About Heat Stress in Dairy Cows

Nutrition adjustments for heat stressed dairy cows

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. According to University of Arizona researchers, 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 (Rhoads et al., 2010). 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.
Three key areas to consider when developing a nutrition plan to cope with heat stress:

Intensively manage high risk groups during heat stress:

• Dry cows - Research from the University of Florida (Tao et al., 2011) 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 (O'Brien et al., 2010) 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 (Moore et al., 2012) 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 (Baumgard et al., 2011) 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:

1. Water is the most important nutrient. Consider adding another water tank to the pen and keep drinking water clean and in abundant supply.
2. Alter feeding times to deliver feed during the coolest part of the day.
3. Increase the number of daily feeding times (mix smaller loads) to keep feed in the bunk cool.
4. Make sure high moisture feed ingredients (distillers and corn gluten) are fed before they experience secondary fermentation (heating).
5. 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.
6. Minimize TMR sorting by increasing feed push up and evaluating particle size.
7. Manage bunker faces to minimize exposure to oxygen. Remove only the amount of feed from the bunker needed to feed cows in the next hour and ensure that removal rates are adequate to prevent secondary fermentation of the silage.
8. Avoid mister spray onto feed. Excessive wetting of the TMR can reduce palatability and accelerate bacterial growth in the bunk reducing feed quality. Keep a clean bunk. Remove all feed refusals daily before any new TMR is fed. Avoid TMR dry matter content less than 50%.
9. 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