Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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October 8, 2012



Sorghum and Sudangrass


Temperatures in the upper 20s hit some locations this past weekend, which may be the first fall freeze for some. Most concerns with the first fall frost deal with managing sorghum species including sudangrass. When these species are injured with the frost, the plant tissue releases prussic acid for a few days, which can be toxic to livestock. If plants are only partially killed, which is likely with a light frost, new shoots may start to grow which have more concentrated levels of prussic acid. Livestock should be removed immediately if a frost occurs and kept out for 5-6 days. If new shoots develop, livestock should be removed until the shoots are the appropriate height to graze (18-20 inches for sudangrass and 24-30 inches for sorghum-sudan), or until 5-6 days after a hard freeze which kills all shoots completely. For more information on managing sorghum species in the fall, see Steve Barnhart’s article at





Because of the concerns with sorghum species, there is the false believe that alfalfa also becomes toxic with the first fall freeze. Alfalfa does not become toxic with the freezing temperatures, but there is a slightly greater risk of bloat, so the standard bloat prevention practices should be used (ie don’t turn hungry cattle out into an alfalfa field where they might gorge themselves on the lush alfalfa).


Because of the shortage of hay this year there will be the temptation to try to get a last cutting of alfalfa this fall. Alfalfa has been under a lot of stress with the drought, which makes the fall rest period even more important this year. Temperatures in the upper 20s will not stop alfalfa growth, with more growth likely in the next week to 10 days. It’s best to wait until after a killing freeze (mid 20s) or until about mid-October to try to take another cutting of alfalfa to minimize the stress on the plants going into the winter.







Green Stem Syndrome


In some fields, soybean stems are remaining green even when soybeans are testing less than 10% moisture, which is resulting in a lot of shattering. This green stem phenomenon tends to occur when the soybeans are under stress during pod fill, which certainly would have been the case this year. Usually there is also a yield loss associated with this, because it occurs when there are fewer pods and fewer beans to fill, resulting in the plant having photosynthates “left over” causing the stems to remain green even after the beans have filled completely and are mature. Green stem syndrome can be induced in plants by removing half of the pods at R6 (full seed size). A freeze may help some to dry the stems down, but it’s better not to wait for these stems to mature because of the increased shatter risk. Fields can be harvested where stems are still green, although it is more difficult, requiring more power and there may be increased problems with plugging. To address combine issues for drought affected corn and soybean crops, see Mark Hanna’s article at: There is some extensive information on the green stem syndrome at this Virginia Tech website:





Integrated Crop Management Conference

November 28-29, 2012



Details will be posted at:



Ag Chemical Dealer Update

December 5, 2012

9:00 a.m. to 4:00 p.m.

Highlander Clarion in Iowa City


Details will be posted at



Crop Advantage Series


January 11, 2013

Catfish Bend Convention & Event Center, Burlington

Featuring Elwynn Taylor and Chad Hart


January 30, 2013

Highlander Clarion in Iowa City

Featuring Craig Johnson, Meteorologist & Executive Director of Iowa Academy of Sciences


Details will be posted at



If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: October 8, 2012
Contact: Virgil Schmitt

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