Placental growth and the ability of sheep to thermoregulate in hot environment

Abstract

Rectal temperatures of a flock of 151 Merino ewes from the semi-arid tropics of northern Australia were measured in March 1991 (Year 1). Two sub-groups of ewes, with different abilities to thermoregulate body temperature, were then classified according to rectal temperature at 16:00 h on 3 consecutive hot summer days: low body temperature status (LBT) ewes had a rectal temperature of ≤39.8 °C on each of the 3 days, and high body temperature status (HBT) ewes had a rectal temperature of ≥39.9 °C. These ewes were mated 20 months later and exposed to a hot summer gestation under typical field conditions. In February 1993 (Year 3) rectal temperatures of ewes were measured again to confirm body temperature status, and on the following day, at 107 ± 0.3 days of gestation, placental and fetal weights were determined at post mortem. Weight (20%, P < 0.01), DNA content (14%, P < 0.05) and protein content (15%, P < 0.05) of the placenta of LBT ewes were greater than for HBT ewes, whereas there was no significant difference in DNA or protein concentrations (mg per g tissue) or DNA:protein ratio. Fetal thoracic girth and fetal heart weight of HBT ewes were lower ( P < 0.05) than for LBT ewes, but there was no significant difference in fetal body weight or fetal crown-rump length. We conclude that placental growth is restricted in sheep which do not thermoregulate well (i.e. HBT) when exposed to a hot environment. The ability of flock managers to identify HBT ewes, which will subsequently experience restricted placental growth, can be used to identify low producing individuals early in their commercial life.