CORN
Crop Observation and Recommendation Network

April 7-14, 2003
C.O.R.N. 2003-8

In This Issue:

A) Crop Establishment: A Key Step In Successful Corn Production
B) Importance of Stand Uniformity in Corn and Tips for Planter Adjustment
C) Purple Fields and Soybean Cyst Nematode
D) Suggestions for Managing Marestail

A) Crop Establishment: A Key Step In Successful Corn Production - Thomison CORN Questions

Although drought was a major factor responsible for low corn yields in 2002, marginal to poor planting conditions due to wet weather and delayed planting can also share much of the blame. Cold, wet soils during or after planting slowed root development. Continuous rains and field operations in wet fields contributed to surface soil compaction which further inhibited root growth. Many fields were too wet when planted, and in-furrow sidewall compaction created another barrier for root growth. These early season problems created a limited root system that was unable to access sufficient moisture when weather turned hot and dry in mid to late June. Poor, nearly non-existent nodal roots led to "rootless/floppy" corn problems in late June and early July, and later in the season contributed to root lodging problems. Although western corn rootworm was often cited as a primary cause of the late season root lodging, the major factor responsible for the problem in most fields was probably poor nodal development due to the adverse environmental conditions described above.

Mistakes made during the planting operation are usually irreversible, and can put a "ceiling" on the crop's yield potential before the plants have even emerged. The following are some proven practices that will help get a crop off to a good start.

1) Perform tillage operations only when necessary and under the proper soil conditions.

Avoid working wet soil and reduce secondary tillage passes. Perform secondary tillage operations only when necessary to prepare an adequate seedbed. Shallow compaction created by excessive secondary tillage can reduce crop yields, especially under drought stress conditions like those that occurred in 2002. Cloddy seed beds and soil compaction caused by spring tillage often contribute to uneven stands. Deep tillage should only be used when a compacted zone has been identified and soil is relatively dry. Late summer and fall are the best times of year for deep tillage.

2) Complete planting by mid-May.

If soil conditions are dry, begin planting before the optimum date. (The recommended time for planting corn in northern Ohio is April 15 to May 10 and in southern Ohio, April 10 to May 10). Avoid early planting on poorly drained soils or those prone to ponding. Yield reductions resulting from "mudding the seed in" may be much greater than those resulting from a slight planting delay. If growers have the equipment capability to plant more than half of their corn acres prior to the optimum planting date, then this should allow planting all the corn acres prior to the calendar date when corn yields begin to decline quickly. During the two to three weeks of optimal corn planting time, there is, on the average, about one out of three days when field work can occur. This narrow window of opportunity further emphasizes the need to begin planting as soon as field conditions will allow, even though the calendar date may be before the optimal date. As a guide, calendar date is more reliable than soil temperature for making the decision on when to begin to plant corn.

3) Adjust Seeding Depth According to Soil Conditions.

Plant between 1-1/2 to 2 inches deep to provide for frost protection and adequate root development. In April, when the soil is usually moist and evaporation rate is low, seed should be planted shallower no deeper than 1-1/2 inches. As the season progresses and evaporation rates increase, deeper planting may be advisable. When soils are warm and dry, corn may be seeded more deeply up to 2 inches on non-crusting soils. Consider seed-press wheels or seed firmers to ensure good seed-soil contact. One risk associated with shallower planting depths is the possibility of poor development of the permanent (or secondary) root system if the crown is at or near the soil surface, some of the permanent roots may not grow under hot, dry conditions (resulting in the "rootless" and "floppy" corn syndromes). Another potential risk from planting less than 1-1/2 inches is shoot uptake of soil-applied herbicides. Seeding depth should be monitored periodically during the planting operation and adjusted for varying soil conditions. Irregular planting depths contribute to uneven plant emergence, which can reduce yields.

4) Adjust Seeding Rates on a Field-by-Field Basis.

When seeding, adjust the seeding rates by using the yield potential of a site as a major criterion for determining the appropriate plant population. 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/acre or more, final stands of 30,000 plants/acre or more may be required to maximize yields.

Lower seeding rates are preferable when droughty soils or late planting (after June 1) limit yield potential. On soils that average 120 bu/acre or less, final stands of 20,000 to 22,000 plants/acre may be adequate for optimal yields. Under drought stress conditions, high plant populations usually do not cause significant yield reduction.

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 early planting dates, the seeding rate should be 10-15% higher than the desired harvest population. Follow seed company recommendations to adjust the population for specific hybrids.

B) Importance of Stand Uniformity in Corn and Tips for Planter Adjustment - Thomison and LaBarge CORN Questions

Cultural practices that promote uniform plant establishment will help maximize corn production. Uneven plant spacing and emergence can reduce yield potential 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 will generally have 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. To decide whether to replant in this situation, growers should compare the expected economic return of the increased yield with both their replanting costs and the risk of emergence problems with the replanted stand.

To improve planter accuracy and enhance uniformity of emergence, consider the following:

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.

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 http://ohioline.osu.edu/agf-fact/0150.html

C) Purple Fields and Soybean Cyst Nematode - A. Dorrance and D. Mills CORN Questions

Purple deadnettle, henbit, shepards-purse and field pennycress have been identified as Soybean Cyst Nematode (SCN) hosts. These weeds have a winter life cycle in Ohio and may serve as sites for SCN reproduction in infested fields during the early or late growing season when soybean plants are absent. The presence of winter annuals also hinder crop rotation's effectiveness in reducing SCN numbers.

Once the field is infested with SCN, it is forever infested , and SCN can only be managed not eradicated. In order to have an effective management strategy, SCN levels must be known for infested fields and that can only be obtained from sampling. To date less than 2% of the cropland acres in Ohio have been sampled. Samples can be sent to the C. Wayne Ellet Plant & Pest Diagnostic Clinic for analysis. Contact your county extension office or the following site for more information. (http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/scn.htm)

Soil samples collected in the spring will have lower SCN counts than those collected in the fall. The SCN management guidelines are based on fall egg counts. Therefore, if your counts come back high, interpret these conservatively and manage these fields accordingly. For example, if your count is 4,500 eggs/cup of soil from a spring sample, plant that field to corn for at least 2 years before soybeans are planted again.

D) Suggestions for Managing Marestail - Loux CORN Questions

Marestail has become more problematic over the past several years. Several factors are responsible for the increase, including: no-tillage; failure to use 2,4-D in soybean burndown treatments; failure to apply herbicides early enough in spring when marestail is small and more easily controlled; and resistance to ALS inhibitors or glyphosate in some populations. Marestail is classified as a winter or summer annual, since it can emerge most of the year. Consequently, an early-spring herbicide treatment may not be adequate unless it contains herbicide(s) with residual or is followed by an effective postemergence treatment. Marestail is a prolific seed producer and the seed are wind-blown, which allows it to spread readily.

Problems with marestail control are most likely to occur before or at the time of planting, especially when plants are more than a few inches tall or after they have bolted (stem elongation). Given the possibility of resistance to ALS inhibitors or glyphosate in marestail, it is difficult to count on any one herbicide to provide complete control. Glyphosate is somewhat weak on larger marestail even in glyphosate-sensitive populations. Some suggestions for managing marestail:

• Apply the burndown when marestail plants are small - less than 4 inches tall if possible. Plants that have bolted can be extremely difficult to control.
  
• Apply at least 7 days before planting and include 2,4-D ester in all burndown treatments. This is especially important where resistance to glyphosate is suspected.
• Use a glyphosate rate appropriate for marestail size. We suggest a rate of 0.75 lb acid equivalent per acre (22 oz of WeatherMax or 32 oz of most other glyphosate products) for marestail up to 6 inches tall. Increase rates for larger plants.
• Control may be improved by including ALS inhibitors such as Canopy SP or XL, FirstRate, Gangster, Gauntlet, or Python in the burndown treatment. However, many populations are ALS resistant, especially in fields where the crop rotation has included primarily non-GMO soybeans over the past 5 or so years.
• A mixture of Sencor (8 oz/A) plus 2,4-D ester is effective for control of marestail up to 4 to 6 inches tall.
• Although marestail can emerge through early June, burndown treatments applied in early May or later should effectively control most of marestail that will emerge in this year's soybean crop. Consider including herbicides with residual activity on seedling marestail in burndown treatments applied before early May. Herbicides with residual on marestail include Sencor, Authority, and Valor. Where ALS resistance in not evident or suspected, ALS inhibitors such ax Canopy, FirstRate, Python, and Scepter also provide residual control.
• Spring tillage will control emerged marestail.
  

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Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Steve Prochaska        Web Editor: Nathan Watermeier

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