Written by Dr. Andy Mueller and Dr. Grace Cun
Heat stress is one of the most predictable challenges on a dairy and one of the easiest to underestimate. Once THI reaches ~68, cows begin losing performance. Reduced intake, lower milk production, depressed components, sorting, loose manure, and declining reproduction are all common signs. During heat stress, cows eat fewer, larger meals, increasing rumen instability while more energy is diverted toward cooling and electrolyte losses.
The goal during heat stress: protect intake, maintain rumen stability, and support hydration.
Practical Nutrition Strategies
1. Optimize Carbohydrates and Energy Density
Many rations get into trouble during heat stress by relying too heavily on rapidly fermentable starch. Excess starch can elevate the risk of a rumen upset, especially when cows slug feed. Instead, shift toward digestible fiber and sugars, targeting roughly 7-9% total sugar (DM basis). Sugar, from sucrose, provides a quick energy source for rumen microbes, and helps support fiber digestion and intake. Fiber digestibility also becomes more important during heat stress. Prioritizing ingredients with higher early fiber digestion (30-hour NDFd) can help reduce rumen fill, improve passage rate, and support dry matter intake. Because intake often declines during summer, maintaining energy density is critical. Work with your nutritionist to balance the overall carbohydrate profile of the diet, prioritize high NDFd feedstuffs, and increase energy density with bypass fat sources when appropriate. Avoid excessive unsaturated fat, which can further depress fiber digestion and milk fat.
2. Positive Dietary Cation-Anion Difference (DCAD)
During heat stress, cows lose more electrolytes, especially potassium (K), making adequate potassium levels critical for maintaining intake, hydration, and rumen stability. Increasing dietary +DCAD, particularly through potassium, is a proven strategy to help support milk production and components during summer.
A more positive DCAD helps maintain rumen pH and a more stable fermentation environment, supporting milk yield, ECM, and milk fat production. Research has also shown that a +DCAD supports more effective biohydrogenation, converting unsaturated fatty acids commonly found in byproducts, oilseeds, and some forages into more saturated forms. This is important because excess unsaturated fats are associated with reduced fiber digestion and milk fat depression.
Potassium carbonate is commonly used to increase +DCAD, but dry sources can create handling, safety, and flowability challenges. Delivering potassium through a molasses-based liquid feed provides a more practical approach, improving palatability and intake consistency while helping support summer performance.
3. Additional Feed Additives to Support Heat Stress Management
Several feed additives may also help support cows during periods of heat stress by improving hydration status, rumen stability, and intake consistency.
Betaine is a naturally derived nutrient, commonly sourced from sugar beets, that functions as both an osmolyte and a methyl donor. During heat stress, betaine helps cells retain water and maintain normal function, which may help support hydration, feed intake, and overall cow performance. In addition, as a methyl donor, betaine may help spare nutrients such as methionine and choline so they can be utilized more efficiently for milk production and other metabolic functions.
Fresh Guard, a blend of propionic acid and sodium benzoate, will help stabilize and condition the TMR during hot weather, reducing heating and spoilage at the feed bunk while helping maintain ration consistency and intake.
4. Water Is the Most Important Nutrient
Milk is roughly 87% water, and during heat stress, water intake can increase by 30–50%. That makes access and availability critical. Keep troughs clean, provide adequate water space, at least 3–4 linear inches per cow, and maintain fast refill rates around 3–5 gallons per minute. If water intake is limited, nothing else in the ration will work as intended.
Management still drives results! Nutrition can help support the cow, but cow comfort is the foundation. Prioritize shade, fans, soakers and frequent feed pushups. Heat stress isn’t one big problem, but a series of small losses, reduced intake, rumen instability, and dehydration, that add up to lower milk and components.
Takeaways
Shift away from rumen pH reducing, highly fermentable starch and toward digestible fiber and sugars to improve intake and rumen efficiency. Increase potassium and +DCAD to help support intake, and rumen stability while adding betaine to aid in cellular hydration during heat stress. Prioritize water availability, cooling, and overall cow comfort, as they remain the foundation of summer performance.
The dairies that perform best in summer are the ones that stay ahead of heat stress. Reach out to your Westway representative to help keep your herd on track.
