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Fend off soybean weeds and diseases

Soybeans often hover in corn's shadow. It's not like this when it comes to pests, though. Glyphosate-resistant weeds and diseases pay plenty of attention to this legume. Meanwhile, some reports link manganese deficiencies to glyphosate applications on soybeans.

The good news is ways exist to mash these maladies. Companies are developing herbicides and traits to complement glyphosate-tolerant systems. Fungicides and disease-resistant varieties help corral diseases. Meanwhile, most studies show no conclusive link between glyphosate applications and manganese deficiencies.

Here are some factors to consider as you plot future soybean strategy.

Herbicide-tolerant traits

“There are weeds that are getting harder and harder to control with glyphosate,” says Bruce Battles, Syngenta Seeds agronomy marketing manager.

That's led to several new soybean herbicides and herbicide-tolerant traits now on the market or in the works. LibertyLink soybeans from Bayer CropScience debuted in 2009. BASF launched Kixor, a new PPO inhibitor compound, in 2010.

Joining these later this decade will be HPPD-tolerant traits from Bayer and Syngenta. These traits give crop tolerance to HPPD-inhibitor herbicides like mesotrione (Callisto) and isoxaflutole (Balance Pro). Officials from both companies say this trait will help complement glyphosate in the herbicide mix and extend the effectiveness of glyphosate.

Dow AgroSciences plans to commercially launch its Dow Herbicide Technology in soybeans in 2015.

“This brings in 2,4-D tolerance,” says Damon Palmer, DHT commercialization leader for Dow AgroSciences. “We are looking to partner it with glyphosate. Growers will be able to enjoy the benefits of glyphosate-tolerant systems and control the weeds glyphosate is increasingly missing like marestail, morningglory, giant ragweed, and common ragweed.”

Monsanto plans to release a dicamba-tolerant trait in 2014, pending regulatory approval. This will be used to complement glyphosate-tolerant soybean varieties and add another mode of action, say company officials. After the dicamba-tolerant trait debuts, Monsanto plans to team it with the Roundup Ready 2 Yield trait.

Also on deck for soybeans pending regulatory approval is Optimum GAT. This is a new glyphosate-tolerant trait with ALS inhibitor activity from Pioneer Hi-Bred and DuPont.

Mark it up

It will be more difficult to keep track of all these different technologies. It's easy to say you'd never apply glufosinate (Ignite) on glyphosate-tolerant (Roundup Ready) soybeans.

In the heat of battle, though, it may be easier to inadvertently apply the wrong compounds than you think. Mixing up herbicides on herbicide-tolerant systems ensures killing of both crops and weeds.

“A flagging system is going to be essential for these technologies,” says Steve Stevens, a Tillar, Arkansas, farmer.

The University of Arkansas has done just that with its Flag The Technology program. It uses color-coded bicycle-type flags conspicuously placed in the field visible from ground and air. Red signifies conventional varieties, white represents Roundup Ready technology, bright green signals LibertyLink systems, and bright yellow represents Clearfield systems.

Frogeye leaf spot

Frogeye leaf spot is a fungal disease creeping in from the South into states like Illinois. Your first step in fighting this fungal disease is to plant frogeye leaf spot-resistant varieties, says Carl Bradley, University of Illinois (U of I) Extension plant pathologist.

Fungicides also can control this disease, but they are most effective on susceptible varieties, says Bradley.

A 2010 U of I trial at Belleville, Illinois, examined fungicide response on a susceptible variety and three others resistant to the disease.

“We saw a 10- to 11-bushel-per-acre response on the susceptible variety,” says Bradley. No statistically significant differences resulted on the resistant varieties.

What about white mold?

Many soybean fields infested with white mold in 2009 went back to corn last year. Since 2011 marks soybean's turn in a corn-soybean rotation, will white mold still be a threat?

Two components of the disease triangle — host and pathogen — will be present this year. Soybeans, of course, are the host. Ditto for the pathogen. White mold inoculum is contained in hardened structures called sclerotia. Mushroom structures known as apothecia sprout from the sclerotia. Spores produced from the apothecia can infect soybean flower petals, thus causing disease. The disease sloughed off soybean plants in 2009 and more sclerotia in infected fields formed. Sclerotia can reside in soil for eight to 10 years or longer.

The third component — weather — is unknown. Similar weather to 2009 (cooler-than-normal temperatures and rainfall prior to and throughout flowering) will prompt white mold. Absent these conditions, though, you can rest easy.

“The risk is up, due to the increased amount of inoculum that's present,” says Bradley. “A lot depends on the weather.”

Fungicides applied at R1 (beginning bloom) and, if necessary, again at R3 (beginning pod) can protect plants if conditions remain favorable for white mold, says Randy Myers, fungicide product manager for Bayer CropScience.

Glyphosate and manganese

Wonder if glyphosate is spurring manganese deficiency in your soybeans? This is an issue that's been raised in scientific journals and reports by some agricultural scientists.

In 2001, Purdue University researchers found Roundup Ready soybean varieties were more sensitive to manganese deficiencies than conventional varieties. But subsequent trials found manganese deficiencies were related to the parentage of the soybean variety and not the Roundup Ready trait, says Bob Hartzler, Iowa State University Extension weed specialist.

Yield traits

Monsanto initially touted its Genuity Roundup Ready 2 Yield soybeans as yielding 7% to 11% above Roundup Ready soybeans. That didn't always occur in 2009. Susceptibility to white mold zapped yields in some varieties. Doubly smarting was the higher price of these varieties compared to Roundup Ready varieties.

The news was better in 2010, though. As more varieties expanded into more regions and white mold diminished, Monsanto officials say Roundup Ready 2 Yield soybeans yields averaged 3.8 bushels per acre more than Roundup Ready varieties in 2010.

For 2011, Monsanto has decoupled its Acceleron seed treatment from Roundup Ready 2 Yield soybeans. Without the treatment, this will slice the extra money farmers pay for the seed over Roundup Ready varieties by about $5 to $6 per bag.

“We still encourage growers to treat,” says Brett Begemann, Monsanto executive vice president of traits and seeds. “The seed treatment is a small price to pay to get the pure value of the genetic potential (for Roundup Ready 2 Yield).”

Concerns regarding glyphosate inducing manganese deficiencies are theoretically possible, he says.

“There are published papers supporting these claims,” Hartzler points out. “The majority of research, however, does not support these concerns. If these things were occurring (with more frequency), we would have more people picking up on it in the field.” 

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