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How heat stress impacts the growth of calves

Gale Bateman II and Mark Hill Published on 09 April 2012

An old song reverberates the line “when you’re hot, you’re hot,” and this idea applies to livestock. Our group has shown that the temperature in the calf nursery impacts the average daily gain.

One of our current research goals is to better understand how and why the environment is influencing growth. We have the ability to automatically measure the temperature of calves every 10 minutes for multiple days, then analyze the information gained.

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We have used this research tool in experiments to measure how the environmental temperature altered calf body temperature and performance. This past year, we measured the temperature of calves in three time periods along with the air temperature inside the calf nursery.

The air temperature inside the calf nursery during the first period ranged from 70º to 90°F (warm); during the second period it ranged from 77º to 98°F (hot); and during the third period it ranged from 52º to 67°F (cool).

In all three periods we observed a rise and fall in both air temperature and body temperature of the calves as the day progressed. This rise and fall in temperature was expected and can be observed in all animals.

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In Figure 1 , the dark blue line is the average body temperatures recorded from calves when air temperatures were warm; the red line is body temperatures when air temperatures were hot; and the green line is body temperature when the air temperatures were cool.

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In all three time periods, the body temperature of the calves was lowest early in the morning and rose throughout the day.

In cool air temperatures, the calves were able to start dissipating this excess body heat in the early evening and, in general, had returned to a basal temperature by the next morning.

However, when the daily low air temperatures were above 77°F, the calves were not able to dissipate the increased heat load over night, and it could be argued that they were “running a fever.” The difference in maximum body temperature between calves in warm and hot conditions was about 1°F.

An analysis of these data revealed that this 1°F increase in a baby calf’s body temperature reduced average daily gain and structural growth by almost 15 percent.

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Figure 2 shows the relationship between air temperature in our calf nursery and average daily gain (blue line) and starter intake (red line) of the calves over 20 research trials with all type of milk replacer programs being fed.

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There was a linear relationship that as air temperature increased, calf average daily gain decreased. Within these calves, for every 10-degree increase in air temperature, calf average daily gain decreased by 5 percent.

When we evaluate these two datasets together, we can start to make some assumptions and possibly some conclusions. Calves exposed to high air temperatures had trouble dissipating heat and were unable to return to their normal body temperature during a 24-hour period.

We know that cattle pant and sweat to dissipate heat. This active process requires energy to be used and, therefore, increases the maintenance energy use of the calf in hot periods. If the amount of energy from the diet used for maintenance increases, there is less remaining to support bodyweight gain.

In addition to the increased maintenance energy expenditures, calves may have less total energy available for all functions during periods of high heat. Almost all animals will experience a voluntary decrease in feed intake during periods of heat stress. Calves are no exception.

The data in Figure 2 also shows the relationship between average air temperature and starter intake of pre-weaned calves. The calves consumed all of the milk replacer they were offered.

However, there is a 1 percent decrease in starter feed intake per 10-degree increase in average temperature. Starter intake did not decrease as much as average daily gain decreased as the air temperature increased, indicating that energy for growth was used to dissipate the heat.

The negative impact of hot weather becomes larger as air temperature increases and may be greatly exaggerated in periods of temperature extremes when calves lose their ability to compensate. However, management may be able to influence the air temperature that determines when calves cannot compensate.

Researchers from both the University of Missouri and Auburn University evaluated putting shade structures over the top of calf hutches. In both experiments, the shade decreased the air temperature in the hutches.

Similarly, we evaluated using fans in our calf nursery. In our experiment, cooling calves with fans reduced their breathing rate and increased average daily gain and feed efficiency by approximately 20 percent. These data are able to show that reducing the heat load calves are exposed to can increase growth.

Nutrition may also impact the calf’s ability to cope with an increased heat load. Researchers from the University of Arizona reported that calves in hot conditions had impaired immune status compared to calves in more desirable temperatures.

They reported reduced immunoglobulin concentrations in serum at 2 and 10 days old from calves under hot temperatures. This could lead to an increase in disease in these calves.

Nutritional supplements that assist in maturing the immune system would help calves cope with this challenge and should reduce disease. Medium-chain fatty acids have been shown to have antimicrobial and antiviral properties.

Supplying them in the diet may help reduce the levels of disease-causing organisms that survive in the gut. Short-chain fatty acids and linolenic acid are known to be immune stimulatory and anti-inflammatory.

In addition, they tend to be deficient in diets fed to livestock (including calves) in North America. When these fatty acids were fed to calves using a commercial supplement, the calf’s ability to generate globulins to vaccines and toxoids increased, their ability to form titers to vaccines increased, fecal shedding of Cryptosproidia and Rotavirus decreased and scouring decreased.

Additionally, their body temperature during hot weather was lowered and average daily gain was increased by approximately 15 percent.

Elevated air temperatures negatively impact the growth of calves. Total feed intake is reduced, energy in the diet must be diverted from growth to maintenance and the immune system may be compromised. Balanced nutrition coupled with management to minimize the impact of air temperature can have a positive impact on calf growth and profitability. PD

Hill is a ruminant nutritionist and researcher at Provimi’s Nurture Research Center in Brookville, Ohio.

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Gale Bateman II
Nurture Research Center
Provimi North America

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