Battling Heat Stress

Texas summers are a challenge to managing dairy cows. Dairy producers in Texas lose nearly $10 million in milk sales annually because of heat stress. During the heat stress months, mean temperatures are over 84^oF with maximum temperatures frequently over 100^oF. Average relative humidity sometimes is 80% (particularly in East Texas), with periods even higher. The combination of high temperature and humidity is especially stressful to the dairy cow. The result is decreased feed intake, milk production, milk fat percentage, and reproductive efficiency.

Feeding During Heat Stress

Water, feed and shade are critical in managing heat stress. Water helps regulate temperature. As the temperature rises above 75^oF, respiration rate increases and sweating occurs. This increases water loss from the lungs and skin.

Larger, high producing cows can consume more than 50 gallons of water per day when the temperature is above 90^oF. Therefore, an adequate water supply is essential. It should be conveniently located, clean, cool and preferably under shade. Try providing a water supply in the holding area of the parlor, at the exit from the parlor, and near the shade and feed to improve cow comfort and performance.

Be sure to check dry matter intake. The cow's first response to heat stress is to reduce dry matter intake, especially as the temperature rises above 77^oF. At 100^oF, dry matter intake may be 25% lower. Usually the cow reduces forage consumption first, if she has a choice. The result can be rumen acidosis and cows going "off feed."

A consistent, well-balanced and palatable ration is essential to maintain intake during heat stress. The summer feed slump can be minimized by using the highest quality forages available. In some cases, the amount of forage is reduced to maintain intake. But milk fat percentage usually decreases as forage consumption declines.

Total mixed rations (TMR) help maintain intake by preventing feed sorting and feed selection, thus reducing acidosis and "off-feed" problems. Some wet feed, such as silage, stimulates intake in a TMR, provided you feed the ration frequently. Wet TMRs spoil rapidly. Therefore, feed only what cows clean up and feed more frequently.

Because intake is reduced during hot weather, the concentration of nutrients in the ration should be increased. Raise the level of protein by at least 1%. Energy can be a problem with less feed intake. The addition of fat to the ration at one-half to two lbs. per head per day improves milk production and bodyweight gain in high producing cows. Some dairy producers claim to have better milk production during hot weather, when feeding Vitamin A at twice the NRC minimum requirement or up to 150,000 units daily per cow.

Increasing mineral consumption also may be beneficial. Work done at Florida has shown an increase in milk production in cows under heat stress when potassium is increased up to 1.5% of the ration dry matter and when sodium is fed at .6% of the dry matter. Feed complete mineral mixes with higher potassium and sodium levels only to the milking cows. If fed to dry cows, these mineral mixes may cause increased udder edema.

Buffers can partially offset the effects of low fiber rations. Frequently buffers are mixed in equal parts of magnesium oxide, limestone and sodium bicarbonate. Feed the mixture according to production. Cows producing 50 lbs. of milk per day receive about one-fourth lb. of buffer per day, while cows producing 100 lbs. of milk receive about one-half lb. of buffer per day. Also, for herds feeding in the parlor, feed hay before milking to reduce acidosis and "off feed" problems due to grain overload.

Cows tend to eat more feed in the cool of the night than during the day. Adjust the feeding rate so less is fed during the day and more at night, thus reducing feed spoilage and improving dry matter intake. Make sure the feed area is lighted.

Also improve palatability of a mixed ration by adding 2-3% molasses. The TMR does not dry out as quickly as when water is added to it. Keep the silage fresh by "squaring" the face of the bunker to help reduce spoilage and avoid cows going "off feed." It's important to keep loose silage picked up to avoid "off feed" problems due to spoiled and moldy silage. If wet brewers' grains are used, have them delivered frequently. Keep feeding areas clean and free from spoiled and moldy feeds.

Finally, provide cows a covered feed area. Not only does it protect the feed from the direct sun and rain, but it makes the animals more comfortable while eating, encouraging greater intake.

Improving Cow Comfort

To improve cow comfort we need to minimize the effect of environmental temperature and provide a way for cows to get rid of body heat. Too many times the only shade available is trees on the far side of a 40-acre pasture. Once the cows get to it, they stay there until milking time or until the sun goes down, not returning for feed or water.

In hot areas with low humidity, radiational cooling gives cows relief at night with evaporative cooling being effective during the day. In areas with high humidity, keeping cows cool is much more challenging. Three possibilities for modifying the environment that are readily available to most dairy operators are shades, fans and sprinkler systems.

Traditionally in pasture systems, trees have served as shade for the milking herd. There is no question as to their preference. However trees, particularly oaks, die quickly when used heavily by cattle. The area under the trees also provides one of the main reservoirs for mastitis outbreaks during the summer months.

In the past 10 years, the use of shades has increased substantially, and most noticeably, the use of moveable or portable shades. The most popular material for portable shade construction has been 80% sun-screen shade cloth, which is both tough and relatively inexpensive. It can be purchased in different sizes with grommets installed for easy installation.

Shade cloth needs to be stretched taut to avoid wind damage. Construct pipe frames one foot longer and wider than the shade cloth. Install the shade material over the cross-pieces for support. Little, if any, loss in the life of the shade will occur provided the material is tight.

A shade that is 20 feet x 30 feet is relatively easy to handle and move. Make the supporting frame a minimum of 12 feet. high, and narrow enough to move through gateways. Brace the frame as well. Portable shades are light weight, easily moved and provide an excellent means of maintaining clean areas. Trees provide better shade for cattle and need to be fenced off.

Fans circulate and exhaust air and are particularly helpful in holding areas. The first recommendation is to minimize the time cows are in the holding area. The second is to locate fans across the holding area moving air parallel to the length of holding area and away from the parlor. For example, center three 36" fans across the front of the holding pen with banks of three every 25-30 feet down the length of the pen. Another possibility for many holding pens is to install an open ridge to exhaust hot air. A 12"to18" ridge vent is generally adequate and effective.

Water sprinklers or sprayers located in feed lanes and holding areas promote evaporative cooling. Any water application in high humidity areas requires the use of fans for effective cooling, otherwise cows stand in a steam bath. Please note that spraying or sprinkling cows is stressed. Misting tends to allow small water droplets to collect on the hair and insulates the cow, reducing heat loss. Sprinklers have large water droplets, which wet cows to the skin for maximum evaporative cooling.

Low pressure nozzles, timers, pressure valves and thermostats are available through irrigation equipment installers, hardware suppliers and nursery suppliers. Low pressure nozzles generally deliver larger water droplet sizes for better hair penetration. Size piping according to the length of the run.

In most dairy operations, operate sprinkler systems on a cycle of 30-90 seconds spraying and 4-5 minutes of fan operation during the hot hours of the day, particularly noon to 6 p.m. Initially mount fans and sprinklers on a temporary basis to allow adjustment based on prevailing winds and effective sprinkling of the cattle. Locate sprinklers just out of reach of the cattle to minimize drift.

Managing the environment during hot weather can add 10% to15% to milk production and is accomplished relatively inexpensively. Reproductive efficiency is also negatively affected with increasing ambient temperatures. It is not unusual for body temperatures to rise to 103to 105F, making it difficult to "settle" cows. It might pay to use a thermometer to determine whether a cow should be bred.

While the above discussion has been directed toward the milking herd, don't forget that dry cows and heifers need shade to avoid early calving, reduce retained placentas and maintain growth.

Monitoring Heat Stress

Many times we hear of the effect of heat stress on production, but how can we measure it? Does it affect all cows the same? Are certain lactations effected more than others?

Using data from 120,000 cows enrolled in the Texas DHI that freshened in 1991, production was evaluated by lactation and month of freshening. Since the effect of heat stress was being evaluated, mature equivalent measures for production were not used. This analysis included all cows, regardless of breed.

Figure 1 shows the average 305-day actual production by month of calving and lactation number. Although fewer cows completed later lactations, the shape of those curves was similar with lactation 2 through 5. Curves were comparable across lactations, except for first lactation animals. Production for first lactation cows varied by only 410 pounds across the year. Highest production for first lactation cows occurred in April and the lowest was in November. In general, there was no difference by month of calving on milk production. One interpretation of this could be that there is consistency across months as to the care and management of heifers.

However, when second lactations were evaluated, production was found to be lowest for cows freshening in September and highest for cows freshening in March. Month of calving had a pronounced effect on 305-day actual production. The difference between a cow freshening in September versus March was 1575 pounds of milk. At $12/cwt, the March freshener produced $189 more in milk than the September freshener. The reproductive consequence of increased number of services and days open for cows calving in the summer was not included in this analysis. The same pattern was true for other lactations.

If you are unable to detect a seasonal difference in month of calving for first lactation cows, but are for second and later lactations cows, you must evaluate the economic consequence of managing your reproductive program. In the past, producers were told that each day open beyond 100 costs $2. You need to evaluate your individual herd situation. Would intentionally delaying breeding of first calf heifers result in increased profitability? In today's dairy operations, you cannot look only at averages to manage your herd. You must rely on your own data.

Records must be used to increase management efficiency. Texas A&M and Texas DHI have developed tools to aid in monitoring these differences. To optimize your profits in today's tight economic times, you must utilize all the power available for herd evaluation.