Your Reading List

Heat Stress Melts $ Off Production

In the last 10 years, there has been a trend among producers to have cows calve later in the spring rather than the colder months of February or March. Good riddance to frozen ears and frostbitten calves, when cows calve on green pastures in April, May and even June. However, one of the biggest drawbacks of such later-planned calving and a later breeding season is summer heat stress, which can be a big problem in getting these cows rebred. While producers can’t control the weather, but they can take precautions to reduce the negative impact of heat stress on breeding stock.

All cattle are said to suffer from heat stress any time the Temperature- Humidity Index (THI); a gradient humidex scale based on 72 F (22.2 C) and 100 per cent humidity is exceeded. A THI of about 80 is considered dangerous for cattle because they are unable to maintain a normal, but life-giving body temperature of 101.5 F (38.6 C).


It’s no secret that heat-stressed breeding cattle will be miserable. They tend to slowdown any grazing activity and thus their feed intake decreases, significantly. Suffering cattle lose their appetite and their normal water consumption increases double or triple. Heat stressed cattle also tend to breath hard or pant rapidly while standing around; they bunch up under shade or if shade is unavailable, they will often lie in each others’ shadows. Most importantly during a hot breeding season, both heat stressed cows and bulls are lethargic and all physical breeding activity seems to stop.

Such changes from normal behaviour of heat-stressed breeding beef cattle are easy to see, whereas the negative effects of heat-stress going on at the same time inside their bodies are largely invisible.

The unseen internal outcome of heat stress upon cow fertility fundamentally alters natural hormone patterns in the beef cow, which are necessary for her normal regular estrus cycle to occur in the first place.

Initially, heat stress causes less luteinizing hormone (LH) to be produced during the estrus cycle and as a result: alters follicular (egg development), delays ovulation (release of the egg) and lowers the quality of eggs prepared for fertilization by the bull’s sperm. Even if this egg is viable and fertilized during hot weather, heat stress increases the internal temperature of the cow’s uterus and can decrease the chance of successful embryo implantation on its uterine wall; leading to greater embryonic mortality. Cows that experience early embryonic loss during the first weeks of pregnancy often appear as repeat breeders in the cowherd.


It’s these cows that cycle again due to the effects of hot weather during the first month of a 60 to 75 day breeding season that will have the biggest negative economic impact on the operation, even if they are successfully bred later on. During this time for every missed reproductive cycle (21 days) that a cow fails to conceive, gross revenues per cow can fall by eight to 10 per cent.

As a consequence, the delay of even one 21-day estrus period in a single beef cow during a heat stressed breeding season translates into about 48 pounds of lost weaning weight in their calves (source: Colorado State University; re: 85 lb. calf at birth must gain 2.27 lbs. per day in order to wean at 550 lbs. in 205 days). Given today’s calf prices of about $1.50 for 500 to 600 lb. calves translates into about $72 per calf of gained or lost revenue, depending upon whether a cowherd has a short or significantly longer breeding season.

To avoid such financial losses due to heat stress, traditional two-month breeding seasons usually fall upon the spring-summer weeks (May and June) of relatively cool weather, so if any of the cows fail to cycle, they have a better chance of rebounding later. Unfortunately, cows that miss their first cycle in a later-timed breeding season (mid-June, July and August) have a lower probability of conception during subsequent cycles, because hot weather patterns are often more frequent and might continue toward the “dog days” of late summer. Fortunately, some breeding cattle adapt somewhat to continued hot temperatures and during the latter part of summer, the nights tend to be cool down, which lessens cows’ heat stress.


Like breeding cows, bull fertility is also severely impacted by heat stress. Even if heat-stressed bulls are interested in breeding the cowherd, excessive heat build up inside their testes can cause temporary sterility or sperm damage to a point where they cannot fertilize the shed egg of the cow.

Subsequently, semen evaluations of heat-stressed bulls have shown an array of sperm production and quality problems: reduced number of live sperm, reduced sperm motility, as well as abnormal altercations to the shape of the sperm cell head and/or tail. Bulls that suffer from any extended period of heat-stress will not fully recover for about eight weeks after such a sperm-damaging incidence; a period which coincides with the nature process of sperm production and maturation.

When such scorcher heat strikes both breeding bulls and cows, there are a few short-term precautions that can be implemented and managed to help reduce hot weather’s negative impact upon their general well-being and reproduction:

Water is essential to cope with heat stress. Lots of cool clean water must be provided (re: about eight litres/50 kg of body weight). Some cattle field studies show that water temperatures over 25 C can actually increase the water requirement for heat-stressed cattle. Provide a large, steady flow of water so several can drink at once.

Cattle should have access to trees or other shade such as open buildings and pole-sheds with light coloured roofs. Windbreaks will provide some shade, but they often reduce air movement and sometimes can contribute to heat-stress.

With pasture management rotate cattle through pastures more quickly. This change can allow cattle to graze more digestible pasture forages, which in turn may lower internal heat from fibre fermentation. If dried-out pastures are supplemented with other feeds such as hay try to feed at dusk or evening which allows cattle to consume forages during lower temperatures, and promotes its dissipation of digestive heat by the cooler temperatures of the next morning.

For long-term precautions against heat-stress, producers might shift their calving/breeding seasons, so the first part of the breeding season avoids the hotter part of the summer (after July). One suggestion involves tightening up the breeding season toward a more desirable 60 day season for the mature cows and an even tighter 45 day season for the first-calf heifers. The cowherd should also be in top health and body condition, so they have at one strong cycle before the bulls are released, which increases the probability of conception in the first couple of weeks in the breeding season.

PeterVittiisanindependentlivestock nutritionistandconsultantbasedinWinnipeg. Toreachhimcall204-254-7497orbyemailat [email protected]

About the author


Peter Vitti is an independent livestock nutritionist and consultant based in Winnipeg. To reach him call 204-254-7497 or by email at [email protected]



Stories from our other publications