Frost Will Nip Some of US Crop, but Most Areas Will Be Spared

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How Mature Is the Crop

Frost damage is proportional to crop maturity, in addition to how long and precisely how cold the temperatures get. 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 considered a severe frost and can kill the plant.

Soybean development is further along in North Dakota than other nearby states, with 86% of the crop dropping leaves compared to a five-year average of 93%. Wisconsin, Minnesota, and South Dakota are at 42%, 60%, and 58% soybean leaf dropping, respectively.

Leaf drop starts in the so-called R6 stage of plant development and accelerates through the R7 stage. When a plant has dropped 60% of its leaves, 50% of its pods are brown, and moisture is at 60% or lower, maximum dry matter accumulation has been reached. At this point frost can no longer damage the crop. More detail on soybean growth stages can be found here.

Corn is similarly protected from frost once maturity is reached. 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.

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The chart on the left compares corn maturity of Michigan, North Dakota, and Wisconsin for this year against the five-year average. There is a large gap in current progress to normal maturity. The chart on the right shows North Dakota’s percentage of soybeans dropping leaves this season (line with markers) just a little behind previous years (blue lines). 

Which Areas Are at Risk of Frost

Corn in North Dakota is much further behind than normal, at only 15% maturity compared to a 57% five-year average. Soybeans in North Dakota may avoid any significant yield damage given their high level of maturity. While there are some areas of the state that will experience freezing temperatures, there are no forecasts for a severe frost. Rolette and Bottineau, the northernmost counties in central North Dakota, were hit by freezing temperatures this week, putting 237,000 acres of soybeans at risk, based on acreage data from the USDA’s Farm Service Agency.

Several counties in Wisconsin are forecast to experience near or below freezing temperatures on Oct. 4 and again on Oct. 11. Wisconsin is susceptible to frost damage with only 16% of corn rated as mature compared to a five-year average of 54%. Gro’s current corn yield forecast of 175.25 bushels per acre for the state would be the second highest on record. With 987,000 acres of corn exposed, representing 25% of planted area, the state’s average yield could come down a little.

Northern Minnesota and some parts of the state’s southwest also will see near or below freezing temperatures on Oct. 4 and Oct. 11. Some 1.1 million acres of corn and 986,000 acres of soybeans will experience minimum temperatures below 1.0 degree C (33.8 degrees F). Crop maturity is well behind normal: Corn is at 22% versus a five-year average of 67%, and soybeans are at 60% versus an 85% average. Yields in most of Minnesota’s at-risk counties, including Beltrami and Nobles, are forecast to be lower than last year for both corn and beans, according to Gro yield models. Any frost damage will further degrade yield or quality.

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The line chart on the left shows the minimum temperature for any county in the selected states for each date. A cold snap is forecast for Oct. 3-5 along with another dip in temperatures on Oct. 11. The map on the right shows the county-level breakdown of minimum temperatures on Oct. 4, according to the GFS forecast. While temperatures are seen approaching freezing, there is no forecast for severe frost in any significant area. 

South Dakota is forecast to see freezing temperatures for several days, but only in the far western part of the state with little corn and soybean acreage. Cropland in Michigan, where corn is at 17% maturity versus a 53% average, will also largely avoid freezing temperatures, which will be limited to the state’s Upper Peninsula.

Nationwide, there are 556,000 acres of corn and 440,000 acres of soybeans that will experience temperatures below 0.5 degrees C (32.9 degrees F) over the next 15 days, according to the GFS forecast. If you extend that threshold to below 1.0 degree C (33.8 degrees F), the area at risk increases to 3 million acres of corn and 2.1 million acres of soybeans.

Signs of damage will start to show up one to two days after a frost in the form of water soaked leaves that eventually turn brown. But, because it is difficult to distinguish dead leaves from living, proper damage assessment may be delayed for five to seven days after a frost event. Satellite-image-based indicators like NDVI, a key part of many yield models, including Gro’s, will be even slower to pick up on the damage as images are compiled over an eight-day period and then additional data processing is required.

A Better Outlook for Core States

Core Corn Belt states of Iowa, Indiana, and Illinois are forecast to avoid minimum temperatures approaching freezing. The 15-day forecast looks past the typical first-freeze date in those states, which falls in the second week of October. If realized, it would be a great relief to farmers who had a significant portion of land prevented from planting, and whose remaining crop is about two weeks behind normal development. Corn yield forecasts improved over the course of August and September. If frost damage is successfully avoided the overall crop will turn out better than some of the pessimistic expectations from midseason.

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The line chart on the left shows the daily minimum temperature forecast for Illinois, Indiana, and Iowa. The forecast stays above freezing for the 15-day GFS outlook. The map on the right shows soil moisture levels across the Corn Belt. North Dakota and northern Minnesota were hit by severe storms recently and excess moisture could delay harvesting. 

A separate risk to crops in North Dakota and northern Minnesota is excess moisture, which could delay harvest and cost farmers some profit. The crops will take longer to dry down to optimal harvest levels and may end up being harvested at higher moisture levels than desired. A light frost would exacerbate the issue. This would require manual drying that adds expense. In addition, disease and pest pressures have been reported in Minnesota, Iowa, and Nebraska.


Farmers this year may have avoided a killing freeze of the like seen in 1993, which sent yields to multidecade lows after a similarly delayed start to the season. Still, 2019 has been a very difficult year and farmers are now faced with harvest complications from the highly varied maturity of their crops. Frost risk is still on the table for some areas that were particularly delayed and there is still a week or two to go until the average first frost date in Illinois, Indiana, and Iowa.

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