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East-Central and Southeast Iowa Crop Information


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May 3, 2010



Stand Evaluations

Corn has really popped up in the last few days, so it is time to start making some corn stand evaluations. The heavy rains may have resulted in some crusting, especially with the mellow soil that many planted into on the tilled ground. On average, corn yields are maximized if planting can be done by May 5-10, although every year is different. In most years we lose about 10-20% of the yield potential when planting is delayed until late May, so potential yields drop about 1-2% per day after about May 10. Potential yields drop off more rapidly if planting is delayed beyond late May.

            Stands of 30,000+ will result in maximum yields. If stands are reduced to 25,000, count on a yield of about 95% of maximum. Corn stands of 20,000 results in yields of about 89% of maximum. This assumes that the remaining stand is fairly uniform. The cost of re-planting and yield loss from late planting needs to be compared to any yield loss from stand losses to make a good decision. The following table can help with re-plant decisions:


Influence of planting date and plant population on corn grain yields in Iowa

          ------------------ Corn Yields (% of maximum) -----------------
Stand     April 20 -     May 5 -     May 15 -     May 25 -     June 5 -
X 1,000   May 5          May 15       May 25       June 5       June 15

   35      100                  96                    87           70                  54
   30        99                  95                    86           69                  53
   25        95                  91                    83           67                  51
   20        89                  85                    77           63                  48

   15        81                  78                    71           57                  44

   10        71                  68                    62           50                  38


This table comes from the latest Iowa research and modeling which is found on page 12 of the new Corn Field Guide (CSI001).

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

The usual method to check corn populations is to measure off 1/1000 of an acre in a row. That is 26’2” in 20” rows, 17’5” in 30” rows, 14’6” in 36” rows, and 13’9” in 38” rows.

For more information on checking corn stands, see the ICM article by Roger Elmore and Lori Abendroth at



Plant Populations


            Many producers are finishing up with corn and switching to planting soybeans now. Even though optimum seeding rates have been increasing every year for corn, recommended seeding rates for soybeans have been going in the other direction. It’s not that today’s varieties perform better at lower populations, but that we have always over-seeded soybeans, partly because of concerns about weed control and because seed costs were relatively low. With today’s better weed control and higher seed costs, it makes sense to seed at rates closer to what is really needed. This is one area where many producers can cut input costs without affecting the yield

Recent work by Palle Pedersen has shown that the optimum final stand for soybeans is 100,000 plants per acre. Optimum seeding rates will vary depending on the seeding method used and germination of the seed, but it will seldom pay to seed at higher than 125,000-140,000 seeds per acre. Because of soybeans ability to compensate for lower stands by branching out and producing more pods/plant and more seeds/pod, yields do not decrease much until populations get below 75,000 plants/acre. See Palle’s fact sheet “Optimum Plant Population in Iowa” at for more information.


Alfalfa Weevil

I haven’t heard of problems with alfalfa weevils this year, but be sure to scout for this pest in alfalfa fields. For details on scouting for and managing alfalfa weevils, see the Integrated Crop News article at  Alfalfa weevil activity is based on Growing Degree Days Base 48.  Growing Degree Days Base 48 for Burlington, Davenport, Cedar Rapids, and Dubuque are posted at


With the rainy conditions and the many windy days that kept sprayers out of the fields, corn has emerged in some fields before the planned nitrogen and/or herbicide applications were made. Liquid UAN solution can be applied over small emerged corn up to around 60-90 lb N/A, without causing yield loss, although some burning will occur. Most herbicide labels do not allow herbicides to be applied using UAN as a carrier after the corn has emerged because of the risk of injury. Corvus is labeled to be applied on up to 2-leaf corn. The label doesn’t forbid applying it post with UAN as a carrier, but states that it is not recommended. Some preemergence herbicides, such as Dual II Magnum can be applied after corn emergence, but will not control emerged weeds. Other herbicides can cause severe injury or death if applied when the corn is spiking, such as Roundup on non-GMO corn. Be sure to check labels. Some may choose to change planned herbicide or nitrogen applications. The following article gives more details on applying UAN and herbicides after corn emergence







The Spring Field Day of the Southeast Research and Demonstration Farm (Crawfordsville) will be on the afternoon of Thursday, June 24, 2010 at the farm.  Certified Crop Advisor (CCA) credits will be available.  In addition, Certified Crop Advisors can obtain additional hours of credit (including soil and water) by attending a special session in the morning followed by the afternoon tour.  More details will be posted soon at



If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: May 3, 2010
Contact: Jim Fawcett

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