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Extension > Minnesota Crop News > Mid-September frost on corn and soybeans

Tuesday, September 16, 2014

Mid-September frost on corn and soybeans

Seth Naeve, Extension Soybean Agronomist, Jeff Coulter, Extension Corn Agronomist, Dave Nicolai, Extension Educator - Crops, and Phyllis Bongard, Educational Content Development and Communications Specialist

corn frost 9-16-14.jpg
Figure 1. Frost effects on corn in Dakota County following a
September 13th early morning frost indicating increased upper
canopy damage in lower elevations of the field.

Many corn and soybean fields in central, west central, and southwest Minnesota were affected by frost during the morning hours of September 13, 2014. As is always the case, the frost damage appears to be highly variable based on local climate conditions, crop maturity, and topographical features. For corn, a killing freeze occurs when temperatures are 32°F for 4 hours or 28°F for minutes. A frost or killing freeze can still occur when temperatures are above 32°F, especially in low and unprotected areas when there is no wind. For soybeans, most reports indicated that the crop was unaffected, 'nipped' slightly at the tops, or (in rare cases) frozen down into the canopy.

Symptoms of frost damage and severity


Figure 2. Frost damage near Nicollet, MN
on September 13, 2014. Note water-soaked
appearance on corn leaves.

Corn leaves are more easily damaged by frost than stalks. In addition, leaves above the ears are more susceptible to injury than leaves below the ear. Leaves damaged by frost initially have a water-soaked appearance (Figure 2), are light green to gray after drying and later turn brown. It is recommended to wait a few days before scouting fields to assess the impact of frost. If the frost is not severe enough to cause premature formation of the kernel black layer, the kernels will continue to accumulate dry matter through translocation of sugars from the stalk and remaining green leaf area (Carter and Hesterman, 1990).


soybean frost 9-16-2014.jpg
Figure 3. September 13th frost effects on more (left) and
less (right) mature soybean plants, demonstrating greater
penetration deeper into the canopy of soybeans with a more
open canopy.

Apart from climatic and topographic variation, soybean management and growth conditions greatly affect frost's damage. Dense and lush soybean canopies tend to fend off severe freezes by holding heat within the canopy, whereas, open canopies tend to allow the cold air access to the lower canopy (Figure 3). Late planting, soybean cyst nematode (SCN), iron deficiency chlorosis (IDC), and/or some conventional herbicides tended to promote open canopies this year in many areas. These fields were likely damaged more by the frost than less stressed fields. Likewise, soybeans nearing maturity tend to see deeper frost damage due to dropped leaves and open canopies. However, injury (in the form of reduced yields) is greatly reduced if frost hits as soybeans near the R7 growth stage (one mature colored pod on the main stem).

Estimating yield loss in corn and soybeans

Due to the variable nature of natural frost, yield loss estimates are quite difficult to come by. Yield and quality reductions depend on the crop stage when frost occurs and the severity of the frost. Frost damage reports varied in Minnesota from just a few soybean leaf trifoliates affected to frost damage of at least the top 6 inches or more in the upper portion of the canopy.


Frost damage to corn reduces grain and silage yields, grain test weight, and silage quality (Table 1). If leaves above the ear are damaged by frost, but leaves below the ear are unharmed, then yield losses are expected to be slightly less than those listed in Table 1 for leaf damage only.

Table 1. Impact of frost damage on corn yield and moisture.

Leaves + stalk damaged by frost1Only leaves damaged by frost
Corn kernel stageSilage yield lossGrain yield lossGrain yield loss2Test weight3Grain moisture4Whole plant moisture5
percent (%)lb/bupercent (%)
R4 (dough)306641---7076
R5 (dent)215523476073
R5.25 (75% milk)153518505268
R5.5 (50% milk)5105534066
R5.75 (25% milk)13254-553763
R6 (mature)000563260
1From Abendroth et al. (2011)

2From Vorst (1990)
3From Hicks (2004a and 2004b)
4From Abendroth et al. (2011) and Lauer (2011)
5From Lauer (2011)


University of Minnesota conducted a series of trials in 2009 and 2010 estimating frost risk based on soybean maturity selection. You may find a summary of results here. Frost was simulated with Liberty® herbicide applications at a 32 oz. rate in early and late September on soybeans with a range of maturities. In addition, a late planting date was also included in 2009.

Under normal planting dates, early and normal maturity soybeans were not affected by frost - even at early September frost dates. Very late maturing soybean varieties saw yields reduced by 15-30%. Late planting (around May 21) drastically increased risk of frost damage in both early and late varieties. These results highlight the large downside risk to planting full and long-season soybeans late in May, while choosing an early season soybean resulted in a small yield penalty in the absence of a terminal frost. Planting a very long-season variety late in May resulted in huge losses if a (rare) very early frost were to take down the crop. In this example, utilizing an adapted line throughout late May forges a nice balance between yield potential and risk aversion.

The most often cited study on frost damage to soybean is Saliba et al., 1982. In this Kansas study, soybeans of varying maturity were grown in pots in the field and transported to growth chambers set to 32°F, where they remained until 80% of the leaves were visibly injured. Plants were returned to the field and yield was determined at the end of the season. As expected, yield data were a bit messy, but yield losses were estimated to be around 37% at R6 and 7% at R7. As one might expect, these yield losses closely follow those noted in defoliation studies.

With the exception of late replants, most frosted fields in Minnesota were probably somewhere between R6 and R7 on Saturday. Unfortunately, the R6-R7 period is a time of rapid yield accrual in soybean. Damage will be dependent on the exact timing of the frost relative to development. Hard hit fields, or replanted areas that were close to R6 might see 30-40% yield reductions, whereas fields closer to R7 will only see minor effects. Fields with few yellow leaves showing prior to the frost will likely see significant yield loss, if a large portion of the canopy was damaged.

Frost effects on crop quality


Corn that is severely damaged by frost often has kernels that are more susceptible to cracking, grain that is less digestible, and silage that has less energy (starch) and more fiber than normal. Grain with severe frost damage and light test weight should be monitored for mycotoxins before feeding to livestock.


Arresting soybean development late in seed fill tends to reduce yield by stopping oil deposition in the seed. Oftentimes, frost-damaged seed is small, flat, and green, with normal protein but low oil concentrations. Mixing severely frosted grain with normal grain has the potential to cause multiple complications for the producer from storage through marketing. Grain from damaged areas can resist field dry-down and can maintain excessive moisture levels through harvest. In addition, elevators may refuse loads with green soybean, or impose stiff dockages. Frost-damaged areas (especially late planted/drowned-out areas) should be isolated and harvested after the unaffected areas. While this can add logistical challenges to large operations, it could reduce risks of damaged grain in storage and will isolate potential dockages to individual loads rather than whole bins. Dr. Charles Hurburgh at Iowa State University has prepared a nice fact sheet related to handling frosted corn and soybean crops.

Frost effects on corn dry-down rates

Typical in-field dry-down rates for corn grain in Minnesota are listed in Table 2. Thus, if a killing freeze occurred on corn at the R5 stage (dent and 60% grain moisture) on September 15, then that corn is expected to have over 38% grain moisture on November 1. In comparison, if a killing freeze occurred on corn at the R5.5 stage (half milk and 40% grain moisture) on September 15, then that corn is expected to reach 25% grain moisture on October 6. Fields where the husks were green and tightly wrapped around the ears at the time of a killing freeze will dry slowly, and ears in those fields should be monitored for mold. In contrast, corn that was more advanced in maturity at the time of the frost (husks opening up and turning brown) will dry more normally.

Table 2. Field drying rates for corn in Minnesota.
DateGrain moisture loss
(% per day)
September 15 to 250.75 to 1.00
September 26 to October 50.50 to 0.75
October 6 to 150.25 to 0.50
October 16 to 310.00 to 0.33
After October 31Very little
From Hicks (2004)


Abendroth, L.J., R.W. Elmore, M.J. Boyer, and S.K. Marlay. 2011. Corn growth and development. PMR1009. Iowa State University.

Carter, P.R., and O.B. Hesterman. 1990. Handling corn damaged by autumn frost. Available at Purdue University.

Hicks, D.R. 2004a. Corn test weight changes during drying. University of Minnesota.

Hicks, D.R. 2004b. The corn crop - frost and maturity. University of Minnesota.

Lauer, J. 2011. Frost. Available at University of Wisconsin.

Lauer, J. 1996. Corn harvest in Wisconsin during cool growing conditions. University of Wisconsin.

Nielson, R.L. Risk of fall frost injury to immature corn grain. Available at Purdue University.

Vorst, J.V. 1990. Assessing hail damage to corn. Available at Purdue University.

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