Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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April 5

April 30, 2013




Alfalfa Weevil 


We have surpassed 200 Growing Degree Days (GDD) (base 48, from Jan. 1) south of Interstate 80, which suggests that it is time to scout south-facing slopes for Alfalfa Weevil  there; north of Interstate 80, the time to start scouting south-facing slopes is fast approaching.  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 it is still best to scout for them.  Alfalfa weevil quit feeding at about 900 GDD Base 48.  You can monitor GDD Base 48 progress at and 


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. This year, it appears that older stands that were intensively harvested and new seedings are most likely to have thin stands.  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.





Corn Planting 


With the soils being warm and the soils in generally good condition, corn planters are starting to roll.  However, it looks like soil temperatures may flirt with 50 degrees over the weekend, and corn does not grow at temperatures below 50 degrees.  If the germination process has started, it is difficult for the plants to stop the process and then start up again.  Fields with a history of pythium seedling rots are especially vulnerable to the rots if the soils are both wet and cold.  Roger Elmore addresses planting issues at

 You can track soil temperatures at 


Stalk Borers


Stalk borers can be a concern for corn grown adjacent to grassy areas, such as ditches and waterways, or in areas where there was a grassy weed or giant ragweed control problem in 2012. Along and south of Highway 34 (Burlington, Fort Madison, Keokuk, and Mount Pleasant area), we are approaching the time for one management strategy, which is to spray an insecticide to those areas immediately before egg hatch to kill the larvae as they hatch. The time of egg hatch is controlled by Growing Degree Days, so the egg hatch will slowly “move north” as more Growing Degree Days accumulate there. Stalk borer activity is based on Growing Degree Days Base 41. Growing Degree Days Base 41 for Burlington, Davenport, Cedar Rapids, and Dubuque, as well as additional stalk borer management information, are posted 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. 





ISU SE Research & Demonstration Farm Spring Field Day – Crawfordsville

June 27, 2013, 1:00 p.m. 

As details become available, they will be posted at


ISU NE Research & Demonstration Farm Spring Field Day – Nashua

June 26, 2013, 1:00 p.m. 

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 30, 2013
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