Metabolic maturity at birth and neonate lamb survival and growth: the effects of maternal low-dose dexamethasone treatment

Abstract

Perinatal mortality is a major contributing factor to reproductive wastage in grazing sheep industries. Enhanced metabolic and endocrine maturity at birth may improve the behavioral competency and thermoregulatory ability of neonates, potentially improving lamb survival over the first 72 h of life. Maternal glucocorticoid treatment in late gestation was investigated as a mechanism for manipulating metabolic and endocrine maturity in the ovine neonate. Multiparous, fine-wool Merino ewes (n = 150) were divided into 3 groups to lamb on pasture. Within each group, 5 single-lamb and 5 twin-lamb bearing ewes were randomly allocated to 1 of 5 treatments. Treatments included a saline control (1 mL), or dexamethasone (2 mg/mL as the sodium phosphate) injected intramuscularly at 1 of 2 dose rates (1.5 or 3.0 mg) at d 130 or 141 of gestation. One-half of the control ewes were injected at d 130 and the remainder at d 141. Dexamethasone treatment had no effect on lamb survival to 72 h after birth, although there tended (P = 0.09) to be a smaller proportion of lambs dying due to dystocia than for control lambs. Heart girth at birth in singleton and twin lambs was reduced (P < 0.01) at the greater dose rate. Further, treatment also reduced birth weight (by about 5%) and presuckling rectal temperatures in twin lambs, but not in singleton lambs. These reductions were also dependent on the sex of the lamb. Dexamethasone treatment did not alter gestation length or lamb presuckling plasma glucose, NEFA, urea, or leptin concentrations, but treatment at d 141 increased (P < 0.05) ghrelin concentrations in singleton and male lambs. Behavioral interactions between ewes and neonatal lambs were generally unaffected, although treatment at d 130 produced lambs that took longer to bleat than lambs of untreated ewes (P < 0.05). Treatment did not affect the concentration of measured blood metabolites or hormones at weaning. Although there were interactions between litter size, lamb sex, and the dose rate and time of treatment on weaning weight, BW recorded 73 d after weaning was unaffected by treatment. Despite changes in birth weight, rectal temperature, lamb behavior, and presuckling plasma ghrelin concentrations, survival in the first 72 h of life, and lamb growth performance were unaffected by periparturient maternal glucocorticoid treatment.