CORN
Crop Observation and Recommendation Network

May 19-26, 2003
C.O.R.N. 2003-14

In This Issue:

A) Delayed Soybean Emergence
B) Challenging Burndown Situations Require Appropriate Herbicide Rates
C) Considerations for Management of Postemergence Herbicides in Corn
D) Estimating N Losses
E) Slug Update
F) Guidelines for Replanting Corn
G) Wheat Powdery Mildew Spreading
H) Physiological Fleck Symptoms on Wheat
I) Wheat Fusarium Head Scab Risk Update

A) Delayed Soybean Emergence - Anne Dorrance and Jim Beuerlein CORN Questions

Several calls last week on delayed and spotty emergence in many fields. Some of this is due to disease (Pythium and/or Phytophthora) as well as cool soils and flooded conditions. The challenge is how to separate all of these factors to make a good diagnosis. When you dig up the seedlings, the roots should be nice and white, the cotyledons will also be a deep green in color. These are signs that everything is fine, just slow. If soils have been wet and waterlogged, you will find weak seedlings. Light tan to brown, sunken discolored roots are signs of root rot. If the roots have a gray cast to them, that is a sign of flooding injury. One thing to note is that the area between the root and the fleshy cotyledons is called the hypocotyl. This region of the seedling can be either purple or green in color. Purple flowered soybeans have a purple hypocotyls while white flowered soybeans have a green hypocotyl. Iowa State University Extension provides a table that gives the days to emergence at average soil temperatures. For soybeans, it takes 21 or more days when soil temperatures are 50 to 55. It takes 10 to 12 days when soil temperatures are in the 60's. Our soil temperatures throughout the state have been hovering between these two - thus we can expect soybean emergence to take between 2 to 3 weeks. If flooding has occurred, emergence could take longer or not at all if the flooding was for a long enough period of time. Obviously if disease is present, this will also reduce emergence, but then you've got to go dig them up to make that decision. On more productive soils where there is substantial vegetative growth- soybean stands of 80,000 to 90,000 plants per acre that are evenly spaced will give 100% of the yield. For lighter soils where there is not as much vegetative growth you will need 100,000 to 120,000 plants per acre.

B) Challenging Burndown Situations Require Appropriate Herbicide Rates - Loux CORN Questions

Be sure to use the appropriate rates of glyphosate or paraquat in no-till fields that have not been planted yet. Where possible, the use of 2,4-D ester is still recommended in these fields, but only where it is possible to delay soybean planting for 7 days after application. Consider increasing glyphosate rates to 1.5 lbs acid equivalent per acre (44 oz of WeatherMax or 64 oz of most other glyphosate products), especially where 2,4-D ester will not be used. Application of glyphosate in nitrogen fertilizer solution (28% UAN) can reduce the effectiveness of glyphosate on large weeds, so use water as the carrier where possible. Even where the field will be tilled prior to planting, there can be some advantage to applying glyphosate a day of so before tillage to make sure large weeds are completely killed.

C) Considerations for Management of Postemergence Herbicides in Corn - Loux CORN Questions

• Where corn has emerged but herbicide has not been applied, consider switching from a preemergence atrazine premix product (Harness Xtra, Bicep, Guardsman, Keystone, etc) to a postemergence herbicide program such as Steadfast ATZ, Option + Distinct, etc. Be aware especially that annual grasses larger than 1 ½ inches tall may not be controlled by atrazine.
  
• Assuming a weed-free start at planting, apply postemergence treatments in corn before most of the weeds, and especially grasses, exceed 4 inches in height. Delaying application past this point can result in corn yield loss due to prolonged weed interference. Where a preemergence herbicide was applied and the weed population has been reduced, timing of postemergence herbicides may not be as critical with regard to weed interference.
  
• Grass control is generally more difficult in corn than broadleaf control, so grass size should be a primary indicator for the timing of postemergence applications in corn. Postemergence grass herbicides should be applied before grasses exceed 3 to 4 inches in height depending upon the product.
  
• Consider including atrazine or other herbicides that will provide at least several weeks of residual activity in postemergence corn programs to control late-emerging weeds.
• Include effective rates of dicamba, atrazine, or bromoxynil with postemergence ALS-inhibiting herbicides (Lightning, Permit, Spirit, etc) to minimize the risk of ALS resistance.
  
• Postemergence grass herbicides for corn (Option, Accent, etc) tend to be most effective when applied with crop oil concentrate (COC), and especially methylated seed oil (MSO). The Option label specifies use of MSO in all treatments. However, COC and MSO may not be allowed in some tank mixtures due to the potential for crop injury, so check labels and local use guides prior to application. Low rates of 28% UAN or ammonium sulfate should also be included with postemergence grass herbicides.
  
• Be aware of limitations on corn height with postemergence corn herbicides. Treatments containing 2,4-D should be applied before corn exceeds 8 inches in height unless drop nozzles are used. Products containing dicamba (Banvel, Clarity, Distinct, NorthStar, Yukon, Celebrity Plus, etc) vary in their labeling with regard to corn size. As a general rule, treatments containing dicamba will be least injurious when applied to corn that is less than about 8 inches tall and prior to the five leaf stage. However, most of these products allow application to considerably larger corn, although they sometimes specify lower rates. Drop nozzles can be used on larger corn to minimize the risk of injury with these products. In addition, make sure the spray boom is high enough above the corn to allow the proper spray overlap and nozzle pattern. Where the spray boom is too low, excess herbicide may be directed into the corn whorl, increasing the risk of injury.
  
• Products containing ALS inhibitors also vary in their labeling with regard to corn size, depending upon the ALS inhibitor and the premix or tank-mix partners. Restriction for these products can be found on the label and on pages 64-65 of the 2003 OSU Weed Control Guide. As a general rule, the risk of injury from ALS inhibitors increases when corn development is past the 5- to 6-collar stage (most critical), or when corn is more than 16 to 20 inches tall. Growth stage is the more critical variable with regard to risk of injury; in some years corn can be fairly short and still at the 5- or 6-collar stage.
  

D) Estimating N Losses - E. Lentz CORN Questions

Producers in areas that have received excess rains the past several weeks may have a concern about nitrogen loss, particularly those who applied N in early spring or the previous fall. Loss potentials increase during mild winters and springs since early-applied anhydrous and other ammonium forms may have converted to nitrate-N. In the nitrate form, nitrogen is susceptible to loss under the right conditions. Most of this loss potential is from denitrification, but leaching may also occur in sandy soils and soils with gravelly subsoils. Denitrification losses may be significant when relatively warm soils remain saturated for three days. Standing water is evidence of saturated soils, but even bare soils that an individual cannot walk across without making footprints are saturated. There is no test that can precisely determine how much nitrogen has been lost, but there are some options available to producers that can assist in making sound N management decisions.

One of these options is the pre-sidedress nitrate test (PSNT). For this test a producer needs to collect 10-15 random soil samples (12-inch depth) from a sampling area no larger than 20 acres, and send to a reputable lab. The lab will determine the nitrate concentration of the sample. Any sample that has more than 25-30 ppm nitrate should have adequate N for the rest of the season. If it is less than 25 ppm the full rate of N needs to be applied. Some states recommend reduced rates of N between 15 and 25 ppm. Ohio State University research, however, showed that our soils are too variable for consistent recommendations between 15 and 25 ppm, and deficiencies may occur later in the season at these reduced N rates. A producer also needs to be aware that the soil nitrate level may change by the time they receive the test results. Most labs have results in several days, but the actual soil nitrate level may be less than the test results if a heavy rain has fallen between soil collection and results received. Also, this test only determines nitrate-N. If a large amount of ammonium type of nitrogen has recently been applied and has not converted to nitrate, the test will not include this N in the results.

For those with Internet access, another tool to estimate N loss is a software program on Ohioline. This program will ask for inputs on location in the state, soil type, saturated soil events, type of N fertilizer, time of application, and so forth. The software uses long term weather averages as defaults in estimating N events. Selecting 'Nitrogen Transformation and Loss' at http://ohioline.ag.ohio-state.edu/lines/farm.html#SWARE will download the program.

If you do not have Internet access, another tool to estimate N loss is a point system developed at the University of Minnesota. This system has been modified to fit Ohio conditions. It also asks a series of questions and assigns a point value depending upon the answer. The probability of a response to additional N increases with more points. The questions and points are given on the back of this page:

FACTOR 1: What N product was used?

Anhydrous ammonia with N-Serv 2 points
Anhydrous Ammonia 3 points
Other fertilizer banded 4 points
Other fertilizer broadcasted 5 points

FACTOR 2: When was the fertilizer N applied?

After April 20 3 points
Before April 20 5 points

FACTOR 3: What has been the predominate soil moisture status in
the field this spring?

Normal soil conditions 1 point
Wet soils 3 points
Standing water/saturated soils 4 points

FACTOR 4: What is the crop's current condition?

Green plants/>12" tall 1 point
Green plants/<12" tall 2 points
Chlorotic plants/<12" tall 3 points
Chlorotic plants/>12" tall 5 points

Total the score for the four factors and use the following guidelines:

Less than 11 No supplemental N recommended
11 16 Evaluate again in 4-7 days
17 or more Add an additional 40-70 lbs. N/acre

The "re evaluation" option is only viable until you no longer have side-dressing options. Illinois research has found that 50 lb N/acre as a supplemental N rate was satisfactory for a wide range of conditions. While a total score of 17 18 would merit a 40 lb/acre N recommendation. A total score of more than 18 may require a higher N rate.

Keep in mind that good judgment is still important when using various methods to estimate N needs. Also, each field needs to be evaluated individually. The PSNT, N software and point value system are intended to serve as guides and not as definitive determination of N needs.

E) Slug Update - Hammond and Eisley CORN Questions

Having experience a significant egg hatch in Ohio, we are now getting reports of slug injury from various parts of the state. Reports indicate that the slugs are quite small and numerous, which suggests they are mostly juvenile gray garden slugs. These reports are to crops planted in late April or early May before the rains came, and the injury is from defoliation. These slugs are pinkish-grayish in color and currently quite small. We currently have not received reports of problems with marsh slugs which are small black slugs covered with clear and moist slime, or dusky slugs, that are usually larger slugs of an orange, reddish to brown color. Growers should expect defoliation to increase as the slugs grow in size over the next few weeks. Slug injury should be relatively easy to see on the plants, and the small slugs can be seen when looking just underneath the residue at or just below the soil surface. Whether to treat is the major question. If the crops appear to be growing out of the injury with new leaves being fairly free from feeding injury, and if the weather forecast is for good growing conditions, treatment should probably be held off. However, if new leaf material is being fed on, and weather continues to be cool, wet, and overcast, treatment might be warranted. The use of Deadline MPs or another molluscicide bait should be used at the recommended rate.

Fields that have not been planted are a different story. The next few weeks are the worst times for slug problems, with the biggest concern being when the crop germinates and begins to emerge. Slugs will be even greater problems if the seeds are planted in less than ideal conditions, resulting in seed furrows that do not close well. Slugs will crawl up and down the seed furrows eating to their heart's content. Growers who have had stand problems caused by slugs in the past should check their fields carefully. If a large number of slugs are present in a field with a history of stand problems, the growers could consider an at-planting time molluscicide application.

For those growers who experience a stand reduction because of slugs resulting in the need for replanting, remember that the slugs will not only still be there, but they will be bigger and consuming greater amounts. Thus, a replanting that is necessary because of slug feeding might need a molluscicide bait application even more so!

F) Guidelines for Replanting Corn - Thomison CORN Questions

The ponding and flooding of corn fields caused by heavy rains during the past week have resulted in questions about replanting. Don't make a final assessment on the extent of damage and stand loss too quickly. For more information on assessing injury to corn from flooding and ponding, check last week's C.O.R.N. article. Replant decisions should be based on strong evidence that the returns to replanting will not only cover replant costs but also net enough to make it worth the effort. The following are some guidelines to consider when making a replant decision.

If the crop damage assessment indicates that a replant decision is called for, some specific information will be needed, including:

Original target plant population/Intended plant stand
Plant stand after damage
Uniformity of plant stand after damage
Original planting date
Possible replanting date
Likely replanting pest control and seed costs

To estimate after damage plant population per acre, count the number of viable plants in a length of row that equals 1/1000 of an acre and multiply by 1000. (Table 1 shows row length needed for various row widths.) Make several counts in different rows in different parts of the field. Six to eight counts per 20 acres should be sufficient.

Table 1. Row length required to equal 1/1000 acre when corn is planted at various row widths.

A major consideration in making a replant decision is the potential yield at the new planting date and possibly different planting rate; this can vary depending on the hybrid used, soil fertility and moisture availability. Tables 2 and 3 show effects of planting date and plant population on final grain yield for the central Corn Belt. Table 2 is a newer chart developed by Dr. Emerson Nafziger at the University of Illinois that includes earlier planting dates and higher optimum plant populations. Table 3 is based on older data from the 1970's, but it still provides a reasonable assessment of potential yield losses, especially for planting dates in June. Grain yields for varying dates and populations in both tables are expressed as a percentage of the yield obtained at the optimum planting date and population.

Table 2. University of Illinois replant chart developed under high yielding conditions (adapted from Nafziger, 1995-96)

Table 3. Central Corn Belt grain yields for corn planted at various dates and population rates,
expressed as a percent of optimum planting date and population yield (uniformly spaced within row).

Here's how these tables might be used to arrive at a replant decision (Table 2 will be used in this example). Let's assume that a farmer planted on May 9 at a seeding rate sufficient to attain a harvest population of 30,000 plants per acre. The farmer determined on May 28 that his stand was reduced to 15,000 plants per acre as a result of saturated soil conditions and ponding,. According to Table 2, the expected yield for the existing stand would be 79% of the optimum. If the corn crop was planted the next day on May 29, and produced a full stand of 30,000 plants per acre, the expected yield would be 81% of the optimum. The difference expected from replanting is 81 minus 79, or 2 percentage points. At a yield level of 150 bushels per acre, this increase would amount to three bushels per acre which would probably not justify replanting costs.

Keep in mind that replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season will be based mainly on plant stand and plant distribution. Later in the season as yield potential begins to decline rapidly because of delayed planting, calendar date assumes increased importance.

It's also important to note plant distribution within the row. Remember that the values in Tables 2 and 3 are based on a uniform distribution of plants within the row! Add a 5% yield loss penalty if the field assessment reveals several gaps of 4 6 feet within rows and a 2% penalty for gaps of 1 3 feet. Yield loss due to stand reduction results not only from the outright loss of plants but also from an uneven distribution of the remaining ones. The more numerous and longer the gaps between plants within the row, the greater the yield reduction.

When making the replant decision, seed and pest control costs must not be overlooked. Depending on the seed company and the cause of stand loss, expense for seed can range from none to full cost. As for the correct hybrid maturity to use in a late planting situation, continue to use adapted hybrids switching to early/mid maturities, if necessary, depending on your location in Ohio. For more details on this issue consult a Purdue/Ohio State publication entitled "Delayed Planting & Hybrid Maturity Decisions". It's available online at: http//www.agry.purdue.edu/ext/pubs/AY-312-W.pdf

You also need to review herbicide and insecticide programs under late planting conditions. For instance, it may be necessary to reapply herbicides, especially if deep tillage is used. However, try to avoid such tillage depending instead on postemergence chemicals or cultivation for weed control. With regard to reapplying a soil insecticide if replanting is necessary - check the label restrictions on the soil insecticide used during the original planting operation. Most soil insecticides cannot be applied more than once per year. Consequently, another product may need to be used when replanting. Also, remember that later planting dates generally increase the possibility of damage from insects such as European corn borer (ECB) and corn rootworm beetle. Therefore it may be appropriate to include the cost of a post emergence insecticide application. To avoid the potential cost of an insecticide application for ECB control, it may be advantageous to replant with a Bt corn.

The cost of replanting will differ depending on the need for tillage and chemical application. The cost and availability of acceptable seed will also be considerations. These factors must be weighed against expected replanting yield gains. If after considering all the factors there is still doubt as to whether or not a field should be replanted, you will perhaps be correct more often if the field is left as is.

The following are some additional on-line sources of information on making replant decisions in will. These include worksheets that provide a step-by-step procedure for assessing the dollars gained or lost from replanting.
Nielsen, Bob. 2002 (rev). Estimating Yield and Dollar Returns From Corn Replanting. Purdue Univ. Cooperative Extension Service publication AY-264-W. Online at http://www.agry.purdue.edu/ext/pubs/AY-264-W.pdf. [URL verified 5/1/03].
Nielsen, Bob and Peter Thomison. 2002. Delayed Planting & Hybrid Maturity Decisions. Purdue Univ. Cooperative Extension Service publication AY-312-W. Online at http://www.agry.purdue.edu/ext/pubs/AY-312-W.pdf. [URL verified 5/1/03].
Univ. of Illinois, Crop Sciences Dept. Replant Decision Aid. Online at http://web.aces.uiuc.edu/aim/IAH/ch2/replant.html. [URL verified 5/2/03].

G) Wheat Powdery Mildew Spreading - Patrick Lipps CORN Questions

Powdery mildew was detected in fields of susceptible varieties a few weeks ago. Powdery mildew requires high relative humidity and temperatures in the 55 to 70 degree F range for spore development and spread. Over the past two weeks the wet weather has provided high relative humidity that favors the disease. Additionally, the forecast for the next 10 days is for continued cool weather and above normal precipitation. We suggest that you scout your wheat fields for powdery mildew at this time.

Yield losses as high as 20 to 25% occur when powdery mildew is present on the second leaf (leaf below the flag leaf) before heading growth stage on susceptible varieties. More than half of the wheat varieties grown in Ohio are susceptible to powdery mildew. Scout fields by pulling 50 tillers randomly from the field and look for the presence of the white mildew lesions on the top leaf (flag leaf) and the second leaf. If 1% of the leaf area has lesions (just 2 to 3 mildew lesions) on these top two leaves then a fungicide application may be justified. Tilt is the better fungicide for control of powdery mildew of the materials available and Ohio has a Section 24(c) special local need label for Tilt (4.0 oz/A) to be applied through Feekes growth stage 10.5 which is full head emergence. This growth stage ends at beginning of flowering.

Most of the wheat fields in southern Ohio are already past the time when a fungicide can be applied because they are already in flower. However, fields in central and northern Ohio are not that far along and a fungicide could be used. It is important to consider the economics of fungicide use so when making the spray decision consider a Tilt application will cost about $10 per acre with application costs additional. At $3.00 per bushel wheat price, the fungicide would have to save about 5 bu/A for it to pay for itself. On a 70 bu/A wheat crop a 10% yield loss would be 7 bu/A. Economics is one of the reasons we suggest growers focus on fields planted to susceptible varieties. Fungicides are not economic when sprayed on resistant and moderately resistant varieties. Specific recommendations for fungicide use on wheat can be obtained at http://ohioline.osu.edu/b785/b785_9.html.

Powdery Mildew on Wheat

H) Physiological Fleck Symptoms on Wheat - Patrick Lipps CORN Questions

When scouting wheat fields for diseases you will likely find varieties with vary small round yellow fleck symptoms scattered over the leaf surface. These flecks are not noticed until the leaves are closely looked at and they are particularly evident when the leaf is held up to the light. These flecks are not caused by disease, but they sometimes are confused with disease symptoms. About half the varieties grown in the state have these physiological flecks, and the intensity of the flecking varies with variety. Over the past week the intensity of the fleck symptoms has increased greatly. This may be due to the overcast weather conditions we have been experiencing. Little is known about the nature of these flecks or the effect they may have on variety performance. It is interesting to note they occur on some of our highest performing varieties.

Physiological Fleck on Wheat

I) Wheat Fusarium Head Scab Risk Update - Pat Lipps and Dennis Mills CORN Questions

The wheat in southern Ohio is in full flower and fields in central Ohio will be in flower this week. The flowering period of the crop (when the anthers are visible on the heads) is the critical time when wheat may become infected by Fusarium resulting in head scab. We have been keeping track of the wheat growth stages through the help of the County Extension Agents and various Ag Industry personnel. We are also monitoring the weather conditions and running our Head Scab Risk Assessment models to help predict when field are at risk of disease development. Our first prediction models are used to determine if Fusarium is producing spores that may infect the wheat. This model uses the 7 day weather data that occurs before flowering. This model has indicated that the wheat in Southern Ohio that has flowered early last week (May 11 to 13) is in risk of head scab development because of the favorable weather for spore development. However, the wheat that went into flower later in the week (May 14 to 18) has less of a risk due to the cooler temperatures. Over the next week we will provide an update on the head scab risk for other areas of the state as the crop reaches the flowering growth stage in various counties. Growers can obtain updated scab risk information from the Ohio Field Crop Disease web site (http://www.oardc.ohio-state.edu/ohiofieldcropdisease). We will have another update in the next C.O. R. N. message next week.

Wheat Head Scab Risk Predictions (Pre-flowering Model) for May 13 to 18

Location Early flowering
Jackson Moderately high risk
Oxford High risk
South Charleston Moderately low risk

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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.

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