Hot weather can present a significant challenge to the physiology of dairy cows, with heat stress having the potential to cause a decrease in feed intake and milk yield. But it doesn´t stop there: heat stress can also have a detrimental effect on fertility, both during and immediately after the period of hot weather.
In this blog we will highlight the impacts of heat stress on reproductive efficiency, starting with a brief physiology refresher:
The oestrus cycle
The oestrus cycle is a 21-day series of events orchestrated by several hormones produced in the brain and reproductive system: the ‘hypothalamic-pituitary-gonadal axis’. In the brain, at the hypothalamus, a hormone called GnRH is produced. This hormone induces another part of the brain (the pituitary gland) to produce follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH stimulates the growth of follicles at the ovary, which eventually results in the maturation of a follicle and the release of increasing amounts of estrogens. The brain subsequently responds by producing a surge of LH that induces ovulation: the mature follicle explodes releasing the egg.
Additionally, LH induces the remains of the ovulated follicle to become a corpus luteum, where progesterone starts being produced. Eventually, after a few days under the influence of progesterone, the uterus, if there is no embryo attached, is induced to produce yet another hormone called PGF2a (prostaglandin), which wipes out the corpus luteum causing progesterone production to decrease. The brain subsequently responds by producing more GnRH to restart the cycle.
How heat stress impacts reproduction
In addition to the process described above, another major hormonal axis is activated during periods of heat stress: the ‘hypothalamic-pituitary-adrenal axis’. This axis controls the production of cortisol – a hormone which enables stress adaptations to take place, but which also influences and disturbs many other hormonal and metabolic processes.
During periods of heat stress, the hypothalamic-pituitary-adrenal axis antagonizes the hypothalamic-pituitary-gonadal axis described above, with research models and studies comparing reproductive performance in hot vs. cool environments showing that cows exposed to heat can incur several negative fertility affects. In summary, research has shown that heat stress1,2:
- Disturbs the size, number, and maturation dynamics of follicles, with effects not only on one cycle but subsequent cycles too
- Lowers the production of estrogens, required at the onset of ovulation, and for a good oestrous duration and intensity
- Lowers the production of LH, required to induce the ovulation and the formation of the corpus luteum
- Lowers the concentration of progesterone, which impairs embryonic viability. This has been found after chronic exposure to heat stress
- Decreases the production of PGF2a in the uterus, which is crucial to wipe out the corpus luteum allowing the cycle to restart when no pregnancy has been established
Beyond these impacts, even if fertilization has happened the viability of the embryo isn’t guaranteed, with further evidence to suggest that the ongoing pregnancy can be compromised under conditions of heat stress – as a result of the decrease in progesterone, plus a direct effect of excessive body temperature on the embryo implantation environment.
It is important to note that heat stress affects reproduction not only at, but before and after ovulation, with negative effects often seen towards the end of the summer months and into the beginning of autumn.
How to combat the effects of heat stress
Considering the severity and wide range of impacts of heat stress on the reproductive system, it isn’t surprising that dairy cow conception rates are often reduced during and after periods of hot weather. In order to mitigate these effects it is essential that cows are managed effectively so that their exposure to excessive heat is limited (click here for more information).
Dietary adjustments should also be considered as a means of helping cows to cope during periods of hot weather. Several studies show that the addition of OmniGen AF can help to reduce the effects of heat stress: cows fed with OmniGen AF3 have been shown to have a lower body temperature and reduced respiration rate, as well as maintain a greater feed intake3 and milk yield4. Additionally, a recently published study conducted on a large commercial farm in Florida, USA has found reproductive performance benefits: this study, carried out in a subtropical climate showed that cows fed with OmniGen AF become pregnant earlier than their control counterparts, achieving an average ten fewer days open (P<0.05)5.
To find out more about how OmniGen AF may help your herd to cope with the effects of hot weather and other stressors please click here.
1Roth, Z., and Wolfenson, D. 2016. Domest. Anim. Endocrinol. 56: S218-27
2Velez Marín, M. and Uribe Velásquez, L.F. 2010. Biosalud 9: 83-95
3Hall, L.W., et al. 2018. J. Dairy Sci. 101: 7095-105
4Fabris, T.F., et al. 2017. J. Dairy Sci. 100: 6733-42
5Casarotto, L., et al. 2020. Anim. Feed Sci. Technol. 267: 114527