Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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May 19, 2008




Hybrid Maturities


If there is still corn to be planted during the last week of May, switching to a hybrid about 5 days earlier in maturity would be something to consider.  Before the last week of May, stay with the hybrids you chose unless you were trying to stretch the season by planting a hybrid 5 days longer than what is common in the area.



Corn Stands

With many corn acres planted under marginal conditions, we may see some issues with low population and/or uneven stands. Re-planting poor stands may be something that growers will be debating about soon. The following table can help in making replant decisions:

Influence of planting date and plant population on corn grain yields

------------------ Corn Yields (% of maximum) -----------------
Stand     April 20 -     May 13 -     May 26 -     June 10 -     June 24 -
X 1,000   May 5          May 19       June 1       June 16       June 28

28–32    100                 99                    90           68                  52
   24        94                  93                    85           64                  49
   20        81                  80                    73           55                  42
   16        74                  73                    67           50                  38
   12        68                  67                    61           46                  35

Numerous gaps of up to 4-6 feet can reduce yields by an additional 5-6%.

The table is based on trials done from 1997-2000 in three locations in Iowa.  Optimum corn stands have increased by at least 2,000 plants per acre since this work was done, so actual corn yields obtained will likely be somewhat lower than what is indicated in the table. One encouraging point is that very little yield penalty was seen with mid-May planting dates. Corn yield potentials did not drop dramatically until planting was delayed into June.

For more information, see Pm-1885 “Corn Planting Guide,” which is also available at and NCR 344 “Uneven Emergence in Corn”  at


Black Cutworms

We are just accumulating 300 Growing Degree Days (GDD) since the first major flight of black cutworm moths into the state, and this morning I began to receive reports of cutting south of I-80.  Once 300 GDD have accumulated, corn should be scouted until it reaches growth stage V-5.  Due to the high price of corn, the threshold for treatment has been lowered by 1%.  Information on the change in thresholds is at  Information on GDD and predicted cutting statewide is at  Finally, I monitor GDD accumulation based on National Weather Service observations at Burlington, Cedar Rapids, Davenport, and Dubuque and post the data for Black Cutworms, as well as a few other insects, at




If using 2, 4-D with Roundup to try to get a better kill on some of the larger weeds, remember the one week planting delay for both soybean and corn.


Some may have planned applications of soil applied herbicides that get delayed due to the rain. Many soil-applied herbicides can be applied after the corn has emerged, but may not have an effect on emerged weeds. Radius (& Balance Pro) should not be applied after corn emergence or severe injury may occur. Obviously Roundup should not be applied after corn emergence on non Roundup-Ready varieties. Mike Owen has an article on herbicide options after corn emergence in the ICM News at





            Stalk borers love giant ragweed as well as corn. If fields with large populations of ragweed are being planted to corn, there is an increased chance of having stalk borer problems. When the ragweed is killed by herbicides or tillage, the stalk borers will move from the ragweed to the corn as it emerges. One time when it is possible to kill stalk borers with insecticides is when they are moving from one plant to another. Once they are inside the corn plants they are protected from the insecticide. For any fields that have ragweed or grass weed problems, consider including an insecticide with the herbicide burndown or make an insecticide treatment as soon as the corn is emerging on tilled fields. For more information and a picture of stalk borers see Marlin Rice’s article in the May 16, 2005 ICM Newsletter at









  9:00 a.m.  – Certified Crop Advisors’ Training

10:30 a.m.  – Drainage Field Day

Noon          – Lunch Available

 1:00 p.m.   – Spring Field Day


Certified Crop Advisors can obtain 5 hours of credit (including 2 hours of soil and water) by attending a special session in the morning followed by the afternoon tour at the ISU SE Iowa Research & Demonstration Farm near Crawfordsville on June 26. This will include a tour in the morning featuring the soil drainage research on the farm.  More details will be posted soon.



If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: May 19, 2008
Contact: Virgil Schmitt

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