Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


October 11, 2006

October 11, 2006




It looks likely that we will get a hard freeze in the next couple of nights. The main concern with this freeze will be its affect on the management of forages, especially sudangrass and sorghum-sudan hybrids. Alfalfa does NOT become toxic after a fall freeze. Alfalfa can be harvested after mid-October, with minimal risk of winter-kill, regardless of whether a hard freeze occurs before then. The following information from Brian Lang summarizes issues related to managing forages in the fall.


Alfalfa usually requires 24 F to completely kill its topgrowth.  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; which ever comes first.  If no killing frost occurs by mid-October and a harvest is desired, harvest the forage.  The short daylengths and cold autumn temperatures will minimize the use of root reserves prior to the “soon-to-come” killing frost.


There is an old, persistent false-hood around that alfalfa becomes toxic following a frost.  Alfalfa does not contain any “toxic” compounds that arise from exposure to frost.  However, alfalfa can cause bloat, and immediately after a frost alfalfa’s bloat potential is higher than normal. An over-simplified and very brief explanation for this is: Bloat is largely caused by a rapid release of soluble proteins into the rumen.  Alfalfa has considerable amounts of soluble proteins.  As cattle eat alfalfa, their chewing action breaks up plant cells and slowly releases the soluble proteins into the rumen.  If the cattle eat recently frosted alfalfa, they eat already-broken cells from the frost which rapidly releases soluble proteins (not a slow release), which increases the chance of bloat.  Once the frosted parts of the plant dry, alfalfa’s bloat potential is back to normal.


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.





November 21 - Ag Chem Dealer Update

Quality Inn & Suites Highlander Conference Center, Iowa City


Greg Tylka, Marlin Rice, Mike Owen, John Sawyer, Jim Fawcett, and Virgil Schmitt will be featured at this year’s dealer update. Online registration is available at



November 29-30 Integrated Crop Management Conference and Expo

Iowa State Center Complex, Ames


Information and on-line registration will be available in late October at :



January 4, 2007 – Crop Advantage Series

Clarion Hotel, 33rd Ave, Cedar Rapids


January 5, 2007 – Crop Advantage Series

Southeastern Community College, West Burlington


Information on these conferences 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 11, 2006
Contact: Jim Fawcett

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