Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


May 3, 2005

May 3, 2005

FREEZE - Crop Implications

The record cold temperatures last night will impact crops that have been planted. The emerged corn should regrow from the frost damage and there aren't too many soybean fields up yet, so most of the concerns are with the alfalfa crop.


Brian Lang wrote this nice summary of how to make decisions regarding the frost damage on the alfalfa. Steve Barnhart will also have an article concerning this issue in the upcoming ICM Newsletter.

Give the frost a chance to clear off, and watch the plant tissue.  If plant tissue turns black (wet, soft, no cell structure), those cells were killed (that part of the plant tissue was killed). The black tissue dries out and turns a tan or white color afterwards.  What remains green is still fine.  So what does this mean?

Established Alfalfa
1) First things first... the frost did not kill the alfalfa plants.  Its just a question of to what degree the foliage got frosted.

2) If the top of a shoot is frost killed, the plant will re-initiate shoot development from a lower axillary bud off of that shoot.  If the tops of most plants are frost killed, leave the field alone.  It will initiate new shoot development for fairly normal growth to first harvest.  Obviously yield will be a little less relative to the degree of frost damage and delayed development.  I believe most fields fit this category.

3) If most of the plant is frosted (more than 70% or so), we move into the realm of speculation on what is best to do.  No specific research to follow.  If we leave the plants alone, they will initiate new shoots and proceed with "normal" growth and development.  First crop will be delayed, it will be reduced, it will contain "dead" stem tissue from the frost kill lowering forage quality some.  However, the plants will use what green tissue they have that survived the frost to more quickly replenish root carbohydrates and reduce stress.  If we cut the alfalfa, we would stimulate a more uniform regrowth for next harvest, but add some stress to the stand because it never got far enough along this spring to replenish root carbohydrates.  This alternative is a poor one unless most of the current stand was frost killed.  In that case, we treat it much like that last cutting in fall... take the "after-frost" harvest ASAP to salvage the frosted plant material.

New Seedings
1) Look to see that the plants are still upright and cotyledons are still green.  Most stands also have the first leaf out (a single "unifoliate" leaf) just starting to unfold.

2) Obvious problem if the new seedings are laying flat on the ground.  Sufficient "frost-kill" to kill the stem tissue below the cotyledons will lay the plant over.  They are dead.  You can re-seed right back into the same field with no problem of autotoxicity.

3) What if the cotyledons show some damage?  Some black or white coloration on the edges of the cotyledons is not a problem.  Even if one cotyledon is completely killed off, the other may be fine and the plant retains its main growing point.  Its just like hail damage to a soybean seedling.  Knocking off one cotyledon does not kill the plant.  However, just like with hail damage, we may have to wait a week and watch for regrowth to be able to interpret the extent of the damage.


Since the growing point on corn is below ground until V6, the corn should regrow from the frost damage. However there are concerns with how this extended cold spell will affect corn stands. In past years when corn is just about to emerge and then the weather turns cold, this has resulted in decreased stands and vigor of the crop. There were a lot of fields that would have been close to emerging when the cold weather hit. With the predicted warm temperatures later this week, it will be easier to tell in 3-4 days if stands have been impacted. It is possible that we may see uneven stands in some fields. The following table can be used to help in making replant decisions. Replanting costs as well as the health of the remaining stand also need to be considered.

Affect of Plant Population and Planting Dates on Corn Yields

Stand X 1,000

April 20 - May 5

May 13 - May 19

May 26 - June 1

June 10 - June 16

June 24 - June 28

28 32






























Uniformity of stand is also a consideration.  Numerous gaps of up to 4-6 feet can reduce yields by an additional 5-6%. For more information, see Pm-1885 "Corn Planting Guide," which is also available at


Any emerged soybean fields are likely toast, but sometimes with a light frost there can be regrowth from axillary buds. The main issue now is whether planting soybeans into cold soils reduces stands and yields. These questions are related to some research from Canada.  Here is the publication link:

According to Dr. Alan Knapp from the ISU Seed Science Lab, discussions about cold or low temperature imbibition shock have been around for a long time.  Most of the work has been done by Bramlage, et al. 1978. Plant Physiol. 61:525-529 and 1979.Crop Sci. 19:811-814.

Dr. Knapp comments... The problem with the latter work is that since seed coat thickness was a variable affecting water uptake, the seed coats were removed.  Similar work exists for corn via 1 research group, but here again the pericarps were removed.  This did not stop people from postulating that such an event occurs and there have been various and sundry "confirmations" of this theory via anecdotal information and such things as are presented in the Ontario web page.  There has been enough said that many people believe such shock does occur. However, most also believe (and it is logical to believe so) that the degree of occurrence and the resultant impact are functions of interactions with actual temperature, seed quality, soil moisture, seed moisture, duration of exposure to stress, mechanical injury, etc.  I do not find anything wrong with the comments from the Crop Scan newsletter regarding the specifics of cold temperature imbibition shock, however, with what I have included above, the interpretation of this relative to planting is difficult to determine because of the small amount of research available, and the interacting factors that probably affect the outcome.

If you are concerned about this, holding off on soybean planting for 2 to 3 days for weather warm-up should take care of the situation.  The research had no problem with 10 degree C soil temperatures.  That's about 50 degree F.  If you think removing seed coats and having limited research on the subject clouds the issue, and the fact that current soil tilth and soil moisture are great, go ahead and plant now.

Spraying Herbicides

The cold will also reduce the effectiveness of burndown herbicides. It would be best to wait 2-3 days to spray until plants are growing again. If applications need to be made now, increasing the rate may overcome potential performance problems.



Georgia was officially added to states with Asiatic soybean rust this year. The rust was found on volunteer soybeans in the extreme SW corner of Georgia, adjacent to Florida. This is the first report of the disease on soybeans in the US this year. Based on past weather patterns, it would be very unlikely for rust spores to blow to Iowa from this region. It will be more concerning if it shows up in Louisiana or eastern Texas, where it would be more likely for winds to eventually blow spores our way. You can follow where rust has been confirmed at the following site:

Check out an update on soybean fungicides in a few days in the next ICM Newsletter at:


If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: May 3, 2005
Contact: Jim Fawcett

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