Farm Progress

Growers need average or late killing frost date so corn crops can dry in the field.

Kent Thiesse, Farm management analyst and vice president

September 6, 2017

5 Min Read

In many years, one of the biggest challenges with the corn and soybean crop in the Upper Midwest is getting the crop mature before the first killing frost. Average first frost dates range from around Sept. 20 in the northern areas of the Minnesota, to around Oct. 15 in southeast Minnesota and northeast Iowa. It appears that 2017 will be another challenging year in many portions of Minnesota and surrounding states for having crop development reach maturity before the first killing frost.

As of Aug. 30, a total of 2,092 growing degree units (GDUs) had been accumulated since May 1, 2017, at the University of Minnesota Southern Research and Outreach Center at Waseca, Minn. This is very near the normal level of GDU accumulation from May 1 until late August; however, the GDU accumulation during the month of August was nearly 20% below normal. This compares to 2,336 GDUs accumulated by the end of August 2016 at the Waseca location. By comparison, the 2017 GDU accumulation at the U of M Research Center in southwest Minnesota at Lamberton from May 1 until the end of August was about 6% behind normal.

The development of the corn and soybean crop in many areas of Upper Midwest remained behind normal in late August. The slower-than-normal accumulation of GDUs coupled with later-than-normal 2017 planting dates has resulted in some significant crop maturity concerns in portions of the region. The GDU accumulation is much further behind normal in some areas central and northern Minnesota. As of Aug. 30, the Waseca research Center had recorded only one day in August of 2017 with a maximum outside temperature of 90° F, and no days above that level. In fact, there were 24 days of August when the max outside temperature did not exceed 80° F, which is quite unusual.

As of Aug. 28, the National Ag Statistics Service (NASS) listed only 33% of Minnesota’s corn crop in the dent stage, which is about 10% behind normal by that date. Iowa listed 41% of the corn at dent stage on August 28, compared to a normal level of 50%, while North and South Dakota had only 18% and 23% of the corn at dent, respectively. For most commonly grown corn hybrids, under normal growing conditions, it takes approximately three weeks from dent stage until the corn reaches physiological maturity. Of course, with a continued pattern of cooler-than-normal conditions that most areas have been experiencing in recent weeks, crop maturity could be delayed even longer. 

Corn is considered safe from a killing frost once the corn reaches physiological maturity, which is when the corn kernel reaches the black layer stage. At that time, it is still usually at a kernel moisture of 28-32%. Ideally corn should be at 15-16% kernel moisture for safe storage in a grain bin until next spring or summer. So even beyond the corn reaching maturity in the coming weeks, some nice weather conditions will be required to allow for natural dry-down of the corn in the field, in order to avoid high corn drying costs this fall. It is likely that a high percentage of the 2017 corn crop will be stored in farm grain storage until spring and summer 2018.

There are also many acres of later planted soybeans in portions of the Upper Midwest that will require favorable growing conditions throughout September, in order to reach maturity before the first killing frost. The normal to above normal rainfall amounts during August in many portions of the Upper Midwest was favorable for soybean growth and pod setting; however, that advantage has been somewhat offset by the extremely cool weather pattern that has persisted during most of the month.

Based on the Aug. 28 USDA Crop Progress Report, 82% of the corn crop and 72% of the soybean crop in Minnesota is rated good to excellent. By comparison, the higher ratings for corn are only 60% in Iowa, 50% in North Dakota and 46% in South Dakota.

The good-to-excellent ratings in the other states for soybeans were: Iowa at 60%, North Dakota at 51% and South Dakota at 47%. North and South Dakota had 18% and 24%, respectively, of their corn crop rated poor to very poor, as of Aug. 28.

Nationally, 62% of the U.S. corn crop and 61% of the U.S. soybean crop was rated good to excellent on Aug. 28, which compares to 75% of the corn and 73% of the soybeans at this point in 2016.

Most crop experts now agree that Minnesota and Iowa’s 2017 crop yields are likely to be highly variable across both states. In the USDA Crop Report on Aug. 10, the 2017 Minnesota corn yield was projected at 183 bushels per acre, which would trail last year’s record yield of 193 bushels per acre, and the 2015 statewide corn yield of 188 bushels per acre. USDA estimates Iowa’s 2017 corn yield at 188 bushels per acre, which would be well below the record statewide average corn yield of 203 bushels per acre in 2016.

Nationally, USDA is projecting a 2017 average corn yield of 169.5 bushel per acre, and a 2017 average soybean yield of 49.4 bushels per acre. The expected 2017 yield levels are well below the record national average yields in 2016 of 174.6 bushels per acre for corn, and 52.1 bushels per acre for soybeans. Many analysts expect the national average yields to decline in the coming months, once harvest is completed, due to the wide variability in the crop conditions that currently exists in many areas of the U.S.  

About the Author(s)

Kent Thiesse

Farm management analyst and vice president, MinnStar Bank

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