Alabama A&M University
S M A L L FA R M S R E S E A R C H C E N T E R
FA C T S H E E T
COLLEGE OF
AGRICULTURAL, LIFE
AND NATURAL SCIENCES
United States Department of Agriculture
Office of Advocacy and Outreach (OAO)
Dairy Calves Feel the Heat, Too
In parts of the country where heat is extremely high this year, heat stress is not only a problem with people, it is with dairy calves as well. It is a top priority of ours to keep cool during the summer months, so we should take the same consideration with our calves as well.
These calves are our future herd and how we care for them now will certainly affect them
later in several ways.
It is an old mind-set that thinks there should be no shade in pastures during the sum-mer
months. Some think that the cows will opt for the shade instead of grazing. This may be true
but those cows may be smarter than we think. Shade offers protection from more than just
the heat.
Have you ever noticed a soda can that “sweats?” Water runs down the sides of the can and
pools on the table. This same thing happens to us when our bodies try to keep us cool.
Remember our discomfort and then think about our calves. When the tempera-ture beats
down on calves their body temperature rises dramatically. Studies have
shown that calves without shade can have a skin temperature of 100oF when the out-side
temperature is 88.5. However, calves that are provided shade have a skin temperature of
96oF when the outside temperature is 85.1oF. This rise in body tempera-ture can cause
dehydration. Calves will try to cool themselves by sweating and panting. They will also
increase the amount of water intake to replace the fluids lost. If there is not an adequate
water supply available, electrolytes such as potassium and sodium are also lost.
Another effect of heat stress is an elevated body temperature. Extremely high body
temperatures can be very dangerous. Mortality increases very quickly with internal body
temperatures of 108oF or above. Another measure of extreme distress is the Black Globe
Humidity Index (BGHI). This index indicates the discomfort level in heat stressed calves.
These high levels of heat stress can also cause a decrease in the calf's immune system.
Immunoglobulins which circulate in the bloodstream to ward off infections are decreased
during extreme heat stress. Thus, remember to provide ade-quate shade for your calves this
summer.
Small Farms Research
Center
Alabama A&M University
4900 Meridian Street
James I. Dawson Building
RM #219
P.O. Box 700
Normal, AL 35762
How can heat stress affect growth rates in calves? Calves grow best in temperatures
ranging from 55oF to 78oF. Temperatures above these can begin to stress the calves. Calves
are usually able to adjust to higher temperatures but this comes at a cost in the form of
reduced growth rates. If an adequate water supply is not readily available calves cooling
ability is reduced which in turn will lower feed intake. Much the same way as heat affects
our appetite in hot weather. This heat-induced lower feed intake produces another problem –
an increase in maintenance requirement for your calves. This comes from the extra energy
required for cooling the animal. What does this mean exactly? Your calf is uncomfortable
1
due to the high temperatures which causes a reduction in feed intake while at the same time her body is requiring
more maintenance to cool her off which results in decrease in her average daily gain. Calves sometimes experience compensatory growth, or a growth surge that allows them to “catch up.” But, if calves are neglected during
critical times she may never experience this growth spurt. The reduced growth rates can haunt you later in your
calf's life by an increased rate at puberty, delayed breeding and calving after 24 months.
Are these effects really worth your concern? Let's look at how heat stress could affect your pocketbook. First, the
cost to maintain an open heifer beyond 24 months is about $1.30 to $1.50 per day, resulting in a total of $45 per
month. If the freshening time is 26 months, you spend at least $90 to maintain her while she is not producing for
you.
Let's remember those heifers that are waiting to reach breeding age. If your heifers calve at 24 months you will
need 66 replacements per 100 cows if you cull rate is 30 percent. If they do not calve until 26 months you will need
72 replacement heifers with the same cull rate. This amounts to approximately $540 for six additional heifers, not to
mention the almost $6,000 to maintain the other heifers.
This will also affect your milk income each day. If a heifer calves at 26 months instead of 24 you will lose income
for each day that she is not producing. On average a heifer will produce 55 pounds of milk per day, or 3,300 pounds
over a two-month period. At $13 per hundredweight, that's $429 over a two month period for just one cow.
You will also have to pay to maintain that heifer for the extra two months. It will cost you $10 per hundredweight,
or $330, in increased feed costs for this period. You will be left with a $3 per hundredweight or $99 “profit.”
However, calving two months late you lose that $99. Plus you will have to add $90 maintenance cost of $99 of lost
milk pro-duction, and you lose $189 per heifer. A 100-cow herd with a 30 percent cull rate would lose almost
$5,700 yearly.
Here's how to prevent heat stress in your calves:
Make sure calves have clean, fresh water available at all times.
Provide an area of shade whether it be trees, hutch or supplemental shade.
Promote good air exchange.
So, remember that while extreme heat can hurt your calves, it may hurt you more where it really counts – your
pocket!
Author: Duncan M. Chembezi, Ph.D. Professor and Director, Small Farms Research Center , Alabama A&M
University
Cooperating Units: USDA Office of Advocacy and Outreach (OAO), USDA/NIFA/ Beginning Farmers and Ranchers
Development Program (BFRDP), USDA/OAO/Outreach Assistance for Socially Disadvantaged Farmers and
Ranchers (OASDFR) Program, Alabama Cooperative Extension Systems, and Alabama A&M University.
2