Heat stress is detrimental to dairy production and affects feed intake, rumination, nutrient digestibility and absorption, which in turn can decrease milk and component yield, say Noah B. Litherland, Dairy Nutritionist and Zach Sawall, M.S. Research Assistant at the University of Minnesota.
Decreased feed intake only accounts for about 35% of the decrease in milk yield during heat stress. Heat stress can also reduce the responsiveness of the liver to key hormones (growth hormone and insulin like growth factor-1) that are important determinants of milk yield. It appears that heat stress may reduce the capacity of the cow's liver to make glucose which makes lactose. Lactose is the key factor determining milk volume, according to University of Arizona research.
The UM specialists say there are three key areas to consider when developing a nutrition plan to cope with heat stress: managing high risk groups, feeding adjustments and feed ingredient adjustments.
Intensively manage high risk groups during heat stress:
Dry cows - Research from the University of Florida showed that cows housed in naturally ventilated barns with fans and sprinklers programmed to cool dry cows once temperatures reached 70°F produced 11 pounds more milk per day in early lactation. Heat stress during the dry period compromises mammary gland development before parturition, which decreases milk yield in the next lactation.
Nursery calves - University of Arizona researchers reported a 12% decrease in starter intake in heat stressed calves (85 to 104°F) vs. control calves housed at 67°F. To increase intake during heat stress, research at the University of Washington reported that when calf hutches were elevated, internal hutch temperatures were cooler than external temperatures, hutch carbon dioxide levels were lower and respiratory rates were lower, particularly during the afternoon.
Fresh cows - Heat stress reduces feed intake and therefore reduces energy intake compromising cow health and performance in fresh cows. Iowa State University housed cows in environmental chambers at either 68°F (control) or a consistent cycle ranging from 85 to 102°F (heat stressed). Heat stressed cows consumed 28% less dry matter intake and had a 29% reduction in milk yield This dramatic effect of heat stress on energy intake would likely be additive in fresh cows that are already in negative energy balance often for the first 40 days of lactation.
Feeding management adjustments:
Water is the most important nutrient. Consider adding another water tank to the pen and keep drinking water clean and in abundant supply, alter feeding times to deliver feed during the coolest part of the day, and increase the number of daily feeding times (mix smaller loads) to keep feed in the bunk cool.