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Watch GDDs as planting weather improves

Jeff Caldwell Updated: 05/07/2013 @ 11:39am Agricultural content creator and marketer.

Though "the weather appears to have straightened out with warmer temperatures and mostly dry conditions" in much of the Corn Belt (according to the Iowa Environmental Mesonet), planting is at just a fraction of where it normally is by this time. Usually, about half of the crop is in the ground, and Monday's USDA Crop Progress report shows just 12% of the crop is planted. There's a lot of ground to make up quickly.

Now that a more "normal" pattern seems to be settling in over the region when it comes to planting conditions, the focus now turns to growing degree days (GDDs). The slow start to spring has GDDs well behind where they typically are at this point, and that's something to watch closely as May unfolds into summer, says MDA Weather Services senior ag meteorologist Kyle Tapley.

"Given the cold spring across the U.S., and the resulting late start to corn planting, it will be important to monitor growing degree days as the growing season progresses," he says. "As you would expect, there is a stark contrast between this year and last year. Last year at this time, growing degree days were 130% to 160% of normal, while this year growing degree days are only 50% to 80% of normal in the western Corn Belt. Some improvement is expected over the next two weeks, however."

The most profound shortfall in GDDs stretches from northern Iowa north into Minnesota, northeastern South Dakota, western Wisconsin and North Dakota. In that area, GDDs are roughly half of where they need to be. It improves moving west to east, ranging from 60% to 70% of normal in Nebraska and Kansas, to 120% or greater in eastern Indiana and Ohio.

There's a "wrinkle" in how GDDs affect the corn you plant, though. The number of GDDs you accumulate doesn't have a static effect on your crop, says Purdue University Extension agronomist and corn specialist Bob Nielsen. Research has shown -- though you need to keep a close eye on GDDs -- even if you plant full-season varieties, you're not necessarily going to lose yield potential if you are forced into a late planting date. This certainly doesn't minimize the importance of tracking GDDs, however. It just means if you're worried about switching to a shorter-season variety, you might want to hold off.

"One should be able to estimate the GDDs remaining from a delayed planting date to the end of the season using long-term climate data, and then choose hybrids with GDD ratings that should mature no later than the date you chose to define as 'the end of the season,'" Nielsen says. "One 'wrinkle' in this concept is that it appears that hybrids mature in fewer GDDs than expected when planted 'late.' Relative to a May 1 planting date, hybrids planted later mature approximately 6.8 fewer GDDs for every day of delay beyond May 1. For example, a hybrid rated at 2,700 GDDs from planting to physiological maturity and planted on May 31 will reach physiological maturity in less than 2,500 GDDs after planting. That response of hybrid development relative to delayed planting means that normal full-maturity hybrids can be safely planted later than one would think and, consequently, means that growers can avoid switching to earlier maturity hybrids until planting dates later than one would think."

Nielsen recommends using a GDD calculator, like the one offered by the Weather Channel here, to track the key early-season crop variable. The tool shows, for example, GDD accumulation of 32 for the first seven days of May, compared to the 30-year average for that time period of 60.

Editor's Note: Graphics courtesy MDA Weather Services.

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