Vol. 20, No. 1, March 1, 2005

In this Issue
Starting up the 2005 CAT Alert season
Soybean aphid prediction for 2005
Asian soybean rust
Asian soybean rust and federally facilitated crop insurance
Applying fungicides for the control of Asian soybean rust
Sprayer tune up
Early spring assessment of wheat stands
2004 Comparison of commercial weed control programs in corn
2004 Economics of commercial weed control programs in soybean
Selection of glyphosate-resistant weeds
2005 Weed Control Guide for Field Crops
Managing higher priced fertilizer inputs

Next issue: April 7

Joy N. Landis, editor and Rebecca Lamb, assistant editor

We are publishing our first issue of the CAT Alert a couple of weeks early this year to better accommodate delivering information you need about soybean rust. This issue offers an update on what to expect about its potential occurrence, tips for effectively applying fungicides and considerations about applying for crop insurance. Also in this issue, a report on a forecasting system for soybean aphid populations, tips for assessing wheat stands and several articles to reference as you plan weed management for 2005.

It’s clear we have a busy season ahead delivering the latest research findings to you for your consideration. This is the start of an 18-issue season. We wish you the best as you begin your field season. 

Christina DiFonzo
Entomology

Soybean aphid (SBA) has been with us since 2000. Every winter, growers ask if researchers can predict what will happen with SBA the next season. For 2005, we believe there is a potential for aphid outbreaks across the Midwest. Is this based on throwing darts at a board? Calling the psychic hotline? A late night party featuring tequila and limes? Or is this prediction based on science? Let’s review….

Dr. Dave Voegtlin, an aphid geek with the Illinois Natural History Survey, runs suction traps across Illinois to capture and count winged aphids. These traps are constructed of PVC with an opening at the top and a fan at the bottom to draw in air from 20 to 30 feet above the ground. Insects are pulled into a collection jar, and the jars are changed weekly. Dr. Voegtlin’s laboratory then sorts, identifies and tallies important aphid species (for example, aphids from small grains or corn) over the season.

Recall that SBA cannot survive the winter in soybean fields. At the end of the season, a generation of winged males and females leaves soybean for buckthorn, the overwintering host. On buckthorn, the female migrants give birth to fat, wingless daughters that mate with the males and deposit eggs along buds. In the spring, the eggs hatch and SBA spends several generations on buckthorn before winged migrants recolonize soybean.

At the end of the 2001 field season, the first full season with SBA and a heavy aphid year, Dr. Voegtlin found SBA in his suction traps in September and October. However, the number of males and females migrants was very low, despite the heavy SBA populations in soybean. In 2002, SBA numbers on soybean during the field season were also fairly low. Yet, at the end of that season, the trap catch of winged migrants in September and October was surprisingly high. Some researchers were able to find overwintering eggs in the field that fall. By the spring of 2003, we knew we had a problem; 2003 was a tremendous aphid year – an estimated 40 percent of the soybean acreage in Michigan was treated. And yet, at the end of the season, the Illinois group trapped virtually no soybean aphid males and females heading back to buckthorn. Was there a pattern to all of this? Dr. Voegtlin thought so. At an Entomology meeting in December 2003, he predicted a low aphid year for 2004, based on the lack of an overwintering population. As predicted, SBA numbers were extremely low during the 2004 season.

The big question – what happened at the end of 2004? Illinois’s suction trap catches of soybean aphid were very high in September and October. According to the pattern of fall trap catches mirroring spring aphid numbers, 2005 could be a big year for SBA. In addition to the trapping data, researchers have found eggs on buckthorn in Ontario, Ohio, Indiana, Illinois and Michigan and likely other states as well. In Ontario, the egg load is particularly high. Eggs are small and difficult to find in nature, so populations must be large for many different people across many locations to find eggs so easily.

Bottom line – 2005 is the year to test our predictive abilities. And just in case this pattern works, Dr. Voegtlin is coordinating a project to build suction traps for several other states, including Michigan. Our tentative plan is to place a trap in the southwest (perhaps Kellogg Biological Station new Hickory Corners), on the MSU campus and in Saginaw County at the bean and beet research farm. This should provide enough coverage across the state to get a general idea of aphid flight in the fall.

Patrick Hart
Plant Pathology

In November 2004 soybean rust was identified on soybeans in at least nine states in the southern United States. It is expected to overwinter on alternate hosts in the southern United States, primarily kudzu, and spread to the Midwest in 2005 (http://www.aphis.usda.gov/ppq/ep/soybean_rust/index.html). The rapid spread of soybean rust and the potential for severe yield losses makes this the most destructive foliar disease of soybean in the United States.

Asian soybean rust, Phakopsora pachyrhizi, significantly reduces yields in Asia, Africa, and South America. In areas where the pathogen commonly occurs, yield losses up to 80 percent have been reported (http://www.apsnet.org/online/feature/rust/). The pathogen had been limited to the Eastern hemisphere until it was found in Hawaii in 1994. Since then, the distribution of P. pachyrhizi includes Africa, Asia, Australia, South America and Hawaii. There has never been a soybean disease in the United States with the potential economic destructive power as soybean rust. Soybean rust will have a major impact on both total soybean production and production costs in the United States and Michigan. A conservative prediction indicated yield losses greater than 10 percent in nearly all the United States soybean growing areas with losses up to 50 percent in the Mississippi Delta and southeastern coastal states. These estimates are for large geographic areas – individual fields would vary from 0 to 80 percent losses. These estimates do not include the cost of fungicides and applications.

Resistant or tolerant soybean varieties are not available, nor likely to be available in the near future, making management with fungicides the only viable option available to growers. The Michigan Department of Agriculture applied and has received approval for at least five fungicides under the Section 18 Quarantine Emergency Exemptions. Application timing will be critical. The first fungicide applications should be made shortly before the rust is expected or immediately after soybean rust symptoms are evident. Unfortunately, identifying soybean rust is difficult in the early stages because of similarity to several other common diseases occurring at the same time. In Brazil, identifying soybean rust early continues to be a challenge for growers. However, after my recent trip to Brazil I think this can be accomplished with proper training

What nobody knows is how soon or how fast the soybean rust will move north this spring and summer. Soybeans are planted in southern states in March and April, and in Michigan, most soybean plantings occur during late May and early June. Even under the most optimum conditions it is unlikely that soybean rust would move into Michigan before the middle of July. But at this point no one really knows what is going to happen or when. The best defense against soybean rust is keeping informed about the problem, so that if it does arrive in Michigan in 2005, we will be prepared. It is important to remember that soybean rust may not make it to Michigan in 2005, or it could come in so late that it does not become an economic problem. For these reasons, growers should follow their normal agronomic practices for soybeans. This is especially true when it comes to buying fungicides. Chemical companies are assuring us that we will have more than adequate supplies of fungicides available for Michigan in 2005, and early stock piling is not necessary.

Detailed information on soybean rust, including alternate host crops, how and where the pathogen overwinters, and the current status of the disease in the United States can be found at http://www.ncpdn.org/.

Click on “first detectors” (See navigation bar on the left of the screen.) and then on Michigan to find links to Power Point presentations on soybean rust. Information on fungicides available for soybean rust, our current state of knowledge on the best way to manage soybean rust with fungicides, and links to other important sites can be found here.

Plant pathologists around the country and at Michigan State University are gearing up so that we will be prepared to deal with soybean rust on a national and regional scale. In 2005, the Field Crops CAT Alert publication will provide weekly updates on the soybean rust situation in the United States and what it means for Michigan. By April, web sites will be up and running that will predict when and where soybean rust will occur. Other sites will provide information on where soybean rust has actually occurred. As the addresses for these sites become available the Field Crop CAT Alert will make this information available, in addition to posting the information at http://www.ncpdn.org/

A manual on fungicide management of soybean rust has been put together by plant pathologists in the North Central Region, and will be available as a hard copy and on the web in the very near future. This rather long document will be reduced to an easy-to-read two to four page bulletin. More information on soybean rust can also be found at the Michigan Farm Bureau, and the Michigan Soybean Promotion Committee web sites. During the coming month Michigan State University will continue to prepare materials that will assist growers in making management decisions related to soybean rust. Michigan State University will also be continuing with research and extension activities over the summer.

Roy Black
Agricultural Economics

Should the emergence of Asian soybean rust as a risk factor in soybean production influence a farmer’s crop insurance purchase decision? Put another way, if a farmer had not previously purchased crop insurance, should the risk of getting rust lead to a re-examination of the insurance purchase decision? This note does not focus on the additional risk exposure, only on the conditions necessary for indemnification of losses should rust occur.

According to the USDA’s Risk Management Agency, which administers the federal crop insurance program:

1)    Asian soybean rust is an insured peril under the federal crop insurance program.

2)    Insured producers are expected to use good farming practices and follow the recommendations of agriculture experts to control this fungus. Cost is not a consideration. This is true regardless of their plan of insurance: individual or group, production or revenue.

3)    Registered fungicides are available to control Asian soybean rust. While disease is an insured peril, damage due to the insufficient or improper application of disease control measures is not covered. However, if it can be documented that “…there are insufficient amounts of chemicals available for effective control, the resulting loss of production is covered.”

4)    Producers are responsible to keep informed of soybean rust outbreaks in their area. RMA expects producers to take recommended measures to control or prevent the disease impact if an outbreak is anticipated or already in the area.

Several points are clear from the discussion. The definition of what is a generally accepted practice becomes crucial when a new risk appears; these are being worked out and fine-tuned. Infrastructure to support detection and action is a big issue, particularly in areas such as Michigan, and there will be some breathing room – initially – as evidenced by the statement “… if there are insufficient amounts of chemicals available for effective control, the resulting loss of production is covered.” But, in the event of a lack of capacity, producers must have good documentation that every effort was made to control the problem and that detection methods were timely.

These are points that should be reviewed with an insurance agent, if insurance is purchased, so there are no surprises about what is expected and steps that must be followed. Agents are receiving updates from insurance companies and are in the best position to assess expectations.

It appears that farm yield and revenue trigger crop insurance policies do reduce the risk exposure associated with Asian soybean rust and there appears to be a good deal of common sense in the discussion as the March 15 insurance sales closing date approaches.

Producers should work with their insurance companies and crop insurance agent as well as document their actions including any advice or counsel from local or recognized Extension personnel or other agronomic experts. RMA requires all insured producers to carry out good farming practices.

Definition of what a generally accepted practice is becomes crucial when a new risk appears and definitions of generally accepted practices are being developed.

We cannot provide a definitive answer of what will constitute a generally accepted management practice; fortunately, insurance agents who have been licensed to sell the federally facilitated insurance products – APH, RA, RA-HO, CRC, GRP, GRIP, and GRIP-HO – are being updated on this point.

The crop insurance policies that use farm yields as a basis for yield guarantees – policies that use the farm’s actual production history – require that farms follow generally accepted management practices to be eligible for indemnification should a natural event outside the farm’s control occur, such as soybean rust. Thus, whether or not indemnification would occur if yields were reduced as a result of rust would depend upon whether the farm followed generally accepted management practices. Definition of what a generally accepted practice is becomes crucial when a new risk appears and definitions of generally accepted practices are being developed.

Applying fungicides for the control of Asian soybean rust

Mark Bernards
Crop & Soil Sciences

The confirmation of Asian soybean rust in the continental United States last fall has elicited a flurry of activity by researchers, regulators and producers. While no one knows how rapidly the disease will spread nor how significant an effect it will have on crop growth and yield, climatic conditions during the much of the growing season in Michigan are suitable for the disease to develop should inoculum arrive in Michigan. Currently, the only method available to control the disease is through the application of foliar fungicides.

Soybean rust fungicides may be classified either as preventative (protective) or curative (eradicant). Preventative fungicides such as the strobulurins (azoxystrobin, pyraclostrobin, trifloxystrobin) and chlorothalonil have the ability to prevent the fungus from infecting plant tissue or to stop spore germination on the plant surface. Protective fungicides must be applied prior to infection, and their efficacy depends upon the plant foliage being adequately coated by the fungicide. Curative fungicides have the ability to inhibit or stop the development of established infections by limiting the reproductive potential of the soybean rust fungus. Curative fungicides, most of which are classified as triazole fungicides, include propaconazole, tebuconazole, tetraconazole and myclobutanil. Curative fungicides are effective if soybean rust is present at low levels in the field, but have little effect if the soybean rust is firmly established in the field.

Soybean rust infects the lower leaves of the soybean plant first. If not controlled, the disease will proceed to infect upper leaves, causing premature leaf death and defoliation of the plant. To prevent the fungus from infecting the plant or to control the disease while it is present only at low levels, it is critical that the fungicides reach the lower canopy and coat as much of the plant foliage as possible. This will require different application techniques than those used for most herbicide applications. We will consider three main factors: spray volume, spray nozzles and spray pressure.

Both aerial and ground applications of fungicides have been effective at controlling soybean rust. However, higher volumes of water must be applied with fungicides than with most herbicides. Aerial applications of soybean rust fungicides should be in a minimum of 5 gal/A, and ground applications in a minimum of 15-20 gal/A. This is about double the rate used for most herbicides, but the extra water helps to insure complete coverage of the plant canopy.

The current recommendation is to select nozzles that will produce a medium-size droplet at the spray volume, pressure and speed you will be using. If the droplet is too fine, it will not penetrate the canopy and may drift. A droplet that is too coarse will penetrate the canopy, but not cover the leaf surface completely. Twin jet, hollow cones and flat fan nozzles are acceptable for fungicide application. However, fungicide deposition trials in peanuts showed that TurboDrop and twin jet nozzles penetrated the peanut canopy more than hollow cones and flat fan nozzles. With twin jet/TurboDrop nozzles two streams are emitted from the nozzle, one at a slight angle forward and the second at a slight angle backwards. It is believed that this spray angle disturbs the upper canopy, enabling the spray to penetrate to the lower parts of the canopy more effectively.

When it is indicated on the label, fungicide labels recommend setting spray pressures at 35 to 40 psi. Some nozzle manufacturers are recommending spray pressures of 60 to 120 psi. As the applicator it will be important that you set the pressure high enough to ensure good canopy penetration, but not so high that the droplet size becomes too small. Nozzle manufacturers can provide information that will help you to determine the proper speed, pressure and nozzle size to maintain medium-size droplets for the nozzles you select.

Good pesticide application practices are critical for effective fungicide application. As a reminder, inspect your sprayers to make sure all the nozzles are the same and replace nozzles that show excessive wear. Calibrate your sprayer to make sure that you are able to accurately apply a uniform amount of product across the field.

And finally, be safe. Stay tuned for more information on soybean rust and fungicides in future CAT Alerts.

Ned M. Birkey
Monroe EANRA

With the possibility of Asian soybean rust coming to Michigan this summer, March is an excellent time to “tune up” field crop ground sprayers. Farmers may want to determine now if their sprayer has the capability to handle new twin nozzles and higher spray pressure that may be required for fungicidal spraying. Fungicide spraying for soybean rust will require more thorough leaf coverage and deeper into the foliage than glyphosate sprays, for example. (See separate article in this issue by Mark Bernards.)

Proper placement of fungicides will be critical to achieve effective control of soybean rust. Plant growth stage, canopy density, fungicide rates, carrier volume, application speed, operating pressure and nozzle selection will all be important factors to achieve proper application.

A ground-based, farm field crop sprayer needs to be calibrated at least once per growing season to be sure that it is good working order and that the nozzles are applying the proper flow rate. With a sprayer in good working order, the spray tank half full of water, and knowing the proper nozzle flow rate, the ground speed and nozzle flow rate can be checked in about an hour or less.

Most Michigan State University Extension county offices have calibration stickers that can be affixed to the sprayer with one simple calibration procedure. The Extension offices also have bulletins dealing with sprayer calibration, and the Pesticide Applicator Core Training manual has some excellent calibration information.

Remember to write down the sprayer calibration date for Michigan Right-to-Farm record keeping. Right-to-Farm also suggests that farmers have a portable spill kit available for mixing and loading in the field.

Martin Nagelkirk
MSU Extension, Sanilac County

Each spring it is important to critically assess wheat stands.  This may be particularly true this year as growers face higher input costs coupled with soft commodity prices.

Plant density is one of the most important indicators of a field’s yield potential and, consequently, may suggest which management practices and production inputs may be warranted.  Stand assessments should begin in early March, particularly if one plans to take advantage of frozen ground for applying nitrogen fertilizer.

Stands having less than optimum populations are most often the result of poor soil conditions for adequate seed germination, planting errors or winterkill.  Of these three, winterkill is the most difficult to assess as appearances can be misleading during early spring.  For example, brown, dried leaves do not necessarily indicate winterkill as these fall-generated leaves can be killed without reducing the plants chance for survival.  On the other hand, an initial flush of green top- growth does not guarantee a plant will survive.  The reason for this is that temperatures are sometimes cold enough to destroy the crown’s ability to grow new roots, but not cold enough to destroy the more resilient shoot regenerating part of the crown.  Fields that appear to be injured should be revisited one or two weeks following green-up to determine if additional injury was incurred due to adverse spring-time conditions such as moisture stress, flooding, or plant heaving.

Where winterkill is suspected, growers or field personnel may want to try a protocol advocated by the University of Nebraska:

§      Remove the top three inches of soil containing plant crowns;
§      Warm samples to room temperature;
§      Wash with cool water to remove attached soil;
§      Cut off leaves and roots to within one inch of the crown;
§      Rinse the crowns with cool water;
§      Place 10 wet crowns in an inflated plastic bag and seal;
§      Place the bags in a lighted room, but not in direct sunlight;
§      Check the crowns in two days, rinse with cool water and re-inflate the bag.

After four days, crowns that are alive should exhibit new root growth as well as new shoot growth. 

Another method, though sometimes less conclusive, is to simply make longitudinal cross-sections of the crown.  If the interior tissue is discolored, damage has likely occurred.

Table 1. provides yield estimates based on live plant densities. The yield potential is listed for wheat exhibiting either high or low vigor, recognizing that plants spared winterkill may yet have sustained injury and, therefore, a loss of vigor and yield.

When assessing numerous fields for winter injury, one or more trends may become apparent:  wheat planted in late fall tends to be less likely to survive and less vigorous than early planted wheat; planting seed overly shallow or deep increases the chance of winterkill; some varieties are more winter hardy than others; no-till wheat may have a higher rate of survival where there is significant residue from the previous crop; and wheat growing in soils that tend to be excessively dry or wet are more likely to experience winter losses.

Growers who experience thin stands for whatever reason may want to give some thought to the following points or suggestions:

§      Wheat having a reasonably uniform stand of five or more vigorous plants per foot of row may still achieve two-thirds of its original yield potential.
§      Contemplate which crops might displace wheat, keeping in mind production costs, market potential, herbicide rotation restrictions and crop rotation implications.
§      Review contractual obligations and marketing alternatives, if any part of the 2005 wheat crop is already sold.
§      Postpone fertilizer nitrogen applications until the wheat stand and yield potential can be assessed.  The exception might be if corn (or other crop having a high nitrogen requirement) is to displace the damaged wheat.
§      Plan for a more aggressive weed control program as wheat stands that are thin or lack vigor will likely experience greater weed competition.
§      If wheat is to be abandoned, the crop insurance agent should be contacted before the crop is destroyed so that an adjuster can appraise and release the acreage. 

Jim Kells and Kathrin Schirmacher
Crop & Soil Sciences

A field trial was conducted in corn in 2004 on the MSU Research Farm to compare weed control, corn injury, corn yield, and economic return of the dominant herbicide programs marketed in Michigan. Major herbicide companies were asked to each submit four weed control programs for the study based on soil type and weed infestation history. Site characteristics and herbicide application timings are described in Table 1.

Table 2 lists the treatment details for the 24 treatments, sorted by application method and need for Roundup Ready hybrids. Weed competition was very high at the research site. The weed-free corn yield was 218 bu/A and the weedy (untreated) yield was 87 bu/A. Thus uncontrolled weeds at the site reduced corn yield by 131 bu/A (60%).

The site received excessive rainfall shortly after planting. A total of 6.1 inches of rainfall occurred within 2 weeks of preemergence herbicide application.

Table 3 lists the weed control effectiveness, weed control cost, corn yield, and gross margins for each of the herbicide programs in the trial. Table 4 summarizes weed control effectiveness, weed control cost, corn yield, and gross margins for each of the six systems included in the trial.

View Table 2
View Table 3
View Table 4

Observations
Excessive rainfall after herbicide application reduced weed control with preemergence programs, which often resulted in lower yields. The most expensive programs involved two applications. The least expensive programs involved one application. Three of the five most expensive programs were Roundup Ready programs. One of the five least expensive programs was a Roundup Ready program. The programs with the highest occurrence of yield loss were conventional treatments involving a preemergence application. Those treatments that caused significant corn injury had the lowest corn yields and gross margins. Four of the five most costly programs had high gross margins. Similarly, four of the five least costly programs had high gross margins. The major factor affecting gross margins was corn yield.

Christy L. Sprague
Crop & Soil Sciences

A field trial in soybean was conducted at the MSU Research Farm in East Lansing to compare weed control, soybean injury, soybean yield, and economic returns of dominant weed control programs being marketed to Michigan growers. Each major herbicide company was asked to submit up to four weed control programs for the study based on soil type and weed infestation history. Site characteristics and herbicide application timings are described in Table 1.

Table 2 describes the herbicide programs selected by each company for the 26-treatment study. Herbicide programs are sorted by application method and the need for Roundup Ready seed. Yield loss due to weeds was extremely high at this location. The maximum soybean yield from this trial was 67.7 bu/A and the weedy (untreated) yield was 21.3 bu/A. Thus, uncontrolled weeds at this site reduced soybean yield by 46.4 bu/A (68.5%).

Immediately after planting and application of the preemergence herbicides the site received 0.53 inch of rainfall, which may have contributed to the persistent injury that was observed from some of the soil-applied herbicides.

View Table 2
View Table 3
View Table 4

Results
Table 3 lists soybean injury, weed control effectiveness, weed control cost, soybean yield, and gross margin for each of the 26-herbicide programs included in the study. Table 4 summarizes soybean injury, weed control, the five most and five least expensive programs, soybean yield, and gross margins for each of the five weed control systems included in the trial.

General conclusions
The immediate rainfall after application of the preemergence herbicides led to persistent injury from some of the soil-applied herbicides. Herbicide programs that did not provide greater than 90% control of all weeds, in particular common ragweed, did not rank among the highest yielding herbicide programs. The programs that did not control common ragweed or programs that had significant injury that persisted throughout the season did not rank among the highest yielding or the highest gross margins. Not all programs that were among the highest gross margins were among the highest yielding programs. Four out of five of the most expensive programs included programs that involved two applications and all five programs that were among the least expensive involved only one application. However, all but two programs with the highest gross margins were sequential herbicide programs (2-pass). Several programs, conventional and Roundup Ready, provided excellent weed control, high soybean yields, and high gross margins.

This study is the results of one year and one year only. Please take this into consideration when forming your own conclusions. This study will be conducted again in 2005 to help strengthen conclusions.

Christy Sprague and Jim Kells, Michigan State University
Chris Boerboom, University of Wisconsin
Kevin Bradley, University of Missouri
Jeff Gunsolus, University of Minnesota
Bob Hartzler, Iowa State University
Bill Johnson, Purdue University
Mark Loux, Ohio State University
Dawn Nordby, University of Illinois
Micheal D. K. Owen, Iowa State University
Bryan Young, Southern Illinois University

It is well known that glyphosate-resistant horseweed (also known as marestail) populations have been selected in Roundup Ready soybean and cotton cropping systems. Resistance was first reported in Delaware in 2000, a mere five years after the introduction of Roundup Ready soybean. Since that initial report, glyphosate-resistant horseweed is now reported in 12 states and is estimated to affect 1.5 million acres in Tennessee alone.

A person could ask if this is any indication of what might lie ahead. On one hand, it has been proposed in a popular advertisement that glyphosate-resistant weeds are unlikely to occur if glyphosate is frequently used, as long as glyphosate is applied at full rates. The comments in this advertisement, in part, are based on several long-term university studies of Roundup Ready cropping systems. However, the question that a person should ask about these studies is whether or not they can prove that resistance will or won’t happen. It is our belief that these studies are not large enough to test if resistance will develop. For example, the University of Wisconsin has a 7-year Roundup Ready cropping system trial. This trial has horseweed in the no-till plots. Despite burndown and in-crop treatments with glyphosate, glyphosate-resistant horseweed has not developed in these plots. Since this trial did not find glyphosate-resistant horseweed, does this mean that glyphosate-resistant horseweed cannot develop? Does it mean that the resistant horseweed in Ohio, Tennessee, or Delaware is not truly resistant? Obviously not. Small-scale trials cannot prove that some event will not occur when a larger scale is considered. In total, these Roundup Ready cropping system trials may only be testing continuous glyphosate use on perhaps 50 acres, which is an extremely small fraction relative to total glyphosate. The true real test to determine if a rare event like glyphosate-resistant weeds will develop is actually being tested on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that are sprayed each year.

Many weed scientists across the Midwest have warned of the potential for additional glyphosate-resistant weeds if a “high selection pressure” is maintained. In this case, “high selection pressure” refers to the repeated use of glyphosate without interruption by herbicides with other modes of action or other weed management practices. This potential was confirmed this week at the North Central Weed Science Society Meeting where glyphosate-resistant common ragweed weed was reported. This is the first report of glyphosate-resistance for common ragweed. It was identified in a Missouri no-till soybean field that has been in continuous soybean production (with some double crop wheat) for many years and in Roundup Ready soybean since 1996. The common ragweed in this field had a high selection pressure for glyphosate with one or more glyphosate applications per year. This is the second example of a glyphosate-resistant weed that has developed in a Roundup Ready cropping system with high selection pressure. We do not know which glyphosate-resistant weed will be the next to develop or when it will occur, but high selection pressure will likely result in additional cases of resistance.

Midwest weed scientists believe in the value that glyphosate and Roundup Ready crops offer to growers. We hope that growers and crop advisors will evaluate how they use glyphosate and the Roundup Ready technologies to gain the value of these technologies without increasing the risk of resistance.

2)    Alternating glyphosate use with other herbicide modes of action between years, and

3)    Incorporating appropriate integrated weed management practices such as cultivation.

Christy Sprague, and Jim Kells, and Kathrin Schirmacher
Crop & Soil Sciences

The 2005 Weed Control Guide for Field Crops, Extension Bulletin E-434, is now available online at the CAT Alert web site. To view the table of contents for the guide, click here.

There were some overall changes to the format of the guide for 2005. Table 1K-Tank-mix Combinations, Additives and Application Timing for Selected Herbicides was removed and replaced by Table 1K-Delayed Herbicide Application in Corn. The new table lists the maximum labeled corn stage for preemergence herbicides should timely application be delayed. Tables 10 (Glyphosate Products Registered for Postemergence Application in Roundup Ready Crops) and 12 (Herbicide Crop Rotation Restrictions) were revised and updated. Table 10 now includes the statement “Surfactant is Needed?” to identify if:

1)      A product is fully loaded with a surfactant (N),
2)      A surfactant is needed (Y), or 3
3)      Additional surfactant may improve control under certain conditions (S).

Table 12 was significantly expanded to include all herbicide products listed in the guide. Seven fact sheets on the control of hard-to-control weeds are included (p.150-157). The fact sheets contain the most current recommendations for control of dandelion, white campion (white cockle), horseweed (marestail), wild carrot, Canada thistle, common pokeweed and hemp dogbane.

Darryl Warncke
Crop & Soil Sciences

Increasing worldwide demand for nitrogen, phosphorus and potassium has brought about significant increases in the costs of these materials for the 2005 growing season, especially potash. The supply of potassium may also be limited. Faced with this situation farmers may have to make decisions on how to get the best return from the fertilizer they are able to purchase. Following are some suggestions.

Soil test!! A soil test indicates the available nutrient status of the soil, the need for lime and how much nitrogen, phosphorus and potassium are needed for the crop to be grown. Test results from samples collected within the last two to three years can be used as a guide. For fields where no soil test information is available collect soil samples, as soon as possible, and have them tested.

Apply lime where needed. Maintaining the soil pH between 6.0 and 6.8 improves nutrient availability and crop growth. In many situations the return on investment is best with lime, both short and long term. When the soil pH is below 6.0, applied fertilizer nutrients or indigenous soil nutrients are not used as efficiently by crops.

Take credit for nutrients contained in manures, composts or other materials applied to crop fields. Nutrients contained in these materials are readily available and can offset the need for purchasing any additional fertilizer. Have manures and compost analyzed for the nutrient content.

Apply phosphorus only where needed. As the result of past buildup, 70 to 75 percent of Michigan farm fields contain adequate or above levels of phosphorus. Now is the time to use some of that phosphorus banked in the soil.

Focus of potassium. Only 20 to 25 percent of Michigan farm fields contain adequate levels of potassium. Increase the amount of potassium included in starter. At a 2-inch by 2-inch placement up to 100 pounds per of K₂O per acre can be applied without concern for injury. Placement of potassium is especially important for corn planted no-till or into a lot of residue. By including some potassium in the band fertilizer the total amount applied per acre may be reduced because of the increased effectiveness.

Band-apply nutrients. Band applied nutrients are used more effectively than broadcast nutrients. In high P testing soils early growth from banded fertilizer comes more from nitrogen and potassium than phosphorus. Overall, reducing phosphorus in favor of nitrogen and potassium may prove beneficial.

Apply P and K first to fields most in need. The crop yield response to incremental additions of potassium or other nutrients can be characterized as the “curve of diminishing returns.” This means that the increase in yield from the first increment of potassium added (say 50 lb K₂O per acre) is greater than the second. And return from the second increment is greater than from the third and so forth. Eventually the cost of an additional increment of potassium is greater than the value of the yield increase. This being the case, it is better to apply or allocate potash first to fields with lower soil potassium values, where there will be a good yield response, than to those fields that have close to adequate levels.

Reduce amount of K applied uniformly across all fields. In fields that need K, applying slightly less, e.g. 25 pounds per acre, than is needed will have only a minimal, if any, effect on net income. Work from soil test results.

Apply preplant nitrogen close to planting time. This will minimize the length of time nitrogen is at risk of lost by leaching or denitrification before crop demand increases.

Use a presidedress soil nitrogen test for corn to determine need for more nitrogen. Some soils can provide significant amounts of available nitrogen, especially where cover crops or animal manures or a previous legume crop is incorporated prior to planting.