1. Introduction
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Summer heat stress HS is a major contributing factor in low fertility among
lactating dairy cows. It is a worldwide problem, which inflicts heavy economic losses and affects about 60 of the world cattle population. Conception rates drop from about
40–60 in cooler months to 10–20 or lower in summer, depending on the severity of Ž
. the thermal stress Cavestany et al., 1985 . The most prominent characteristic of summer
infertility is its multifactorial nature, since hyperthermia directly alters and impairs the cellular functions of various partsrtissues of the reproductive system. Furthermore,
exposure of cattle to thermal stress elicits indirect responses, which may also have an impact on reproductive processes. Such responses include redistribution of blood flow
among body organs, reduction in food intake, respiratory alkalosis, etc. Although the impact of the various direct and indirect effects of HS on reproductive processes has
never been quantified, it is believed that the direct effect of hyperthermia in impairing cellular functions is the predominant one. The substantial rise in milk yield in recent
years has aggravated the low summer fertility syndrome, because of the concurrent rise in metabolic heat production. The various cooling procedures used on farms have not
been able to improve fertility substantially, and the conception rate of lactating cows in the summer, even in farms equipped with cooling systems, is still pronouncedly below
Ž .
that in the winter Hansen, 1997 . Traditionally, low summer fertility is associated mainly with the warm months of the
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year usually June, July, August and September in the northern hemisphere . However, Ž
. fertility remains lower in autumn October and November , than in winter, although
ambient temperatures drop and the cows are no longer exposed to HS. A delayed effect of summer HS on autumn fertility is evident, and it accounts for about one-third or more
of the low summer fertility syndrome.
In the last decade, considerable efforts have been dedicated to shedding light on the HS-induced impairment of processes in the reproductive system, and of the functioning
of various parts of it. Ultrasonography, cell culture, in vitro oocyte maturation and fertilization are among the means of achieving improved understanding of mechanisms
by which thermal stress negatively influences bovine fertility. This review focuses basically on immediate and delayed effects of HS on follicular development, dynamics
of follicular waves, steroidogenic capacity of theca and granulosa cells, corpus luteum Ž
. CL development and function, and secretion of progesterone and gonadotrophins, and
briefly reviews oocyte quality, embryonic development and uterine function under HS. In the second part of the review, several hormonal strategies are discussed, which aim to
optimize reproductive functioning and to improve fertility of cattle under HS conditions.
2. Follicular dynamics
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