Dr. Dan Talks Agronomy

Cold weather covered the Corn Belt this week and some farmers, especially in low-lying areas of the northern reaches, had frost. So what damage may it have caused, and will replanting be required?

It has been a long, cold spring for certain, but no one expected to see mid-April weather during mid-May with temperatures dropping into the 30s and 20s with a layer of frost covering the ground at sun-up. With today's large planters and multiple planters per farm, a lot of acres get sown quickly. The seed seemed to come up quickly with the soil in good condition this spring.

So what are the prospects for a crop that experienced frost after it emerged? It all depends on the crop and its height, landscape, temperature and duration of the frost. Don't rush to judgment. Wait a few days to see how the crop responds. If a hard frost hit, you can see permanent symptoms of damage after four days. For soybeans, just look at the top of the seedling to see if its growing point has turned dark. For corn, cut open the growing point and see if it is necrotic or brown.

CROPS DIFFER

Soybeans are more vulnerable than corn because of the location of the growing point. Corn's growing point is located below the surface until V6 when it begins to elongate. Corn that has emerged is small, probably just spiking or at V1 or V2. Sure, leaves will be frost-damaged and appear water-soaked, but the seedling will recover and grow back with no impact on yield at this young stage.

Soybeans are another story. Their growing point is located at the top of the plant and is vulnerable to frost. Once the growing point dies at this young stage, there is no recovery.

LANDSCAPE MAKES A DIFFERENCE

As we saw in northeast Nebraska this week, heavy frost hit low-lying areas while hills were not affected. This limits the number of acres affected in a field in hilly terrain. Another factor is crop height and residue cover. Air temperature is warmer near the soil surface and residue helps trap a little heat. Smaller seedlings nearer to the ground and protected by residue's insulation may have been unaffected. However, temperatures near the soil surface can be colder than a few feet above because air movement mixes warm and cold air -- or they can be warmer if the soil contained a lot of heat energy built up during previous warm days.

TEMPERATURE AND DURATION MAKE A DIFFERENCE

Frost damage depends on how cold it got and how long it lasts. Frost can occur when air temperatures fall to 30 to 32 F, but that probably is not cold enough to create ice crystals and damage cells in a plant near the soil surface. However, if air temperatures drop to 26 to 28 F, that is cold enough to form ice crystals within cells, causing damage.

The other factor is duration. A light 30 to 32 F frost that occurs before sunrise and lasts for an hour is less likely to cause damage than one that lasts four hours. The greater the duration, the more time to form ice crystals. Corn can survive short-duration, light frosts even though leaf tissue is damaged. Cold temperatures below 28 F can be lethal even if the growing point is below ground and the frost lasts for several hours.

REPLANTING DECISIONS

Don't rush to replant without due consideration. Soybeans are easy to replant because they can be seeded over the top, and any new seedling will not necessarily compete because of soybeans' compensatory nature. However, replanting corn means disking out the damaged crop first or replanting over the top with early and late-planted corn seedlings competing with each other.

Before replanting, consider the yield potential of the existing crop compared to replanting costs and yield of a replanted crop. Replant costs include time, fuel, seed costs, and penalties associated with hybrid selection when the best genetics are no longer available. Yield potential of a replanted crop will also be influenced by planting date.

Read more about corn and soybean replanting decisions here: http://www.agry.purdue.edu/…

http://extension.missouri.edu/…

Dan Davidson can be reached at AskDrDan@dtn.com

Follow Dan Davidson on Twitter @dandavidsondtn

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