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.
University of Minnesota specialists also say to make sure high moisture feed ingredients (distillers and corn gluten) are fed before they experience secondary fermentation (heating), and consider using a bunk stabilizer such as propionic acid. These commercially available products reduce growth of yeast and molds in silage when added during the ensiling process.
Additional management techniques include minimizing TMR sorting by increasing feed push up and evaluating particle size, managing bunker faces to minimize exposure to oxygen by removing only the amount of feed from the bunker needed to feed cows within the hour, and avoiding mister spray onto feed. The specialists say excessive wetting of the TMR can reduce palatability and accelerate bacterial growth in the bunk reducing feed quality.
Finally, keep a clean bunk. Remove all feed refusals daily before any new TMR is fed. Avoid TMR dry matter content less than 50%. Also plan on lower dry matter intake during heat stress and work with your nutritionist to adjust diets accordingly. An example is to increase the nutrient density of the diet to reflect actual dry matter intake, which is typically reduced during heat stress.
Feed ingredient adjustments:
Litherland and Sawall recommend increasing the energy density of the diet and reducing the heat of fermentation by substituting carbohydrates with rumen inert fat. Supplemental saturated fatty acids at 1.5 or 3.0% of diet dry matter have been shown to increase milk yield, milk fat content and yield, and reduce peak rectal temperatures in heat stressed cows.
They also say to increase delivery of key minerals, including phosphorous, potassium and sodium. Supplementation to increase dietary potassium from 1.0% to 1.5% in heat stressed cows in Arizona increased milk production. Research also indicated that increasing the dietary cation-anion difference for lactating cows increased milk yield when sodium (sodium bicarbonate) and potassium (potassium carbonate) were added to the diet. Avoid excessive concentrations of dietary sulfate and chloride in lactating cow diets especially during heat stress.
Supplement TMR with yeast culture. Research has shown feeding yeast culture improved daily milk yield by 2.6 pounds per cow during heat stressed conditions in California.