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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2007-07

Dates Covered: 
April 3, 2007 - April 9, 2007
Steve Prochaska

Taking a Second look at Plant Populations for Corn

Authors: Peter Thomison

With commodity prices for corn higher than they’ve been in many years, some growers are taking a second look at adjusting plant populations to increase grain yields. According to the National Agricultural Statistics Service, final stands for corn in Ohio averaged 26,200 plants/A in 2006. Since the early 1970’s corn populations have increased 45%. Newer hybrids are more stable under stress with improved resistance to lodging, diseases and insect pests. Studies have demonstrated that the superiority of modern hybrids is fully expressed only at higher plant populations.

Most corn agronomists recommend adjusting seeding rates by using the yield potential of a site as a major criterion for determining the appropriate plant population. In the OSU Agronomy Guide, higher seeding rates are recommended for sites with high-yield potential with high soil-fertility levels and water-holding capacity. On productive soils, with average yields of 160 bu/A or more, final stands of 30,000 plants/acre or more may be required to maximize yields, whereas on soils that average about 150 bu/A, a final stand of 28,000 plants per acre may be needed to optimize yields. Lower seeding rates are preferable when droughty soils or late planting (after June 1) limit yield potential. On soils that average 120 bu/A or less, final stands of 20,000 to 22,000 plants/acre may be adequate for optimal yields. Under moderate drought stress conditions, high plant populations generally do not cause significant yield reduction on most Ohio soils.

In 2006, we initiated an evaluation of corn response to plant population across a range of production environments to determine if our recommendations need adjustment. Our 2006 work included evaluations of plant population on different planting dates. Since weather plays a major role in determining grain yields and corn response to population, we plan to repeat these studies for two or more years to provide a better assessment of varying plant population. Our tests were conducted at eight locations with high yield potential (200 bu/A + under favorable conditions). We considered final stands of 24, 30, 36, and 42,000 plants/A with corn planted no later than May 8. Our preliminary 2006 results suggested that final stands of 36,000/A were required for optimal yield at several locations. However, higher plant populations were associated with greater lodging. Averaged across eight locations, stalk lodging (stalks broken below the ear) was 3, 10, 14, and 19% at final stands of 24, 30, 36, and 42, 000 plants/A, respectively. Averaged across eight locations, plant population effects on grain moisture were small, 20.3, 20.1, 20.1, 20.0 at 24, 30, 36, and 42, 000 plants/A, respectively. Effects of late planting (early to mid June planting dates) on corn response to population were investigated at three of these eight locations. Results indicated little benefit from increasing seeding rate and a significant yield loss at plant populations above 30,000 plants/A. For corn planted late in 2006, averaged across the three locations, grain yields were 159, 161, 133, and 138 bu/A at 24, 30, 36, and 42, 000 plants/A, respectively. The lack of response to plant population was related to stalk lodging which ranged from 59% at 24,000 plants/A to 97% at 42,000 plants/A.

With USDA predicting the highest corn plantings in 60 years, some grain analysts predict a longer corn harvest season because of the time and capacity demands on harvest machinery, drying facilities, transport and storage. As a result, some of the corn crop is likely to remain in the field longer than normal (and that’s without a wet fall like 2006 to contend with). If a grower plans to rely extensively on field drying that can delay harvest, there may be little benefit from using high plant populations above 30,000 plants/A. We recently completed a study that evaluated effects of plant population (24, 30, 36, 42,000 plants/A) and harvest dates (early-mid Oct., Nov. and Dec.) on the agronomic performance of four hybrids. Although a positive yield response to increasing plant population was observed on the first harvest date, with harvest delays, major yield losses occurred at plant populations above 30,000 plants/A, due to increased stalk lodging. See Table 4-10 in the OSU Agronomy Guide ( for details.

Planting rate or population can be cut to lower seed costs but this approach typically costs more than it saves. Most research suggests that planting a hybrid at suboptimal seeding rates is usually more likely to cause yield loss than planting above recommended rates (unless lodging becomes more severe at higher population levels). When planting occurs in cold soils, usually very early planting dates, the seeding rate should be 15% higher than the desired harvest population. Follow seed company recommendations to adjust the population for specific hybrids.

For more information on recent research concerning plant population, check out the following article by Dr. Emerson Nafziger at the University of Illinois that reviews recent and on-going Illinois studies of plant population in corn.

Nafziger, E. 2007. Should we increase corn plant populations? The University of Illinois Pest Management & Crop Development Bulletin. (No. 1 Article 12/March 23, 2007) available online at

Starter Placement and Rate, How Far Away and At What Rate?

Authors: Robert Mullen, Edwin Lentz

Starter fertilizers can be an effective way to supply early nutrients to your corn crop, but how much can you apply and how far away from the seed should you be?

Before we get into the actual discussion of starter placement and rate, we will first discuss starter response and the need for starter fertilizer. Early-season response to starter fertilizer in the form of more vigorous growth is common, but the early-season response does not necessarily translate into a yield gain at the end of the season. For starter applications, we would recommend a 2x2 placement simply because we can supply more nitrogen (N), which is normally our most limiting nutrient. Application of starter N will allow us to delay side-dress applications (if soybean planting or inclement weather get in the way). Inclusion of phosphorus (P2O5) and potassium (K2O) in a starter package may be warranted if soil test levels for those two nutrients are below the established critical values (Tri-State Fertilizer Recommendations). Starter response is also affected by the tillage regime utilized. No-till systems result in cooler, wetter soils and are more likely to benefit from starter fertilizers than tilled soils.

Pop-up or 2x2-Starter Fertilization?

With the advent of larger planting equipment affecting our ability to add 2x2 attachments due to weight restrictions or machine folding operations, pop-up, or in-seed furrow, applications of fertilizers may be the only possibility. Application of pop-up materials is much more restricted because we are putting salts and possibly ammonia forming fertilizers near the germinating seed. So how much fertilizer can be placed with the seed? The general rule of thumb used by Ohio State University is 8 lb of N + K2O per acre with the seed. The main factor determining whether or not salt will be an issue is soil moisture at the time of planting. The wetter the soil the more salt that can be applied without any adverse affects on seed germination. Urea-based nitrogen sources and diammonium phosphate (DAP) can be problematic when applied with the seed due to the formation of free ammonia during their breakdown. This too is dependent upon soil moisture, and the wetter the soil the less likely there will be a problem. The recommendation in place by Ohio State University does have a safety factor because we do not have a really good way to determine soil moisture at the time of planting.

Two by two (beside and below the seed) starter applications allow us to supply additional salt fertilizers and ammonia creating fertilizers with less concern (it is rate dependent though). You need a buffer zone between the seed and the fertilizer material especially for ammonia generating nitrogen sources (urea-ammonium nitrate (UAN) and liquid urea) and placing the material over 2.5 inches away from the seed acts as the buffer zone. Ohio State University recommends a maximum of 50 pounds of N and 50 pounds of K2O in a single application for corn (you can obviously put down less than this). Additional N can be applied, but the distance away from the seed should increase. There are no hard and fast rules that dictate how far away the material should be as a function of rate, so use some caution when making your decision.


Corn Planted into Wheat Fields

Authors: Ron Hammond, Bruce Eisley

We are receiving comments suggesting that many growers are planning on abandoning their wheat fields and planting corn into those fields this coming spring. Reasons for switching to corn include poor wheat stands and high corn prices. In these fields, growers should consider potential insect problems that might occur, depending on the cropping practices used. If wheat is tilled into the soil prior to corn planting, the potential for greater seedcorn maggot problems exist. If tilling wheat, a seed treatment would be appropriate for preventive maggot control. If growers plan on planting directly into standing wheat and then killing the wheat with a herbicide, the potential for other insect problems is high. The main insect of concern would be armyworms. The likelihood of an armyworm problem is dependent on whether the moths migrate in large numbers from southern states. But if this occurs, a large population that begins in wheat fields might move to corn following destruction of the wheat. Thus, growers who plant into standing wheat should follow the CORN newsletter on the current situation with armyworms, and plan on scouting their corn fields if the situation arises.

Growers should also be reminded that if changing to corn, they need to take into account the previous crop from 2006 ( before wheat was planted) . That crop could play a role in insect problems that might occur. Corn that is planted into wheat could have a problem with corn rootworm larval injury if 1) the field was in corn in 2006, 2) the field was in soybean in 2006 and trapping for the western corn rootworm variant had high numbers (one week of trapping with > 5 beetles/trap/day), or 3) the field was in soybean in 2006, was not trapped for the western corn rootworm variant but the field is located in an area that had high trap counts for western corn rootworm variant (see map We would suggest that if any one of the above criteria is met then the field needs to be treated with either a granular or liquid insecticide, a high rate of a seed treatment insecticide (Cruiser 1250 or Poncho 1250) or one of the corn hybrids with the Bt corn rootworm trait be used in these fields.

Weed Growth and Potential Insect Problems

Authors: Ron Hammond, Bruce Eisley

Many growers are reporting heavy weed growth in fields. Numerous insect pests are associated with the presence of weeds, the main one being black cutworm. In fields with winter annuals, especially chickweed, growers should be aware of potential cutworm problems and thus, plan on monitoring their cutworm situation during the early stages of corn growth. Additionally, some growers might choose to till under heavier weed growth. As we get into later April and early May, the tillage and burial of weeds might cause in increase in the population of seedcorn maggots. Although insect increases are usually associated with the tillage of old alfalfa fields or even wheat fields (see article on corn planted into wheat fields), large seedcorn maggot populations and subsequent injury have been seen with tillage of significant weed growth. In cases with either corn or soybean, growers would be advised to consider an insecticide seed treatment.

Soybean Aphid in Ohio

Authors: Ron Hammond, Bruce Eisley

Last week we found newly hatched aphids on buckthorn on the campus of Ohio State University in Columbus. The soybean aphid eggs hatched immediately following bud break on the buckthorn. These newly discovered aphids suggest that aphids have overwintered successfully, and our predictions for the presence of aphids this summer is on track. We will be following this buckthorn in Columbus and other locations in the state over the next few weeks to obtain a better indication as to the potential for future problems this summer. Growers should continue to read this newsletter for future developments on the soybean aphid front.

Corn Planter Adjustment - Last Minute Tips

Authors: Greg LaBarge, Peter Thomison

Uneven plant spacing and emergence may cause significant yield losses in corn. Seed should be spaced as uniformly as possible within the row to ensure maximum yields and optimal crop performance. Corn plants next to a gap in the row may produce a larger ear or additional ears (if the hybrid has a prolific tendency), compensating to some extent for missing plants. Skips can reduce yield in fields where the intended population is at or below the optimum, while doubles increase yield when populations are less than optimum. Reduced plant stands will yield better if plants are spaced uniformly than if there are large gaps in the row. As a "rule of thumb", yields are reduced an additional 5 percent if there are gaps of 4 to 6 feet in the row and an additional 2 percent for gaps of 1 to 3 feet.

Uneven corn emergence generally has a greater impact on grain yield than uneven plant spacing. Uneven emergence affects corn performance because competition from larger, early-emerging plants decreases the yield from smaller, later-emerging plants. If the delay in emergence is less than two weeks, replanting increases yields less than 5 percent, regardless of the pattern of unevenness. However, if one-half or more of the plants in the stand emerge three weeks late or later, then replanting may increase yields up to 10 percent. Emergence delays of 10 days or more usually translate to growth stage differences of two leaves or more. When two plants differ by two leaves or more, the younger, smaller plant is more likely to be barren or produce nubbin ears. Weeds also tend to be a greater problem in those areas of a field characterized by skips and gaps in the corn rows, and slow, erratic corn emergence.

Corn sometimes emerges unevenly because of environmental conditions beyond the control of growers. However, timely planter servicing and adjustment, as well as appropriate management practices, can help prevent many stand uniformity problems. The following are some tips for improving the uniformity of seed placement during planting.
1. Keep the planting speed within the range specified in the planter's manual.
2. Match the seed grade with the planter plate.
3. Check planters with finger pickups for wear on the back plate and brush (use a feeler gauge to check tension on the fingers, then tighten them correctly).
4. Check for wear on double-disc openers and seed tubes.
5. Make sure the sprocket settings on the planter transmission are correct.
6. Check for worn chains, stiff chain links, and improper tire pressure.
7. Make sure seed drop tubes are clean and clear of any obstructions.
8. Clean seed tube sensors if a planter monitor is being used.
9. Make sure coulters and disc openers are aligned.
10. Match the air pressure to the weight of the seed being planted.
11. Make planter adjustments and follow lubricant recommendations when using seed-applied insecticides (e.g. Poncho and Cruiser)
Most of the corn seed that will be planted in Ohio in 2007 is treated with seed-applied insecticides (e.g.. Poncho and Cruiser). While these seed insecticides can help reduce stand losses from soil insects, it is critical that corn growers make planter adjustments and follow lubricant recommendations when using these seed-applied insecticides. Unless these precautions are followed, the extra chemical loading on the seed may adversely affect the “plantability” of seed. Vacuum planters may under seed and finger pickup planters may over seed. To improve planter accuracy, talc or graphite should be used according to the planter manufacturer’s recommendations. With vacuum planters, it will probably be necessary to raise the vacuum to achieve more accurate seed drop.

For more information on planter adjustments to improve stand establishment in corn, consult: "Tips to Reduce Planter Performance Effects on Corn Yield" OSU Extension Fact Sheet AGF-150-01

Dr. Bob Nielsen at Purdue University has a good article entitled “Planter Maintenance: Less Down Time, More Yield ” (Chat 'n Chew Café, 21 Feb 2005). It includes links to the service support Web pages at Case-IH, Deere, and Kinze and is accessible online at

Weed Management Resources

Authors: Mark Loux

This is just a reminder that a number of weed management resources developed by OSU can be downloaded from our website, Resources available here include pdf versions of:

- OSU Extension bulletin, “Weed Control Guide for Ohio and Indiana”

- “Biology and Management of Horseweed (marestail)”, part of the GWC series of publications developed through the cooperation of state extension specialists in weed management.

- fact sheets on the value of preemergence herbicides in Roundup Ready soybean, control of lambsquarters, and management of giant ragweed populations that have not been effectively controlled in Roundup Ready soybeans in previous years.

In addition, users of the website can find a list of links to previous C.O.R.N. articles on weed management. This list includes previous years’ articles on use of burndown and residual herbicides, and the principles outlined in these will usually still be current.

Common Chickweed Control in Summer Forage Seedings

Authors: Mark Loux

The recent warm weather has been good for chickweed growth, and dense chickweed populations are present in many fields where forages were seeded last summer. Chickweed should be controlled as soon as possible to reduce the risk of forage stand loss, improve the vigor of the spring forage growth, and improve the quality of the first cutting. Pursuit and Raptor are the most effective options for chickweed control in alfalfa seedings, as well as for control of other winter annual broadleaf weeds. Unfortunately, the options for control of chickweed in mixed seedings of alfalfa and orchardgrass are not very effective. While Pursuit and Raptor can adequately control chickweed, they will also injure or kill the orchardgrass. Buctril or Butyrac should have little effect on orchardgrass and have the potential to be used in a mixed seeding, but they are largely ineffective for control of chickweed. So, the decision for producers of grass/alfalfa stands may be whether to tolerate the chickweed and hope it has minimal effect on the forage stand, or apply Pursuit or Raptor and hope the herbicide has minimal impact on the orchardgrass.

Applying Wheat Herbicides in 28% - How Late Is Safe?

Authors: Mark Loux

Since wet weather has prevented traffic in many wheat fields, application of herbicides and nitrogen fertilizer may occur when wheat is at a more advanced growth stage compared to some other years. Be sure to stage wheat prior to herbicide applications, and consult the “Weed Control Guide for Ohio and Indiana” and herbicide labels for information on herbicides and growth stage. The figure on page 149 of the 2007 guide serves as a quick reference on application timing relative to wheat stage. One of the questions relative to this subject is – how late in spring can wheat herbicides be applied using 28% UAN as the spray carrier? Stated another way, does application of herbicide in 28% UAN increase the risk of wheat injury and yield loss compared with application in water, when wheat is advanced in growth stage?

Herbicide labels often do not provide much information on application in 28% UAN. We conducted one study on this in 2003, where we applied various herbicides in water or 28% when wheat was in the joint or boot stage. Herbicides applied included Harmony Extra, 2,4-D amine, Buctril, and Harmony Extra plus 2,4-D amine. None of the herbicides reduced wheat yield when applied in water or 28% UAN on April 16 to wheat in the joint stage. The major treatment effect we observed for application later in the spring, to wheat in the boot stage on May 9, was a yield loss of about 20% when Harmony Extra or Harmony Extra plus 2,4-D was applied in 28% UAN. Aside from this, for the May 9 application, there was a trend for minor yield loss with the combination of Harmony Extra plus 2,4-D applied in water, and Buctril applied in 28% UAN. Keep in mind that the wheat growth stage on May 9 was far enough advanced that several of these herbicides should not have been applied in water or 28% UAN. For example, 2,4-D labels specify application to wheat prior to joint or boot stage, depending upon the product.

These results indicate that wheat should tolerate application of herbicide in 28% UAN well into April, as long as the restrictions on herbicide labels relative to wheat growth stage are followed. For products that are applied with surfactant, consult labels about the surfactant rate when applying in water vs 28%, since surfactant rates may be reduced to minimize the risk of injury when applying in 28%.

A final note on chickweed control in wheat – Harmony Extra or Express are the only effective options for control. As with forages, herbicide should be applied as soon as possible where chickweed is a problem.

Urease Inhibitors, How do They Work and Should You Consider Them?

Authors: Robert Mullen, Edwin Lentz

As nitrogen prices remain high the question of nitrogen stabilizers always comes up. Urease inhibitors are one type of stabilizers available to producers. As the name implies, they are only effective for urea or urea blends such 28% solution (urea-ammonium nitrate). To get the most benefit from a urease inhibitor, one needs to understand how they work and what situations may provide the most economic return.

When urea goes into soil solution it requires exposure to the enzyme urease to form ammonium-nitrogen. The first product on the way to the stable ammonium ion is ammonia, which is a gas subject to potential volatilization loss (floats away from the soil surface). Urease inhibitors deactivate the enzyme preventing ammonia/ammonium from forming until rain (or irrigation) incorporates the material into the soil. Any urea based fertilizer that is incorporated or injected below the soil surface should not require a urease inhibitor. Surface applications can benefit from urease inhibitors, but the need for the inhibitor is directly related to rainfall. Surface application of urea-based fertilizer followed quickly by rainfall is not likely to lose much nitrogen by volatilization, but you have to trust the weather forecast. Essentially, the urease inhibitor acts as an insurance policy in case rainfall does not come quickly enough. How long the insurance lasts depends on the amount of the active ingredient of the inhibitor, which generally is proprietary information of the supplier.

Tillage is also a major player in determining the need for a urease inhibitor. Tillage systems that maintain a large amount of residue on the soil surface have an abundance of urease and do not allow incorporation of the urea-based fertilizer. Therefore, surface applications of urea-based fertilizers are more likely to benefit from urease inhibitors. However, banding products such as 28% solution will greatly reduce the potential for volatilization.

The need for urease inhibitors is also dependent upon the rate of nitrogen being applied. The potential for urease inhibitors is typically greatest when the amount of nitrogen being applied is lean. High rates of nitrogen are less likely to show a positive economic response to urease inhibitors because some loss can be tolerated with no agronomic penalty. Lean rates of nitrogen can benefit because the rate of nitrogen applied may be limiting and we desire to keep all that was supplied in the soil system.

Time of year is also another factor to consider on the need for a urease inhibitor. Nitrogen fertilization on wheat generally occurs when temperatures are relatively cool and there is a high probability for a rain event before hot, dry conditions -- in other words, a low potential for volatilization loss. However, surface applied urea-based products on corn often occur when conditions may be hot and dry and an extended time may pass before a rain event.

In summary, consider rainfall, tillage, and nitrogen rate when deciding upon the inclusion of a urease inhibitor.

Anhydrous Ammonia Placement

Authors: Robert Mullen, Edwin Lentz

The wetter than normal spring may have delayed some early season applications of nitrogen in the form of anhydrous ammonia, and the warmer weather may have you thinking more about planting then putting on that nitrogen. But how do you manage your ammonia application? Like the discussion above, placing anhydrous ammonia too near the germinating seed can kill the seed and cause stand problems, so some caution is warranted. If you put anhydrous ammonia deep enough into the soil (6-8 inches below the surface) you can plant immediately after the application, but be certain about the depth. What we are trying to avoid is placing the seed in the ammonia saturation zone. As anhydrous ammonia is applied, the liquid form that is injected quickly becomes a gas and expands. The amount of expansion that occurs is a function of soil moisture, and the greater the moisture level the smaller the zone will be. Soil moisture is a function of soil texture, and finer textured soils have greater moisture holding capacities. Anhydrous ammonia can also be applied on the diagonal to avoid applications that kill entire rows of corn. If the weather really works against you and you are not able to supply or anhydrous ammonia prior to planting, wait until the corn has emerged to make your application. Delayed applications will not be detrimental as long as the nitrogen is applied in a timely fashion.

What Happened to Soybean Rust?

Authors: Anne Dorrance

Soybean rust was found in Georgia, Florida, and Alabama on kudzu leaves until the hard freeze that took place in mid to late February 2007. Texas also had one soybean field positive for rust back in February, but this field was harvested and rust can no longer be found there. Visits to the public website will see some counties are red, others are a red/hatch combination. This is why some of you are hearing that rust was there and then it was gone. The host, kudzu, was by frost and it has not been re-infected yet. The only site positive for kudzu in northern Florida is Jacksonville. Last week, Alabama was reporting dry conditions. In other words we are back to square one. On the public website, green boxes indicate that active scouting is taking place in those counties. Sentinel plots of soybean and other hosts have been planted and the scouting and monitoring of soybean rust has started. Based on requests, for the 2007 season, we will have a short weekly report on the finds of the past week each Monday. Ohio’s sentinel plots will be planted in mid-to late April to be ahead of the neighbors.

Crop Insurance Dates for Corn and Soybeans

Authors: Steve Prochaska

If certain crop insurance products have been purchased, the earliest date corn can be planted for coverage is April 6 and the last plant date is June 5. For soybeans to retain coverage April 21 is the early date and June 20 is the last plant date. Contact your crop insurance agent to verify or clarify your policy coverage .

Archive Issue Contributors: 

Peter Thomison (Corn Production), Anne Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Robert Mullen (Agronomy) and Mark Loux (Weed Science). Extension Agents: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Keith Diedrick (Wayne), Todd Mangen (Mercer), Bruce Clevenger (Defiance), Greg LaBarge (Fulton), Mark Koenig (Sandusky), Gary Wilson (Hancock), Ed Lentz (Seneca), Harold Watters (Champaign), Mike Gastier (Huron), and Steve Prochaska (Crawford).

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.