CORN
Crop Observation and Recommendation Network

April 1-7, 2002
C.O.R.N. 2002-7

In This Issue:

A) Increasing Soy Profits in 2002
B) Soybeans and Inoculants
C) Managing Marestail
D) New Herbicide Labels

A) Increasing Soy Profits in 2002 - Jim Beuerlein & Anne Dorrance CORN Questions

Two ways to significantly increase soybean profits in 2002 are to improve the crop's health and to reduce seed cost. Using fungicide treated seed on five well drained, highly productive test sites in a corn-soy rotation increased profit by $6.00 to $15.00 per acre in 2001. A seeding rate study at the same sites which used two varieties and six seeding rates indicated that maximum yields were produced with seeding rates of 150,000 to 175,000 seeds per acre and harvest populations of only 125,000 plants per acre. Reducing the seeding rate by 50,000 seeds per acre can save up to $10.00 per acre which is equal to about two bushels of yield. Often stand loss can be reduced by 50,000 plants by using an appropriate combination of seed fungicide treatments which is only half the cost of 50,000 seeds. Therefore, the net benefit of reducing seeding rates and adding fungicides is an extra bushel profit per acre. An additional benefit is a healthier root system that may add even more bushels when fungicides are used.

Root rot diseases are by far the biggest yield reducers in Ohio and Phytophthora is the most important of the root rot diseases. A two-pronged attack can increase plant stands and improve root health. 1) Select varieties with high levels of partial resistance that will give a good level of protection against all strains of Phytophthora and then 2) treat the seed with Allegiance or Apron XL to protect it until the partial resistance kicks in just after emergence. Low rates of Apron XL (0.16 fl oz/cwt) or Allegiance (0.2 to 0.75 fl oz per cwt) have no effect on Phytophthora. Fields which have been replanted and those where stem rot has developed are priority fields for seed treatments. Rates of seed treatments effective for Phytophthora are Allegiance (1.5 fl oz/cwt) and Apron XL (0.64 fl oz/cwt).

B) Soybeans and Inoculants - Anne Dorrance & Dennis Mills CORN Questions

Research at Ohio State University by Dr. Jim Beuerlein, has shown that application of inoculants to soybean seed resulted in increased yields. Inoculants are comprised of a living organism, a bacterium called Rhizobium. This bacterium forms a symbiotic relationship with the soybean roots, which results in nodule formation. The bacterium captures nitrogen and converts it to forms that can be utilized by the soybean plant. Because inoculants are living organisms they are subject to heat, changes in moisture and some of the ingredients in seed treatments. These factors can greatly reduce cell numbers, making them ineffective. Many fields in Ohio need fungicide seed treatment, but not all seed treatments control every disease. Know what disease is causing or likely to cause problems in your field and select the proper seed treatment. This information is available from the Ohio State University Extension bulletin 639 "Seed Treatment for Agronomic Crops" available at county Extension offices, on Ohioline (http://Ohioline.osu.edu) or the Ohio Field Crops Disease web site (http://www.oardc.ohio-state.edu/ohiofieldcropdisease). The optimum method is to treat seed first, then allow the seed treatment to dry. The inoculant is then applied shortly before planting. To maintain the viability of the inoculant once it is applied to the seed it is important to keep it cool and protect it from dramatic changes in the environment. Read the label and follow all precautions and directions when handling seed treatments and treated seed.

C) Managing Marestail - Mark Loux CORN Questions

Marestail is once again becoming increasingly problematic in Ohio soybean production. We remain fairly concerned about its potential to become resistant to herbicides with several sites of action and therefore extremely difficult to manage. Some facts about marestail:

• The common name is officially horseweed, but we generally refer to it as marestail to avoid confusing it with the other "horseweed" (giant ragweed).
  
  
• Marestail is classified as a winter or summer annual weed, and can emerge throughout much of the year. The majority of its emergence occurs in fall, but over the past several years we have observed considerable emergence in spring.
  
  
• Marestail can be difficult to control in late spring when it is more than several inches tall. It can be somewhat tolerant of low rates of 2,4-D (1 pint or less) that can be used within a week of soybean planting. It can also be difficult to control with glyphosate, depending upon its size, weather conditions, and glyphosate rate.
  
  
• About half of the marestail populations we have screened to date appear to be resistant to ALS inhibitors. This is unfortunate, since preplant application of Canopy XL, Python, and FirstRate/Guantlet has typically helped with control of emerged plants and provided residual control. Classic, Synchrony, and FirstRate are the only effective postemergence options, with the exception of glyphosate in Roundup Ready soybeans. All of these herbicides except glyphosate are ALS inhibitors.
  
  
• A number of marestail populations in the eastern United States have been characterized as glyphosate-resistant (not controlled with rates of 4 lbs acid equivalent/A or more), further limiting herbicide options for managing this weed. We recently completed the first run of a greenhouse experiment to characterize the response of 50 marestail populations to glyphosate. While all of the populations were injured initially, a number survived the 26 oz rate of Roundup UltraMax with final control ratings of only about 60%. A few were able to survive the 104 oz rate, but remained severely injured. While we are hesitant to characterize these populations as glyphosate-resistant, it reaffirms that marestail can be a difficult weed to control with glyphosate and that we definitely have variable response among populations.
  
  
• We are currently conducting field studies to develop the best management strategy for marestail. Based on the above facts and other information available, consider the following when trying to manage marestail:
  
  
• Marestail is primarily (but not always) a weed of no-till systems, and tillage can reduce its potential to be a problem.
  
  
• In no-till systems, a late-fall or early-spring application of 2,4-D will control emerged plants that can be the majority of the population some years. The 1-pint rate will probably be effective at these times, but may not be adequate as plants increase in size in the spring. At least one 2,4-D label allows application of 1 lb/A of 2,4-D up to 15 days before planting (see last week's C.O.R.N.).
  
  
• When using a mixture of 2,4-D ester plus glyphosate for burndown close to planting, use a minimum glyphosate rate of 26 oz/A of UltraMax or 32 oz/A of other glyphosate formulations.
  
  
• When applying glyphosate for burndown without 2,4-D, use the following rates based on marestail size: up to 6 inches - 26 oz UltraMax or 32 oz of other glyphosates; 6 to 12 inches - 40 oz UltraMax or 48 oz of others; 12 to 18 inches - 52 oz UltraMax or 64 oz of others.
  
  
• Early preplant application of Sencor or Valor plus 2,4-D ester can control small, emerged plants and provide some residual control (we do not have good information on the length of residual control - our current field studies should help remedy this). Similar results can be obtained with a mixture of 2,4-D ester with Canopy XL, Python, FirstRate, or Guantlet, with the exception of ALS-resistant populations (we suspect the majority of the marestail to be ALS-resistant at this point).
  
  
• Consider a mixture of Sencor, 2,4-D, and Gramoxone Max for burndown of emerged marestail, especially in populations that appear to be ALS-resistant and tolerant of glyphosate.
  
  
• We strongly recommend using other herbicides in addition to glyphosate in Roundup Ready soybean fields, to slow the development of glyphosate-resistant marestail populations. This strategy can include the preplant use of 2,4-D, Canopy XL, Sencor, Valor, Python, and FirstRate in place of or mixed with glyphosate. It can also include mixtures of glyphosate with FirstRate or Classic in postemergence applications.
  
  
• Postemergence herbicides should be applied when marestail is less than 6 inches tall.

D) New Herbicide Labels - Mark Loux CORN Questions

Raptor (BASF) - Our recent article on weed control in forages did not include Raptor, which is now labeled for use in alfalfa. The activity and use of Raptor in alfalfa will generally be similar to Pursuit. Both are broad-spectrum herbicides with activity on grass and broadleaf weeds. Pursuit has a greater residual life in soil than Raptor. Consequently, Pursuit is likely to provide a month or more of residual weed control, which is a benefit compared to the extremely short residual activity of Raptor. However, application of Raptor is likely to result in greater safety to a sensitive crop planted the following year. Pursuit and Raptor have similar activity on winter annual weeds, which are a primary target of spring applications. Raptor is more effective on lambsquarters and ragweeds than Pursuit, and generally more effective on grasses, and might therefore be a better choice than Pursuit later in spring or early summer when these weeds are present.

Option (Aventis) - Option is now labeled for postemergence application to field corn. The active ingredient in Option is foramsulfuron, which is a sulfonylurea herbicide and an ALS inhibitor. A safener is also included in the formulation. Option is similar to Accent with regard to activity on annual and perennial grasses, and is labeled for control of most annual grasses up to 3 or 4 inches tall. Option also controls a number of broadleaf weeds if they are small (2 to 3 inches), but should generally be tank-mixed with a broadleaf herbicide in most situations where broadleaf weeds are present. The label currently states that any crop can be planted 60 days after application.

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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

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