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Heat stress on dairy cows can be very serious. Not only does heat stress negatively impact dry matter intake and milk production, it also has adverse consequences on foot health, reproduction, milk quality, and the incidence of disease. During periods of heat stress, the goal is to keep a cow’s body temperature as close to normal as possible.

Detrimental effects of heat stress include:

 

Production

·         Decreased dry matter intake

·         Decreased milk yield

Herd Health

·         Decreased rumen pH, which leads to increased subclinical acidosis

·         Increased laminitis

Reproduction

·         Decreased duration and intensity of heats

·         Smaller follicles

·         Decreased conception rates

·         Decreased blood flow to uterus and fetus

Milk Quality

·         Increased somatic cell count

·         Increased bacteria

 

Heat stress is a function of the following conditions:

·         Temperature

·         Humidity

·         Sunlight exposure

·         Air movement

 

Cows begin experiencing mild heat stress when the temperature-humidity index (THI) exceeds 72 (see Table 1 for examples of THI).

 

Critical Components of Heat Abatement

Every effort should be made to prevent heat stress. Three key areas that can have a tremendous impact on alleviation of heat stress include shade, cooling, and air flow.  The key to keeping cows cool, according to Dr Dennis Armstrong (University of Arizona), is “wet the cow to the skin and blow air across her.” 

Shade – Natural or artificial shade prevents solar radiation from reaching cows. Feed bunks should be shaded to encourage feed intake during hot weather. The height and width of a shade structure affects natural air movement. Building structures less than 40 ft. wide should have a 12-ft. minimum eave height. Structures greater than 40 ft. wide should have a minimum 16-ft. eave height.

Cooling – The use of sprinklers can help cool cows. When applying water, the hair coat should be wet to the skin. When water evaporates, cooling occurs. Cooling guidelines include:

·         Combine air movement (usually with the use of fans) with sprinkler use.

·         Adjust sprinklers to put out 50-100 gal. of water per cow daily.

·         Soak cows for 1-2 minutes at 5- to 10-minute intervals.

·         Run fans continuously when using sprinklers and misters.

Air flow – The use of air flow (ventilation) over cows is extremely important for evaporative cooling of cows. Fans are preferred over natural air flow, especially when using sprinklers. An air velocity of 400-600 ft. per minute over the cow is recommended. Typically, a fan can effectively blow air over a length equal to 10 times its diameter.

Critical placement of fans includes the holding pen, maternity pen, close-up pen, and along the feed bunk of all lactating groups. At the feed bunk, fans should be placed such that air blows over the cows’ backs. Fans should be placed at about a 30-degree angle. Fans should be well maintained. Dust accumulation on fan motors can significantly affect the efficiency of operation. Clean fans are more effective in cooling cows.

In the evening, a cow’s body temperature does not begin to drop until 4 hours (and possibly even longer) after the air temperature begins to drop.  So, leave the fans on well into the evening hours to help cows cool down faster. 

 

Feeding Strategies to Minimize Heat Stress

Feeding management and diet manipulation can be used to help minimize heat stress.

 

Feeding management tips:

·         Ensure unlimited supply of clean, fresh, cool water is available at all times. Cows may consume 30-40 gal. per head daily. Cows should have access to water immediately after milking/exiting the parlor.

·         Clean and check waterers regularly.  

·         Feed larger meals at night, when the temperature drops. Cows are likely to consume more feed after sundown, when their body temperature begins to drop

·         If your TMR is too dry, consider adding water to increase consumption and help minimize sorting.

·         Keep the feed bunks clean. Hot feed and moldy feed deter feed intake

·         Use a proven silage inoculant, which can aid in preventing secondary fermentation in the feed bunk

 

Ration modification tips:

·         Feed high-quality forages. 

·         Consider increasing the amount of digestible fiber (NDF) in the ration. This can be accomplished by substituting soy hulls or citrus pulp for forage NDF. 

·         Add fat to increase ration energy density. Avoid excessive levels of oilseeds (unsaturated fat), as they depress fiber digestibility.

·         Consider using ENERTIA^®, which will help improve energy density while avoiding fiber digestion depression.

·         Feed bicarb or S-carb at 4-8 oz. per head daily.

·         Increase ration crude protein, especially the RUP fraction.

·         Increase sodium to 0.45% (with buffer).

·         Increase magnesium to 0.3-0.35%.

·         Add Prosponse^® yeast culture to help enhance fiber digestibility.

·         Consider increasing ration potassium and ration DCAD using DCAD Plus, a source of potassium carbonate. 

·         Consider using Alliance Animal Health’s Mold Control Pak V or Ration Guard^®* to help reduce mold growth in TMR.

·         Consider using ADM Thermal Care R to assist feed digestion, gut health, immune function, and heat dissipation.

 

Summary

Ration modifications should include adding buffer and feeding elevated levels of potassium, sodium, and magnesium to replace lost electrolytes. Evaluate levels and types of fats used in the ration. Feed high-quality forages to maximize fiber digestibility. Don’t underestimate the importance of bypass protein and amino acid quality. Providing recommended levels of RUP, with proper levels of lysine and methionine, will definitely benefit cows experiencing heat stress.   And, don’t forget to ensure an unlimited supply of clean, cool, fresh drinking water.

 

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