Limiting Effects of Summer Heat Stress in Cattle

July 12, 2009 07:00 PM

By Bruce Carpenter, Texas AgriLife Extension

Texas is a hot place in the summer, and anyone who has to be outside everyday looking after cattle or doing other ranch work certainly knows that. Chances are, if you're feeling heat stress, so are your cattle.

Though cattle sweat relatively little, they can cool themselves in other ways: They respire (pant) to cool themselves and we've all probably noticed altered grazing patterns in the summer, or maybe if it is hot enough, and they have a chance, you may even notice them standing in a pond or dirt tank. These things all help the animal cope but are they enough?

That depends on a few other factors and how they interact. First, what stage of production are the cattle in? Are cows safely pregnant by June or July, or are they attempting to breed in those months? Bull fertility can suffer from heat stress as well. In what region of the state are the cattle located? Regions with both heat and high humidity are more stressful than those where lower humidity can allow some nighttime cooling. Breed type may also affect an animal's ability to cope.

Heat Index. Meteorologists use a formula to calculate a heat index to describe how hot the environment actually feels to people and to provide some guidelines for human activity and the risk of heat-related health problems. Researchers at Oklahoma State University have further refined this concept to develop the Oklahoma Mesonet Cattle Stress Index (which can also predict cold stress).

Cows and Heat Stress. Probably the biggest concern regarding cows are potential negative affects on fertility. Looking at cows in general (i.e. without heat-stress), most studies agree that fertilization is the rule, and failure to conceive is the exception (though it can and does happen; see bull section below). It is thought that about 30% of embryos die between conception and day 14; and another 5-10% or so during pregnancy recognition (day 14-19). After placental attachment at about day 42, losses become minimal. Total pregnancy loss in beef cows is thought to range from a low of about 42 % to a high of about 72%.

The point is that early embryo losses are high enough already without adding heat stress to the equation. Heat stress appears to exasperate losses during the same critical periods described above. Two studies showed that heat stressed dairy cows lost the majority of embryos before either day 7 or day 14 of conception.

Another study reported that when rectal temperatures increased from 101.3 degrees to 104 degrees post-artificial insemination, that pregnancy rates fell from 42% to 0%. Some reasons for embryo death may include changes in uterine environment, changes in proteins critical for pregnancy, and reduced corpus luteum function by the ovary.

Failure to conceive can be another reason for reduced fertility in cows. Heat stress may cause ovulation or conception failure may due to reduced follicle quality or suppressed estrus.

During heat stress cows sometimes fail to display normal estrus behavior or may show estrus more during the nighttime hours. The latter could be a consideration for AI programs. Overall reproductive rates from one OSU trial are shown in Table 1.

Table 1.

Effects of Imposed Heat Stress
on Reproduction in Beef Cows
  Control Mild Severe
Day Temp. (F)
Night Temp (F)
Rel. Hum (%)
Rectal Temp (F)
Pregnancy (%)

Source: Biggers, 1986;Oklahoma State University

Bulls and Heat Stress. It has been well documented that bulls experience reduced semen quality during summer months in many regions of the Southern U.S. Oklahoma researchers reported reduced motile sperm, reduced sperm production, and an increase in the percent of abnormal and aged sperm. In this study bulls were maintained in controlled chambers at 73 degrees for 8 weeks. Heat stressed bulls were then subjected to 88 to 95 degree temperatures for 8 weeks, followed by 8 more weeks of 73 degrees. Heat stressed vs. control bulls
consumed 35% more water, respired 55% more, and had a rectal temperature one degree higher.

Producers with early fall breeding programs or with fall bull sales which include semen testing should be mindful of the potential effects of summer heat stress. It takes sperm cells about 60 days to mature in the testis. So for example, bulls that begin breeding on October 1 could have reduced semen quality due to heat stress which may have occurred back in August. It is also likely that bulls may reduce their breeding activities during times of heat stress.

Heat Stress and Calves. Calves that are heat stressed consume less feed and likely suckle less. Data on 8,000 calves from Texas shows reduced weaning weights for calves born May – September (Table 2).

Table 2
Effect of month of birth on adjusted weaning weight of calves (Sprott, 2000)

Month of Birth
Trial Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Trial 1, Burleson Co. 1976
Trial 2, Webb Co. 1969
Trial 3, Calhoun Co. 1976-79

Managing Heat Stress. The most economically important effects of heat stress are on fertility and calf vigor. Therefore, the first management step for most of Texas and other hot climates, is to allow neither summertime breeding seasons nor summertime calving seasons. There may be regional exceptions where nighttime cooling is a regularity and/or summertime rainfall is likely.

Other management strategies:

  • If cattle don't have access to shade, you may want to consider erecting some structures.
  • Fly control may be important (i.e. ear tags, etc.) as cattle may avoid shade as a means of avoiding flies.
  • Make sure cattle have adequate water at all times. Water consumption may double that of winter, approaching 2 gallons per hundred pounds of animal weight per day (i.e. 20 gallons for a 1000 lb animal).
  • And finally, while no breed is immune to heat stress, select breeds of cattle that best tolerate your environmental conditions.

Bruce B. Carpenter. associate professor and extension livestock specialist, is headquartered at the District VI Texas AgriLife Extension Center in Ft. Stockton.

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