Click on the thumbnail image to go to a full sized graphic interpretation of the effects of heat stress.
Under the influence of heat stress, the duration and intensity of oestrus are reduced. There is a clear decrease in motor activity and other manifestations of oestrus such as mounting. Nobel et al (1997) found that Holstein cows during the summer have 4.5 mounts per oestrus versus 8.6 per oestrus in winter.
Higher incidence of silent heat and anoestrus is therefore one of the most often reported findings in cows exposed to high ambient temperatures.
The mechanisms through which heat stress influences the function of hypothalamic-hypophyseal-ovarian axis are not completely understood.
FSH secretion from the pituitary gland does not appear to be impaired in animals exposed to high ambient temperatures.
In contrast a clear reduction in the pulse and amplitude of LH release has been observed in cows exposed to heat stress.
Thus during high ambient temperatures:
On the other hand a reduction in LH pulse and amplitude leads to:
Development of larger number of bigger follicles probably leads also to an increased rate of double ovulations and twin calving (Wolfenson et al., 2000).
Low progesterone concentrations in the circulation of cows have been associated with compromised reproductive function and reduced pregnancy rates (Butler et al., 1996; Lamming et al., 1989; Mann et al., 1995; 2001).
Whether insufficient progesterone secretion by the corpus luteum contributes low fertility in cattle exposed to heat stress is debatable. A recent work published by Wolfenson et al. (2002) analysed progesterone production in vitro by theca and granulosa cells obtained from cows in cool and hot seasons as well as progesterone concentrations in general circulation. This study demonstrated that under chronic summer stress conditions, progesterone production was markedly reduced, especially by luteinised theca cells. Results presented indicated a 25% decrease in plasma concentrations of progesterone in cows in summer compared to those in winter. These authors postulated that heat-stress-induced damage to follicular function was carried over to the subsequently formed corpus luteum.
The formation of gametes and development of early embryonic stages have been shown to be highly temperature sensitive.
Heat stress causes hyperthermia of the scrotum and testes which can lead to poorer morphological and functional semen quality. Hansen (1997) reported deterioration of bull fertility caused by heat stress during the summer months. Heat stress has less severe effects on semen quality of zebu bulls than it does on bulls of European breeds and this phenomenon is associated not only with the generally more efficient thermoregulation observed in zebu cattle but also specific adaptations that enhance the local cooling of blood entering the testis (Brito et al., 2004).
Heat stress through delayed ovulation and follicular persistency can lead to the ovulation of an aged, poor quality oocyte which is associated with low fertilisation rate and embryonic mortality (Sartori et al., 2000; Al-Katanani et al., 2001; Roth et al., 2001). High temperature also negatively affects pre-attachment embryos (Ryan et al., 1993; Ealy et al., 1993). The resistance of embryos to these effects increases as the embryos develop (Ealy et al., 1993; Sartori et al., 2002, Hansen et al., 2001).
Marked differences were noted between bos taurus and bos indicus on the developmental potential and quality of oocytes and embryos. Higher resistance of embryos derived from bos indicus cows to heat stress were shown by Paula-Lopes et al. (2003) and Hernandez-Ceron et al. (2004) and summarised by Hansen (2004).
Heat stress compromises uterine environment with decreased blood flow to the uterus and increased uterine temperature which can lead to implantation failure and embryonic mortality.
These effects are thought to be associated with the production of heat-shock proteins by the endometrium during heat stress and reduced production of interferon-tau by the conceptus. Moreover heat stress can affect endometrial prostaglandin secretion leading to premature luteolysis and embryo loss.
Malayer and Hansen (1990) found also clear differences between Brahman and Holstein cows in endometrial responses to culture at elevated temperature.
It is very clear that the negative effects of heat stress on reproduction is the result of the direct influence on reproductive functions and embryonic development and indirect influences mediated through changes in energy balance.
In heat stressed dairy cows dry matter intake is reduced. This prolongs the period of negative energy balance and negatively influences the plasma concentrations of insulin, IGF-I and glucose (Jonsson et al., 1997; Ronchi et al., 2001).
This results in: