SUMMARYAn experiment was conducted at Toorak Research Station, Julia Creek, in semi-arid northern Australia (141° E, 21° S) during 1990 to determine the relationship between placental and fetal weight in sheep after placental growth had been restricted by chronic heat stress during mid-pregnancy. Placental and fetal weight were measured in single bearing ewes housed either in a thermoneutral environment throughout pregnancy, or in a heated room between the 30th and 80th days of pregnancy followed by a thermoneutral environment until the 140th day of pregnancy. The placental weight of the heat-treated ewes was lower than that of the control ewes at the 80th (P< 0·05) and 140th (P< 0·01) days of pregnancy. Fetal weight and dimensions did not differ significantly between control and heat-treated ewes at the 80th day of pregnancy, although fetal weight (P< 0·01) and fetal dimensions (P< 0·05) for the previously heat-treated ewes were lower than those for the control ewes at the 140th day of pregnancy. Placental and fetal weight at the 140th day of pregnancy were correlated (P< 0·05) with the rectal temperature of ewes measured at 08.00 and 16.30 h during the period of heat-treatment, but not with the change in rectal temperature between 08.00 and 16.30 h. It was concluded that restricted placental growth in heat-treated ewes retarded fetal growth during late pregnancy even in the absence of heat treatment, and it is suggested that selection of ewes which can maintain normal rectal temperatures during periods of heat stress would produce lambs of normal birthweight in a hot climate.

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.

The aim of this experiment was to determine whether the thermal responses of sheep exposed to a hot environment were similar in different years. Rectal temperatures of sheep were measured during the hot months (between November and March) of three consecutive years (1990/91, 1991/92, 1992/93) in a flock of Merino sheep (n = 151) in the semi-arid tropics of northern Australia. Mean daily maximum temperatures in these months were 36 + 0-4, 39 ±0-3 and 37 + 0-3 °C in Years 1, 2 and 3, respectively. In Year 1, rectal temperatures were measured on 3 consecutive days; the repeatability was 0-47 at 08.00 h and 0-64 at 16.00 h. In Year 1, two sub-groups were identified according to measurements of rectal temperature, low body temperature (LBT; n = 35) sheep, whose rectal temperature was ^ 39-8 °C at 16.00 h on each of the 3 consecutive measurement days, and high body temperature (HBT; n = 32) sheep, whose rectal temperature was ^ 39-9 °C. The rectal temperatures of LBT and HBT sheep were then measured during the subsequent 2 years. Rectal temperatures of HBT sheep were significantly higher than those of LBT sheep at both 08.00 and 16.00 h, in both Year 2 and Year 3. Rectal temperatures at 16.00 h in Year 1 were correlated (P < 0001) with rectal temperatures at 16.00 h in both Year 2 (r = 0-79) and Year 3 (r = 0-82), respectively. It was concluded that measurement of rectal temperature in sheep exposed to a hot environment in any one year is an accurate index of their rectal temperatures during subsequent years. INTRDDIirTIONl^'s r e s P o n s e ' s t n e s a m e in subsequent years has not been investigated. The aim of this experiment was to Grazing sheep which are exposed to a hot environment determine whether the thermal response of sheep to a typically experience elevated rectal temperatures hot environment was similar over 3 consecutive years, during the afternoon (Brown 1971(Brown , 1974 McCrabbe/ al. 1993fl). Elevated rectal temperatures are a conse-MATERIALS AND METHODS quence both of increased ambient temperatures and . . , ,,, .