Heat affects dairy cow performance negatively more than cold in North Dakota due to the rise in temperature and humidity.
Those negative effects include a drop in milk and fat yield, as well as an increase in health and fertility problems, so make adjustments to management and feeding practices to help dairy cows get through the challenging periods of heat stress.
The combination of the effects of temperature and humidity is measured by the temperature humidity index (THI). Until recently, heat stress for dairy cows was thought to begin at a THI of 72 F. However, research using cows with high milk production has determined that heat stress can begin at a THI of 68. At this level, body temperature reaches 101.3 F and the respiration rate is 60 breaths per minute, which are the threshold for milk and reproductive losses.
To diminish the effects of heat stress, cows eat less dry-matter feed, drink more water and attempt to evaporate water from lungs and skin.
Heat-stressed cows eat less to avoid excessive heat from digestion. When a cow's dry-matter intake (DMI) decreases, the amount of nutrients available to support high milk yield also is limited. In addition, cows expend more energy, which often is seen as increased panting activity, in an effort to stay cool. If cows are refusing food, they are more likely to sort their food for smaller particle sizes, which, in turn, can increase the risk of subacute ruminal acidosis.
Higher respiration rates increase the amount of carbon dioxide the cows lose from the blood and the bicarbonate they excrete in their urine, so less bicarbonate goes to saliva. Heat-stressed cows also have less chewing activity and fewer ruminal movements due to the decreased feed/forage intake. Hence, the rumen’s buffering capacity is reduced and the cows become prone to developing ruminal acidosis.
Increased sweating makes the cow lose electrolytes, mainly potassium and sodium, that are important for the overall animal cation-anion balance.
Strategies that might help cows better cope with extreme weather conditions include:
- Adjust the nutrient concentration in the diet to account for the drop in DMI. Increased protein concentration is important for maintaining high milk yield with low DMI; however, keeping the amount of protein supplied in balance with the cow’s needs is essential to controlling diet costs and limiting nutrient losses in manure. Reducing rumen-degradable protein and balancing for amino acids can be particularly helpful in diets for cows under heat stress. Make sure to check that potassium and sodium concentrations are at least at 1.5 and 0.5 percent of ration dry matter, respectively, to facilitate electrolyte equilibrium and restore minerals lost due to increased respiration and perspiration. Also adjust ration magnesium levels accordingly (0.35 to 0.40 percent).
- Include buffers such as sodium bicarbonate or sodium bentonite. They might help prevent rumen pH drop and problems related to acidosis, including milk fat depression.
- Add or increase inert fats to help maintain energy intake when heat depresses DMI. In addition, fats produce less heat than fiber or starch when they are digested.
- Adjust feeding frequency so cows are fed more than once a day. This could contribute to feed freshness.
- Pay attention to the feed quality; watch for molds or feed heating, and clean the feed bunk more frequently.
- Feed animals several hours before or after the daily THI peak. Early morning and late afternoon feeding can avoid the peak THI coinciding with the peak of digestion heat, which often occurs four hours after feeding.
- Make sure cows have ample drinking water during heat stress. Cows should have free and easy access to fresh, clean water. Watering units must be working well, without leaks, and they have to be cleaned often to avoid algae or other contaminants. Providing more water space per cow can be beneficial in the summer months.
- Use fans and sprinklers to keep cows cool. Also maintain adequate cow density, and provide ventilation and shade.
Summer conditions can be tough on high-producing dairy cows, but adequate feeding practices can help mitigate the effects of heat and humidity on cows’ well-being and performance.