Heat stress effects on fetal development during late gestation in the ewe.

Forty-eight crossbred ewes were subjected to heat stress of short or long duration during the last third of gestation in a completely randomized design to determine the effects of maternal heat stress on lamb birth weight, lamb conformation and subsequent preweaning growth rate. Ewe treatments were spring range pasture, confinement on slotted floors in a heated room maintained between 28 to 38 C or in a slotted floor barn with feed intake equal to the feed intake of the 38 C housed ewes. Treatment exposure averaged 25 and 53 days for short and long duration, respectively. Lamb birth weights of heated, range and restricted fed ewes were 3.18, 4.57 and 4.16 kg, respectively. After adjustment for sex and multiple birth effects, birth weight of lambs from hot environment ewes were smaller (P<.01) for both durations of treatment. Lamb birth weights from heated ewes were smaller than those of lambs from restricted fed ewes (P<.05). Relative kidney and liver weights were greater in lambs from heated ewes (P<.05). Relative muscle weights and relative linear equivalent bone lengths were not different among ewe treatments. Lambs from heated ewes had 30 and 56-day weights similar to those of lambs from range and restricted fed ewe groups. Small Iambs resulting from maternal heat stress of 25 or 53 days duration are proportional dwarfs and occur independently of ewe level of nutrition.