Corn Plant Populations and Seed Costs

Plant populations continue to increase and, unfortunately, so do seed costs.  Certainly hybrids today withstand the stress of higher seeding rates better than ever before. In fact, higher seeding rates are one component that drove annual yield increases the last several decades. 

The current questions are where and when does the yield responsiveness to increased seeding rates plateau or stop? If we consider net return, we arrive at this point once increased seeding rates no longer cover the additional seed cost.

Since 2006, we have conducted plant population research across 32 sites in Iowa.  Figure 1 displays the yield distribution pattern from these trials expressed as a percent of the maximum yield across the 32 locations.  On average, maximum grain yields occur between 34,500 and 37,000 plants per acre (ppa); although there is significant variation across locations and years (Figure 1). This population range, 34,500 to 37,000 ppa, is 2000-3000 ppa greater than what was found in plant population research 5 to 10 years ago.

Seed costs have increased substantially.  But, don't forget that transgenic traits add a measure of yield protection — herbicide and insect resistance allowing improved weed and insect management - as they add to the price of seed.  Is there a point where adding another 1000 seeds per acre does not return the cost of the increased seed?
We answer that question by comparing a range of seed costs actually paid by producers in 2009 with expected yields at different plant populations based on our 2006 to 2008 data (Figure 2).  As seed prices increase, return to seed decreases. The best net returns occur with plant populations between 30,000 and 35,000 ppa. 

Not every seed that is planted develops into a plant.  Our recorded losses from seeding to plant survival range from 4 to 7 percent.  In general, increasing seeding rates by 5 percent will insure that the proper plant population is achieved.  We recognize that plant survival depends on many factors and may vary from field to field.

Plant populations between 30,000 and 35,000 ppa optimize yield while maximizing net income.  We continue to process these data and are also conducting additional plant population trials this year. Please stay tuned.



Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production. Elmore can be contacted by email at or (515) 294-6655; Abendroth can be contacted by email at or (515) 294-5692.

Northern Corn Leaf Blight Showing up In Iowa 06/15/2015

Diseases Showing Up in Iowa Corn

By Alison Robertson, Department of Plant Pathology and Microbiology

Northern corn leaf blight

Northern corn leaf blight (NCLB) (Figure 1) has been reported from several fields in southern Iowa. You will remember that this disease was widespread in Iowa in 2014, and severe on susceptible hybrids. Since the fungus survives the winter in corn residue, we likely have above normal inoculum present. Cool weather with frequent precipitation favors infection of corn by the fungus and disease development. New lesions may develop every 4 days (Muiro et al., 2010) when conditions are favorable (susceptible hybrid, cool and wet weather). Warm dry conditions will slow or halt disease development until favorable conditions return.


It will be very important this growing season to scout fields that are planted to NCLB-susceptible hybrids. If the disease is present on 50% or more of the plants in the field, the hybrid is scored susceptible and cool, wet weather is forecast, a foliar fungicide application may be required. In 2014, applications at V5-V6 reduced NCLB, but applications made at R1 were more effective at protecting the canopy through dent. In 2014 however, July was dry and NCLB development slowed or stopped before starting up again in August through September.  If 2015 remains cool and wet, NCLB will win the “Disease of the Year” award for a second consecutive year.

Anthracnose leaf blight

On corn following corn fields, many of you have likely noticed anthracnose leaf blight (Figure 2) on the lowest leaves of the corn plant. This is not unusual.  Anthracnose leaf blight is usually present in wet springs, but does not need to be managed. Research at Iowa State University and University of Wisconsin has shown that there is no relationship between anthracnose leaf blight and anthracnose stalk rot, although both are caused by the same pathogen. Corn will rapidly grow out of the disease, and the affected lower leaves, which do not contribute to yield, will die and fall off the plant within a couple of weeks.



Common rust

Common rust (Figure 3) has also been observed but at very low incidence (few plants in a field). Most hybrids have good resistance to common rust; inbreds do not, thus seed production fields should be scouted and a fungicide applied if disease is present.



Alison Robertson is an associate professor of plant pathology with research and extension responsibilities in field crop diseases at Iowa State University. Alison Robertson may be reached at or 515-294-6708.


This article was published originally on 6/16/2015 The information contained within the article may or may not be up to date depending on when you are accessing the information.


ISU Helps Answer the Question....

Corn Fungicides: To Spray or Not to Spray?

By Alison Robertson, professor, and John Shriver, research assistant, with the Department of Plant Pathology and Microbiology

Across the state of Iowa, much of the crop is reaching V5 to V6 and thoughts of an early fungicide application have probably crossed some people’s minds.

Every year, we evaluate and compare registered fungicides applied at either V5 to V6, R1 alone, or At V5 to V6 plus R1 for foliar disease management and effects on yield. In 2014, we tested 8 products at 6 locations in Iowa (Table 1). A randomized complete block design with 4 replications was used. At each location, two non-sprayed checks were included.

The 2014 growing season was predominantly cool and wet with above average rainfall in June, August, and September. Northern corn leaf blight (NCLB) was reported prior to tasseling in southwest and central Iowa, which was considerably earlier than usual. Over the past decade, NCLB has been reported after silking at various locations across the state. Different hybrids were grown at each location in the trial, but all had moderate resistance to NCLB.

Results by location

Northwest Research Farm in Sutherland, Iowa: NCLB severity in the upper canopy (ear leaf and all leaves above) two non-sprayed controls was 8.4% and 7.8% (Table 1). Although an application of Priaxor at V6 did not reduce NCLB severity compared to the non-sprayed controls (P < 0.1), applications of Custodia, Fortix and Stratego YLD at V6 did reduce disease. All treatments that included an application of fungicide at R1 reduced NCLB severity (P < 0.1),;however, applications at V5 followed by R1 were not different from application at R1 only. In general, treatments that were applied at R1 resulted in greater yields than the untreated check and V5 applications alone (P < 0.1) (Table 2).

Northern Research Farm in Kanawha, Iowa: NCLB severity in the two non-sprayed controls was 10.5% and 9.3% (Table 1). A reduction in NCLB severity was detected from all fungicides applied at all timings. No difference between application timings was detected, that is, applications made at V5 were as effective as applications made at R1, and at V5 plus R1. This is likely due to the fact that NCLB development started early prior to the crop tasseling. There was significant lodging in the plots due to a wind storm that occurred mid-July. No evidence of an effect of fungicide on yield was detected (Table 2).

Northeast Research Farm near Nashua, Iowa: NCLB severity was less than 1%. In this part of the state, precipitation was normal to below normal. In general, an application of fungicide at V6 reduced NCLB compared to the non-sprayed controls. Applications of Fortix at R1, Headline Amp at R1, Quilt Xcel at R1, Stratego YLD at V5 + R1, Stratego YLD at R1 all increased yield compared to the non-sprayed control (P < 0.1) (Table 2).

Ag Engineering/Agronomy Farm in Boone, Iowa: In the two non-sprayed control, NCLB severity was 14% and 16% (Table 1). In general, an application of fungicide at V5 did not reduce disease severity, apart from an application of Aproach. An application of fungicide at R1 and double applications of fungicide (V5 plus R1) reduced disease severity. Greater yields occurred with applications of Fortix at R1, Headline AMP at R1, Stratego YLD at R1 and Stratego YLD at V5 plus R1 (P < 0.1) (Table 2).

Southwest Research Farm in Lewis, Iowa: Disease severity in the two non-sprayed controls was 10.5% and 9.3% (Table 1). No effect of an application of fungicide at V6 on NCLB severity was detected, except for Priaxor (3oz/A) (P < 0.1).  An application of fungicide at R1 significantly reduced NCLB severity. Double applications of fungicide (at V5 plus R1) were not different from the single application at R1. No evidence of an effect of fungicide on yield was detected (Table 2).

Southeast Research Farm in Crawfordsville, Iowa: NCLB severity in the two non-sprayed controls was 7.8% and 8.4% (Table 1).  All applications of fungicide reduced disease severity except for applications of Priaxor or Stratego YLD at V5, and Aproach at V5 plus Aproach Prima at R1. Greater yields compared to the non-sprayed control occurred with Fortix, Headline AMP Quilt Xcel and Stratego YLD all applied at R1, and with a two applications of Stratego YLD made at V5 and R1 (P < 0.1) (Table 2).


All fungicides effectively reduced NCLB at all locations in Iowa in 2014. Although applications at either V5 or at R1 reduced disease, applications at R1 were most effective at reducing disease. No additional disease control occurred with applications at V5 and again at R1.

With El Niño in effect, it looks like 2015 has a chance to be cool and wet. Since the NCLB fungus overwinters in corn residue, NCLB could be a disease risk in 2015. It will be important to scout fields, particularly those planted to hybrids that are rated susceptible or moderately susceptible to NCLB. If disease is present anywhere on more than 50% of the plants in a field, a fungicide application could be a prudent decision. Applications made between VT and R2 should protect the crop. Avoid applications between V12 and V18; since, this increases the risk of arrested ear development.

Acknowledgements: Thanks to managers and staff at outlying ISU research and demonstration farms for managing these trials and applying fungicide treatments at their respective farms.