CORN
Crop Observation and Recommendation Network

May 29 to June 3, 2001
C.O.R.N. 2001-15

In This Issue:

A) Estimating Nitrogen Losses
B) Herbicide Injury Warning to Corn Producers
C) Delayed Burndown Applications in Roundup Ready Soybeans
D) Don't Rush Corn Replant Decisions
E) Don't be too Concerned About Leaf Purpling in Corn
F) Wet Weather and Diseases on Wheat: Mildew, Glume Blotch and Head Scab
G) Soybean Management
H) Insect Field Report
I) Field Days and Other Events

A) Estimating Nitrogen Losses (Ed Lentz) CORN Questions

As we approach sidedress time, producers may want to evaluate their nitrogen program - particularly early preplant, weed-and-feed, and fall systems. Producers who used spring anhydrous with N-serve probably have had minimal N losses (5-10% loss). Nitrogen from fall anhydrous probably has converted to nitrate-N and susceptible to loss depending upon the local environmental conditions. The dry weather early in the season, may have allowed volatilization losses from surface applied urea products, such as UAN (15-20% potential N loss). Wet conditions the past two weeks may have caused denitrification in some fields. Denitrification losses depend upon the number of days soils are saturated and the drainage of the field. In well-drained fields, 10 - 20% N loss may have occurred from spring anhydrous without N-serve and 20 - 30% loss from UAN or urea applications. In poorly drained fields, N losses between 20 - 50% may have occurred with anhydrous ammonia and 30 - 60% with UAN or urea. 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 (discussed in last week's newsletter). Any field that has more than 30 ppm nitrate-N should have adequate N for the rest of the season. If it is less than 30 ppm the full rate of N may need to be applied. Some states recommend reduced rates of N between 15 and 25 ppm. Ohio State University research, however, has shown 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.

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 below:

The "re-evaluation" option is only viable until you no longer can sidedress. While a total score of 17-18 may merit 40 lb/acre N, a score of more than 18 may require higher rates. However, Illinois research has found that 50 lb N/acre was satisfactory for a wide range of conditions.

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.

B)Herbicide Injury Warning to Corn Producers (Mark Loux and Jeff Stachler) CORN Questions

Early planted corn in Ohio that was frozen off in early April may be more sensitive to herbicides than expected. Corn that was completely frozen off is physiologically more advanced than is observed at this time. If a corn plant had 1 to 2 leaves at the time of being frozen off and the corn plant is now showing 4 collars (V4), that plant may actually have 5 or 6 collars (V5-V6). The only way to determine the exact stage of these corn plants is to split the plant into two pieces longitudinally. However, before 7 or 8 collars (V7-V8) the nodes are not clearly visible after splitting. These corn plants are also shorter than normal, so add at least 2-4 inches of height above what is observed. Therefore, these fields of corn will be more sensitive to growth regulators, such as 2,4-D, Clarity, Banvel, and premix products containing these herbicides and ALS-herbicides, such as Basis, Basis Gold, Accent, Accent Gold, Hornet, Celebrity Plus, Beacon, Spirit, Northstar, and Exceed. Clarity and 2,4-D products should not be broadcast applied once the corn has reached 8 inches. These products can be applied with drops after this height and until 36-inch corn for Clarity products and until tasseling for 2,4-D products. Clarity products can be broadcast applied up to 24-inch corn if applied at less than 6 oz/A. The ALS products should not be broadcast applied once the corn has reached the 6-collar stage (V6), except for Basis which must be applied before the 3-collar stage (V3) and Basis Gold and Accent Gold which can only be applied up to 12-inch corn or the 6-collar stage (V6) whichever is more restrictive. Most of these products can be applied with drop nozzles after the 6-collar stage (V6) up to about 36-inches, except Basis, Basis Gold, and Accent Gold as mentioned above.

We have some general concerns about the current health of corn in many fields and the risk of corn injury. We see the scenario as follows: corn growth has been at a stand-still in many fields due to waterlogged soils and cool temperatures, but will probably start a phase of extremely rapid growth once soils dry somewhat and warm temperatures occur. Weed growth is likely to be extremely rapid at that time also. As soon as field conditions allow, then, there will undoubtedly be a rush to apply postemergence corn herbicides. However, due to the recent cool, overly wet conditions, the ability of corn to metabolize (detoxify) herbicide and avoid injury is reduced. We suspect that application of growth regulators to corn that is in this condition will lead to brittle stalks and other symptoms once corn begins the rapid growth that is likely to accompany warm temperatures. We have also observed occasionally in the last few years that ALS inhibitors can cause injury to corn when they are applied at the end of a sustained cool period. Our assumption is that, along with some drying of soil, several days of warm weather will be required for corn growth to resume to the point that the detoxifying mechanisms are working adequately. It may work out that corn growth will resume well before soils are dry enough to handle application equipment, which should alleviate injury concerns somewhat. However, we offer the following suggestions:

• Allow corn to recover from the extended cool, wet conditions before applying herbicides. This may require a few warm days in some fields. Look for an improvement in the color of corn and any increase in size that will signify better plant health. Weed control is also likely to improve when weeds are actively growing under warmer temperatures.
  
• Even where stress on corn has been relatively minor (due to more limited rain), delay herbicide applications until after one morning's low temperature is greater than 50 degrees and the daytime temperature is greater than 70 degrees. Most injury seems to occur in the first day that warm temperatures return.
  
• Be sure to check the size and growth stage of corn in each field before applying herbicide. Corn that is growing rapidly may become too big for certain herbicides or herbicide mixtures, or rates of herbicide.
  
• Although weed size may seem to require a considerable amount of herbicide, resist the temptation to apply too "hot" a mix of herbicides to corn. A better strategy would be to plan for a second application, possibly post-directed, if needed.
  
• Corn that has suffered hail damage may also be more susceptible to injury from herbicides. If possible, delay herbicide application until corn has started to show new growth following hail damage. Otherwise, apply under favorable weather conditions.
  
• The good news - by the time we can actually apply postemergence herbicides to corn, there may be little need for much residual activity. By early June, weed emergence is past its peak and corn should be able to outgrow later-emerging weeds without risk of yield loss.
  

C) Delayed Burndown Applications in Roundup Ready Soybeans (Mark Loux and Jeff Stachler) CORN Questions

Many Roundup Ready soybean fields were not treated with glyphosate at the time of planting this year. The strategy in these fields was to make an early postemergence glyphosate application, and follow with a later postemergence application as needed. This can be an effective strategy in some no-till fields, but wet weather has delayed the first postemergence application considerably. Consequently, we have observed a number of soybean fields with large weeds. Be sure to adjust glyphosate rates accordingly. It appears to us that the often-standard 32 oz rate of Roundup Ultra, Glyphomax, and Touchdown IQ (or 26 oz rate of Roundup Ultra Max) will not be sufficient in this situation. We suggest increasing rates to 48 or 64 oz of the 4 lb/gal formulation, or 39 to 52 oz of Ultra Max. In our opinion, adjusting the glyphosate rate to get effective control of large weeds with the first application is a better strategy than relying on the second glyphosate application to "finish off" weeds that were only suppressed initially.

D) Don't Rush Corn Replant Decisions (Peter Thomison) CORN Questions

As fields begin to dry out this week, many growers will be evaluating their corn stands to determine whether to replant. 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, consult 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).

*Adapted from NCH-30 "Guidelines for Making Corn Replant Decisions"

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.

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.

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.

Concerning insect control, if insecticides were applied in the row at initial planting, consider re-application if tillage is used before replanting. Also remember that later planting dates generally increase the possibility of damage from insects such as European corn borer, corn rootworm beetle, and black cutworm. Therefore, understand 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 yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.

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.

Sources: Guidelines for making corn replant decisions. 1990. National Corn Handbook. NCH-30; Illinois Agronomy Handbook. 1995-96. University of Illinois.

E) Don't be too Concerned About Leaf Purpling in Corn (Thomison) CORN Questions

During the past week I have received several reports concerning leaf purpling in corn. Weather conditions (cool, saturated soil conditions) are probably responsible for much of this abnormal leaf purpling. While leaf purpling is often associated with phosphorus deficiencies, any stress that inhibits root growth and development can lead to purpling of the leaves. Root growth can be hindered by factors such as cool soil temperatures, crop injury, soil compaction, drought stress, nutrient deficiency, soil-borne diseases, root-feeding insects, current year's herbicide, or herbicide carryover. Development of the permanent (crown or secondary) root system is more sensitive to temperatures than is that of leaves and is more likely to be adversely affected by temperature swings than leaf tissue.

If utilization of sugars (produced by photosynthesis in leaf tissue) is reduced by a restricted, slowly growing root system, then large amounts of sugar build up in the leaves. Such an "overload" of sugars in above ground tissue triggers production of anthocyanin, a purple pigment, and results in purple plants.

Leaf purpling may also be influenced by hybrid genetics - or more specifically, differences in hybrid sensitivity to varying environmental conditions. During early vegetative growth some corn hybrids are more likely to show purpling following periods of cool, cloudy weather or hot, dry conditions. Certain hybrids may show more purpling than others if they contain more anthocyanin producing genes. Genetic purpling itself will usually have no effect on grain yield. Leaf purpling should slowly disappear as temperatures return to normal levels.

The cause of the leaf purpling, not the purpling itself, will determine whether yield loss will occur by harvest this fall. If the cause of the root restriction is temporary (e.g., cool temperatures), then the purpling should disappear as the plants further develop and yield losses should be minimal if any. Much of the purpling we've seen recently should disappear as soils warm up and dry out. If the cause of the root restriction continues to affect plant growth all season (e.g., soil compaction), then the purpling may continue for some time and some yield loss may result.

F) Wet Weather and Diseases on Wheat: Mildew, Glume Blotch and Head Scab (Pat Lipps and Dennis Mills) CORN Questions

Disease prevalence has changed drastically in the state at this time. Because of the timing of the wet weather and disease development in relation to the growth stage of the crop there is little producers can due about disease problems from this time forward. Fungicides like Tilt and Quadris can be applied through Feekes' growth stage 10.5 only. Thus, these fungicides will be off label when the flowering period ends. There may be time for a fungicide application onto some fields in northern Ohio. Know the variety, its susceptibility to disease and scout the fields for disease before making any fungicide application.

Powdery mildew continues to develop in many fields in north central Ohio. At Wooster the flag leaves of susceptible varieties have 10 to 20% leaf area covered with mildew lesions. The threshold for a fungicide application is 1% on the leaf below the flag leaf on susceptible varieties. Fields with powdery mildew on the flag leaf will likely have 10 to 25% yield losses if the disease continues to develop during the grain filling period. The only thing that will stop mildew development now is hot (above 80 F) weather. For more information on powdery mildew see the following web link: www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/mildew.htm

Stagonospora leaf and glume blotch and Septoria leaf blotch are becoming common in many wheat fields. Both of these diseases have had a slow start this year because of the dry conditions in April and early May. Recently, the consistent rain showers have favored the spread of spores of these fungi by splashing them up onto leaf surfaces. Septoria leaf blotch appears more common than usual this year because it is favored by the cooler temperatures. Stagonospora leaf and glume blotch is becoming a serious threat to the wheat in southern Ohio and is showing up in northern Ohio as well. Stagonospora will continue to develop in fields throughout the rest of the season as long as it keeps raining. Stagonospora was present on the leaf below the flag leaf of about 50% of the tillers examined of susceptible varieties at Wooster, OH late last week. We suspect Stagonospora glume blotch to be severe during grain fill if we receive 2-3 days of rain over the next 2 to 3 weeks. For more information and pictures of these diseases see the following web links: www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/septoria.htm and www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/stagonospora.htm

Head Scab is favored by wet conditions during the 7 days preceding flowering and several days of rain during the flowering period if the temperatures are above 60 F. We have had plenty of rain in most areas of the state for head scab development, but the temperatures have remained below the 60 F threshold. We have collected spores of Fusarium fungus from the air above corn residues last week. There is a likelihood of some disease developing in areas with sufficient spores and not so favorable temperatures. It is impossible to determine the level of disease that may develop at this time because so much depends on the weather that occurs after flowering. We are using a disease forecasting model to estimate the potential risk from head scab in different regions of the state. So far it appears that the risk of severe scab development is low in southern (Below I-70), northeast and in the northern third of Ohio. Southern and northeast Ohio were dry the week before flowering which probably prevented spore development. The northern third of Ohio was wet, but the temperatures were too cool for significant spore development. West central and Central Ohio (north of I-70 and south of State Rt. 30) appears to have had the right combination of environmental conditions for scab development. We would indicate that counties in this area have a moderate risk of scab.

We have developed a web site for our wheat head scab risk assessment validation, which can be viewed on the Ohio Field Crop Disease web page: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/. Just click on the area entitled '2001 Wheat Head Scab Risk' for a description of the two models and a map of Ohio showing risk areas. We have collected sufficient data to complete Risk Assessment Model 1 that incorporates weather data for the 7 days prior to flowering. We will be updating the risk assessment maps for Model 2 as we accumulate post flowering weather information.

May 28, 2001 - Head Scab Risk Assessment
Southern Ohio: low risk
West central and Central Ohio: moderate risk
Northwest and north central Ohio: low risk
Northeast Ohio: low risk

G) Soybean Management (Ed Lentz) CORN Questions

Similar to last year, wet and cool conditions since Mother's Day have prevented some producers from planting soybeans and poor emergence in stands planted earlier. For those that are not planted, a full-season variety would be preferred over an early variety (2.7 to 3.5 for northern Ohio and 3.0 to 4.0 in southern Ohio). Data collected from Northwestern O.A.R.D.C. branch has shown that a group 2.1 variety planted end of May yielded 15 to18% less than a group 2.6, 3.0, and 3.6. Seeding rate should possibly be increased for late May plantings -- approximately 200,000 pure live seeds per acre should be adequate.

Soybeans yield well at a wide range of populations. In general for 7 to 8 inch row spacing, a population of 1 to 2 plants per row foot should not need replanting.
On-farm research in Northwest Ohio has shown that soybeans had similar yields with final populations between 125,000 and 200,000 plants per acre. Keep in mind that soybeans planted in early May also tend to yield better than late May/early June plantings. Research at the Northwestern O.A.R.D.C. branch has shown approximately 20% reduction in yield for the late May planting date compared to early May. This difference would be less for southern Ohio. If populations justify replanting, make sure the problem for early stand loss has been determined and corrected before replanting.

H) Insect Field Report (Bruce Eisley) CORN Questions

Corn - Cutworm damage is still being reported in some areas of the state. The cutting is above ground and worms range in size from 1" to over 2" in length. The larger worms are done feeding while the smaller worms will continue to feed.

Armyworm is being reported in some corn fields. Fields no-tilled into cover crops or grassy fields need to be checked. Also check fields located next to wheat fields as armyworms may start to move from wheat into the corn. Threshold for armyworm in corn was included in last weeks newsletter.

Wheat - Reports of armyworm in wheat are coming in from all areas of the state. The number of worms is quite variable from one field to the next so each field needs to be checked. Some fields were treated last week and others will be treated this week. The worms range in size from 3/4" to 1- 1/2" in length. The larger worms have almost completed their feeding cycle and will not do much more damage whereas the smaller worms will be around for a few days and will continue to feed. Rescue treatment for armyworm in wheat is recommended if 6 or more armyworm larvae are found per 1 foot of row or if head cutting occurs.

I) Field Days and Other Events CORN Questions

"Just in Time" Crop Management Decision Making Information

CCA and pesticide applicator credits
For additional information contact OSU Extension at 740-349-6900 or
e-mail www.ag.ohio-state.edu/~lick/

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

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: Clark Hutson        Web Editor: Tom Rosati

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868

  | C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |