CORN
Crop Observation and Recommendation Network

September 23 - September 29, 2002
C.O.R.N. 2002-32

In This Issue:

A) Fall Herbicide Treatments For Winter Annual Weed Control
B) Consider Imazaquin Carryover Risk When Selecting Corn Hybrids
C) Phosphorus And Potash Recommendations For Corn And Soybeans
D) It Is Presently Too Late To Harvest Alfalfa

A) Fall Herbicide Treatments for Winter Annual Weed Control - Mark Loux, Jeff Stachler, and Anthony Dobbels CORN Questions

Many weeds that emerge in late summer or fall and survive through the winter, including common chickweed, purple deadnettle, cressleaf groundsel, and marestail, are more easily controlled in the fall when they are small. Biennial weeds that emerge in spring and early summer, such as poison hemlock and wild carrot, are also more easily controlled in the fall. Control of these weeds in the fall has the following benefits:

• prevents the dense mat of chickweed and other winter annuals that slow soil drying and warming and interfere with planting and tillage in the spring
• reduces the weed cover that can harbor insects
• dandelion is effectively controlled in the fall, which prevents it from interfering with early crop growth the following spring. Herbicide treatments in spring are somewhat variable in their effectiveness on dandelion
• control of purple deadnettle and several other winter annuals prevents them from serving as hosts for soybean cyst nematode
• may reduce the rate of development of herbicide resistance in marestail and other weeds that appear to become readily resistant to glyphosate and ALS inhibitors
• prevents winter annuals from producing seed, reducing problems in the future

Fall herbicide treatments are an extremely effective tool for managing winter annuals, biennials, and simple perennials (dandelion, curly dock, and chicory). However, fall treatments should probably be used primarily where these weeds are a problem. If spring herbicide treatments have been managing winter annuals, biennials, and simple perennials well enough in certain fields, and their populations do not appear to be increasing, there may be little benefit to fall treatments in these fields. Fall treatments may not reduce the need for a burndown treatment the following spring, especially if crop planting is delayed. Producers that use a program consisting of preplant herbicides with residual (Canopy XL, Python, Sencor, Valor, Backdraft, etc) followed by postemergence herbicides will generally maximize the period of residual activity by applying in spring rather than fall.

The dry weather we have experienced in late summer may have resulted in fewer winter annuals in many fields, but fields where they have been a problem over the past several years should be considered as good candidates for fall herbicide treatments. Some general suggestions to maximize effectiveness of these treatments follow:

• Optimum time of application for control of emerged winter annuals is between mid-October and mid-November. We have applied as late as the first week of December under very cold conditions, and still achieved effective control. However, we suggest applying herbicides under relatively warm conditions if possible. Apply by early November when trying to control perennial weeds.
• When using residual herbicides (Canopy, Backdraft, Python, etc) with the goal of controlling weeds as long as possible into the following crop, delay application until mid-November to minimize herbicide degradation.
• We recommend including 2,4-D with all treatments, to reduce the risk of herbicide resistance and help control dandelions and a few other weeds. A rate of 1 pint/A should be adequate with most treatments.
• Treatments with glyphosate should be applied with ammonium sulfate. Include surfactant if recommended by the glyphosate product label. Most other treatments should be applied with crop oil concentrate.
• If possible, wait a minimum of a week or so after crop harvest to apply herbicides, which allows time for crop residue to settle and maximizes contact of spray with weeds.

We have conducted studies on the effectiveness of fall herbicide treatments over the past several years, and these have included most of the treatments that manufacturers are currently promoting. Some brief information on their effectiveness follows. This list does not include every possible combination of herbicides, but should provide an idea of relative effectiveness. Note on glyphosate formulations - a rate of 0.75 lb acid equivalent per acre corresponds to the following: Roundup WeatherMax - 22 oz; Roundup UltraMax - 26 oz; Roundup Original, Glyphomax, Touchdown, and most other glyphosate products - 32 oz.

Glyphosate or glyphosate plus 2,4-D: Very effective treatment for control of dandelion and winter annuals. Possibly the most effective treatment for fields where dandelion is extremely problematic. Can be used prior to corn or soybeans. Primary disadvantage is the lack of residual activity into the following spring, which means that two in-season herbicide applications are likely to be necessary. The minimum rate for glyphosate applied alone in fields with dandelion, wild carrot, and other simple perennials and biennials should be 0.75 lb acid equivalent per acre. When applied with 2,4-D, the glyphosate rate can be reduced unless the population density of these species is extremely high.

2,4-D (1qt/A): Economical treatment for dandelion and most winter annuals, but will not control chickweed. Can be used prior to corn or soybeans. Primary disadvantage - no residual activity into the spring. Populations of wild carrot can be resistant to 2,4-D, and control of these will be unacceptable. Cressleaf groundsel and curly dock may not be completely controlled, because they have tolerance to 2,4-D.

Backdraft (glyphosate plus Scepter) plus 2,4-D: Similar to glyphosate plus 2,4-D, with the addition of residual activity into the following season. Scepter appears to antagonize the glyphosate/2,4-D occasionally, reducing control of dandelion primarily. Use before soybeans only.

Canopy XL plus Express plus 2,4-D: Effective treatment for dandelion and winter annuals. Provides residual activity into the following season - the length of this residual varies with rate of Canopy XL applied. Disadvantage - possibility of developing ALS resistance (so use of 2,4-D with this treatment is essentially mandatory) and lack of control of ALS-resistant marestail in the spring. Use before soybeans only. Low rates of Canopy XL may not completely control 2,4-D-resistant wild carrot.

Python plus 2,4-D plus glyphosate: Similar to glyphosate plus 2,4-D, with the addition of residual activity into the following season on certain broadleaf weed species. Python will not provide residual control of ALS- resistant marestail into the spring. Can be used before corn or soybeans.

Sencor plus 2,4-D: Effective on many winter annuals, but rarely provides more than 80% chickweed control in OSU studies (usually conducted in very dense chickweed areas). Sencor appears to antagonize the 2,4-D on dandelion occasionally, reducing the control. Sencor does not control wild carrot. Expect shorter period of residual control compared to CanopyXL or Scepter. Can be applied in fall before corn or soybeans.

Valor plus 2,4-D: Variable on most winter annuals in OSU studies, and ineffective for dandelion and chickweed control when applied in the fall. Provides residual control into the spring, but not on giant ragweed. Can be applied before corn or soybeans.

Simazine plus 2,4-D: Effective on winter annuals and dandelion, but a 2,4-D rate of 1 quart/A may be necessary for effective control of dandelion and large purple deadnettle in problem fields. Use in the fall before corn only.

B) Consider Imazaquin Carryover Risk When Selecting Corn Hybrids - Mark Loux CORN Questions

Take steps to determine the risk of imazaquin (Scepter, Backdraft, Squadron,
Steel) carryover prior to selecting corn hybrids for next year. Labels for imazaquin-containing products indicate the following: corn can be planted 9.5 months after application if at least 15 inches of rainfall is received from 2 weeks prior to last application through November 15. If this requirement is not met, plant only corn hybrids that are tolerant to imidazolinone herbicides (Clearfield hybrids) the following spring.

The fields that are probably most at risk for imazaquin carryover are those where it was applied just prior to the extended dry period that we began to experience in mid-June. Carryover is less likely in fields where imazaquin was applied earlier in spring, since many of these fields could have received substantial rainfall within 4 to 6 weeks after application and soil moisture was plentiful during this period. Aside from the effect of rainfall, imazaquin tends to persist longer in low pH (<5.5) and finer-textured, high organic matter soils. Low pH will therefore increase the risk of carryover in a dry year. Using Clearfield hybrids will eliminate the risk of crop injury from imazaquin carryover.

Most seed companies sell at least a few Clearfield hybrids. However, if it appears that you are at risk from imazaquin carryover, consider contacting your seed supplier soon to avoid any problems purchasing the appropriate Clearfield hybrid for your area.

C) Phosphorus and Potash Recommendations for Corn and Soybeans - Ed Lentz CORN Questions

In many areas the dry weather has caused lower than expected yields for 2002. Since phosphorus and potash rates were applied for higher yields, producers have asked: will these nutrients be available for future crops, and may I reduce application rates for 2003? Phosphorus and potash not removed by the present crop will generally accumulate (build-up) for future crops. However, a soil test will be needed to make adjustments to your overall program. Understanding the relationship between soil levels and crop response should help determine whether your soils have more than adequate, adequate, or deficient levels of phosphorus and potash for next year's corn or soybean crop.

Generally corn or soybeans have more than adequate phosphorus when levels are above 40 ppm (80 lb). Additional applications at or above this level will accumulate in the soil but not benefit the crop, and may even cause environmental concerns. At adequate levels (15 - 30 ppm/30 - 60 lb), additional phosphorus should be applied to equal the amount removed by the crop. For corn, crop removal equals the yield goal multiplied by 0.35; for soybeans, crop removal equals the yield goal multiplied by 0.83. Between 30 - 40 ppm, application amounts less than crop removal would be recommended. If soil levels were below 15 ppm, then applications would include crop removal and a program that raises the overall soil P level. For example, if soil P levels were 10 ppm (20 lb), then a 160 bu/A corn crop would require 85 lb/A P and a 50 bu/A soybean crop would require 65 lb/A.

Potash recommendations follow the same philosophy as phosphorus except consideration is given for soil cation exchange capacity (CEC). Since applied potash may be held more tightly by soils with high CEC, potash rates increase as the soil CEC increases. Regardless of CEC and yield goals, corn and soybean yields would not respond to additional potash at soil potash levels above 200 ppm (400 lb). Corn and soybeans grown on soils that have a CEC < 10 would not respond to additional potash when the soil test level > 150 ppm (300 lb). Tables on page 14 in the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat & Alfalfa guide provide potash rates for corn and soybeans at various yield goals and soil CEC. Many private soil testing laboratories have the capability to make recommendations from this guide upon request. If you are not sure whether a private lab follows these recommendations, a computer program using the tri-state recommendations may be downloaded from the Internet at http://ohioline.osu.edu/lines/farm.html and selecting the software option.

In summary, by understanding your soil tests, phosphorus and potash rates may be reduced without lowering corn and soybean yields. This may be one way to lower input costs following a year of poor crop performance.

More details on soil fertility may be obtained in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, Bulletin E-2567, available through you county Extension Office. Recommendations from this publication incorporate over 40 years of data from field calibration and correlation studies.

D) It Is Presently Too Late To Harvest Alfalfa - Gary Wilson CORN Questions

Producers often want to harvest the fall growth from alfalfa stands, but fall harvesting usually increases the risk of legume heaving and winter kill, and interferes with accumulation of root reserves required for winter survival and growth the following spring. The timeline to have the last regular harvest completed is September 7 in northern Ohio, September 12 in central Ohio, and September 15 in southern Ohio. DO NOT harvest during late September and October. During this period, forages are actively storing reserve carbohydrates in the crowns and roots. These energy reserves are used by the plant to survive over winter, to develop cold tolerance, and to initiate spring growth and growth after each cutting. Research from Iowa indicates alfalfa yields were 0.61 tons per acre lower the following harvest year where a harvest was made during the previous fall. An Ohio study showed plots with alfalfa harvested in the late fall on October 20 showed severe damage with 38 percent of the plants heaved compared to only 5 percent heaved when no crop was harvested on October 20.

If a late fall harvest must be made, it should be delayed until a killing frost (25 degrees F. for several hours). Suggestions to minimize the problems of this late harvest are as follows:

• Alfalfa stand should be well established - not a new seeding.
• Field should be will drained.
• Soil must have a high level of fertility which is maintained with annual fertilization. Fall application of fertilizer, especially potassium, is very important.
• Cut four inches high for this late cut to provide some insulation against winter hazards and to assist in snow retention.
• If possible, also mulch with strawy manure or other appropriate mulching material following the late harvest. This will provide winter protection and reduce heaving caused by alternate freezing and thawing.
• Snow cover acts as an insulator of the soil and plants. Thus this practice of late harvest may be more feasible in areas of northeast Ohio which historically receive larger, longer lasting quantities of snow.

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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: Ed Lentz         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.

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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.

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