As we move into summer, it is important to recognize the danger
that high heat and humidity pose to cattle. Cattle are more sensitive to heat than humans,
and reach the critical temperature for negative effects when the Temperature-Humidity
Index reaches the high 80s.
Temperature, humidity, and the amount of direct sunlight are the
primary factors that determine an animal’s body temperature. However, other factors
such as precipitation, wind, amount of night cooling and exposure to fescue endophyte are
also very important.
Precipitation can cause problems because high humidity reduces
the ability of cattle to use evaporation to dissipate heat. Evaporative cooling occurs
when sweat or moisture evaporates away from the respiratory tract or skin. Evaporation is
the primary means that cattle have to cool themselves at temperatures over 70°F. If winds
are calm or cattle congregate behind a windbreak, their ability to be cooled is reduced.
Night temperatures that remain above 70°F increase the danger of heat stress because of
little or no night cooling. Cattle that are not acclimated to hot weather are also a
greater risk if weather changes rapidly or if the cattle are moved to an environment with
greater heat stress.
Cattle that have eaten endophyte-infected fescue may have
increased body temperature and be predisposed to heat stress. Following removal from
endophyte-infected fescue pastures, feedlot cattle may experience severe health problems
related to summer toxicosis. Upon arrival in the feedyard, the most notable clinical signs
attributable to previous fescue ingestion were long, rough hair coats and obvious signs of
heat stress as ambient temperatures increased during the day. Although not all cattle were
affected equally, mortality rates 24 to 48 hours after extreme heat stress reached as high
as 10% but diminished in three to five days, followed by a lingering intolerance to heat.
Another factor that plays a role in heat stress is hide color,
with black-hided cattle at greater risk than cattle with light-colored hides. Breed plays
a role in that Bos indicus breeds (Brahman and others) handle heat better than do Bos
taurus (European) breeds. Brahma-type cattle have more sweat glands, the glands are more
widely spaced, and have a slightly larger surface area created by skinfolds allows better
heat dissipation. European breeds have a faster metabolic rate as indicated by faster
gains than Bos indicus, which makes them less heat tolerant.
For cattle in a feedlot situation, special attention should be
given to newly-arrived cattle and heavier cattle that are approaching finished-weight.
Iowa researchers found that non-shaded lots facing south, southwest, or west had higher
death loss than lots facing east or southeast during a period of severe heat stress.
In addition to its affect on health and production, heat stress
also affects reproduction. Heat stress dramatically lowers conception rates, influences
estrus behavior, modifies hormone function, alters the oviductal and uterine environments,
and delays or interrupts early embryo development in dairy cattle. Heat stress occurring
on the day of estrus or within 1 to 7 days postestrus is particularly deleterious to
embryo survival. Other researchers have found that follicular growth and development and
luteolytic mechanisms were compromised in heat-stressed cows; as a result luteolysis was
delayed, and second wave dominant follicles did not ovulate. Conception rates plummet
during periods of extremely high temperatures and humidity. In Florida, the most
significant decline in conception rates occurred during the months of June, July, and
August, and recovery was slow, requiring an additional 2 months, before achieving pre-heat
stress rates.
Work with dairy cattle has shown that as environmental
temperatures rise, an animal’s attempt to regulate her temperature results in blood
flow being redirected away from the body core toward peripheral tissues for dissipation of
excess body heat. Consequences of this response to the reproductive tract include
decreased blood flow to the uterus, reduced nutrient exchange, and alterations in volume
and content of oviductal and uterine secretions. Because of the redirection of blood flow
away from the uterus, cows that are in the last trimester of gestation during a period of
heat stress will have less nutrient exchange between the dam and fetus during the period
of greatest fetal growth. Therefore, calves that are born shortly after periods of heat
(August, September for the Northern Hemisphere) are lighter at birth than calves born at
other times of the year.
Managing
Periods of Heat Stress
During periods of heat stress, it is important to
have ample water available. When temperatures reach 80 degrees, cattle need two to three
gallons of water per 100 pounds of body weight. And the flow rate should be able to
accommodate 5 to 10 gallons per animal per hour.
If cattle must be handled, work them from midnight
to 8 AM. Do not move or work cattle after 10 AM. While it may make sense to work cattle
after sundown, wait until the cattle have had at least six hours of night cooling before
working.
Shades that are 10 to 12 feet tall and provide 20 to
30 square feet or more per head have been shown to reduce heat stress and to increase feed
intake, weight gain, and performance of cattle. Shade reduces the heat gain resulting from
solar radiation even when air temperature is not reduced. During periods of heat stress,
cattle seek out the coolest spots and are unwilling to leave these areas. Shades should
therefore be placed over feed and over areas where the producer wants the cattle to spend
time. Shades should have a north-south orientation to allow drying under the shades as the
shaded area moves throughout the day.
For cattle confined in a lot, enhance airflow by providing mounds
for cattle to stand on. Move cattle away from windbreaks and wind dead spots in the
feedlot. Sprinklers in can be used to combat heat stress. In geographic areas where
humidity can be high, a large water droplet is required to wet the skin; fine mists or fog
systems are not as effective in these areas. Sprinklers reduce heat stress by increasing
evaporative losses, by reducing ground temperature and reducing radiant heat gain, and by
reducing dust. Sprinkling should be intermittent, otherwise high humidity may result and
there may be little opportunity for evaporation. A 1- to 2-minute shower followed by 20 to
30 minutes of drying and evaporation is common.
Heat stress is caused by a combination of environmental and
animal factors that result in reduced performance as the animal attempts to cool itself.
In extreme cases, death is possible. The main objective for cattle managers during periods
of heat stress is to keep the cattle’s body temperature from climbing to dangerous
levels.
Drought
Strategies
