New research with today’s hard-working, high-producing dairy cow shows that she begins to feel the effects of heat stress at THI levels as low as 68.1 Click here to view the current THI chart.
This four-point adjustment may not seem like much, but to a cow it means a great deal. It also means that managers must respond to heat stress sooner than previously thought if a dairy is to proactively decrease the effects of this costly challenge to cow health and performance.
Physical response
A cow’s body does its best to cool itself when heat-stress conditions hit, but there’s only so much she can do on her own. At a THI of 68, respiration rates exceed 60 breaths per minute, rectal temperatures rise above 101.3º F and milk production starts to decline.
By the time THI reaches 80 to 89 cows’ respiration rate climbs higher than 85 breaths per minute, rectal temperatures will exceed 104º F and death losses begin to occur.
Other physical responses to heat stress include reduced dry matter intake and rumination, potassium deficiency, reduced length and intensity of estrus, compromised embryo development and quality, as well as a lower ability to maintain pregnancies. In addition, increased incidence of health problems like mastitis, retained placenta, metritis and ketosis have also been reported during summer months.2
Nutrition solutions to combat heat
In the face of heat stress, the diet and nutrition management play key roles in minimizing its effects.
“It’s important to keep dietary potassium levels up during heat stress,” explains Kevin Lefeber, dairy consultant with CP Feeds, Valders, Wis. “We want to make sure we’re replacing the potassium she’s losing to milk production and in her response to heat stress.”
DCAD Plus® Stabilized Potassium Carbonate is a key tool he uses to help accomplish this goal.
That’s because cows are more prone to potassium deficiencies under heat-stress conditions. Potassium is the main component of sweat, which cows lose more quickly through perspiration, salivary action and urination when it gets hot. Metabolic acids also increase during heat stress, but can properly be buffered by increasing ration DCAD with DCAD Plus.
Take action
While many of Lefeber’s customers incorporate DCAD Plus into rations year ’round, special emphasis is placed on its inclusion before heat stress begins. DCAD Plus should be added to rations at least a month prior to the onset of heat-stress conditions.
“We like to make sure we’ve got proper ration DCAD levels in place in May before temperatures get too hot,” he says. “Then we keep it in at least through September or October when it begins to cool down again.”
For best results, feed DCAD Plus to reach a dietary potassium level of 1.7% dry matter or greater. Keep in mind the optimum DCAD range for high-producing cows is +35 to +45.
To ensure accuracy, test forages with a wet chemistry macromineral analysis of sodium, potassium, chloride and sulfur to properly balance for DCAD. For help with balancing rations in the midst of heat stress, check out this DCAD calculator or read "8 Tips to Manage Heat Stress".
1 Zimbleman RB, Collier RJ. Heat Hits Cows Sooner Than We Thought. Hoard’s Dairyman April 25, 2011;281. Available at: http://www.hoards.com/E_animalhealth/ah17. Accessed March 12, 2012.
2 Bilby TR. “Strategies to Improve Reproduction during summer. ”Paper presented at: Mid-South Ruminant Nutrition Conference; April 8, 2008; Arlington, Texas.