Many areas of North Dakota experienced their first fall frost of the season the weekend of Sept. 5-7. While most North Dakota Agricultural Weather Network (NDAWN) stations stayed close to a minimum temperature of 32 degrees Fahrenheit, a few stations recorded lower temperatures with some upper 20s observed. The Dickinson NDAWN station fluctuated between 32 and 33 degrees Fahrenheit for five hours between 3 and 7 a.m. the morning of Sept. 6. The Hettinger station ranged between 32 and 30 degrees Fahrenheit for four hours early Saturday morning.

“Today marks five days out from the frost and freeze events of last weekend,” says Clair Keene, NDSU Extension small grains agronomist. “Based on the temperatures observed, farmers should assess their crops for damage now in order to make timely harvest and marketing decisions going forward.”

Corn assessment

Corn can be killed at 32 degrees Fahrenheit if the temperature stays at freezing for a few consecutive hours, usually thought to be three to four hours, says Keene.

Corn plant death occurs more quickly at colder temperatures, with exposure at 28 degrees or less for just 15 minutes being sufficient to kill most tissues. When temperatures are just above freezing, 33 to 40 degrees Fahrenheit, for multiple consecutive hours, damage is likely to be highly variable and strongly influenced by the topography of the field. Cold air is heavier than warm air, and as air cools, it flows to lower positions on the landscape. If a corn field has rolling hills or is adjacent to a low creek bed, farmers should expect to find more damaged plants in the swales or along the drainage points.

The weather the day before and after the frost occurs also influences the extent of damage, shares Keene. If it was warm and sunny the day before the frost and also warms up quickly with sunny conditions the morning after the frost, the corn is less likely to sustain as much damage than if conditions were cloudy and cool, with highs less than 50 degrees before and after the frost.

The tops of corn plants are the most likely to be damaged by frost. When checking corn fields for frost or freeze injury, it is helpful to delay assessment for five to seven days after the frost event so that tissue that was killed has time to turn brown and dry out.

“If assessing fields less than five days after the frost, farmers may underestimate the damage because tissue might still look green when it is actually dying,” says Keene.

The severity of the frost and the corn’s developmental stage are key factors in determining potential yield losses. The following estimates can help farmers assess risk and make informed decisions:

• R4 (soft dough stage):
• Killing frost (leaves and stalk dead): Up to 55% yield reduction
• Light frost: Around 35% yield reduction
• R5 (dent stage):
• Killing frost: Up to 40% yield reduction
• Light frost: About 25% yield reduction
• R5.5 (50% milk line):
• Killing frost: Up to 12% yield reduction
• Light frost: About 5% yield reduction
• R6 (black layer/physiological maturity):
• Killing or light frost: 0% yield reduction

Soybean assessment

“Research shows soybean yield losses are most severe when frost occurs before plants reach full seed (R6) stage,” says Ana Cardedo, NDSU Extension broadleaf crops agronomist. “At R7, when one normal pod on the main stem shows mature color, yield impact is minimal. Once soybeans reach physiological maturity (R8), when 95% of pods have turned their mature color, frost will not reduce yield, though harvest timing remains important.”

Farmers can estimate possible yield reductions due to frost based on soybean maturity stage:

• R5 (beginning seed): 75%-80% yield reduction
• R6 (full seed): 20%-40% yield reduction
• 5 (late seed fill): 10%-20% yield reduction
• R7 (beginning maturity): 0%-5% yield reduction
• R8 (mature): 0% yield reduction

Soybeans killed at R6 or R7 should be left in the field to dry and harvested along with mature soybeans once seed moisture is at the desired level, recommends Carcedo.

Weather conditions during the field-drying period strongly influence the color of the seed coat. R6 soybeans dried under sunny, dry conditions developed a golden-yellow seed coat similar to that of mature soybeans (R8). However, when plants are exposed to freezing temperatures, snow or rain without sufficient sunlight, beans often retain a greenish cast caused by chlorophyll.

“Green soybeans are discounted at elevators because chlorophyll removal adds cost to processing, although oil quality is usually less affected,” shares Carcedo. “Even under poor weather conditions, frost-damaged soybeans left in the field may continue to ripen naturally. Research has shown that frost-damaged plants often mature and change color more quickly than undamaged plants.”

The Sept. 11 edition of the NDSU Crop and Pest Report, available at ndsu.ag/cpreport, shares detailed information to help farmers estimate corn and soybean yield loss based on growth and/or maturity stage at the time of frost.

NDSU Agriculture Communication – Sept. 12, 2025

Source: Clair Keene, 701-231-7405, clair.keene@ndsu.edu

Source: Ana Carcedo, 701-831-5796, a.carcedo@ndsu.edu

Editor: Kelli Anderson, 701-231-6136, kelli.c.anderson@ndsu.edu