Heat Stress Also Affects Dry Cows

by Richard Goings, Ph.D., Dairy Specialist

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Effects of Heat Stress

• Core body temperature rises
• Respiration and heart rate increases
• Water and electrolytes are lost
• Blood pH declines
• Hormonal levels change including more cortisol
• Decreased DMI and milk production
• Increased risk of laminitis

The bad effects of heat stress on milking cows are well known. Heat stress also affects the dry cow, only milk production drops do not occur until after calving. The dairy cow's ideal temperature is 30-70 degrees. Above 75 degrees the dairy cow has great difficulty in controlling internal body temperature. That is because the heat of digestion from rumen fermentation adds to the heat load.

As the core body temperature rises, more blood flows to the skin, respiratory rates increase, sweating occurs (limited in cows) and cows tend to stand rather than lay down. Increased blood flow to the skin results in reduced mammary blood flow (less milk) and reduced internal blood flow which slows the rate of passage of digesta. Slower rates of passage results in lower dry matter intake. Increased respiratory rates caused respiratory alkalosis (reduces blood bicarbonate levels resulting in reduced buffering capacity from salvia). Sweating and increased urination (result of respiratory alkalosis) contributes to the loss of both potassium and sodium. Due to these changes in metabolism, maintenance requirements (energy) will increase by 7-25%, water intake will increase by up to one-third and feed intake will be reduced. Even if energy intake is maintained, volatile fatty acid production will be reduced due to changes in gastrointestinal motility.

Heat stress in the dry period results in reduced calf birth weights (10%), reduced milk production (12%), lower IgG concentrations in colostrum and increased incidence of periparturient disorders (dystocia, retained placenta, etc.). The University of Missouri recently reported on the effects of a thrice-weekly one-hour treatment of water spray and fan cooling to pre-fresh cows. Results were more consistent dry matter intake pre-calving and significantly higher milk production after calving with supplemental cooling.

Managing the heat stressed dry cow:

1. Provide shade. The sun's radiant energy adds to the heat load. However, don't cut out ventilation using an enclosed building to provide the shade.
2. Control horn flies. Flies cause the cows to "bunch-up".
3. Don't let dry cows to lose weight. If more ration energy is needed, it is probably better to feed higher quality forages than more grain or supplementing fat particularly for far-off dry cows.
4. Don't feed excessive protein (>12-13%). Metabolizing the extra protein adds to the heat load.
5. Consider supplemental cooling especially around calving time.
6. Don't eliminate salt. Sodium and potassium add to a positive cation:anion balance which is undesirable ahead of calving and salt (sodium) has been implicated with udder edema. However, the cow's need for these cations increases with heat stress.

For more information, please call us at 1-800-553-1712 or email us at contactus@vigortone.com. You can reach Dr. Goings at rgoings@vigortone.com.