Forecast Frosts Could Take Toll on Immature Midwest Crops

Corn and soybean crops across much of the US Midwest are about two weeks late in maturing, thanks to this spring’s record precipitation and planting delays. Now, the season’s first freezing temperatures are forecast in the upper Midwest in coming days, threatening to stress plants and reduce yields in one of the most frustrating growing seasons in many years.

Yields already were expected to be sharply lower than last year, largely because of the late planting. Even a light frost, coming at a time when freezes typically occur in some regions, could further stunt prospects for this year’s harvest. The current forecast from the National Oceanic and Atmospheric Administration’s (NOAA) Global Forecasting System (GFS) shows the first freezing temperatures of the season in the Dakotas and Minnesota in the next four to 10 days. This is around the typical time for first frosts.

The chart on the left shows the GFS forecast for minimum temperature in four northern states. Freezing temperature is expected in North Dakota and Minnesota on Sept. 28. The map on the right shows the county level minimums for a given day. Oct 4. Is displayed, showing freezing temperatures across a wide swath of the Corn Belt. 

It’s a race to maturity for corn and soybeans. Only 5% of corn in North Dakota is mature as of Sept. 22 compared to a five-year average of 37%; in South Dakota the figure is 12% versus an average of 44%; and 8% is mature in Minnesota compared to a 44% average. Soybeans in North Dakota are a bit more advanced at 67% dropping leaves, behind the 5-year average of 83%, but in South Dakota only 30% are dropping leaves compared to an average of 73%.

Light frost damage harms the leaves of the plant and hinders photosynthesis. If this occurs when the plant is still transferring dry matter to the seeds, yield can be reduced as the plant accelerates the maturation process. Temperatures near 32 degrees Fahrenheit (zero degrees Celsius) for a few hours will cause damage, while it takes only a few minutes at temperatures near 28 degrees F (minus 2.2 degrees C). A period below 28 degrees for several hours is known as a severe frost and can kill the corn stalk. Such a killing freeze stops everything and growers are left with whatever yield has accumulated to that point.

This season has been compared to the Great Flood of 1993 for similar planting delays and lost acreage. But that year also suffered from a combination of factors, including early season flooding, the remnants of an earlier massive volcanic eruption in the Philippines, and a devastating frost, resulting in the lowest corn yields in Iowa, Minnesota and North Dakota for nearly two decades.

The Mount Pinatubo eruption in 1991 injected enormous amounts of dust into the stratosphere, leading to a decrease in temperatures, especially in the Northern Hemisphere. The US had the coldest July on record in 1992 and the climate effects carried over into the 1993 season. The cold, combined with planting delays, resulted in a particularly late maturing crop. Severe frost on Sept. 14, 1993, ended the growing season in Nebraska’s panhandle, the western Dakotas, and the northern High Plains. Wisconsin was hit by freezing temperatures on Sept. 30, followed by Minnesota and northern Iowa on Oct. 2. Freezing temperatures covered all but the southern Corn Belt on Oct. 10.

Corn yields in Iowa, Nebraska, Minnesota, and the Dakotas were down an average 32% in 1993 from the prior year, while Illinois and Indiana yields fell 11%. For soybeans, the average loss in yield was 25% for Iowa, Nebraska, Minnesota, and the Dakotas, compared to a gain of 7% in Indiana and no change in Illinois.

This year’s forecast doesn’t anticipate anything as severe as 1993, but the late maturing crop is still vulnerable to a sudden cold snap. South Dakota is currently showing high soil moisture, which will delay drying in the field and may put more crop at risk. (Click here to go to an interactive display of soil moisture readings.)

Corn reaches maturity when the kernels form a black layer at the tip. At this point no more dry matter is added to the kernels and frost will not reduce yield. Soybeans do not have as distinct a line of demarcation for maturity, but the University of Nebraska estimates that maximum dry matter accumulation has been reached when 60% of leaves have dropped, 50% of pods are brown, and moisture is 60% or lower and the pods may be shrinking. Expected yield loss is proportional to the growth stage when the frost occurs and the amount of leaf tissue killed.

Animal feed is the best use for frost-damaged corn, according to the University of Wisconsin Extension. The quality discount imposed by processors will be higher than from feed operations. The various wet and dry ethanol operations will not want frost-damaged corn as it can lead to low yields of starch, dry mill grits, and DDGs, in addition to causing variability in ethanol production.