Climate change means more heat stress. Dairy herds under heat stress mean less milk production, more illness, less dry matter intake, decreased fertility, increased embryonic loss and poor growth.

Farmers will observe an increase in respiration rates, less time lying down and decreased rumination. Animals will have increased mineral loss, increased risk of acidosis and increased energy requirements.

Behaviors exhibited during heat stress can cause additional problems. Increased standing and walking can cause foot problems and lameness. The immunoglobulins in colostrum are decreased, so calf health suffers. Calves are born smaller due to the lack of nutrients available for fetal growth.

Heat stress happens when the body can no longer maintain thermal balance. Taking steps to prevent this from happening or to decrease the intensity and duration of any heat stress episode will pay off in herd health benefits.

“The animal is working harder,” said Larry Chase, professor emeritus, Cornell University Department of Animal Science. “Has anyone ever gone into a hutch when it is very, very hot outside? Think about cows in a pasture. Would you rather be a white cow or a black cow” on hot and humid days?

Animal response

The impact of heat stress depends on many factors. The actual temperature and humidity, nighttime coolness and the length of duration of a heat stress period all play a role. Air flow and ventilation, shade, the availability of water, animal size, milk production and even cow color can also have an effect on how severely heat stress is felt.

Heat stress has traditionally been said to occur if the temperature-humidity index (THI) rose above 72, but with today’s increase in milk production, THI of 68 is a better measure. The previous value is “outdated given the level of production we’re seeing in herds today,” Chase said.

A decrease in dry matter intake is often blamed for the loss of milk production. While there is a correlation, intake reduction isn’t the only reason milk loss occurs. No matter whether cows are on a high-forage or a high-grain diet, changes in the rumen occur during heat stress.

“It counts for maybe a maximum of 50 percent of a change in milk,” Chase said of decreases in dry matter intake. “The milk response is partly intake and partly metabolic.” Heat stress “tends to lower the pH of the rumen. It changes some of the rumen environment. You are affecting rumination of the microbial population.”

While changes in the milking herd are often a primary focus, there are lasting impacts to calves and heifers. Because nutrients are shifted from the fetus during heat stress, calves will be smaller. They are prone to diseases, and their future production and reproduction is compromised. Future milk production of heifers is compromised too.

“We need to think about more than just the lactating cow,” Chase said. “The impact if we cool the cows in the dry period” will be seen “in milk production in the next lactation” and in overall animal comfort and health.

Predicting events

If a farmer knew heat stress was going to happen, could measures be put into place to prevent it? Could dairy farmers, alerted in real-time to an increase in THI, use that information to protect the herd through changing daily management practices?

Stephen Jascourt of MDA Information Systems LLC would like to enable farmers to do just that. MDA’s forecast systems could be developed to pinpoint exactly where areas of heat stress are likely to occur on a daily or hourly basis.

By utilizing dew point temperatures and creating a heat index for the day, forecast systems could pinpoint exactly where the THI will be elevated and when, precisely, a heat stress event will occur on the farm. Forecasts could also be generated which would allow farmers to make individualized cropping decisions based on field level data.

“If we’re not sure there’s really going to be a demand for the product, then we won’t develop it fully in advance,” Jascourt said.

But if there is a demand to develop the model, such a system would enable farmers to see small-scale differences on the field level. Real-time data could allow farmers to assess situations across their farms.

Theoretically, this would allow farmers to move pastured herds from a field location where heat stress would occur to another pasture where the heat stress threshold won’t be met.

“What would you want to know if you could know it? What do you need for dairy operations and long-term planning?” Jascourt asked. “What range of uncertainty can dairy producers work with?”

Mitigating climate change’s impacts should be on every dairy manager’s to-do list. As climate change impacts are felt on dairy farms, farm management practices will need to be adapted to maintain herd health and productivity. And the technology to assist farmers in doing so can be developed.

“The trend seems to be going one way,” and an increase in the number of days of heat stress is happening, Chase said. “We need to increase heat stress mitigation.”  PD

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food system topics.

ILLUSTRATIONS: By Kristen Phillips.