| Extension Extra | ExEx 2018 |
| SDSU Cooperative Extension Service South Dakota State University/US Department of Agriculture |
SDSU Department of Animal
and Range Sciences |
Effects of High Heat and Humidity
on Reproduction in Cattle
by Bill Epperson, Extension veterinarian,
and Doug Zalesky, Extension beef specialist
High temperatures and humidity in the summer can result in behavioral
and physical changes in cattle that affect breeding and reproductive success. These
changes are commonly referred to as "heat stress" and include:
Advanced heat stress is characterized by incoordination, collapse, and death. In addition to these outward signs, changes within the reproductive organs of
bulls and cows occur that may result in decreased fertility. The threshold
environmental
temperature at which fertility changes may occur has been poorly defined. Temperature
effects on fertility depend on the duration and magnitude of heat stress. High
environmental temperatures that persist for weeks may be especially detrimental to
reproductive efficiency. Cool nights will help decrease the heat stress and may offset the
effects of high daytime temperatures.
Other important factors that help alleviate heat stress include:
Appropriate use of water/water misters to increase heat loss through
evaporation.
Effects of Heat Stress on Bulls
Signs of heat stress have been observed in bulls maintained in 90 F environments. Environmental temperatures of 100 F can be dangerous and may produce advanced signs of heat stress.
An abundance of experimental data indicate heat stress can damage semen quality. Changes after thermal stress can be observed on a semen evaluation and include alterations in the shape of the sperm cell head and tailpiece.
Applying external insulation to the bull's scrotum can raise the scrotal skin temperature 1-4 F. This experimental technique mimics an acute heat stress condition. Insulation of the scrotum of yearling bulls for 24-72 hours has resulted in a decrease in normal sperm. This decrease begins 1-2 weeks after the heat stress event. When the heat stress is alleviated, semen quality continues to decline for an additional 1-4 weeks.
Depending on the duration of heat stress, semen quality returns to pre-stress levels approximately 4-8 weeks after the heat stress event. Experiments where animals were exposed to hot conditions for longer periods of time (88 F - 95 F for 8 weeks) yielded similar results, with recovery occurring within 8 weeks after heat stress subsided.
The practical outcome of heat stress on bulls is not well defined. Changes in semen quality may decrease fertility, especially if the bull is only marginally fertile before heat stress. Experiments have shown that bulls differ in their apparent susceptibility to heat stress. Individual bulls may produce very acceptable semen in the face of heat stress, while others are more severely affected.
If heat stress occurs and 1- 2 weeks later a bull is expected to breed a large number of cows, as in a synchronized breeding program, a decrease in pregnancy rates could occur. However, under most conditions, with a normal bull, it would be unlikely, that acute heat stress alone would radicand, compromise herd fertility. Data is lacking to allow an accurate assessment of fertility, in the face of heat stress under the variety of management conditions encountered in the field.
It is commonly thought that heat stress is present only when animals are exhibiting the typical physical signs. It is important to recognize that effects of heat stress on semen quality are not immediate, but become evident after the heat stress event and may be present for a significant period of time.
Heat stress effects on bulls may be minimized with good breeding management. These practices include:
Effects of Heat Stress on Reproduction in Cows
Environmental temperatures greater than 86 F may alter hormone patterns in cows. A shorter duration of estrus and less intensive signs of estrus may result.
Heat stress at and immediately after the time of breeding (7-10 days) may result in a lower conception rate. This is particularly true if there has been no or little adaptation to the high temperature. This change appears to stem from an increased rate of early embryonic death, not from a failure of the egg to be fertilized.
With early embryonic death due to heat stress, the cow returns to estrus in 21 days. The developing embryo was lost before the maternal recognition of pregnancy, so the cow continues uninterrupted through her estrous cycle. The decreased fertility appears to be much more of a problem in lactating dairy cows, but some depression of fertility may be noted in heifers as well.
While most concern regarding heat stress is traditionally directed at the bull, it is important to realize that the reproductive efficiency of cows may be affected by heat stress.
This is particularly true in regard to the time near breeding and may be a significant consideration when employing a synchronization program.
In a synchronization program, timing the initial estrus to avoid hot weather and/or implementing management practices to decrease the impact of heat stress, as outlined previously, may be appropriate.
Issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30. 1914. in cooperation with the USDA. Richard A. Battaglia, Director of CES, SDSU, Brookings. Educational programs and materials offered without regard to age. race, color, religion, sex, handicap, or national origin. An Equal Opportunity Employer.
Date last modified: January 04, 2005