Lamb birthweight in sheep differently acclimatized to a hot environment

Abstract

Two groups of ewes were selected from a flock of 151 ewes in the semi-arid tropics of northern Australia, according to their rectal temperature measured at 1600 hours on each of 3 consecutive days when maximum daily ambient temperature was 36¦ 0.5¦C. Low temperature status (LRT) ewes had a rectal temperature of <39.8¦C at 1600 hours on each of the 3 days, while high temperature status (HRT) ewes had a rectal temperature of >39.9¦C. These ewes were mated 8 months later and exposed to a summer gestation under normal grazing conditions. Maximum and minimum daily ambient temperatures during pregnancy were 37¦0.3¦C and 21¦0.3¦C respectively. Rectal temperature measured at both 0800 and 1600 hours for HRT ewes was higher (P < 0.01) than for LRT ewes during pregnancy. LRT ewes produced lambs of higher (P < 0.05) birthweight and ponderal index than HRT ewes. The physiological basis for the difference in fetal growth in ewes which respond differently to a hot climate has not been identified, but possible mechanisms are discussed.