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Decisions about managing frost damaged and immature crops need to be based on severity, crop maturity, available resources, insurance, livestock health, and other factors. The following resources will help growers assess their options for damaged or delayed fields. See the links at the right to contact Extension educators directly.
When immature crops are damaged by frost, grain yield, drying rate, and grain quality can all be affected. Growers who are covered by crop insurance should check with their company rep before they do any harvesting.
The effect of frost on yield depends on how much leaf area is killed and the stage of development when the frost occurs. When only a portion of leaves are killed, those not killed can and will continue to function and contribute to yield if good growing conditions exist after the frost. For this reason, as well as high moisture, it is best to not harvest crops earlier than necessary.
University studies indicate that soybeans are injured by frost until they reach physiological maturity, which is generally attained at "yellow pod" or R7 growth stage. When the terminal growing point is damaged (by frost), re-growth can occur from the vegetative buds (growing points) located in the leaf axils on the main stem. However, the plant is also trying to fill the pods, and the crop is running out of time to compensate for the loss of foliage. Partly freezing when the soybeans have green pods 3/4 inch long, at one of the four uppermost nodes on main stem (R4 ) can cause a yield reduction of 70–80%. Frost at the R6 growth stage and earlier, assuming that the plants did not completely freeze at this time, will result in a yield reduction and damaged beans with greenish color and wrinkled seed coats. Protein content should not be affected by frost; however oil content will generally be reduced if frost occurs before R6 (full seed)
Fields should not be harvested until pods have a mature color, even if a killing frost occurred when the pods were green or yellow. Studies have shown that beans on frost-damaged plants will mature and change color at least as early and perhaps earlier than undamaged soybeans, however, the frosted, dead leaves will remain on the plant, and some beans may retain their green color.
The majority of the corn in Minnesota is at or beyond the half-milk stage. A hard killing frost at the half-milk stage will reduce final grain yield by around 8-12%. The impact of frost on corn yield decreases as the crop gets closer to maturity (no visible milk line and a black layer present just under the tip of the kernel). For corn that is nearly mature (milk line barely visible and no clearly defined black layer present), a killing frost will have little impact on grain yield (see Maturity, frost, and harvest moisture considerations for corn).
Corn prematurely killed by a frost will dry in the field at a rate that is similar to corn that reaches maturity before a frost (see The corn crop—frost and maturity). On average, we can expect a drying rate of 0.5 to 0.25% per day between October 6 and 15, and 0 to 0.33% per day between October 16 and 31 in Minnesota. The grain moisture of corn naturally drops as the crop matures, and corn that is prematurely frozen is sometimes described as soft and wet if sun, wind, and warn weather are not present to facilitate good in-field drying.
After a cool growing season, both corn and soybeans may have high moisture at harvest. Soybeans, which are traded at 13% moisture, can be harvested at moisture contents up to about 18% without too much damage, although artificial drying is required. If harvesting immature, green soybeans, make sure to remove as much chaff and green plant material as possible before binning the beans. Immature beans can be stored without significant molding, but concentrations of green chaff can lead to heating in storage. Although it is commonly stated that green soybeans will eventually turn yellow in storage, the color change observed in a U of M laboratory study was minimal. It might still be worthwhile to store green soybeans for a few months after harvest though, to avoid the high discounts that are applied when large quantities of green beans are delivered during harvest.
Energy costs are often of special concern. For corn drying, some ideas for reducing energy costs include making more silage, letting corn dry in the field as much as possible, harvesting late and holding wet corn for silage, or using a slow cooling method. For details on these energy-saving methods, visit Drying, Handling, and Storing Wet, Immature, and Frost-Damaged Corn.
Immature and frost-damaged corn will have marginal quality, so it’s important to manage equipment carefully to minimize further quality degradation. Set combines carefully, to balance the need to get small kernels with kernal damage. Manage the fines and chaff, which can increase mold problems in storage. Dry grain to uniform moisture levels, a tricky business because harvest moisture is likely to be somewhat uneven after a cold, short growing season. Dry corn as gently as possible, even if it is tempting to crank it up for higher dryer capacity. Also, use slow cooling methods after gas-fired drying to minimize quality problems. If possible, aerate stored grain to cool it to 20 to 30F for winter storage (in the upper Midwest).
Where frost injury occurred without complete kill, additional yield and forage quality accumulation is likely for the rest of the growing season from surviving plant parts. However, where these crops were completely killed by frost before reaching optimum grain or even forage harvest maturity, harvesting as forage is a viable option. In addition, based on the delayed maturation of corn and soybeans to date, chances are good that there will many acres of these crops that will receive a killing frost before reaching maturity, so harvest as forage may still be one of the better options as the growing season plays out.
Frost-killed, immature corn should be allowed to stand in the field until reaching proper moisture for chopping and ensiling. Ensiling immature, high moisture corn may require additions of an inoculant for fermentation and preservatives and conditioners to overcome high moisture and low carbohydrate levels. Corn in the blister stage may be as high as 80% moisture. Grazing is an option that may provide the means to capture the most nutritive value from standing, immature corn that is too wet to ensile.
Like immature corn, soybeans will also be too wet to ensile immediately after a killing frost. However, in contrast to corn, soybeans intended for harvested forage should be mowed and conditioned and left in a swath as soon after the killing frost as possible to retain leaves. It is too late to consider mowing-conditioning soybeans that may have been fully killed by the recent frost for silage or hay, but if a killing frost should occur before sufficient maturity for grain, ensiling after immediately mowing-conditioning into a swath and wilting or grazing may be viable options. Silage is preferred to hay for soybean because hay requires considerable wilting, is dusty, and stems are brittle, increasing feed bunk refusal. If grazing is considered, watch for bloat, which has not been reported by may be a possibility.
Note that some herbicides used on crops have label restrictions that prevent the crop being used as a forage.
A severe frost could mean that there are soybean and corn fields that will not be harvested. Crops in these fields will probably be plowed under. If this action is taken, there are serious questions about the nutrient value of the destroyed crop. There are no research projects that provide data specific for this situation. However, we can apply information taken from other sources and adapt it to the damaged crop.
The phosphorus and potassium in the destroyed crop will become available to subsequent crops over time. So, it is more important to sample fields when the destroyed crops have been plowed under.
We estimate a N credit for soybeans if destroyed at three states of development. These are:
An additional credit of 40 lb. N per acre should also be applied if a crop requiring nitrogen in a fertilizer program follows soybeans that are either harvested or destroyed. This credit is a consequence of factors other than the incorporation of soybean residue into the soil.
Soil sampled for nitrate–nitrogen is a recommended practice where damaged crops are plowed under. This sampling should be delayed as long as possible this fall. This delay will provide for a more accurate measure of mineralized N.
If a farmer has purchased crop insurance, yield losses caused by the recent frosts may result in an indenmity payment, but that payment depends on the type of insurance purchased. To understand how potential indemnity payments are calculated, check out this brief paper: Estimating indemnity payments from crop insurance (24 K PDF). We show how indemnity payments are calculated under three types of crop insurance (1) Multi-Peril Crop Insurance (MPCI), (2) Crop Revenue Coverage (CRC), and (3) Revenue Assurance with “fall harvest price option” (RA-HPO). For a longer discussion of crop insurance options and how to choose between them, check out this earlier paper: Utilizing Federal Crop Insurance (81 K PDF).
Call the Farm Information Line at 1-800-232-9077.
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