Maternal Performance, Metabolic Status, and Calf Size at Birth for Beef Cows with Historically Divergent Fetal Growth Fed at a Similar Nutritional Plane During Late Gestation

Abstract Even within single populations, there is substantial variation in calf birth weight. In beef cows with historically divergent fetal growth, we hypothesized that altered maternal metabolism and uteroplacental nutrient transport capacity would explain fetal growth differences when females experienced similar nutritional management during late gestation. Spring-calving, multiparous MARC II cows managed in multi-sire pastures were bred via natural service. To be enrolled in the current study, 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), hip height, 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 (using the estimated day of gestation based on pregnancy diagnosis and the group average calf birth weight of 39 kg for all females). Dam BW and BCS were determined, and jugular blood samples were obtained on d -113, -65, and -29 ± 10 relative to calving and immediately post-calving. At birth, calf jugular blood was obtained (pre-suckling), and calf BW and size were measured. Data were analyzed with historical dam fetal growth group, calf sex, and date of calving as fixed effects. Gestational metabolites were analyzed separately from post-calving metabolites and included day and fetal growth group × day using repeated measures. Historical dam fetal growth group did not affect (P ≥ 0.25) dam BW and BCS during gestation or post-calving. Gestational circulating glucose, non-esterified fatty acids, and triglycerides were not affected (P ≥ 0.21) by the fetal growth group × day interaction or fetal growth. Fetal growth group did not affect (P ≥ 0.51) dam post-calving circulating metabolites. Calf BW, shoulder to rump length, heart girth, abdominal girth, flank girth, and cannon circumference were greater (P ≤ 0.05) at birth for calves born to HIGH dams, but shoulder height and cannon length were not affected (P ≥ 0.13). Neonatal calf circulating metabolites were not affected (P ≥ 0.35) by fetal growth group. Even with similar nutritional management during late gestation, mature beef cows with historically divergent fetal growth had calf birth weights that were 13.2% different. Despite fetal growth differences, maternal performance and metabolic status were not affected, suggesting that the efficiency of uteroplacental nutrient transport capacity may be involved.