Heat stress remains a costly, persistent challenge in U.S. beef production. Annual losses exceed $370 million annually, and the economic impact often begins before producers see obvious clinical signs. What feels like mild heat and humidity to humans may still impact cattle comfort and performance.
Heat stress reduces feed intake, lowers daily weight gain and decreases feed efficiency in feedlot cattle. Extreme cases lead to greater treatment costs and sudden mortality. In warm regions such as Texas, unmitigated heat stress may result in up to 37 pounds of lost bodyweight per animal during the feeding period.
Pasture cattle face similar challenges with diminished grazing time and reduced feed intake. Heat stress is responsible for reduced breeding efficiency and early embryonic loss and can negatively impact bull fertility by lowering semen quality for weeks after a heat event. Warm-season heat coincides with peak fly activity, compounding animal stress. Biting insects spread disease and cause animal discomfort. Bunching traps heat among animals, and defensive behaviors such as kicking and head throwing cost valuable energy.
Some behavioral indicators that cattle are experiencing heat stress include:
- Increased standing time
- Restlessness
- Shade and water seeking
- Bunching
- Increased respiration (more than 90 breaths per minute is an alert; more than 130 indicates an emergency)
- Panting and open-mouth breathing
- Drooping heads
- Excessive salivation
Time spent under hot conditions increases respiration, diverts glucose away from performance (weight gain or milk), suppresses immunity, disrupts rumen function and impacts nutrient utilization. Nutritional strategies targeting one or more of these physiological responses can help cattle maintain performance and reproductive efficiency during heat events.
Nutritional tools to mitigate heat stress
Nutritional additives cannot eliminate heat stress but can support the biological systems that are affected, helping to maintain productivity and reproduction.
Capsaicin, the active compound in capsicum, is responsible for the characteristic “heat” of chili peppers and has gained attention as a nutritional tool to help cattle cope with heat stress and inflammation. It interacts with receptors that influence vasodilation (blood vessel expansion) and visceral sensory pathways tied to appetite. By promoting vasodilation, capsicum increases blood flow to the skin, improving heat dissipation and contributing to greater animal comfort. Research in finishing Holstein heifers has found that capsicum supplementation can stimulate feed and water intake, enhance rumen volatile fatty acid production and shift feeding behavior toward smaller, more frequent meals. These shifts in thermoregulation and feeding behavior support animal comfort and rumen fermentation when heat stress pushes cattle toward depressed and erratic intake patterns.
Aspergillus oryzae fermentation extract is a postbiotic feed additive derived from a domesticated aerobic fungus that provides enzymatic components that break down starches, proteins and fiber. These compounds support and enhance beneficial ruminal fungal populations. Research has demonstrated that A. oryzae extracts increase feed intake and significantly improve fiber digestibility, leading to better animal performance (increased weight gain and milk production). Because heat stress often reduces feed efficiency and disrupts gut barrier function in both grazing and feedlot cattle, supplementing with A. oryzae fermentation extract may help stabilize rumen function and improve nutrient utilization during periods of elevated environmental stress.
Garlic has been suggested to have repellent or antifeeding effects on horn flies, stable flies and ticks. Garlic contains high levels of organosulfur compounds, such as allicin. The volatilization of allicin is what gives garlic its distinctive smell and flavor. In grazing systems, supplementation with garlic has been found to reduce fly abundance on beef cows by 47%, decreasing defensive behaviors such as belly kicking and head throwing by 41%. A tropical study found that garlic supplementation prevented further tick infestation in grazing cows by 60%-69%; however, it did not significantly reduce the number of ticks present before garlic supplementation was initiated. Across studies, responses to garlic varied, influenced by environmental conditions, breed, product form (powder versus extract) and dosage.
Feed-through fly control products such as s-methoprene, diflubenzuron and tetrachlorvinphos help reduce populations of horn flies, stable flies and house flies (depending on the product). These compounds pass through the digestive tract and remain active in the manure, where they interrupt the development of fly larvae. When used alongside garlic supplementation, feed-through products can form a complementary strategy. The feed-through component can limit overall fly populations, while garlic provides repellent and antifeeding effects against remaining insects.
Using garlic in free-choice minerals can help maintain adequate mineral consumption when the formulation contains less-palatable ingredients such as magnesium oxide or chlortetracycline. This becomes especially valuable in regions where the breeding season overlaps with peak fly activity. In these situations, incorporating garlic offers a dual advantage by increasing mineral palatability and encouraging consistent consumption, while also helping cattle cope with fly pressure.
Tips for using feed additives to mitigate heat stress
- Start early: Nutritional tools are supportive, not corrective. Build additives into cow minerals or balancers before heat events peak. Lost performance or missed breedings are difficult to recover from.
- Follow dosage guidelines: Feeding rates differ by product potency and mode of action. Your nutritionist can help determine additive inclusion rate.
- Monitor product freshness: Some additives are volatile (garlic oils and capsicum) and may lose potency over time. Follow manufacturing date guidelines to ensure efficacy.
- Verify mineral intake: When additives or feed-through products are delivered through free-choice minerals, confirm cattle are consuming the recommended amount. A standard 2-4 ounces per head per day recommendation means 100 cows should consume 13-25 pounds of mineral a day.
Feed additives cannot replace foundational heat-stress management practices. Shade, airflow, abundant water and strategic feeding schedules remain essential. Flies may still migrate from the neighbors, and plant-derived products may not replace traditional feed-through fly control. However, targeted nutritional tools that address rumen function, appetite and thermoregulation can help cattle maintain performance, fertility and overall resilience during the hottest months.
References omitted but are available upon request by sending an email to the editor.
