RELATIONSHIP BETWEEN PLACENTAL DEVELOPMENT AND CALF BIRTH-WEIGHT IN BEEF-CATTLE

Relationships between circulating concentrations of estrone sulfate and progesterone in the dam prepartum and weight of the calf and placenta at birth were evaluated at first parity in Hereford x Angus heifers (n=27) bred to a purebred Angus bull. Blood samples were collected from the tail vein of all heifers at 14 day intervals throughout the last trimester of gestation, and a complete placenta was obtained from 22 heifers at parturition. Estrone sulfate was measured as free estrone after incubation with sulfatase enzyme, arid both estrone and progesterone levels were quantitated by radioimmunoassay. Maternal plasma estrone sulfate concentrations increased (P < 0.01) quadratically from Day 100 to Day 10 prepartum (Day 100, 6.4 ng ml-1; Day 10, 19.0 ng ml-1), and were correlated negatively (r = - 0. 84, P < 0.01) with changes in plasma progesterone for the same period (Day 100, 11.4 ng ml-1; Day 10, 6.2 ng ml-1). Birth weight was greater for male than female calves (38.3 vs. 32.9 kg, P < 0.01), but maternal plasma estrone sulfate and progesterone concentrations and placental weights were not affected (P > 0.1) by the sex of the fetus. Calf birth weight was correlated positively with maternal plasma estrone sulfate concentrations between Days 10 and 1 prepartum (r = 0.65, P < 0.01) and with the dry weight of the cotyledons (r = 0.76, P < 0.01, intercotyledonary membranes (r = 0.48, P < 0.05) and total placenta (r = 0.71, P < 0.01). Three neonatal calf deaths occurred; dams for two of the calves had a 50% reduction in plasma estrone sulfate concentrations for the last 20 days of gestation and a reduction in cotyledonary surface area. Collectively, these results suggest that variation among dams (i.e. either of inherent or pathological origin) for total mass and (or) function of the placentomes within the placenta may influence birth weight, and possibly neonatal viability, of calves born to first parity beef heifers. Furthermore, estrone sulfate concentrations in the maternal circulation provide an index of fetal-placental-maternal well-being and placental insufficiency in cattle.