Heat stress’s reproductive ramifications

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Animal agriculture loses around $2 billion a year due to heat stress.  History has taught us that heat stress does not have to last a long period of time to have a profound impact on production. A publication by Dr. T. R. Bilby reminds us that in 1999 Nebraska producers lost more than $20 million in cattle deaths and performance losses and that in just two days in July of 1995 a combination of heat and humidity caused the deaths of over 3,700 cattle in a thirteen county area of western Iowa. These economic losses were a result of heat stress reducing such things as milk production, heifer growth, increasing cow and calf mortalities, health-care costs, and reductions in reproductive performance.

Reproduction is the main factor limiting production efficiency in the cow/calf industry.  Infertile females can be categorized into three groups: females that fail to become pregnant, become pregnant but fail to calve, or become pregnant late in the breeding season and fall out of the annual production cycle. At an 85% pregnancy rate each exposed female that fails to become pregnant cost the producer an estimated $94. In addition for each 1% deviation from an 85% pregnancy rate, a $6.25 increase or reduction in that value is seen (ex: 84% = $100.25 and 86% = $87.75).

In a time of drought, many factors compound the impact of infertility within a herd. Heat stress is an issue that has been recognized within the dairy industry for decades, but is often overlooked in the beef industry, until a time of drought. The impact of heat stress can be seen in males and females, and without proper management could have devastating reproductive ramifications. 

Bulls

While most people consider the impact of heat stress on female fertility, the impact of heat stress in our bull herd during the breeding season should also be taken into account. Semen quality is shown to decrease when bulls are exposed to ambient temperatures of 86°F for 5 weeks or 100°F for 2 weeks. This is seen with a decrease in sperm concentrations, motility, and an increase in abnormal sperm cells in each ejaculate. Following a period of heat stress sperm quality does not return to normal for approximately 2 months.

Cows and Heifers

Following a summer of heat stress and drought it is likely that producers will see a reduction in reproductive performance. Overcoming a low plane of nutrition due to drought stricken pastures is compounded by heat stress, causing a reduction in fertility.  The effects of heat stress can be seen throughout all stages of the pregnancy but are most prevalent early in gestation.

Heat stress will reduce the quality of oocytes (eggs) available for fertilization, resulting in a reduction in conception rates.  Embryos of heat stressed cows have reduced developmental competence in early stages, but heat stress can also have a major impact on embryo growth up to 17 days. Pregnancy is recognized by the females on day 18 of her 21 day estrous cycle. Thus, with a reduction in embryo growth, the maternal recognition of the pregnancy may be compromised. In addition, the uterine environment is compromised due to redirection of blood flow, away from the core of the body, as a cooling mechanism. This also contributes to early abortions. In addition, heat stress will reduce the dominance of ovulatory follicles and can cause an increase in twinning during the subsequent calving season.

With heat stress causing a reduction in embryo quality and oocyte competence, in addition to a reduction in semen quality and viability, producers are likely to see a reduction in reproductive performance following a summer of record temperatures and extreme drought. Providing shade, cool clean water, and increasing the bull to cow ratio may be management practices that should be considered when cattle are experiencing heat stress.

Source: Kalyn Bischoff


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Jesse McGaha    
Oklahoma  |  August, 21, 2012 at 09:08 AM

Thanks for the updated information and it is true, the heat stress and the extreme drought has caused a lot of problems in all of agriculture for many parts of the USA. We had a great spring and grass and clover pasture was super great until about May this year and now, with almost no rain, it is very dry and the grass is not growing for now, and we have had some oak trees die from the lack of water and from extreme heat last year and this year too. Keep the news coming to the readers of this publication, great to get updates too! Thanks, and have a great year.

Ken Dreger    
Bertram Texas  |  September, 03, 2012 at 11:11 AM

Question: Have any of the ranchers tried the "Cow Jackets" that our friens downunder in AUS use on their cows? I have looked into this for a friend who has a ranch in Oregon and it seems like these "Jackets" protect the cows from loosing water in the summer, lowering the stress caused by over heating, and in the winter prevent heat loss which reduces the amount of food intake they must consume to retain body fat. I know it really sounds funny, but these are animals and they do get cold and warm like the rest of us and if there is a way to keep them at a better temp that can improve overall weight gain over a reduced time frame I would think that the costs may be something a rancher would look into? Any ideas?

Mike Howell    
Mississippi  |  September, 14, 2012 at 08:56 PM

Heat stress is something we deal with every summer in the south. Fall born calves stall out after weaning due to the heat and humidity and spring calving herds typically do not have cows that lactate to their expectations. I have a registered Angus herd and the genetic lines developed in the mid-west or northern plains do not do well in the south because they do not shed off well. In the south, fall is a good time to calve heifers because the heat and humidity of the summer will hold birth weight down. BSE work on bulls should be delayed, when heat stress has been experienced, for 60 days after temperatures moderate. Heat stress is also a big factor in early embyronic death in southern cows. These will be the one that checked pregnant in the early summer then when the temps cool off in September, producers will notice some of these cows cycling again.


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