Whom will heat stress hit hardest?

Whom will heat stress hit hardest?

Climate prediction models can help farmers plan ahead and work to reduce the potential impacts a changing environment will have on the dairy industry.

by Tamilee Nennich, Guillaume Mauger, Yoram Bauman and Eric Salathe
The authors are at Purdue University, West Lafayette, Ind.; University of Washington, Seattle; Sightline Institute, Seattle, Wash.; and University of Washington, Bothell.

heifers at a dry lot manger

Heat stress has impacts that extend well beyond just the amount of milk produced, hampering milk components, reproduction and animal health. Its reach may be even greater as improvements in genetics and management increase the baseline production level of lactating cows.

Heat stress has significant impacts on milk production. As weather patterns change, temperatures can rise enough to cause heat stress in cattle, with impacts on both individual farms and the industry as a whole. Regardless of the cause of climate change, there is evidence that weather patterns are shifting in different areas around the U.S. Understanding the projected changes in climate can help farmers be more proactive in planning for the future needs of their herd.

Complex models are currently available to help predict changes in weather patterns. Researchers at the University of Washington have been working with a model that allows them to predict changes in temperature in various locations across the U.S.

Higher temperatures, less milk
Daily temperatures, and particularly the temperature-humidity index (THI), have an impact on production, reproduction and the health of lactating cows. As the temperature-humidity index rises above 70, cows become heat stressed. A THI of 70 would correspond to a temperature of 70°F and extreme humidity (100 percent relative humidity), or to a temperature of 84.7°F with no humidity (0 percent RH). Recent research from the University of Arizona even suggests that high-producing cows may experience heat stress starting at a THI of 68 or lower. In the absence of heat abatement strategies, research has identified a clear relationship between the THI and milk production, with dry matter intake and milk production shrinking as the THI goes up.

Researchers from the University of Washington have used climate models to estimate both the potential milk production losses and the resulting financial implications that could result from escalating heat stress. The models show that the potential impacts on milk production vary by location across the U.S. Some areas are expected to be impacted more by rising temperatures than others.

Calculating production losses
The model for this project was based on historic minimum and maximum temperatures from the National Oceanic and Atmospheric Administration (NOAA) going back to 1950. To estimate daily temperatures for the 2050s and 2080s, the model combined both historic data and global climate model output. A moderate global climate model, indicating a 5°F temperature gain globally over the next century, was selected for the development of the temperature estimates.

Additional inputs to the model included the number of cows per square kilometer across the U.S. An average milk production of 66 pounds per cow and a milk price of $15.90 per hundredweight were used to conservatively estimate additional financial losses.

Future climate predictions were obtained from global model output for the 2050s and 2080s using a middle-of-the road projection of future greenhouse gas emissions. The model calculated potential milk production losses on a daily basis according to projected minimum and maximum temperatures and morning and afternoon humidity. Apart from the wide geographic coverage, a key improvement over previous studies is the use of daily data for calculations — studies using only monthly data may discount important short-term heat events that are not present in the long-term average.

South takes hardest hit
The calculations showed that estimates of climate change led to noticeable milk production losses, and that these losses varied by location, with some areas much more sensitive to warming than others. Added heat could further compound summer production losses that these areas already see by another 10 to 15 pounds per day. Some locations, such as Okeechobee County, Fla., and Maricopa County, Ariz., are already frequently in excess of the THI threshold of 70°F. In such locations, future warming will dramatically raise the duration and severity of heat exposure. In other regions, such as Tillamook County, Ore., temperatures are cool enough that additional warming doesn’t substantially tip the scales towards greater heat stress.

Annual milk loss estimates across the entire U.S. are shown in the figure below. Note that these losses in milk production are averaged over the year and would be expected to be much greater (about three to four times greater) during the summer months.

Change in temperature and change in milk production comparison charts

Estimates of economic loss due to reduced milk production indicate that a higher incidence of heat stress over the course of the 21st century could lead to more than three times the losses currently resulting from heat stress. This could exceed over $2 billion per year for the entire U.S. dairy industry by 2100. These estimates demonstrate that heat stress has, and could continue to have, a significant financial impact on the dairy industry. In percentage terms, the impact is likely to be measurable but modest: about 6 percent for the country as a whole, with greater impacts in locations already experiencing heavier losses due to heat stress.

The economic loss estimates were determined for each county in the U.S. For more information, county-level estimates of economic losses can be found at: http://on.hoards.com/LOSS_counties.

Although our model focuses specifically on milk production losses, the impacts of rising temperatures would extend much beyond just the pounds of milk that would be lost. Models like this can begin to demonstrate some of the impacts that could result if heat abatement strategies are not used and provide indications as to areas in the U.S. where heat stress may increase and be of greater concern in the future.

This article appears on page 363 of the May 25, 2014 issue of Hoard's Dairyman.

Return to the Hoard's Dairyman feature page.