Heat stress in dairy cattle is an ongoing challenge that can have significant effects on milk production, animal health, and overall farm efficiency. As we move into 2024, it’s more important than ever to implement effective strategies to combat heat stress, ensuring your herd remains healthy and productive.
Heat stress in dairy cattle leads to a cascade of negative outcomes, including increased water intake, rapid respiration, elevated body temperature, altered hormone profiles, reduced dry matter intake, decreased milk production, depressed milk fat and protein, impaired reproduction, and heightened susceptibility to metabolic diseases.
This can result in severe issues like metritis, mastitis, ruminal acidosis, lameness, and even death. The devastating effects of heat stress extend beyond the immediate period, impacting long-term herd health and productivity.
Understanding Heat Stress in Dairy Cattle
Heat stress occurs when the heat produced in a cow’s body from rumen fermentation, digestion, and metabolism exceeds its ability to dissipate it.
This condition is primarily influenced by environmental temperature and humidity, measured using the Temperature-Humidity Index (THI), and the cow’s level of milk production.
As THI rises, the cow’s body initiates responses to dissipate heat and reduce internal heat production, but when these mechanisms fail, core body temperature increases, leading to heat stress.
The Role of THI in Heat Stress
As THI initially increases, cows attempt to manage heat through increased water intake, peripheral blood flow, and sweating.
However, when THI exceeds 68, the cow’s ability to cope is overwhelmed, leading to increased respiration rates, which are a reliable indicator of both heat stress severity and rising core body temperatures.
| Temperature–Humidity Index | 68–71 | 72–79 | 80–89 | 90+ |
| Heat stress level | None – Mild | Mild – Moderate | Moderate – Severe | Severe |
| Respiration rate (breath/min) | > 60 | > 75 | > 85 | 120 – 140 |
| Rectal temperature (oC (oF)) | > 38.5 (101.3) | > 39.0 (102.2) | > 40.0 (104.0) | > 41.0 (105.8) |
*Based on R.B. Zimbleman and R.J. Collier, 2011: see Hoards Dairyman, April 25, 2011, page 281 for THI chart.
When the measures to dissipate heat are insufficient, the body reduces heat production by decreasing milk production and reducing feed intake.
This is achieved by decreasing blood flow to the mammary gland and altering hormone profiles, which shifts nutrient partitioning away from milk production.
Additionally, rumen motility and nutrient absorption decrease, further impacting overall productivity.
The Critical Importance of Cooling
Recent studies have shown that only 35% of the observed drop in milk production during heat stress is due to reduced feed intake.
The remaining loss is attributed to the increased energy required for active heat dissipation.
This underscores the limited potential for diet manipulation to recover lost milk production during heat stress periods and highlights the necessity of investing in effective cooling technologies.
Research conducted on commercial dairies in Israel, where summers are intensely hot, demonstrated that aggressive cooling—such as 10 periods of 45 minutes of cooling per day, with cycles of wetting and air movement—can nearly eliminate the effects of heat stress compared to moderate cooling methods.
Without aggressive cooling, the rise in respiration rates during heat stress reduces bicarbonate levels in the blood and saliva, decreasing rumen buffering and leading to ruminal acidosis.
This, in turn, increases the incidence of laminitis and lameness, as seen in the rise of hoof problems and treatments required in the fall. Furthermore, the negative energy balance caused by reduced intake and increased energy expenditure during heat stress significantly impairs fertility, extending well beyond the heat stress period.
Optimizing TMR Rations During Heat Stress
To mitigate the effects of heat stress, it is essential to fine-tune Total Mixed Rations (TMR) to minimize heat production and decrease the risk of acidosis.
This not only helps maintain dry matter intake and milk production but also supports reproduction during this challenging period.
The following TMR formulation tips are based on scientific research and are designed to help animals cope with the effects of heat stress.
While these strategies can alleviate the negative impacts, it is important to recognize that aggressive cooling remains the most effective method for managing heat stress.
- Increase Ration Density: Boosting ration density within the limits of safe rumen function delivers more nutrients per unit of dry matter consumed, increases feed efficiency, and reduces the heat produced during fermentation and digestion.
- Feed High-Quality Forages: Prioritize feeding high-quality forage, particularly alfalfa, during heat stress. High-quality forages have lower fermentation heat and higher buffering capacity, helping to maintain rumen pH. Limit the use of lower-quality forages and straw.
- Replace Starch with Digestible Fiber: Substituting starch with high-energy fiber sources like soybean hulls, distillers grains, and citrus pulp helps maintain energy levels while reducing the risk of acidosis.
- Lower Starch with Bypass Fat: Fat contains more than twice the metabolizable energy of grain, allowing for starch reduction while increasing ration density. However, adding higher levels of bypass fat during heat stress has not yet shown significant benefits.
- Decrease Protein Solubility and Degradability: During heat stress, rumen ammonia levels and protein degradation increase, likely due to reduced rumen motility. Reducing soluble protein and increasing bypass protein can mitigate these effects.
- Add Yeast: Incorporating live yeast or yeast culture into the TMR improves rumen fermentation, reduces the risk of acidosis, and has been shown to lower core body temperature and maintain milk fat content during heat stress.
- Increase Sodium Bicarbonate and Add Potassium and Magnesium: Sodium bicarbonate compensates for reduced saliva buffering, neutralizes VFAs and lactate, and maintains rumen pH. Increased sodium and potassium intake replenishes lost electrolytes, and magnesium balances the increased potassium levels. These adjustments should be made in consultation with a professional nutritionist.
- Add Zinc Methionine: Heat stress causes cows to stand more to aid cooling, leading to increased hoof and leg problems. Zinc methionine supports hoof health, reduces lameness, and lowers somatic cell counts, thereby reducing mastitis risk.
- Consider Bypass Niacin and/or Biotin: Preliminary research suggests that rumen-protected niacin can reduce body temperature in heat-stressed lactating cows. Biotin improves hoof hardness, reduces lameness, and enhances milk production in high-grain diets.
TMR Management Tips for Heat Stress
Effective TMR management during heat stress involves more than just formulation adjustments. It requires vigilance in feed ingredient management, feeding schedules, and ration consistency to maintain animal health and productivity.
- Bunker Silo Management and Feed Spoilage: The heat of summer accelerates feed spoilage, particularly in moist ingredients like silage. Spoiled feed increases the rate of spoilage in the entire ration, further depressing intake. Impeccable silo face management and cycling of wet byproducts at a faster rate are crucial during heat stress. Avoid any feed with mold or mycotoxins, as these are particularly problematic during heat stress.
- Batch Feeding Twice Per Day: Feed spoilage escalates rapidly with time. Feeding twice daily reduces the time feed remains in the bunk, minimizing spoilage and reducing the risk of acidosis. If once-daily feeding is the only option, retain a portion in the mixer for the next milking to reduce spoilage and sorting.
- Monitor Particle Size and Mix Uniformity: Adequate physically effective fiber is necessary to stimulate rumination, but during heat stress, reduced rumen motility complicates this. Ensure long fiber additions do not lead to sorting, as this increases acidosis risk. Regular use of the Penn State Particle Separator helps maintain a uniform ration resistant to sorting.
- Add Water to Control Sorting: If sorting persists, adjust the mixer knives for a more uniform cut and consider adding water to bind the ration together. Start with 5–10 lbs per cow, monitor results, and increase water if dry matter intake improves. However, avoid increasing moisture content beyond 55%, as this can reduce intake and increase sorting.
- Feed After Afternoon/Evening Milking: Maximizing dry matter intake during heat stress is best achieved by feeding in the late afternoon or evening when temperatures decline. Batch enough feed to last through the morning milking, and top up the remainder during the day.
- Ensure Constant Feed Availability: During heat stress, cows eat less frequently but in larger meals, increasing the risk of acidosis. Ensure cows always have access to feed to prevent binge eating and associated health risks.
- Push Up Feed Multiple Times Daily: Increase the frequency of feed push-ups to reduce sorting and ensure a consistent supply of relatively unsorted ration during peak intake periods following milking.
- Clean Water Availability: Cows under heat stress can more than double their water intake to replace moisture lost through sweating and to aid in cooling. Ensure an unlimited supply of clean water is available in multiple locations, particularly after milking, and clean waterers daily to a standard that meets human drinking water quality.
Effectively managing heat stress in dairy cattle requires a comprehensive approach that includes environmental controls, strategic TMR adjustments, and vigilant management of feeding practices.
By implementing these strategies, dairy farmers can mitigate the impacts of heat stress, ensuring their herds remain healthy and productive even during the hottest months.
Feel free to share these tips with fellow producers to help them navigate the challenges of heat stress in 2024.
