Heat stress significantly affects cows’ fertility. When the ambient temperature rises, heat disrupts hormonal balance, reduces egg quality, and sharply decreases the chances of conception. Dairy farmers in warmer climates or during hot summers therefore see their reproductive rates decline every year. In this article, we answer the most frequently asked questions about heat stress and fertility in cattle.
How does heat stress affect hormone balance in cows?
Heat stress disrupts the hormonal balance in cows by suppressing the production of luteinizing hormone (LH) and progesterone. These hormones are essential for normal ovulation and for maintaining pregnancy. When body temperature rises, the pulsatile release of LH from the pituitary gland decreases, disrupting follicle maturation and ovulation.
The hypothalamic-pituitary-gonadal axis, the central regulatory system for reproduction, is particularly sensitive to thermal stress. When the animal’s core temperature rises, the body prioritizes cooling mechanisms and reduces the energy flow to reproductive processes. This is an evolutionary survival strategy, but in modern dairy farming, it has direct economic consequences.
At the same time, levels of cortisol—the stress hormone—rise during heat waves. Elevated cortisol levels, in turn, inhibit progesterone production by the corpus luteum, which increases the risk of early embryonic death. Hormonal disruption is therefore not a single problem, but a chain of negative effects that reinforce one another.
Why does the fertilization rate decrease at high temperatures?
The conception rate decreases at high temperatures because heat negatively affects both egg quality and the uterine environment. Oocytes that develop during a period of heat stress are structurally compromised and less suitable for successful fertilization. In addition, estrus signs become less pronounced, making it more difficult to inseminate cows at the right time.
Under normal circumstances, a cow in heat exhibits clearly visible estrus behavior: she stands still when mounted, is restless, and is more active. Under heat stress, this behavior is shorter and less pronounced. The duration of estrus can shrink from an average of eighteen hours to less than eight hours, which greatly reduces the chance of successful insemination at the right time.
In addition, the uterine environment is less receptive to an embryo during heat stress. Blood flow to the uterus changes because the body diverts blood to the skin to cool down. This reduces the supply of oxygen and nutrients to the embryo during the first critical days after fertilization.
When does heat stress begin to cause measurable damage to reproduction?
Measurable reproductive damage caused by heat stress begins at a temperature-humidity index (THI) of 68. Above this threshold, the conception rate in dairy cattle decreases noticeably. At a THI above 72, the effects on embryonic survival and estrus expression are significant, and at a THI above 78, the situation is considered severe heat stress with clear consequences for the entire reproductive cycle.
It is important to note that the damage does not occur only during the heat wave itself. Egg cells have a maturation period of about three weeks. This means that a cow inseminated in July uses egg cells that began maturing as early as June. Heat stress in the lead-up to insemination may therefore have already compromised egg quality, even if the temperature has already dropped by the time of insemination.
Nighttime temperatures also play a role. Cows recover from daily heat stress largely at night. In regions or during periods when nighttime temperatures also remain high, thermal stress accumulates, and reproductive damage increases more rapidly and becomes more severe.
Which reproductive parameters are most severely affected?
The reproductive parameters most severely affected by heat stress in dairy cattle are conception rate, embryo survival rate, and calving interval. Together, these three factors largely determine a herd’s reproductive efficiency, and all three deteriorate significantly during prolonged heat.
Specifically, this involves the following parameters:
- Chance of fertilization: decreases sharply due to reduced egg quality and a less favorable uterine environment
- Signs of estrus: shorter and less intense, causing the optimal time for insemination to be missed
- Embryonic survival: Early embryos are particularly sensitive to elevated body temperature during the first seven days after fertilization
- Progesterone concentration: reduced due to inhibition of the corpus luteum, which impairs fertility
- Intercalving period: extended because multiple inseminations are required for a successful pregnancy
- Pregnancy rate per insemination: noticeably lower in the summer months compared to the fall and winter
The combination of all these effects makes heat stress one of the most significant factors affecting reproductive performance on a dairy farm. Furthermore, the consequences are not always immediately apparent: a failed pregnancy following summer insemination is not detected until weeks later during a pregnancy check.
How can nutrition support fertility during heat stress?
Nutrition can support fertility during heat stress by enhancing cellular protection, stabilizing energy balance, and alleviating stress on the endocrine system. Natural betaine plays a special role in this process: as an osmolyt, it protects cells from dehydration and shrinkage by maintaining intracellular water balance, even at elevated body temperatures.
Betaine acts as what is known as a compatible osmolyt. When a cell is exposed to heat or osmotic stress, betaine helps the cell maintain its normal volume and function without toxic side effects. This is relevant for reproductive cells and the uterine lining, as well as for the animal’s overall metabolism.
In addition, betaine acts as a methyl donor in one-carbon metabolism. It provides methyl groups that are necessary for the synthesis of carnitine, creatine, and choline. During heat stress, the demand for these metabolites increases because the body exerts extra effort to protect cells and limit tissue damage. An adequate supply of betaine through the diet directly supports these processes.
Other nutritional measures that support fertility during heat stress include increasing the energy density of the diet, providing sufficient fresh drinking water, and optimizing mineral intake. Cows that eat less during heat stress benefit from palatable, energy-rich feed that limits a negative energy balance. A negative energy balance exacerbates hormonal disruption and prolongs the time until the first ovulation after calving.
What are the economic consequences of heat stress on fertility?
The economic impact of heat stress on cow fertility is significant. A longer calving interval, more inseminations per pregnancy, and increased culling costs collectively result in a direct loss of income per animal. In regions with hot summers, losses per cow per year can amount to hundreds of euros, depending on the duration and intensity of the heat waves.
The costs are broken down into several categories:
- Higher insemination costs: more semen straws and vet visits per successful pregnancy
- Longer calving interval: fewer calves per cow per year and lower lactation efficiency
- Early embryonic loss: hidden losses that are not detected until late in a pregnancy checkup
- Higher replacement costs: cows that repeatedly fail to become pregnant are replaced, which entails purchase and rearing costs
- Reduced milk production: A longer intercalving period shifts the peak of the lactation curve and reduces annual milk yield
In addition to the direct financial losses, there are also indirect consequences, such as a higher workload for the livestock producer and poorer herd health due to a more irregular calving pattern. Farms that actively invest in heat stress management—through both barn climate control and feeding strategies—systematically reduce these losses and maintain more stable reproductive performance throughout the year.
How Jodoco Helps with Heat Stress and Fertility in Cows
At Jodoco, we understand that heat stress in dairy cattle is a complex challenge that requires a scientifically grounded approach. Our solutions address the core of the problem: protecting cells, stabilizing metabolism, and supporting reproductive processes during thermal stress.
Our approach offers concrete benefits for dairy farmers and their advisors:
- Osmotic cell protection through natural betaine, which protects cells from dehydration and shrinkage at high temperatures
- Methyl donor support for more efficient metabolism and better hormonal balance during heat waves
- Optimizing energy balance through products such as Acibet G, which stimulate rumen fermentation and limit negative energy balance
- Scientific evidence based on our own R&D and collaboration with national and international research institutions
- Flexible administration via feed or drinking water, tailored to the farm’s specific practices
Our Jodobet natural betaine is the core product for ruminants exposed to heat stress. It supports osmotic balance, strengthens intestinal integrity, and contributes to a more stable reproductive cycle. Want to learn more about how betaine works to protect against heat stress? Or are you looking for a suitable solution for your business or client portfolio? Contact us, and we’d be happy to work with you to develop a tailored approach.
