Managing and Feeding Lactating Dairy Cows in Hot Weather (B 956) University of Georgia Extension Hot and humid environmental conditions stress the lactating dairy cow and reduce intake of the nutrients necessary to support milk yield and body maintenance. In Georgia, weather conditions are sufficiently hot and humid to reduce performance of dairy cows for five months or more each year. This publication presents methods that can be used to minimize the stress on dairy cows during hot weather and enhance production during the hot summer months. 2017-04-18 16:30:36.277 2006-06-02 14:26:48.0 Managing and Feeding Lactating Dairy Cows in Hot Weather | Publications | UGA Extension Skip to content

Managing and Feeding Lactating Dairy Cows in Hot Weather (B 956)

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Joe W. West, Extension Dairy Scientist

The modern dairy cow is similar to a factory, consuming and processing large quantities of raw materials (feedstuffs) and producing a large volume of high quality product (milk). It is essential that the flow of these raw materials (nutrients) not be reduced or interrupted if milk yield of the cow is to be maintained. Hot and humid environmental conditions stress the lactating dairy cow and reduce intake of the nutrients necessary to support milk yield and body maintenance. In Georgia, weather conditions are sufficiently hot and humid to reduce performance of dairy cows for five months or more each year. This is very costly to production, but is a great opportunity to improve profitability if the dairyman successfully meets the challenge presented by heat stress.

This publication presents methods which can be used to minimize the stress on dairy cows during hot weather and enhance production during the hot summer months.

What Is Heat Stress?

The primary factors that cause heat stress in dairy cows are high environmental temperatures and high relative humidity. In addition, radiant energy from the sun contributes to stress if cows are not properly shaded. As the environmental temperature increases, the difference between the temperature of the cow's surroundings and her body decreases, and her reliance on evaporative cooling (sweating and panting) to dissipate body heat increases. However, high relative humidity reduces the effectiveness of evaporative cooling and during hot, humid summer weather the cow cannot eliminate sufficient body heat and her body temperature rises. The tremendous amount of body heat that the high yielding dairy cow produces is helpful in cold climates but is a severe liability during hot weather.

One way to measure the combined effect of temperature and humidity is use the temperature-humidity index (THI). Table 1 contains the THI that results from different combinations of temperatures and relative humidities. A THI exceeding 72 is sufficient to cause minor heat stress and cause a response such as reduced feed intake in the cows. Note that with high relative humidity, the temperature that causes a THI of 72 can be quite low. Note also the danger zone for THI (Table 1). When the THI is in this range, livestock are at risk and environmental modification will be necessary to avoid sharp declines in production, and in some cases to avoid death losses.

Table 1. Table of temperature-humidity indexes (THI)1 at varying temperatures and relative humidities.
Temp(F) — — — — — Relative Humidity, % — — — — —
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
— — — — — THI — — — — —
70 64 64 64 65 65 65 66 66 66 67 67 67 68 68 68 69 69 69 70 70
71 64 65 65 65 66 66 66 67 67 67 68 68 68 69 69 70 70 70 71 71 Heat stress begins.
72 65 65 65 66 66 67 67 67 68 68 69 69 69 70 70 70 71 71 72 72
73 65 66 66 66 67 67 68 68 68 69 69 70 70 71 71 71 72 72 73 73
74 66 66 67 67 67 68 68 69 69 70 70 70 71 71 72 72 73 73 74 74
75 67 67 67 68 68 68 69 69 70 70 71 71 72 72 73 73 74 74 75 75
76 67 67 68 68 69 69 70 70 71 71 72 72 73 73 74 74 75 75 76 76 Sharp drops in production occur.
77 67 68 68 69 69 70 70 71 71 72 72 73 73 74 74 75 75 76 76 77
78 68 68 69 69 70 70 71 71 72 73 73 74 74 75 75 76 76 77 77 78
79 68 69 69 70 70 71 71 72 73 73 74 74 75 76 76 77 77 78 78 79
80 69 69 70 70 71 72 72 73 73 74 75 75 76 76 77 78 78 79 79 80
81 69 70 70 71 72 72 73 73 74 75 75 76 77 77 78 78 79 80 80 81
82 69 70 71 71 72 73 73 74 75 75 76 77 77 78 79 79 80 81 81 82 Danger Zone
83 70 71 71 72 73 73 74 75 75 76 77 78 78 79 80 80 81 82 82 83
84 70 71 72 73 73 74 75 75 76 77 78 78 79 80 80 81 82 83 83 84
85 71 72 72 73 74 75 75 76 77 78 78 79 80 81 81 82 83 84 84 85
86 71 72 73 74 74 75 76 77 78 78 79 80 81 81 82 83 84 84 85 86
87 72 73 73 74 75 76 77 77 78 79 80 81 81 82 83 84 85 85 86 87
88 72 73 74 75 76 76 77 78 79 80 81 81 82 83 84 85 86 86 87 88
89 73 74 75 75 76 77 78 79 80 80 81 82 83 84 85 86 86 87 88 89
90 73 74 75 76 77 78 79 79 80 81 82 83 84 85 86 86 87 88 89 90
91 74 75 76 76 77 78 79 80 81 82 83 84 85 86 86 87 88 89 90 91
92 74 75 76 77 78 79 80 81 82 83 84 85 85 86 87 88 89 90 91 92
93 75 76 77 78 79 80 80 81 82 83 84 85 86 87 88 89 90 91 92 93
94 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94
95 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
96 76 77 78 79 80 81 82 83 85 86 87 88 89 90 91 92 93 94 95 96
97 77 78 79 80 81 82 83 84 85 86 87 88 89 91 92 93 94 95 96 97
98 77 78 79 80 82 83 84 85 86 87 88 89 90 91 93 94 95 96 97 98
99 78 79 80 81 82 83 84 85 87 88 89 90 91 92 93 94 96 97 98 99
100 78 79 80 82 83 84 85 86 87 88 90 91 92 93 94 95 97 98 99 100
1THI = td - (0.5 x RH)(td - 58), where td = dry bulb temperature (degrees F) and RH = relative humidity in decimals.

The effects of hot, humid weather are costly to the dairyman in many ways. In addition to reduced milk yield (15 to 40%), cows have lower milk fat content, impaired reproductive performance, and greater susceptibility to health problems during hot weather. Feed intake declines and the energy which is consumed by the cow is used less efficiently for milk production. There are many tools available to help the dairyman combat heat stress, but management must be excellent if performance is to be maintained. Aspects of hot weather management for lactating dairy cows that will be addressed include: (1) management of the cow’s environment, (2) feed ration management, (3) feed bunk management, and (4) cow management.

Practical Management to Reduce Heat Stress

Shades and Cooling

The easiest and most obvious way to help heat-stressed cows is to provide adequate shade. Direct sunlight adds a tremendous heat load to the cow, but heat energy that is reflected from areas exposed to the sun such as concrete floors, barn walls and other exposed surfaces also add to the cow’s heat load. Shading reduces the black globe environmental temperature (a measure of temperature and radiant energy) and lowers the rectal temperature and respiration rate of cows, increasing feed intake and milk yield (Table 2). Gains in milk production of 10 to 20 percent occurred where shaded and unshaded cows were compared.

Table 2. Effect of shade on heat stress indicators in lactating dairy cows.
Measurement Shadea No Shade % Change Shadeb No Shade % Change
Black globe temp., degrees F 86.2 101.8 -18.1 84.4 105.8 -25.3
Rectal temp., degrees F 101.7 103.3 -1.6 102.6 105.4 -2.7
Respirations/minute 78 115 -47.4 83 133 -60.2
Daily feed intake, lb. 45.6 37.0 +23.2
Daily milk yield, lb. 42.8 37.5 +14.1
aAdapted from Collier, et.al., 1981. J. Dairy Sci. 64:844.
bAdapted from Scheider, et.al., 1984. J. Dairy Sci. 67:2546.

In times past when dairy herds were smaller and most cows were pastured, adequate shade was provided by trees. In larger herds the greater cow density has killed many trees. Mudholes created under the trees increase the incidence of mastitis and slow milking parlor throughput because of the greater time required to clean dirty cows before milking. Because of these negative effects, use of trees for shading has declined.

Numerous shading alternatives are available to the producer. Mesh shadecloth is lightweight, available in numerous sizes, has reinforced grommets that make installation easy, and can be used in portable or permanent installations. A commonly used shadecloth is an 80 percent mesh, which means 80 percent of the sunlight is blocked. Because 80 percent of the sunlight is reflected and 20 percent passes through, shadecloth is not as effective as other types of shading, but it is far better than no shade at all. The passage of some sunlight through the mesh may help to dry the area underneath the shade. Ease of installation and relatively low cost lends shade cloth to many uses. Portable shades using mesh shadecloth and placed in pastures can be moved so that mudholes are not created.

Loose housing barns provide shade for cows without the cost of freestalls or the concrete floors. These barns have a sand base mounded in the center of the barn to minimize accumulation of moisture. The bedding must be cleaned and maintained regularly to prevent pitting and fresh sand must be added as needed. However, separate feeding facilities must be maintained.

An option which is becoming quite popular is a feeding/cooling barn with or without freestalls. These barns provide shade and the convenience of drive-through feeding. These barns have high roofs which are steeply pitched to minimize the transfer of heat energy from the metal roof to the cows and to encourage air flow, venting hot air out the ridge vent and drawing in cooler air from the eaves.

In the hot, humid environment of the southeast shading alone does not provide adequate heat stress relief. Additional cooling in the form of fans and sprinklers is usually beneficial and is easily installed in these barns. Sprinkling (not misting) the cow with water to fully wet her body and using fans to evaporate the water cools the cow and encourages greater feed intake and milk production. Research shows an 11 percent increase in milk yield when cows were cooled with fans and sprinklers compared with shading alone. Sprinklers and fans are usually placed next to the feedbunk so that the feeding area is the coolest place on the farm, helping to encourage greater feed intake.

Shading Concerns and Precautions

Orientation of the shading structure relative to the path of the sun is important to minimize intrusion of sunlight under the structure during summer. Notice that little sunlight penetrates underneath a structure with an east-west orientation in June, while the north-south orientation allows a great deal of sunlight penetration. In December, the east-west orientation allows some sunlight under the structure, providing warmth in the cooler months. An east-west orientation is most desirable so that the barn is cooler in summer while allowing sunlight to warm the barn during cold months.

Overcrowding under shades reduces their effectiveness. A minimum of 38 to 48 square feet per cow is needed to minimize heat build up between the animals. Cows should have access to adequate shading at all times; in the holding pen, at the water trough, at the feed bunk, and while resting.

Highly reflective roofs reflect sunlight and minimize heat transfer to the cattle underneath. White galvanized metal or aluminum roofing is very reflective. A layer of insulation underneath the roofing also minimizes heat transfer. Roofs high enough to minimize heat transfer but low enough to reduce intrusion of sunlight are necessary. An eave height from 11.5 to 14.5 feet is desired.

Water from sprinklers must not be blown into freestalls by fans, which creates a mastitis hazard, or onto the feed, which increases spoilage. Sprinkler water can be confined to desired areas by choosing the right nozzle and placing nozzles correctly in the barn. Also, having fans and sprinklers run in sequence (for example 13.5 minutes for fans, 1.5 minutes for sprinklers) so that fans and sprinklers do not run at the same time will keep water from being blown where it is not wanted. A cooling system which is thermostatically controlled ensures that the system runs as long as temperatures are high, and not just until the evening milking shift is over. If cows are sprinkled with water in the holding pen they must be dry by the time they are milked. High capacity fans directing air at floor level help to dry cows’ udders prior to milking.

Feeding and Nutritional Management During Hot Weather

Water is the most important nutrient for the cow. It should always be available, should be fresh and clean, and there may be advantages to providing cool water during summer. Waterers should be cleaned r

Status and Revision History
Published on Jan 1, 1995
In review Feb 3, 2009
Re-published on May 1, 2009
Reviewed on Jan 16, 2014
Reviewed on Mar 28, 2017