Heat Stress

Heat stress in dairy cattle occurs when a cow’s heat load is higher than her capacity to lose heat, leading to reduced animal welfare and—in extreme cases—death.

Heat stress effects on fertility include:

An increase in the number of open days.
Reduced conception rate, ranging between 20% and 30% compared to the results obtained in the winter months^1,2.
More cows suffering various types of anestrus.
Decreased reproductive success because of potential direct effects on the ovaries, uterus, gametes, embryos and early fetus.

Heat stress disrupts reproduction in two general ways³:

Physiological adjustments: redistribution of blood flow and energy deficits.
Elevated internal body temperature directly disrupts endocrine mechanisms and cellular physiology (ovarian cells, germ cells, embryo or other).

Impact of heat stress on endocrine mechanisms:

It reduces LH levels in cows, which interferes with maturation and ovulation of dominant follicles.
GnRH is depressed and FSH is elevated during heat stress.
Follicular estradiol secretion is depressed during heat stress, which disrupts expression of estrus, gonadotropin surge, ovulation, transport of gametes and fertilization.
In case dominant follicles ovulate, subsequent plasma progesterone concentrations are reduced, which limit endometrial function and embryo development.

In high-producing dairy animals with high metabolic demands, these effects are exacerbated⁴.

The bovine thermal comfort zone ranges from 5°C to 25°C, and 80% their body temperature ranges from 38.4°C to 39.1°C⁵. Heat stress is a combination of temperature and humidity. Above 25°C and 80% relative humidity, cows suffer from heat stress, diminishing their health status and zootechnical performance.

Heat stress acts in more than one way to reduce fertility in lactating dairy cows:

Indirectly, by reducing dry feed intake, which inhibits gonadotropin-releasing hormone (GnRH) and luteinising hormone (LH) secretion from the hypothalamic-pituitary system.
Directly, by compromising the uterine environment, causing embryo loss and infertility.

Heat Stress Effects on Reproduction in Dairy Cows

Effect on the Reproductive Pattern

Reduced duration and intensity of estrus.
Decreased mounting and other manifestations of estrus.
Holstein cows are mounted 4.5 times per estrus during the summer versus 8.6 times per estrus in winter⁶.

Effect on Embryo Quality and Development

Gamete-formation is temperature sensitive. Plasma LH concentrations are lower in cows exposed to high ambient temperatures.
Disrupting the follicle and its enclosed oocyte seem to be pivotal in the complex mechanisms that impair fertility due to heat stress².
Adverse effects of heat stress appear during the period from oocyte maturation to preimplantation development.
Most detrimental effects of heat stress could be the result of oxidative stress caused by the temperature elevation:
- Direct damage to organelles or induced DNA damage (leading to apoptosis)
- Altered gene expression
- Reduced developmental competence in preimplantation embryos

Semen Quality

Hyperthermia of the scrotum and testes can lead to poor morphological and functional semen quality, thus causing deterioration in bull fertility.

Uterine Environment

Heat stress may decrease flow of blood to the uterine environment. Exposure to high temperatures during pregnancy may cause blood to be diverted from some organs, such as the uterus, to others involved in heat regulation, such as the skin, lungs and respiratory tract.

Prolonged negative energy balance is one of the main causes of anovulation in dairy cows, especially early post partum⁷. Reduced feed intake results in:

Poor follicular development
Low estrus LH expression
Poor oocyte quality²

Three Main Strategies

Biotechnical Options

Genetically Develop More Heat-Tolerant Dairy Breeds

This can improve both the reproductive ability and adaptability of cows to warmer climates⁸.

Embryo Transfer

This can bypass the harmful effects heat stress has on the fertility of lactating dairy cows⁹.

Management

Nutritional Modification

Decreased feed intake in heat-stressed cows causes a negative energy balance. Also, the energy requirements of heat-stressed cows are higher, because their core body temperature is increased and their heat-dissipation processes are inefficient.

Feeding these animals high-quality forage supplemented with concentrates, additional fats and feed additives can help mitigate some of these adverse effects¹⁰.

More fiber and/or fewer concentrates in the ration will increase rumination, and this will compensate for rumen and metabolic acidosis. You can also provide bi-carbonate in the ration to compensate the rumen acidosis.

Environmental Modification

Shade protects from direct solar radiation but does not alter the air temperature or the relative humidity of their surroundings.

Evaporative cooling strategies are costly, but are very useful for alleviating heat stress in animals. Lack of proper ventilation can lead to high moisture levels, manure gases, pathogens and dust concentrations, creating an adverse environment for dairy cows.

However, the selection of cooling systems to improve cow health and performance may vary between humid and arid areas. Options should optimize the combination of increased air velocity and conductive cooling to improve cow well-being by reduce their body temperature¹¹.

Hormonal Therapy

Hormonal therapy does not address the causes of heat stress, but it can ameliorate some of its direct effects on the endocrine reproductive axis. It can also help to overcome the decrease in reproductive performance in cattle seen during summer and early autumn.

Timed Artificial Insemination (TAI) Protocol

Breeding synchronization protocols for insemination at fixed time-points may reduce the calving-conception interval and the number of services per conception.

The optimal hormone treatment to combat the effects of heat stress should:

Be suitable for cows in a fixed time-point insemination program and avoid the need for heat detection.
Resolve anoestrus, including ovarian cysts or anovulatory follicles.
Have a short duration.

Timed Artificial Insemination in Dairy Cows

One of the most popular treatments, the OvSynch protocol, consists of the following treatments: GnRH (day 0), prostaglandin F2α (day 7), and GnRH (day 9), with artificial insemination 16-20 hours after the second GnRH treatment. It has been shown that TAI protocol can reduce losses in reproductive efficiency in cattle caused by poor summer estrus detection.

GnRH Administration at Estrus

Induction of ovulation with GnRH not only decreases the incidence of delayed ovulation, but also provides additional luteotrophic support to the early corpus luteum.

GnRH or Human Chorionic Gonadotropin (hCG) Administration Post Artificial Insemination

Administration of hCG or GnRH post insemination supports the function of the yellow body in the ovary, thereby maintaining higher progesteron levels resulting in higher pregnancy rates.

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