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Index

• Dear Reader
• Start Weed-free Before Planting No-till Soybean
• Eight Years Later - What We've Learned About Managing Weeds in Roundup-Ready Soybeans
• Will Soybean Rust Overwinter In the U.S.
• Should You Inoculate Your Soybeans
• Virginia Ag Expo - Aug. 12
• Virginia Small Grains Assoc. Annual Field Day
• American Soybean Assoc. News

Dear Reader,

I stress that much of your yield potential for soybeans are established at planting. Variety selection is important, but getting a good, uniform stand at the intended plant population is even more critical. Our continued research with precision planting indicates that we need to focus more on our planting process.

Good luck to all in the coming month! I hope this is a beginning to a great season.

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Start Weed-free Before Planting No-till Soybean

But, the system is not without its faults. Due to its relatively wider application window, we've tended to wait longer into the growing season to spray. This was the topic of my June 2003 article, "Delayed Weed Control Can Lead to Lower Yields." In that article I highlighted the effect of weed competition on yield when applications were delayed. For more details, see Soybean Update Vol.6, No.4 at: www.vaes.vt.edu/tidewater/soybean/soybeanup/soybeanup.html.

That article references Nebraska research that concluded to avoid a 5% or more yield loss, glyphosate needs to be sprayed by the 1-leaf stage (V1) or about 9 days after soybean emergence (DAE) for 30-inch rows, by the V2 stage or 15 DAE for 15-inch rows, and by the V3 stage or 19 DAE for 7 1/2 - inch rows. The days after emergence will likely vary with climate and planting date. Soybean will put on new leaves much faster if planted in June versus May. Although the research was conducted in Nebraska, I feel would relate to Virginia conditions as well. The nice thing about it is that it focuses on soybean stages and not days after emergence. This makes it more adaptive to different climates and cropping systems.

So, if you drilled your soybeans, you have around 19 days after soybean emergence to spray, right? Well, not exactly. The Nebraska research began with weed-free fields. Both the soybean and weeds emerged pretty much at the same time. Here lies one problem with the Roundup-Ready weed control program. Many producers have decided to forego a burndown application and solely use glyphosate after soybean emergence in an effort to save one application and money. But, are you really saving money? Keep in mind the above research. While the research refers to soybean stage and time after soybean emergence, it really indicates the time after weed emergence. Therefore, you can probably deduct the amount of time that the weeds were present before soybean emergence from the stated days after emergence above.

For example, if weeds had emerged 10 days before soybean emergence, then you would need to spray 5 or 9 days after soybean emergence instead of 15 or 19 to avoid a yield loss. This strategy may be a good one (if you spray within 5 to 9 days after soybean emergence), but more weeds will likely emerge. Then another application will be needed.

A 5% yield loss of 30 bushels/acre equals 1.5 bushels. At $7.00, this is $10.50 per acre. Most of your weed control money is tied up in seed. Therefore, a burndown application followed by a timely postemergence application will likely pay for itself. Even if you decide to wait until after emergence, you'll probably have to make a second application to avoid weed competition and yield loss.

The below article is from the April 27 issue of Ohio State's Crop Observation and Recommendation Network. It has some very good comments about our Roundup-Ready soybean system. This newsletter can be found at http://corn.osu.edu/.

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Eight Years Later - What We've Learned About Managing Weeds in Roundup Ready Soybeans

Glyphosate may be the best herbicide ever developed, and the ability to use it in corn and soybeans has certainly changed our expectations of weed control. Especially in soybeans, Roundup Ready systems are among the most consistently effective when properly implemented. We hear much less about a number of weeds than we used to, including Canada thistle, hemp dogbane, common cocklebur, and several annual grasses (e.g. yellow foxtail) that were becoming more problematic in the mid-1990's. On the flip side, a number of weeds can still be a problem in Roundup Ready systems, especially when producers decide that the flexibility of glyphosate allows them to throw basic weed management principles out the window. The result has been an increase in a number of weeds that can take advantage of poor management or a lack of diversity in herbicide use in Roundup Ready systems. Some examples are dandelion, marestail, lambsquarters, and winter annuals. So - what have we learned about glyphosate and weed management in Roundup Ready soybeans after eight years of use?

1. Weeds can and will develop resistance or reduced sensitivity to glyphosate. Use of glyphosate to the exclusion of other types of herbicides may be a primary cause of resistance, and rotation away from glyphosate for a year does not necessarily prevent resistance from occurring or adequately mitigate existing resistance problems. We have confirmed glyphosate resistance in marestail in Roundup Ready soybean fields following a year of corn. A common theme in these fields seems to be use of only glyphosate in the year the soybeans were grown.
2. When it comes to annual weed control, bigger is not better. Even given the flexibility of glyphosate and its ability to control large weeds at high rates, a number of issues arise when weeds are allowed to become too large before application. Problems with letting the weeds get too large include: A) possibly reduced control, especially if rates are not increased, which leads to a need for re-sprays and increases the risk of resistance; B) the stalks of several weed species (e.g. ragweeds, marestail, lambsquarters) can be infested with insects, possibly reducing herbicide effectiveness, and the potential for stalk damage increases the longer weeds remain in the field before herbicide application; C) reduced sensitivity of some lambsquarters populations to glyphosate appears to be expressed to a greater degree in larger plants (more than 4 to 6 inches tall); D) large weeds may prevent herbicide particles from reaching smaller weeds lower in the crop canopy; and last but not least, E) weeds that emerge with the crop can reduce yield when they are allowed to reach a size of more than 4 inches (corn) or 6 inches (soybeans).
3. Skipping the early-season burndown application can be a big mistake in no-till soybeans. Delaying the first herbicide application until mid to late May allows a number of weeds, including dandelion, chickweed, purple deadnettle, and annual bluegrass, to flower and produce seed. We have attributed the increase in these weeds across the state to the failure to apply early enough in spring to prevent seed production. This is based partly on our research in producers' no-till fields in the early 1990's, when an early-season application of 2,4-D ester plus glyphosate (or 2,4-D plus residual herbicides if applied early enough) was critical for the success of weed management programs. Most of the fields did not have a real problem with winter annuals or dandelions because seed production was prevented. In addition, glyphosate and 2,4-D are much less effective on dandelion after seed production, because movement of herbicide to the root decreases at this time.
4. There are still plenty of good reasons to use other herbicides in combination with glyphosate in Roundup Ready systems. A primary reason to integrate other herbicides is to reduce the overall dependence upon multiple glyphosate applications, thus delaying the development of herbicide resistance. In burndown treatments, the addition of 2,4-D to glyphosate can greatly improve control of dandelion, marestail, and lambsquarters, among other weeds, and help mitigate the effects of cold weather or time of day on glyphosate activity. The addition of CanopyXL can improve control of dandelion, marestail, and other weeds, and FirstRate/Amplify can improve control of marestail and ragweeds. In postemergence glyphosate applications, the addition of Classic can improve marestail, morningglory, and dandelion control, and the addition of FirstRate/Amplify can improve marestail, morningglory, and giant ragweed control and provide some residual control of ragweeds.
5. There are still plenty of good reasons to use a residual herbicide, such as CanopyXL, Sencor, Valor, etc, in Roundup Ready soybeans, including the following: 1) seedling dandelion and marestail control; 2) early-season giant ragweed control/suppression to minimize the competitiveness of this extremely competitive weed during the first month of soybean growth and reduce the need for a second postemergence glyphosate application; 3) residual control of lambsquarters, which has developed reduced sensitivity to glyphosate in some fields; 4) minimizing weed/crop interference during the first month on soybean growth, so that unplanned delays in the postemergence glyphosate application do not result in crop yield loss; and 5) control of weeds during the first month or more to compensate for slow soybean development due to poor soil conditions or weather.

The bottom line for Roundup Ready soybeans: glyphosate is not a substitute for basic weed management skills, and there is no substitute for a multiple-pass, diverse herbicide program that creates flexibility in postemergence application windows, protects crop yield, and provides effective control and prevents seed production of winter annual, summer annual, and perennial weeds, reducing future weed populations.

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Will Soybean Rust Overwinter in the United States?

Brazil had some of the most intensive cropping systems that I've ever observed. It was my understanding that in some areas, they were growing not just 2, but 3 crops per year (soybean, corn, wheat). But, this intensive system might just be the reason that soybean rust has developed so rapidly in that part of the world. With soybeans growing throughout most of the year, there is a continuous inoculum source for the disease.

When you look at the size of Brazil (it's bigger that the continental 48 U.S. states), it's pretty mind boggling how much territory soybean rust has covered in just a couple of years. In addition, the disease seems to have been more destructive in Brazil than in other countries where the disease has been established. The real question is should we expect the same rapid spread and damage in the U.S.?

My gut reaction is that no, we won't. I based this largely on the U.S. cropping system and climate. First, we don't grow more that 2 crops of soybean per year; therefore the inoculum level should be much lower than in Brazil. On the other hand, if you look over the entire U.S., our soybean growing season is pretty long. For example, in the mid-southern states (MS, LA, AR, TX), they have moved to an early production system in which they plant in early April. For the rest of the country, planting continue in May. Then, in the southern states, we plant soybeans following small grain harvest in June and sometimes July. So, we have soybeans growing in the U.S. from April to November. Although rust would not persist on soybeans year-round, seven to eight months of soybean production could allow for a significant buildup of inoculum. As explained later in this article, I'm particularly concerned about our double-crop production system.

Still, only in our most southern locations would the cooler U.S. climate allow for the disease to overwinter. This was the subject of recently published research that evaluated whether or not soybean rust could establish itself in the U.S. and persist year-round. This research, "Assessment of the potential year-round establishment of soybean rust throughout the world," was conducted by S. Pivonia and X.B. Yang of Iowa State University and published in Plant Disease Vol. 88, pp 523-530. Threats of soybean rust to U.S. have been assessed in terms of yield losses or economic damage, but this is the first publication to assess the potential establishment of the disease once it arrives. Below is a summary of that research.

The researchers use a computer-based system that integrates information about the response of soybean rust to its environment and its potential for survival in that environment. The system used long-term monthly meteorological data to estimate how temperature (high or low) and lack of moisture would restrict rust survival throughout the year.

Below are some assumptions, based on past research, which the model used to develop its conclusions:

• Uredospores of soybean rust were reported to germinate at temperatures between 46 and 91° F.
• Average nighttime temperatures below 57°F prevented or greatly inhibited rust development.
• Soybean rust did not survive 5 days when kept at temperatures of 39 to 41oF and below.
• When weekly precipitation was 4 inches or more, dry stress would not develop.
• When relative humidity was higher or equal to 90% for 6 hours or more during the day, no dry stress developed.
• Trace levels of primary rust lesions were developed on soybean with 6 hours of dew and increased 10-fold when the dew period was increased to 8 hours.

• Soybean rust fungus survival may be impossible due to cold temperatures in regions with latitudes higher than 23 to 30° N and S (Virginia soybean growing areas are 36° or higher.
• In coastal areas where winter temperatures are milder, rust would survive at higher latitudes.
• The areas in the world where rust can occur in a growing season are:
1. Areas where the disease can survive year-round if a suitable host prevails.
2. Areas where seasonal rust epidemics are dependent on long-distance dispersal of the inoculum from the source area
• The main regions that have not been affected by rust, but have the potential to overwinter the disease are: northern South America, Central America, Mexico, the Caribbean, and coastal regions in southern Texas and parts of Florida.
• The model assumed that after 9 to 10 weeks of average temperatures of 45° F or less, rust would not survive. However, if that temperature threshold was lowered to 39° F, then the coastal regions of Mississippi and Louisiana would be included in the soybean rust survival regions.
• In the U.S., the possible overwintering zone is up to latitudes 28 to 30° N in Florida and the western Gulf of Mexico coastal regions. The majority of the U.S. soybean growing area is between latitude of 30 and 48° N.

OK, so what does all this mean? Quoting from the article's abstract, "If the disease is to establish in the U.S., it is likely to be restricted to parts of Florida and southern Texas during the winter in the frost-free areas or areas where the fungus could overcome short periods of below-freezing temperatures. Occurrence of rust epidemics within the U.S. soybean belt would depend on south-to-north dispersal of uredospores." This is generally good news.

Host availability was not considered in this research. Soybean rust has a wide host range that includes several legume species that are common in the U.S. Most notable are kudzu, vetch, and lupine. This might send up a red flag, but even if the host is available, temperature extremes and lack of moisture could still restrict its survival. Therefore the conclusions of this research should still be viewed as credible.

I'm still concerned about our double-crop plantings. The intensively-managed wheat and double-crop soybean system is many of our producers' bread and butter. Without the other, neither crop is as profitable. My concern is this. Let's assume that rust only overwinters in Florida and Texas and the uredospores blow into Virginia every year. Due to a latent (dormant or undeveloped) period, the disease may not cause a huge problem in our full-season soybeans. However, inoculum could build up in the full-season soybeans, and then hit our double-crop plantings hard.

I have no ideas whether any of these scenarios will occur. The extent and spread of the disease will not be known until it has entered the U.S. The best that we can do is to keep a close eye on our soybeans and contact your county extension office when you see anything suspicious.

Most experts do not expect soybean rust to be a major problem in the first year of its introduction. Knowing what I know about the disease, I would tend to concur. But, after it has established itself in the most southern states, we should be ready the following year.

Still, we do not want the disease to creep in unnoticed. With the help of growers and county agents, we have already established some early soybean production system plots throughout the state. We'll use these plots to monitor any early infection. We will also be intensely monitoring most of Virginia this summer and will report any activity. If the disease is found, we will notify everyone as soon as possible so control measures can be implemented if necessary.

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Should You Inoculate Your Soybeans?

I still stand by my past answer, which is based on my and other research on Mid-Atlantic soils. You will not usually get a yield response to inoculant if soybeans have been grown in the past 3 years. However, some recent research and new developments have caught my attention.

Several states are now recommending inoculation even for land that has only been out of soybean production for one year. In Ohio, Jim Beuerlein, OSU Soybean Specialist, has conducted 60 inoculation trials in producer fields that were selected because they had a very low potential for response to inoculants. The average yield increase was 1.9 bushels per acre. Some producers experienced 3 to 8 bushels increase. On about 1/2 of the trials, no yield increase was obtained, but was attributed to dry soil or waterlogged soil for several days. So, maybe optimal soil conditions are the key to getting a response from inoculants. Remember though, our soils are not mid-western ones and they can dry out quickly, especially in double-crop plantings.

On the next page are results from a study from Cornell University that show a positive response to inoculants in rotated soybean land. Only at the higher yield potentials did a yield response occur; a yield response was not evident under more typical Virginia yields.

So, does this mean that we should begin inoculating our soybeans across the board? I don't think so; I've yet to get a yield response with a number of experiments conducted over the past several years. Most other soybean agronomists tend to agree that soybeans do not need annual inoculation. Still, the data makes me re-think my recommendations and at least, continue to test.

There have also been some new developments with the Bradyrhizobium soil bacteria. Some new strains have been developed that tend to produce more nodulation under controlled conditions. But yield responses in the field have not been consistent.

Another new development has come from Nitragin Inc. in the form of a new product that contains the Bradyrhizobium japonicum bacteria in combination with a new soybean growth promoter and an additive to enhance bacteria survival. Optimize is the name given to the new product. This liquid formulation must be custom applied on seed at a retail facility to ensure uniform application and grower convenience. According to company data, Optimize has increased plant stand, vigor, nodulation, and yield. In 22 trials in the U.S., yield was increased by about 5 bushels per acre or 10.1%. Note that this again was under high yield conditions.

How does it work? The patented promoter is a natural compound produces by the bacteria. Normally, a "message" is sent from the soybean root to the bacteria in the soil. After recognition by the bacteria, the bacteria begin producing the promoter. Afterwards, nodulation occurs. Because Optimize already contains the growth promoter, the thought is that nodulation takes place earlier; therefore, one could assume more nitrogen fixation.

While this sounds good, it has not been extensively tested by universities throughout the U.S., and as far as I'm aware, not in Virginia at all. We plan on putting in several studies this year. I hope we can gather some data under Virginia conditions. If anyone is interested in testing this product on their farm, please contact me.

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Virginia Ag Expo Scheduled - Aug 12

More information will follow. We look forward to seeing you there.

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Small Grain Field Day

The Virginia Small Grains Association Field Day will be held on May 20, 2004 at the Eastern Virginia Agricultural Research & Extension Center (EVAREC) in Warsaw. If you have questions or need additional information please contact Bob Pitman, Superintendent, EVAREC at 804/333-3485, rpitman@vt.edu or Ellen Davis, Executive Director, Virginia Small Grains Association at 804/843-4456, eded@inna.net.

8:00 a.m.	Registration
8:30	Welcome and announcements	Bob Pitman
8:35 - 8:40	Move to first stop
8:40 - 11:20	Three concurrent tours will take place. Topics are shown below.
	No till wheat	Wade Thomason
	Clearfield wheat, weed control	Tom Pridgen, Siyuan Tan,
	Bread wheat	Jason Kenner
	Wheat increases	Carl Griffey
	Barley increases	Wynse Brooks, Mark Vaughn
	Seed/foliar treatments	Erik Stromberg
11:20 - 11:30	Welcome and remarks	Skip Jubb, Bob Pitman
11:30 - 11:40	Comments by new VAES Director	Craig Nessler
11:40 - 12:10	Market outlook for wheat and other grains	Wayne Purcell

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American Soybean Association News

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Sincerely,

David L. Holshouser

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