HEAT-STRESS DURING MID-PREGNANCY IN SHEEP AND THE CONSEQUENCES FOR PLACENTAL AND FETAL GROWTH

An experiment was conducted at Toorak Research Station, Julia Creek, i n semi-arid northern Australia(141' E,21-degrees-S)during 1990 to dete rmine the relationship between placental and fetal weight in sheep aft er placental growth had been restricted by chronic heat stress during mid-pregnancy. Placental and fetal weight were measured in single bear ing ewes housed either in a thermoneutral environment throughout pregn ancy, or in a heated room between the 30th and 80th days of pregnancy followed by a thermoneutral environment until the 140th day of pregnan cy. 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 o f pregnancy. Fetal weight and dimensions did not differ significantly between control and heat-treated ewes at the 80th day of pregnancy, al though fetal weight (P < 0.01) and fetal dimensions (P < 0.05) for the previously heat-treated ewes were lower than those for the control ew es at the 140th day of pregnancy. Placental and fetal weight at the 14 0th day of pregnancy were correlated (P < 0.05) with the rectal temper ature 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 hea t-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.