Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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April 5

April 5, 2012 


 Frost (Maybe)

 It appears that areas along and north of I-80 are at risk of light frost injury tonight.  Some articles that address the issue, if it occurs, are:


Alfalfa Weevil 

We have surpassed 250 Growing Degree Days (GDD) (base 48, from Jan. 1) which suggests that it is time to scout for Alfalfa Weevil.  In a normal spring, we would not reach this point in northeast Iowa until May.  The easiest way to scout for Alfalfa Weevil is to start with a sweep net just to survey a field.  If there are some alfalfa weevil in the net, then follow the proper scouting procedure in  (in the paragraph above Table 1) to determine if the economic threshold is reached.   Alfalfa Weevil problems in eastern Iowa have been few in recent years, but the warm winter may have allowed for higher overwinter survival of the pest, and alfalfa harvest is still a ways off.  Alfalfa weevil quit feeding at about 900 GDD Base 48.  You can monitor GDD Base 48 progress at 

Evaluating Alfalfa Stands 

There do not appear to be widespread problems with winter-kill in alfalfa, but there will always be some winter-kill, especially on older stands, so stands should be evaluated. Dr. Barnhart, ISU Extension Forage Specialist, posted basic information on stand evaluation of alfalfa and other forages at: A publication that helps assess root health is available at:    This publication also provides the other option for stand evaluation called the “Stem-Count Method”. This is actually a better evaluation method then for plant counts, but you have to wait for stands to reach 6 to 10 inches in height for the assessment to be reliable. An ideal stand has 55 or more stems per square foot. Consider replacing stands that are less than 40 stems per square foot.



For those who may not have their spring seeding of small grains done yet, potential yields decline for both oats and wheat about 10% per week for each week planting is delayed after April 15, and an additional 15% per week if planting is delayed beyond May 1. A nice fact sheet on growing spring wheat in Iowa is at  



In general, try to complete seeding of cool season forage grasses and legumes before May 1 as seeding after May 1 increases the likelihood that seeds will germinate but less frequent rainfall will allow the soil to dry out before roots are deep enough to reach moist soil, killing the seedlings.



Winter Rye Cover:  Time to Kill It 

For those using winter rye as erosion control cover, not as a forage crop for harvest, it’s time to kill it off.  You don’t want it to get too tall for a couple of reasons:   1) the more growth, the harder to kill, 2) the more growth, the more alleopathic compounds produced that could interfere with the following corn crop yield.  And we could add a third possible reason.  Not knowing if we will get normal rainfall in April, killing off the winter rye earlier will conserve soil moisture.  If you have 25 minutes, an excellent webinar on Managing a Winter Rye Cover Crop by Jeremy Singer is available for viewing at:    It’s the third webinar below the “2011 ILF Webinar Archives” heading.  The discussion of spring management on killing off the rye ahead of corn planting is about half-way through the presentation.  In addition to the webinar, Dr. Singer has a publication on this subject at: 



No-till Fields 

The warm temperatures have produced a bumper crop of winter annual weeds, as well as perennials, in some no-till fields. It probably makes more sense to take care of those soon with some 2,4-D and/or Roundup, and come back later with the residual soil herbicides. The larger weeds may take more than the pint/A rate of 2,4-D.  Corn can be planted 14 days after a quart of 2,4-D and soybeans can be planted 30 days after the quart rate of 2,4-D.  For more information, see the article from Bob Hartzler and Mike Owen at 

Thistles in Pastures 

Musk thistle and bull thistle are much easier to kill if they are sprayed when they are in the rosette stage, before they begin to bolt in the spring. Although 2,4-D and dicamba can do a good job of killing emerged thistles, products that have greater soil activity, such as Milestone (3-5 oz/A), Forefront (1.5-2.0 pt/A), and Grazon P&D (2-4 pt/A), usually provide superior control, partly because they will also control seedlings  that emerge after application.


For the most effective control of Canada thistle, it is best to wait until the thistles have a chance to grow some and are about to put on buds, which is usually in late May or early June. Canada thistles have extensive underground root systems. It is easy to kill the above ground part of the plant, but much more difficult to kill the root system so they will not be back the next year. Milestone, Forefront, and Grazon usually give the most consistent control. Higher rates are needed than for biennial thistle control. Crossbow is not a good product for Canada thistle control, but is very effective on multiflora rose and other woody and broadleaf weeds. A fact sheet based on Canada thistle trials conducted in Johnson and Keokuk Counties is attached. 



With the soils being warm and the soils in generally good condition, it has been difficult for many to not plant until the date allowed by crop insurance.   We need to remember that we had a freeze on May 9, 2010.  Any March planted corn in 2012 would likely need to be re-planted if we have a freeze in May of this year. Most of the corn that got frozen two years ago still had the growing point below ground, so it did re-grow, but some found out it did not completely recover from that stress and so yields were reduced. For more discussion, see Roger Elmore’s article at

 You can track soil temperatures at 




ISU SE Research & Demonstration Farm 25th Anniversary Celebration – Crawfordsville

June 21, 2012 

A tentative agenda, which will be updated as details are finalized, is at


ISU NE Research & Demonstration Farm Spring Field Day – Nashua

June 28, 2012 

As details become available, they 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: April 5, 2012
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