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Farmers track growing degree days as well as calendar for crop progress

By Jonathan Knutson Forum News Service Sure, area farmers are watching the thermometer and calendar. But they're keeping tabs on growing degree days, too: Growing degree days, also known as heat units, influence decisions on everything from irrig...

Corn near Willmar
A corn field is seen Monday east of Willmar. Corn needs more heat than most other crop species. Upper Midwest crops generally are faring better on growing degree days now than they were one year ago. (GARY MILLER | TRIBUNE)

By Jonathan Knutson
Forum News Service
Sure, area farmers are watching the thermometer and calendar. But they’re keeping tabs on growing degree days, too: Growing degree days, also known as heat units, influence decisions on everything from irrigation to the timing of pesticide applications and have a huge impact on both yields and quality.
“It takes energy for crops, or even an insect, to grow, and heat units supply it,” said Daryl Ritchison, assistant North Dakota state climatologist.
Upper Midwest crops generally are faring better on growing degree days than they were a year ago at this time, reflecting the late spring that delayed planting and hampered plant growth in 2014. Area fields that hadn’t received enough growing degree days were hurt by the September 2014 frost.
Growing degree days have drawn more attention in North Dakota, western South Dakota, northwest Minnesota and Montana as corn has spread north and west across the region.
“Different crops need different amounts of heat, and corn needs more than most other species,” Ritchison said.
Many factors, including moisture and light availability, affect the healthy progress to maturity of corn and other cro ps. But temperature is the most important factor, and growing degree days are a crucial measure of it.
How to measure it
Growing degree days - also known as growing degree units, heat units, thermal units and thermal time - measure heat accumulation within a specified temperature range. Little, if any, growth occurs when the temperature falls below a threshold or base temperature, while plants are stressed if the upper threshold temperature is exceeded. The key is accumulating the right amount of heat day by day for plants to move properly from one point to another in their life cycle.
The threshold temperatures vary by crop. With corn, for instance, the lower base is 50 degrees and the higher base is 86 degrees. Other crops grown in the region generally have both a lower and higher base than corn, which needs more heat.
Growing degree days for a crop are calculated by subtracting its individual lower threshold temperature from the average daily air temperature. The latter, in turn, is calculated by averaging the daily maximum and minimum air temperatures measured in a 24-hour period.
There are many sources of information on Growing degree days, including state extension services, state National Agricultural Statistic Services offices and state climate centers. The North Dakota State Climate Office, for instances, operates the North Dakota Agricultural Weather Network, which provides growing degree days for locations across the state, as well as for a handful of sites in South Dakota, Montana and Minnesota.
One of the best sources is Useful to Usable, or U2U, a U.S. Department of Agriculture-funded research and extension project that seeks to make Midwestern farms more resilient and profitable, given the changing climate. It offers an online interactive GDD tool available at www.mygeohub.org/groups/u2u/gdd .

Related Topics: WEATHERCROPS
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The Tribune publishes Records as part of its obligation to inform readers about the business of public institutions and to serve as a keeper of the local historical record. All items are written by Tribune staff members based on information contained in public documents from the state court system and from law enforcement agencies. It is the Tribune’s policy that this column contain a complete record. Requests for items to be withheld will not be granted.