Moreover, supplementation with progesterone during the
first follicular wave improved fertility.27
Summer heat stress is followed by a gradual restoration
of oocyte competence for fertility28,29 and oocyte competence
for chemically activated development.30 These observations
are symptomatic that heat stress can affect follicular
development early in the 16-week period that is required for
a primordial follicle to reach dominance.31 Indeed, there is
experimental evidence for delayed effects of heat stress on
function of the follicle and oocyte. In one study,32 exposure
of lactating Holsteins to heat stress affected follicular steroid
production 20–26 days later. An experiment in Gir cattle
(Bos indicus) indicated that carry-over effects of heat stress
can persist for up to 133 days after initiation of heat stress
(i.e., 105 days after the end of heat stress).3 Cows were exposed
to heat stress for 28 days and oocytes aspirated and subjected
to in vitro maturation and fertilization at weekly intervals for
up to 133 days after the beginning of heat stress. Heat stress
did not affect follicular dynamics during the 28-day heat
stress period but increased the diameter of the first and second
largest follicles, as well as the number of follicles over
9 mm, from days 28 to 49 after initiation of heat stress.
Moreover, the percent of oocytes that became a blastocyst
was reduced by heat stress even at the end of the experiment
at 133 days after initiation of heat stress