Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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September 21, 2011





First Fall Frost


Some fields in the northern part of the area experienced a frost last week, but most of EC and SE Iowa has not yet seen temperatures below 32. Most of the corn and soybeans are now mature or close enough to maturity that a freeze would have little if any impact on the yields. The main concern with the fall’s first frost is with the management of forage sorghum species and alfalfa.


Alfalfa does NOT become toxic after a fall freeze. Alfalfa usually requires 24 F to completely kill its top growth.  Temperatures above 24 degrees F will cause visible damage, but the plant will continue to grow using the remaining leaf area. The main reason not to harvest alfalfa after a light frost is that the harvest would remove all of the leaf area, and the plant's continued development would be entirely at the expense of root reserves.  To optimize plant development and its over-wintering ability, allow the plant to grow until a killing frost or mid-October; whichever comes first.  If no killing frost occurs by mid-October and a harvest is desired, harvest the forage.  The short day lengths and cold autumn temperatures will minimize the use of root reserves prior to the "soon-to-come" killing frost. Although alfalfa is not toxic after a frost, it sometimes does increase the chances for bloat problems when cattle eat recently frosted plants. The risk returns to normal when the frosted plants dry. For more information on the fall management of alfalfa, see Steve Barnhart’s article at


Sudangrass and sorghum-sudan hybrids require 28 F for a killing frost, however even a "light" frost requires special management. Prussic acid accumulates in the frosted tissue within a few hours after thawing and wilting. A "light" frost may damage just the tops of plants. If this occurs, delay grazing or harvest a few days after frost to allow the prussic acid to dissipate from the plant tops.  Livestock can be returned to frost injured sudangrass (18 inches or taller) and sorghum-sudan (28 inches or taller) after 5 to 7 days.


Sometimes a "light" frost enhances development of young shoots from the base of the plants. If this occurs, delay sending livestock to graze this forage since these new shoots would be high in prussic acid. Ideally, wait for the new shoots to get to a proper grazing height, but more than likely a complete killing frost will occur before that would happen.  Once a complete killing frost occurs, wait at least 10 days (wait until the frosted tissue is drying out) before grazing or harvest.


If haying the forage, the curing process decreases the prussic acid content as much as 75%, which removes the feeding concern.  However, haying these forages this late in the season is nearly impossible because of poor dry-down conditions. If green-chopping the forage, chop only as much forage as the cattle will consume in 4 to 5 hours. Never green-chop the forage and let it sit on the wagon overnight.  If ensiling, harvest at proper moisture for your storage structure to ensure good fermentation. This takes a minimum of 4 weeks.  The fermentation process will reduce the prussic acid content.  Since immature plants can contain higher prussic acid levels, leave this forage ferment for at least 8 weeks before feeding.  Never allow horses to graze sorghums or sudangrass at any time.




Fall Stalk Nitrate Test


It has been another challenging year for management nitrogen in corn with excess rainfall received in the spring. One way to fine-tune nitrogen management is to use the fall stalk nitrate test to determine if nitrogen levels were optimum for corn production. The ideal time to cut the stalk samples is 1-3 weeks after black layer, which would be right now for many fields, but samples can also be taken immediately after harvest, which makes taking the samples even easier. Collect 15 8-inch stalk samples starting 6 inches above the soil. If this is going to be done after harvest, just make sure the cutter bar is set high enough where samples are to be taken so this segment can be cut after harvest. Samples can be sent to the ISU lab, or the lab of your choice.  The instructions for the test are in the following publication. If you chose to send your samples to ISU, the following link provides the ISU sample submission form.




Harvesting Lodged Corn


Corn harvest has begun in the area, with some reporting yields that are better than expected. One thing I will remember about this season is the huge number of corn fields that have been flattened to some degree by the various windstorms that have gone through the state. Fortunately most of the flattened corn did not have green snap, so at least there is corn to harvest, but it will not be much fun to harvest those fields. Some have started harvesting these flattened fields and found that they do need to harvest all the corn from the same direction. For some tips on harvesting lodged corn, see Mark Hanna’s article at For other issues regarding this fall’s harvest season see Charles Hurburgh’s article at



Will Ear Rots and Mycotoxins Be a Problem?


There is a greater risk of ear rots with the hot, dry weather much of the area experienced this summer. I have seen some Aspergillus (fungus that can produce aflatoxin) in the area. However the presence of the Aspergillus does not necessarily mean that there has been any alflatoxin produced. Fortunately the cooler night temperatures that we had from around August 7 on should have helped to reduce mycotoxin production. Fields with ear rots should be harvested first to try to minimize the problem. For more information on scouting for ear rots see







Ag Chemical Dealer Update

December 9 - Iowa City (Highlander)


Crop Advantage Series Conference

January 26 - Iowa City (Highlander)



If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: September 21, 2011
Contact: Jim Fawcett

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