“Heat stress affects cows in a number of ways, but the most significant lingering effects take aim at reproduction,” says Dr. Todd Bilby, Texas AgriLife Extension dairy specialist. “It takes almost two months under more temperate conditions for reproduction to return to normal.”
That’s because heat stress has negative effects on growing ovarian follicles. Keep in mind that the follicle emerges 40 to 50 days prior to ovulation. Heat stress that occurs at any time during this period can negatively affect ovarian follicular growth and functionality. Furthermore, either due to higher temperatures or changes to follicular function, the oocyte (egg cell) may be compromised.
And it’s also not unusual for fewer fertilized eggs to develop into embryos during periods of heat stress. In 1999, researchers reported a decrease in the number of Holstein oocytes that developed to the blastocyst (which becomes the embryo) stage during July and August compared to cooler months.1
Similarly, the effects of heat stress also linger with dairy bulls since semen quality doesn’t return to normal for up to two months following heat stress, notes Dr. Bilby.
Given these facts, don’t let up on heat abatement management strategies just because cooler temperatures may be around the corner. Read “8 Tips to Manage Heat Stress” for more about heat abatement strategies.
Bump up nutrition management
In addition to keeping fans and sprinklers running, take a look at your late-summer nutrition program and feed management to keep heat stress effects from hanging around.
- Push up feed frequently, suggests Dr. Bilby, and be on constant watch for signs of acidosis. “The heat-stressed cow is prone to acidosis,” he says, “and those effects linger after heat stress passes, too.”
- Feed the highest-quality forage available to help provide as much buffering capacity as possible because heat-stressed cows tend to ruminate less, and therefore secrete less salivary buffers. Ensure a neutral rumen pH is achieved by adding ARM & HAMMER® Rumen Buffers to rations.
- Incorporate supplemental bypass fat sources like MEGALAC®-R Omega-3 and Omega-6 Essential Fatty Acids into lactating rations, which offer the dual benefit of rumen bypass fat with two essential fatty acids in a single source. “Research shows that you can increase milk production during the summer with supplemental fat sources,” explains Dr. Bilby. “By using a rumen bypass fat you get the extra energy without the thermal side-effects.”
- Lastly, manage DCAD levels. Cows require additional minerals during heat stress to overcome the decline in dry matter intake and increased losses of electrolytes. In addition, cattle sweat is rich in potassium, and high temperatures sharply increase potassium losses. Also note that the absorption of certain minerals like calcium, phosphorus and potassium may be reduced during heat stress. Watch dietary sodium and magnesium levels, as well.
Click here for more on properly balancing ration DCAD during heat stress using DCAD Plus® Stabilized Potassium Carbonate. Also check out the DCAD Calculator to confirm proper DCAD levels.
Don’t forget about dry cows
Dry cows are often the forgotten part of the heat stress solution equation. In addition to the stress of getting ready to calve, these cows also undergo social change stress and dietary change stress. Plus, these animals generally go from a facility with heat abatement to facilities without it—and then back again. “That’s a lot of stress that can carry over into milk production and reproduction performance,” says Dr. Bilby.
Research conducted in Florida in 2009 found that housing dry cows in a barn with fans and sprinklers increased milk yield in the subsequent lactation by 16.5 pounds of milk per day compared to cows housed in a similar barn without fans and sprinklers.2
“A dry cow cooling program doesn’t have to be as intense as for lactating cows, but there are definite economic and reproductive benefits to cooling these cows,” Dr. Bilby says.
“Remember that cooling dry cows also results in improved reproduction in the next lactation,” he notes.” If a cow is heat-stressed when she calves, the ovarian follicles that begin to develop shortly thereafter will also be compromised. If the voluntary waiting period is 50 to 60 days, that’s the follicle that’s going to ovulate.”
One last hint to prevent the effects of summer heat stress from carrying over into the fall: Don’t be too quick to stop cooling.
“At the beginning and the tail-end of the season cows feel heat stress just like in the middle of summer,” cautions Dr. Bilby. “Whether turning systems on or off, don’t go by the date on the calendar, go by the temperature.”
1 Bilby TR. Strategies to improve reproduction during summer. In Proceedings. Dairy Heat Stress Road Show, 2011:22.
2 Hansen PJ. Cooling strategies during heat stress. In Proceedings. Dairy Heat Stress Road Show, 2011:13.