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Prospects for Frost Damaged Corn and Soybeans

University of Minnesota Extension, Stearns County News
September 17, 2014         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties


Prospects for Frost Damaged Corn and Soybeans
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (09/12/2014) — Writing this on Friday morning September 12, it may not have gotten cold enough Saturday morning to totally kill corn and soybean crops. It might create more variation across fields.

SOYBEANS are at the beginning of the R6 stage when one pod at one of the four top nodes contains green seeds that totally fill the pod cavity. The nodes are the locations on the stem where a pod or cluster of pods is attached. Killing the crop at the R6 stage, could result in as much as 50% yield loss.  We have plenty of soybeans in the area that are not this far along yet.

At the R6.5 stage pods all the way to the top of the plant are filled with green seeds that totally fill the pod. At this point 85% of the crop might be made and killing the plant could result in a yield loss of about 15%. Yields on many fields have already been limited by late planting and wet weather. 

At R7 one normal pod on the main stem has reached a mature pod color. 50% or more of the leaves might be yellow at this time. This is the point of physiological maturity where 100% of the potential yield should be there. 

I’ve watched soybeans in previous years where plants were totally green prior to frost and at or near the R6.5 stage. Where pods stayed green after the frost, they should have a fairly good chance of ripening to a yellow color with favorable weather. Time will tell. Where fields or parts of fields had a golden brown color at the time of frost, soybeans were likely close to full yield compared fields and parts of fields where green leaves froze.

CORN clues might be gained from an article posted a couple of years ago by state Extension corn agronomist Jeff Coulter titled, “Yield and Harvest Considerations for Frost Damaged Corn.”

The future of the corn crop is determined by the extent of frost damage to leaves, stalk, ear shank and kernels. If the shank and kernel connection is still alive, nutrients can still move to the kernels from the stalk and any remaining green leaf material. If the shank connection is killed by frost, it will start to discolor and might have a spoiled smell within a few days to a week after the frost. If the kernel connection is killed, a premature black-layer will start to form inside the kernel tip from 4 to 5 days to a week or so after the frost. In this case, black layer is a poor indicator of kernel or plant moisture.

When the stalks and leaves are damaged by frost for corn in the dough stage, there might be a silage yield loss of 30% or a grain yield loss of 66%. If only the leaves are damaged, the grain loss might be 41%. If you’re able to combine grain, the test weight could be less than 47 pounds.

In the dent stage, where stalks and leaves are damaged, there might be a corn silage loss of 21% or a grain loss of 55%. If only the leaves are ruined, there might be a 23% grain loss. The test weight might be in the 47 pound range.

At 3/4 milk line, corn killed by frost could have a silage loss of 15% or a grain loss of 35%. If only the leaves are damaged, a grain loss of about 18% and the test weight could land around 50 pounds.   At half milk line, corn with leaves and stalks damaged by frost might have a silage loss of 5% or a grain yield loss of 10%. Corn with only leaves killed at half milk line might have a 5% grain loss and a 53 pound /test weight if dried properly. Half milk line is pretty significant point where yield and market quality for grain can still turn out pretty well.

How fast can we expect frozen immature corn to dry down? The grain moisture at dent stage is about 60%. With typical drying rates starting at September 15, this corn grain could still be 38% moisture on November 1. By contrast, corn grain at half milk line on September 15 would have kernel moisture at about 40%. If killed by frost at half milk line on September 15, with drying rates based on average weather, this corn grain could be 25% moisture on October 6. Where husks are green and tightly wrapped when a killing frost occurs, there is a concern about the chance of mold growth. 

Typical grain moisture loss per day from September 15 to 25 is 0.75 to 1%; from September 26 to October 5 0.5 to 0.75%; from October 6 to 15 0.25 to 0.5%; from October 16 to 31 0 to 0.33%; and after October 31 very little. Obviously weather conditions are a major factor.

SORGHUM AND SUDAN GRASS is more sensitive to frost. A heavy canopy might still protect it from a light frost. If taller than 18-24 inches and killed by frost, by the time is dries enough to be chopped for storage, prussic acid should not be an issue. If less than 18-24 inches wait 2 weeks to graze or green chop after a killing frost. 

Crop farmers with crops that might not make good market grain might visit with farmers with cattle needing silage feed to see whether they agree on a price for making use of some of this crop. The crop insurance company should be consulted so proper steps can be taken for insurance purposes.  



Daniel Martens
Extension Educator, Ag Production Systems
(320) 968-5077
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