Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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May 13, 2009


Stand Evaluations

Corn is finally emerging, so soon it will be time to be making stand evaluations and unfortunately some re-plant decisions. The crusting as well as cool soil temperatures will likely result in some stand losses in the area. On average, corn yields are maximized if planting can be done by May 5-10, although every year is different. Last year some corn planted in early June actually yielded better than April planted corn, but let’s hope 2008 doesn’t become the norm for weather. In most years we lose about 10% of the yield potential when planting is delayed until late May, so potential yields drop about 1% per day (2 bu/A/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 24,000, count on a yield of about 94% of maximum. Corn stands of 20,000 results in yields of about 81% 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

------------------ 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

The table is based on trials done from 1997-2000 in three locations in Iowa.

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

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


Seed Treatments?


This may be another area where input costs can be cut. As we get later into May with warmer soil temperatures, it becomes less likely that fungicide seed treatments for soybeans will pay off. Insecticide seed treatments do a nice job of controlling overwintering bean leaf beetles, but they are a greater problem with the early planted soybeans which are already in the ground. The cold winter should also reduce our bean leaf beetle populations. See Erin Hodgson & Rich Pope’s article at Insecticide seed treatments are not very effective for controlling soybean aphids, because the treatment does not last long enough for killing aphids in August.


Soybean seed inoculants have been advertized a lot lately. Although the nitrogen-fixing bacteria are important for soybeans, it seldom pays to inoculate seed where soybeans are being planted in soils where soybeans have grown in the last 3-5 years. I’ve seen three fields in the past 20 years where there have been yield reductions because the soybean seed had not been inoculated. In every case soybeans had never been grown on the field before, and the field was isolated from other soybean fields. Palle Pedersen has had many trials out in Iowa and has yet to see a yield response to inoculating seed  in a corn-soybean rotation. See his article at One situation where an inoculant might pay off this year is in fields that were flooded for more than a week last year, which could have reduced the rhizobia bacteria population.


Incentives for Organic Agriculture


There is money available for farmers that are interested in transitioning to organic agriculture. Up to $20,000 per year is available from a new program in EQIP called “organic conversion assistance.” The deadline for signing up at the NRCS offices is May 29. See Kathleen Delate’s article in the ICM News for more info at









Certified Crop Advisors can obtain 5 hours of credit (including 3.5 hours in soil and water management) by attending a cover crop and residue management session in the morning followed by the afternoon tour at the ISU SE Iowa Research & Demonstration Farm near Crawfordsville on June 24. 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 13, 2009
Contact: Jim Fawcett

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