It's July and one of the hottest months of the year! As summer’s heat and humidity grow, so do the resulting production losses and health risks for dairy cattle - challenges that often persist beyond the return of cooler weather in the fall. Today’s dairy cows begin to experience heat stress at lower temperatures than many people realize. Recently updated guidelines indicate that, for cows making 35+ kg of  milk, production and reproductive losses begin at an average daily Temperature Humidity Index (THI) of 68 (23°C at 40% RH, or 20°C at 85% RH). Even in herds averaging less than 30 kg of milk, remember that the high producing cows will be negatively affected, and so in turn will the bulk tank average.

Focus on Facilities First: Heat stress, like most challenges faced by today’s dairy producer, is one that is most effectively addressed with a multi-pronged approach. The largest and  most cost-effective opportunities to reduce heat stress are facility-based. Ensuring that cows have adequate shade and  abundant water provision are attainable goals for all dairies.

Beyond those considerations, dairies in humid climates (typical of the northeast and mid-Atlantic regions) can most effectively cool cows by: repeatedly wetting cows down and blowing air over them on a cycle that increases in frequency with a higher THI. Blowing hot, humid air over hot cows is ineffective.

Ration Formulation Considerations:

1. Ensure that the dairy ration fed during hot weather is rumen friendly. Heat-stressed cows are more likely to experiencerumen health problems.
2. Daily eating patterns may be altered by hot weather, increasing the risk of slug feeding.
3. Hot cows stand more, and will often pant as a means of trying to cool off. The more they pant, the less they chew their cud, and these two behavioral changes combine to reduce the amount of saliva that is produced and swallowed. This in turn means less bicarbonate enters the rumen to function as a buffer, and a greater risk of sub-acute rumen acidosis results
4. Heat-stressed cows also eat less. In an attempt to compensate for this, past nutritional approaches often included increasing ration energy density, commonly achieved (at least in part) by feeding more grain. Given the rumen health risks already present, feeding more grain (starch) is generally ill-advised. Instead, ration changes should focus on feeding less total and/or rapidly fermentable starch, more fermentable fiber, and potentially more fat, as diets so formulated should not add to the risk of acidosis. Brown mid-rib (BMR) forages and high-fiber (or low starch) byproduct feeds like soy hulls fit well with this nutritional approach. Feeding lower starch rations may reduce feed efficiency, but this measure tends to be poorer for heat stressed cows to begin with. Furthermore, this approach should help minimize the risk of a significant nutritional contribution to the increase in lameness cases many herds experience in late summer or early fall.
5. Strive to feed appropriate protein levels. Overfeeding protein (or feeding excess protein relative to the amount of fermentable carbohydrates in the ration) can increase MUN (milk urea nitrogen) levels. If MUNs are significantly elevated, they may further contribute to the reduced conception rates typical of heat-stressed cows. Excess ration protein may also unnecessarily increase ration costs, depending on the source(s) used.

Ration Additives: A variety of research-proven feed additives are available that may help with milk production and/or cow health during hot weather. However, nothing works everywhere (except good management!), and for many of the products listed below, the research data has yielded  mixed results, with some studies showing a benefit to feeding the product whereas others do not.

• Live yeast and yeast culture: products from several manufacturers have shown improved rumen function, milk production, and/or feed efficiency when fed to dairy cows under heat stress conditions.
• Aspergillus oryzae (fungus extract): Research finds that cows fed this may better tolerate hot weather, with cows making more milk and/or having slightly lower body temperatures in some trials.
• Seaweed/kelp (Ascophyllum nodosum) meal: shown benefits in some studies, but not ·others, when fed to heat-stressed cows
• Niacin: shown benefits in some studies, but not  others, when fed to heat-stressed cows
• Sodium bicarbonate: Several studies have shown benefits to increasing ration DCAD (dietary cation-anion difference) levels, particularly in early lactation cows. It helps offset the reduced saliva production and rumen buffering experienced by heat-stressed cows. Cows lose more potassium as they sweat more during hot weather.
• Potassium carbonate:  can help offset this loss and increase the cows’ blood buffering capacity, and often contributes to higher milk production or improved butterfat percentage in the process.
• Rumensin®: feed additive labeled to improve milk production efficiency in dairy cows that typically generates a  strongly positive economic return when it is fed. Research to-date shows that this improvement in milk production efficiency is maintained in heat-stressed cows.

It's important to consider the use of any of these additives with input from a knowledgeable nutritionist. The additive(s) that are most likely to be appropriate and/or cost-effective  may vary somewhat from herd to herd, and over time, depending upon feeding strategies, ration composition, desired response from the cows, milk component concentrations, and milk price.

Adapted from article by Dr. Tom Bass, Renaissance Nutrition, Inc., 2012