Hot, humid weather during the breeding season hurts beef cow reproduction, especially if it turns hot and muggy quickly or early.
University of Nebraska researchers analyzed 10 years of temperature, humidity and reproductive data from a herd of crossbred cows in southeast Nebraska that were bred by bulls in pastures. They compared the number of calves born in the spring with weather conditions during the previous June-July breeding season.
"This study suggests conception rates are clearly lower during summers when it's hot during the breeding season than during cooler summers," says Terry Mader, an animal scientist in Concord.
Cattle producers have known intuitively that hot, muggy days are tough on cattle. However, this was the first study to quantify the relationship between environmental conditions and reproduction in beef cattle under typical pasture breeding conditions, Mader says.
"This shines light on a hidden cost of production," he adds.
In general, for each 1 degree F. that temperatures are above normal, conception rates are reduced 1%, the analysis shows. The problem is even greater in years with a combination of above normal temperatures and humidity.
"If you even have slightly above normal temperatures and humidity, the conception rate could easily be 1 to 3% lower during the season," Mader explains. "On 100 cows, that could amount to $1,000 to $2,000 in lost income if those cows don't conceive."
In Nebraska, a major beef producing state with roughly 2 million cows, a 1% reduction in conception rate means 20,000 cows don't get bred.
"If a weaned calf is worth $600, that's $12 million in lost income to the state's cow-calf industry," Mader says.
Heat and humidity trigger stress, which can disrupt a cow's reproductive cycle. If a cow does conceive and it stays hot, the embryo may not attach to the uterus.
Hot, humid weather during the first 30 days of the typical 60- to 75-day breeding season can potentially have the greatest economic impact even if the cows get bred later, Mader says. Research elsewhere showed that for every reproductive cycle or 21 days that a cow fails to breed, per cow profits decrease 10% on average.
"The first 30 days of breeding are significant because it's normally the coolest time when typically one-half to two-thirds of cows get bred," Mader says. "Those cows may breed later but, if it's hotter than normal, you're reducing the probability they are going to conceive."
The Nebraska study showed that if hot weather persists through the first 60 days of breeding, the impact on reproduction continues. Under these conditions, conception rates can be reduced by up to 2% for each degree temperature is above normal.
Meanwhile, producers can take several steps to minimize heat and humidity stress on their cow herds this season, including:
- Minimize cattle activity and movement during breeding season, especially when it's hot. Physical activity can raise a cow's body temperature 1 to 2 degrees.
- If you must move or work cattle, do it early in the morning when it's cooler.
- Provide plenty of clean water as it's one of the most useful tools for cooling cattle. If cattle have to walk long distances for water, consider additional watering sites.
- Provide shade or a place for cattle to cool off when possible. The best body surface areas to cool animals with water are around the face and legs.
- Control flies to discourage bunching and physical activity associated with cattle fighting flies, which exacerbates heat stress.
- Control parasites.
- Black-hided cattle are more susceptible to heat stress than lighter hided cattle.