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

Relationships between circulating concentrations of estrone sulfate an d progesterone in the dam prepartum and weight of the calf and placent a at birth were evaluated at first parity in Hereford x Angus heifers (n=27) bred to a purebred Angus bull. Blood samples were collected fro m 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 a fter incubation with sulfatase enzyme, arid both estrone and progester one levels were quantitated by radioimmunoassay. Maternal plasma estro ne 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 1 0, 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 posi tively with maternal plasma estrone sulfate concentrations between Day s 10 and 1 prepartum (r = 0.65, P < 0.01) and with the dry weight of t he 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 i n plasma estrone sulfate concentrations for the last 20 days of gestat ion 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 placentom es within the placenta may influence birth weight, and possibly neonat al viability, of calves born to first parity beef heifers. Furthermore , estrone sulfate concentrations in the maternal circulation provide a n index of fetal-placental-maternal well-being and placental insuffici ency in cattle.