fetal growth. It is still unclear whether greater uterine blood flow of the dam drove fetal growth or if fetal growth potential invoked these physiological responses. USDA is an equal opportunity provider and employer.