Cool Weather Slows Crop Maturity

September 4, 2017


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 September 20 in the northern areas of the Minnesota, to around October 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 August 30, a total of 2,092 growing degree units (GDU’s) had been accumulated since May 1, 2017, at the University of Minnesota Southern Research and Outreach Center at Waseca, MN. 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 percent below normal. This compares to 2,336 GDU’s accumulated by the end of August in 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 1st until at the end of August was about 6 percent 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 GDU’s, together with later than normal 2017 planting dates in many locations, 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, compared to southern regions of the State. As of August 30, the Waseca research Center had recorded only one day in August of 2017 with a maximum outside temperature of 90 degrees F, and no days above that level. In fact, on 24 days in August the maximum outside temperature did not exceed 80 degrees F, which is quite unusual.

As of August 28, the National Ag Statistics Service (NASS) listed only 33 percent of Minnesota’s corn crop in the “dent” stage, which is about 10 percent behind normal by that date. Iowa listed 41 percent of the corn in the “dent” stage on August 28, compared to a normal level of 50 percent, while North and South Dakota had only 18 and 23 percent of the corn, respectively, listed in the “dent” stage by that date. For most commonly grown corn hybrids, under normal growing conditions, it takes approximately three weeks from the “dent” stage until the corn reaches physiological maturity. Of course, with a continued pattern of cooler than normal conditions, such as 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. When the corn reaches “back layer”, it is still usually at a kernel moisture of 28-32 percent. Ideally corn should be at 15-16 percent 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 the Spring and Summer of 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 August 28th USDA Crop Progress Report, 82 percent of the corn crop and 72 percent of the soybean crop in Minnesota is rated “good-to-excellent”. By comparison, the higher ratings for corn are only 60 percent in Iowa, 50 percent in North Dakota, and 46 percent in South Dakota. The “good-to-excellent” ratings in the other States for soybeans were Iowa at 60 percent, North Dakota at 51 percent, and South Dakota at 47 percent. North and South Dakota had 18 percent and 24 percent, respectively, of their corn crop rated “poor-to very poor”, as of August 28. Nationally, 62 percent of the U.S. corn crop and 61 percent of the U.S. soybean crop was rated “good-to-excellent” on August 28, which compares to 75 percent of the corn and 73 percent of the soybeans at this point in 2016.

Most crop experts are now agreeing that Minnesota and Iowa’s 2017 crop yields are likely to be highly variable across both States. In the last USDA Crop Report on August 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 is estimating 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.



Note — For additional information contact Kent Thiesse, Farm Management Analyst and

Vice President, MinnStar Bank, Lake Crystal, MN.  (Phone — (507) 381-7960);

E-mail —


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