The effect of mid-pregnancy shearing and litter size on lamb birth weight and postnatal plasma cortisol response

Abstract

The study was conducted to determine the effect of mid-pregnancy shearing on lamb birth weight, body dimensions and postnatal plasma cortisol response to a stressor. Fetal exposure to excess maternal glucocorticoids at specific stages of pregnancy may influence postnatal function of the hypothalamic-pituitary-adrenal (HPA) axis. This in turn may have implications on the growth, development and survival of lambs. Romney ewes of various ages, bearing singletons ( n = 47) or twins ( n = 48) were shorn or not (control) on day 80 of pregnancy. Ewes were managed as a single flock throughout pregnancy under general commercial farming conditions. Within 12 h of birth the lamb's live weight and body dimensions were recorded. On day 39 of lactation the plasma cortisol levels of the ram lambs in response to handling or castration was assessed. A random sample of ram lambs born to the control (6 singleton and 9 twins) or shorn (7 singleton and 11 twins) ewes were allocated to either a handling or castration treatment. Singleton and twin lambs born to shorn ewes were 7% heavier at birth than their counterparts. Lambs born to ewes shorn in mid-pregnancy recorded longer ( P < 0.05) crown rump (53.1 cm versus 51.7 cm) and forelimb (30.8 cm versus 30.0 cm) measurements than the counterparts born to control ewes, respectively. However, when the lamb's live weight was considered in the model these differences were not significant. Mid-pregnancy shearing had no effect on the cortisol response of singleton or twin-born ram lambs to handling or castration. The postnatal plasma cortisol response of twin lambs was higher ( P < 0.05) than that of singletons (30.8 mmol l −1 min versus 14.4 mmol l −1 min), regardless of the treatment. Mid-pregnancy shearing resulted in increased lamb live weights and body dimensions in both singleton and twin lambs at birth—while plasma cortisol response of lambs to handling or castration was unaffected. These data suggest that mid-pregnancy shearing had little effect on HPA axis development in singleton or twin offspring and the increase in lamb birth weight caused by mid-pregnancy shearing did not result in altered postnatal HPA function.