Ag Weather Forum

Crops Chasing Growing Degree Days

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Growing¬-degree-day totals are already running well behind average, due to the very cool temperature trend of spring 2019. (NOAA graphic)

The magnitude of just how slow the 2019 crop season has been shows up in emergence rates. USDA's weekly progress report for the week ended June 16 placed the corn emergence rate at 79%. That is by far the lowest emergence rate going back to 1998. This pace is 13 percentage points less than the 92% rate logged in 2013, and 15 points below the 94% tallied in 2011.

Soybean emergence is way, way low as well, at only 55% as of June 16, 2019. That is 11 percentage points less than the 66% rate recorded in 2013, and 18 points below the 73% emergence figure in mid-June 2008.

By my reckoning, there's still a full one-fifth (20%) of the corn crop and almost half (45%) of the projected soybean crop that has still not emerged. Talk about a slowdown! And, with this tremendous lag in progress, there is widespread concern about how much time crops will have to grow, pollinate, flower, fill ears, fill pods, mature, and dry down before harvest.

As country music legend Jerry Reed sang in the movie "Smokey and the Bandit," this crop has a long way to go and a short time to get there. With that in mind, we are already behind the curve when it comes to growing degree day numbers. As of mid-June, most of the primary U.S. crop areas had growing degree day totals running at least 100 GDDs lower than average, with 200 GDDs below average noted in the Dakotas, Minnesota and Wisconsin.

This lack of growing degree days translates to a late year for crops and the need for at least mild conditions the rest of the summer. However, July and August offer no more than normal temperatures for the largest row-crop producing states. That's due, in part, to the longer-term effect of the saturated soils. DTN long-range forecaster Stephen Strum described things this way in a subscription service review on Wednesday, June 12:

"The spring pattern will likely have an impact on the summer pattern this year as there is a strong correlation between soil moisture and temperatures across the main agricultural areas of the Plains and Midwest. The relationship between soil moisture and temperature breaks down elsewhere, but the very wet central U.S. does favor a cooler summer in those areas, at least through the first half of the summer season."

Wet soils can have a thermostat-like effect on the ability of ground to heat up. Warmer conditions draw out moisture, which cools the air. That low-level moisture can also fire up new thunderstorms as well, of course. The end result is that, while dry soils can help the air get even hotter (as we saw in 2012), wet soils can get in the way of heat. And, in the case of the 2019 crops, they're also getting in the way of growing degree day totals.

Bryce Anderson can be reached at

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