Heat stress can have a major impact on dairy cattle depending on location, duration of heat stress, environmental modification, and humidity. Environmental factors that increase heat stress include high temperature, direct sunlight, and high humidity which a cow must dissipate. Milk yield and dry matter intake add to heat stress challenges. Evaluating heat stress, metabolic responses, feeding guidelines, ration formulation, and water requirement are discussed below.

Evaluating heat stress

Heat stress can be defined using temperature-humidity index (THI) with THI values of 75 to 78 considered stressful and values over 78 extremely stressful as dairy cows are unable to maintain normal body temperature. The THI is calculated from the wet and dry bulb air temperatures for a particular day using the following formula:

THI = 0.72 (W + D) + 40.6

The W is wet bulb and D is dry bulb temperature in degrees centigrade. Dairy manager and consultants can evaluate heat stress level on the dairy farm using the following guidelines and observations.
Body temperatures over 102.6 degrees F (normal body temperature is 101.5 degrees F)

· Panting rates greater than 80 breaths per minute

· Reduced activity and more standing

· Reduced feed intake

· Reduced milk yield

· Lower fertility and conception rates

· Drop in milk fat and milk protein content

Cows attempt to reduce heat stress by conduction (movement of heat to cool surfaces such as water ponds), convention (air movement and fans), radiation (night time cooling), and evaporation (sweating and panting). High producing cows in early lactation generate more heat stress with greater declines in milk production. Cows producing 31.6 kg of milk generated 48.5 percent more heat while cows at 18.5 kg of milk produces 27.3 percent more heat compared to dry cows.

Metabolic responses

Maintaining blood pH is a critical homeostatic control point relying on concentrations of carbonic acid and bicarbonate in the blood. Metabolic acidosis and metabolic alkalosis involve bicarbonate. Respiratory acidosis and respiratory alkalosis are related to partial pressure of carbon dioxide which can stimulate respiration when dissolved levels of carbon dioxide are low. Renal compensation to alkalosis during heat stress was increased urine pH. Hyperventilation decreases dissolved carbon dioxide and the cow secretes bicarbonate in the urine to balance the lower levels of dissolved carbon dioxide. The kidney excretes hydrogen ions to balance the resorption of bicarbonate. Metabolic acidosis occurs if available sodium is reduced, lowering bicarbonate and increasing plasma chloride.

Reduced metabolism in cattle under heat stress was related to reduced thyroid hormone secretion and less gut mobility lead to high gut fill. Plasma growth hormone concentration also declined. Rumen pH values can drop 0.2 unit (from 6.0 to 5.8 for example) not related to dietary or feeding patterns due to lower blood flow to the rumen. More blood flow is diverted to the surface area of the cow to dissipate heat reducing nutrient absorption from the rumen and lower gut. Mineral salts (such as sodium bicarbonate and sodium chloride) can increase the dilution rate of rumen fluid increasing rumen pH and maintaining a favorable acetate to propionate ratio of rumen fluids.

Rumen microorganisms that synthesis B vitamins, amino acids, and fatty acids can be reduced under heat stress. Lower levels of short chain fatty acids (produced from rumen acetate and butyrate) in milk fat has been reported compared to increases in C16 and C18 fatty acids (from dietary or body reserve sources). Rumination is depressed when cow are experiencing dehydration and heat stress. Higher levels of lactic acid can also build up in the rumen of dairy cattle experiencing heat stress inhibiting rumen motility and drop rumen pH. Nutrient digestibility is improved due to longer feed retention time in the digestive tract and slower rate of passage. This response is related to lower feed intake (dry matter). Feed intake changes are reflected in Table 1.

Feeding guidelines

The dairy manager should recognize challenges that can occur and reduce their impact.

Challenge 1. Dry matter intake will decline due to associated heat of fermentation and less cow feeding activity. Depending on nutrient concentration of the current ration, increasing nutrient density may be possible to maintain target nutrient needs (Table 2).

Challenge 2. Meeting high nutrient requirements can be achieved using the following strategies or combination of changes.

· Increasing forage quality (higher NDF digestibility and faster rate of passage)

· Shifting from forage NDF sources to by-product NDF sources (such as citrus pulp, soy hulls, or corn gluten feed)

· Increasing feed grain and starch levels while avoiding rumen acidosis risks

· Adding fat/oil to the ration to increase energy density. Fat/oil sources must be considered to avoid negative rumen and fiber digestive efforts associated with unsaturated fatty acids that are more available in the rumen (such as extruded oil seeds and corn distillers grain). Rumen inert and saturated fatty acids can be better choices.

Challenge 3. Protein form and level can impact heat stress responses. Danish workers reported that higher protein diets (19 percent compared to 23 percent) reduced milk yield by 3.1 pounds. Higher levels of milk urea nitrogen, blood urea nitrogen, and rumen ammonia can occur as microbial yield of protein can be reduced. Higher uterine nitrogen products and/or uterine ion balance could reduce reproductive success. Utah workers also found higher levels of degradable protein reduced milk yield under heat stress.

Challenge 4. Dietary cation-anion difference (DCAD) can be beneficial when dairy cows are under heat stress. Mississippi workers reported values over + 200 meq/kg increased dry matter intake, rumen pH levels, blood pH, and urine pH. Electrolyte balance, especially sodium and potassium, are lowered under heat stress. Sweating can increase potassium losses while sodium is excreted in the urine. Mineral guidelines are listed in Table 3.

Challenge 5. Feed bunk management can impact dry matter intake.

· Avoiding secondary fermentation in the feed bunk (indicated by higher feed temperature and undesirable odors) can stimulate feed intake.

· Feeding fermented forage (such as corn silage) can extend feed bunk life due higher levels of organic acid and lower feed pH.

· Adding water (2 to 5 liters) can increase total dry matter intake, reduce sorting, and avoid feed fines and /or dust that can reduce palatability. Adding water may increase secondary fermentation risk.

· Applying a propionic acid based feed stabilizer can to the TMR at mixing time can extend bunk like and feed palatability.

· Removing stale and/or hot feed (weigh back) is critical before adding fresh feed. Offering ration dry matter during the night period (cooler time), multiple feedings, and fresh feed after each milking can be improve dry matter intake.

· Soaking cows and placing fans over the feed bunk are effective ways to improve cow comfort, but avoid getting the feed wet.

· Placing rubber mats or belting next to the feed bunk manger under shade can encourage cows to remain at the feed bunk longer.

Building ration for heat stressed cows

The following nutrient guidelines can improve cow response to heat stress:

· Maintain 20 to 22 percent effective NDF in the ration to maintain cud chewing and forage raft in the rumen.

· Limit total crude protein to less than 18 percent on a dry matter basis with 6.5 to 7 percent as RUP (rumen undegraded protein).

· Maintain 28 to 30 percent NDF, 18 to 20 percent ADF.

· Target 24 to 26 percent starch and 4 to 6 percent sugar to adjust the rate of carbohydrate fermentation in the rumen and favor microbial growth.

· Raise DCAD over 250 meq per kg.

· Increase total ration potassium to 1.4 to 1.6 percent, sodium to 0.4 to 0.5 percent, and magnesium to 0.35 percent.

Several feed additives can be beneficial. Sodium bicarbonate/sodium sesquicarbonate are effective rumen buffers. Add 0.75 percent sodium bicarbonate to the total ration dry matter. If dry matter declines, maintain the target level (225 grams for example) of the desired sodium bicarbonate intake. Offering free-choice sodium bicarbonate plus recommended levels in the ration may be beneficial (measure intake avoiding levels over 50 grams per cow per day in the group). Potassium carbonate is another effect product to increase dietary potassium while delivering buffering capacity and increasing DCAD. Yeast culture and fungal products can stabilize rumen pH and improve the rumen environment while maintaining fiber digestion. These products can also reduce lactic acid levels in the rumen that can drop rumen pH. Monensin (Rumensin brand name) can also reduce rumen lactic acid levels, protein degradation, and methane production increasing available energy resources.

Water requirements

Water quality, quantity, and availability are factors that can assist cows to reduce heat stress and response to heat stress challenges. Table 4 summarizes water requirements based on milk yield and heat stress. The following guidelines are useful water management guidelines:

· Cool water is desirable (not cold).

· Heat stress increases water requirements by 1.2 to 2 times normal levels.

· Provide ten centimeters (four inches) of space per cow.

· Offer water as cows exit the parlor.

· Place water under shaded areas

Selected References

Hansen, P.J. 2005. Managing the heat-stress cow to improve reproduction. Proc. of 7^th Western Dairy Mgmt Conf. Reno, NV. p. 63-76
Hutjens, M.F. 2004. Feeding guideline. Hoards Dairyman Press. Fort Atkinson, WI. pp.
Kadzere, C.T., M.R. Murphy, N. Silanikove, and E. Maltz. 2002. Heat stress in lactating dairy cows: a review. Livestock Prod Sci. 77:59-91
West, J.W. 2005. Heat stress affects how dairy cows produce and reproduce. Proc of 7^th Western Dairy Mgmt Conf. Reno, NV.

Table 1. Impact of heat stress on increase in maintenance energy needs and milk yield response.

Table 2. Impact of changes in nutrient concentrations when changes in dry matter occur.

Table 3. Mineral guidelines for cows under heat stress and at thermal neutrality.

Table 4. Water requirements for large breed dairy cattle at various temperatures.