DIETARY STRATEGIES TO HELP DAIRY COWS WEATHER THE HEAT

Maybe surprising to some of you, heat stress does actually occur here in northern New York. As I write this, it is 80°F at 8 am, and it is expected to reach 96°F later today with a heat index of 106. Unlike the southern US, which experiences chronic heat stress, we experience it intermittently during the summer. Even in traditional temperate regions like Chazy, the rising temperature-humidity index takes a toll on feed intake, milk production, and overall herd health if we don’t implement management practices to mitigate it. While the use of fans, soakers, and shade remains our primary focus for heat abatement, selected nutritional strategies can also help our cows deal with the heat and higher humidity.

Maintaining feed intake during hot weather is half the battle. This is a battle we typically win, unless we end up with 3 or more nights in a row without nighttime cooling. Recently, a group of researchers from the US and around the world worked together to conduct a meta-analysis of 53 studies on lactating cows to figure out which nutritional strategies, particularly feed additives, can help offset intake and production losses caused by heat stress (DOI: 10.3168/jds.2025-27591). A meta-analysis gives us more confidence than a single study in determining which strategy gives the best or most consistent response. What they found was that feed additives are not a silver bullet, but they are a valuable tool in the “heat-abatement toolbox”.

Among all the categories they evaluated, yeast and yeast-derived products, especially those based on Saccharomyces cerevisiae, delivered the most consistent results. Live yeast increased intake (6.7%) and milk (5.7%), while the postbiotic and cell wall forms increased milk (3.4% and 16.3%, respectively) but not intake. The mechanisms behind the production gains likely include improved rumen fermentation, greater fiber digestibility, improved microbial stability, and reduced inflammation. Cows fed yeast had lower respiration rates and improved antioxidant status, suggesting a reduction in physiological stress. We use a yeast-derived product in our lactating cow diets, and this research reinforces its value during periods of heat stress.

Minerals, vitamins, and their blends were also beneficial, increasing milk by 4 to 8% depending on the formulation. Chromium yeast showed the strongest response for improving intake and milk production, likely due to its role in glucose metabolism and energy balance under stress. Zinc had a more modest response and has a role in immune function and epithelial integrity. Niacin had little effect on intake and milk production, and its primary impact appears to be on improved thermoregulation (i.e., lower respiration and panting). This reinforces that not all feed additives will directly contribute to milk in the tank, but they can still support overall physiology and health during times of stress.

Plant-derived bioactive compounds, known as phytochemicals, improved milk production by about 3 to 4% when they were considered together (i.e., honeycomb flavonoids, essential oils, polyphenols, capsicum, and fenugreek). However, the results varied widely depending on the compound used, as they have different modes of action. The polyphenol-rich and byproduct-based products performed the best. The phytochemicals did not consistently reduce body temperature or respiration, suggesting that their effect is more likely metabolic than thermoregulatory.

As you work to figure out what summertime feeding strategy is right for you, keep in mind that response to feed additives can vary among herds. Differences in heat load, facilities, stage of lactation, and ration formulation all influence the results. The best return on investment will likely come from combining effective heat abatement with good nutritional management and use of feed additives that have research support behind them.

— Heather Dann

dann@whminer.com