LAMB BIRTH-WEIGHT IN SHEEP DIFFERENTLY ACCLIMATIZED TO A HOT ENVIRONMENT

Two groups of ewes were selected from a flock of 151 ewes in the semi- arid tropics of northern Australia, according to their rectal temperat ure measured at 1600 hours on each of 3 consecutive days when maximum daily ambient temperature was 36 +/- 0.5-degrees-C. Low temperature st atus (LRT) ewes had a rectal temperature of less-than-or-equal-to 39.8 -degrees-C at 1600 hours on each of the 3 days, while high temperature status (HRT) ewes had a rectal temperature of greater-than-or-equal-t o 39.9-degrees-C. These ewes were mated 8 months later and exposed to a summer gestation under normal grazing conditions. Maximum and minimu m daily ambient temperatures during pregnancy were 37 +/- 0.3-degrees- C and 21 +/- 0.3-degrees-C respectively. Rectal temperature measured a t 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 < 0.0 5) birthweight and ponderal index than HRT ewes. The physiological bas is for the difference in fetal growth in ewes which respond differentl y to a hot climate has not been identified, but possible mechanisms ar e discussed.