Effects of Heat Stress on Dairy Cattle Reproduction


Follicular Growth/Luteolytic Mechanisms

Early Embryonic Development

Oviductal/Uterine Environment

Conception Rates

Signs of Heat Stress

Management Practices


The purpose of this website is to provide a general overview on how various aspects of reproduction are compromised due to heat stress. Included is information relating to heat-stressed cattle and changes in estrus, follicular growth and luteolytic mechanisms, early embryonic development, oviductal and uterine environment, and conception rates. Also listed are suggested management practices to aid in overcoming the costly effects of heat stress.




In times of heat stress, estrous is reduced in duration and intensity. Cows exposed to high-temperature conditions may become physically lethargic. Therefore, a cow is less likely to exhibit standing estrus and other behaviors associated with estrus during peak temperatures. Furthermore, cattle are more prone to exhibit signs of estrus during cooler temperatures, which predominately occur at night. With these reductions and changes in estrus behavior, producers have fewer opportunities to witness their cattle in estrus. In one study, dairy cows in estrus during the summer displayed only 4.5 mounts per estrus compared to 8.6 mounts per estrus displayed in the winter.

Hormone Changes

Follicular Growth and Luteolytic Mechanisms

The number of follicles in heat-stressed dairy cows does not vary from non heat-stressed cows. The main difference: follicles in heat-stressed cows are not ovulated because of the lack of recruitment in the first follicular wave. In order for follicles to grow and exhibit dominance, they first must have LH receptors on the granulosa cells. A lack of LH receptors causes a decrease in the conversion of testosterone to estradiol. A lack of estradiol results in an increase in the number of subordinate follicles. These subordinate follicles will never gain dominance and therefore will not be ovulated.

Follicular growth rate increases during the second follicular wave in heat-stressed dairy cows. As a result, heat-stressed cows enter the second follicular wave earlier than non-heat stressed cows. Furthermore, aged follicles may be ovulated due to the increased duration of time the oocyte remains in the ovary. Decreased fertility in these oocytes is displayed.

The function of the corpus luteum in cattle is to produce progesterone required for maintenance of pregnancy. If no pregnancy occurs in cattle, estradiol is the hormone responsible for inducing luteolysis. Estradiol concentrations are reduced in heat-stressed cattle, preventing the corpus luteum from regression. If luteolysis does not occur, the animal will remain in a sustained luteal phase because progesterone has a negative feedback on GnRH production. With the sustained presence of progesterone, estrus will not occur and neither will ovulation or pregnancy.

Hormone Changes

chart 1: This chart represents how follicular growth varies between waves.  It also shows  how heat stress decreases dominant follicles during the first wave and grows more rapidly during the second wave.

Chart 1: This chart illustrates varied follicular growth between waves. It also shows how heat stress decreases dominant follicles during the first wave and grows more rapidly during the second wave.

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Early Embryonic Development

The early stages of embryonic growth are critical for embryo survival. Embryos are most susceptible to heat stress during the first two days of pregnancy. At day three of pregnancy embryos become increasingly resistant to heat stress. It is at this time (8-16 cell stage) that the embryos start producing heat shock proteins. Heat shock proteins (HSP) are induced by heat. They act like "chaperons" and make sure proteins are in their correct shape at the right place and time. They also help new or distorted proteins to fold into shape and shuttle them from one compartment to another. Furthermore, HSPs play a role in transporting old proteins out of cells. Heat stress reduces protein synthesis. As a result, embryonic death from heat stress is most prevalent before heat shock proteins are produced.

This table shows the effects of heat stress on early embryonic development. Notice an increase in the % Live Embryos on day 7 in comparison to day 1.

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Oviductal and Uterine Environment

Heat stress causes decreased blood flow to uterine and oviductal areas which may reduce nutrients and increase biochemical waste products at the tissue level. In the early embryonic stages of development, HSP production increases. The increase in HSP production alters the binding activities of steroid receptors. The reduction of the steroid receptors diminishes the action of steroid hormones, predominately progesterone and estrogen on the reproductive tract. The lack of these hormones may cause embryonic death during pregnancy.

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Conception Rates

High temperatures can lead to a loss of pregnancy in heat-stressed dairy cows. Heat stress at and immediately after the time of breeding results in lower conception rates. Heat-stressed dairy cows tend to have a decrease in dry matter intake thus reducing the amount of energy in their diet. In order to maintain pregnancy it is critical to sustain a healthy diet. During times of heat stress, cattle tend to cool themselves by blood vessel dilation near the surface of the skin in order to release heat. This results in decreased blood flow to the reproductive tract and reduces the efficacy of counter-current heat exchange between capillary vessels.

During the summer months conception rates may drop below ten percent. One artificial insemination study demonstrated a five percent decrease in conception rates for every ten degree increase in envrionmental temperature over fifty degrees Fahrenheit.

Chart 1: This chart shows varying conception rates in dairy cows depending on their location (Florida-sub tropical climate, Arizona-desert southwest, and Minnesota- temperate climate). It also illustrates that during the summer months that conception rates decline significantly.

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Signs of Heat Stress in Dairy Cows

• If the respiration rate is over 80 respirations per minute for 7 or more out of 10 cows.
• If the rectal temperature is 102.5 degrees F. or above for 7 or more out of 10 cows.
• If dry matter feed intake drops 10 or more percent in hot weather.
• If milk production drops 10 or more percent in hot weather.
• Increased breathing rate
• Increased water intake
• Increased sweating
• Decreased feed intake
• Decreased milk production
• Change in milk composition, e.g. fat % and protein % declines
• Change in blood hormone concentration, e.g. increased prolactin
• Changed behaviour:
• Seek shade
• Crowd together
• Refusal to lie down
• Change orientation to sun
• Stand in water
• Stand next to water trough

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Management Practices

• Factors critical to the correct design of cooling systems must include climate and relative humidity when evaluating the economics of a cooling system investment.

• Studies evaluating the effects of heat stress on embryonic survival support use of cooling during the immediate postbreeding period.

• Successful cooling strategies for lactating dairy cows are based on maximizing available routes of heat exchange, convection, conduction, radiation, and evaporation.

• Systems designated for heat relief: air movement (fans), soaking cow’s body surface, evaporation (high pressure mist), facilities to minimize transfer of solar radiation.


Shade: Total heat load could be reduced from 30 to 50 % with a well-designed shade. Cows in a shade vs. no shade environment had lower rectal temperatures (38.9 degrees C and 39.4 degrees C), reduced respiratory rate (54 and 82 breaths/min), and yielded 10% more milk. Suggestions: Mature dairy cow requires 3.5-4.5 m2 of space beneath shade with north-south orientation to allow for penetration of sunlight beneath shade for drying ground.

Holding-Pen Cooling: Holding pen is where dairy cows experience the most heat stress. Cows under sprinklers and fans in holding pens show reduced body temperature of 1.95 degrees C. Sprinklers should be run according to temperature and humidity. Fans should be mounted overhead and blow downward at a 30 degree angle.

Exit-Lane Cooling: Exit sprinklers installed in lanes attached to milking parlor. If properly installed, sprinklers should wet the top and sides of the cow, keep udder dry, and not interfere with postdip.

Free-Stall Cooling: The most effective cooling system in free-stall barns was a spray and fan system. In addition, good natural ventilation should include 4.3m sidewalls, opened 75 -100%. Feed-line cooling, using spraying fans and misters has also been successful in cooling cows in the bedded stall.

Nutrient Adjustment: Changes in ration formulation to meet the nutrient needs in lactating dairy cows with a goal of pregnancy include increased nutrient density, altered mineral and water deliveries and altered digestive tract function. Supplemental rumen-active fat has advantages over starch-based concentrate to increase energy density of diets.

Insemination Practices: Adjustments to avoid estrus detection such as timed AI and embryo transfer via in vivo-derived embryos are two protocols with results suggesting significant increases in conception rates in heat-stressed dairy cows. Timed AI programs demonstrated improved pregnancy rates versus AI without timing.


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Krsitin Thompson

Andy Peterson

Ann Huenink

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