Science From The Field

Steps For Success With Continuous Soybeans

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Being hooked on soybeans isn’t necessarily a bad habit. Although most research shows a yield advantage to growing soybeans one year at a time in a crop rotation, there are management steps that can help to make the most of consecutive soybean crops and minimize yield drag. Learning those steps can give growers another planting option when facing volatile grain markets or wild weather patterns.

“The general feeling is that second year soybeans will suffer a 5% to 10% yield loss compared to soybeans in the first year of a crop rotation, but that doesn’t have to be the case,” says Mary Gumz, Ph.D., eastern cornbelt agronomy manager for Corteva Agriscience.

“A very intentional and deliberate management approach will minimize any yield loss and make second-year soybeans a success,” she continues. “The most important step is to evaluate fields for potential problems and either don’t go back with soybeans on those fields where problems exist or carefully select varieties that have high levels of resistance to those potential threats.”

Gumz recommends avoiding continuous soybeans on poorly-drained soils due to their greater risk of Pythium, Phytophthora, Fusarium and other seedling diseases, as well as sudden death syndrome (SDS) and brown stem rot.

TAKE THE TEST:

It’s also important to know the soybean cyst nematode (SCN) populations in fields. That’s best done with soil tests that show the number of SCN eggs in the soil.

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University research shows that SCN can reduce soybean yields up to 30% without any visible symptoms, so soil testing is important. “Fields with low SCN egg counts could be good candidates for back-to-back soybean production, but even those fields should be monitored with regular soil tests as SCN populations will increase under continuous soybean production,” adds Gumz.

Where the SCN population is a threat, she says planting resistant varieties is the best management tactic. Pioneer research shows this can reduce SCN reproduction by 70% to 80%. Most SCN resistant soybean varieties use PI 88788 source resistance.

“Unfortunately, some HG types (races) of the nematode can still reproduce with that source of resistance,” explains Gumz. “If that’s the case, a second source of resistance (Peking) is available in some varieties, but these are generally available only in Early Group 3 and earlier varieties. Even if you can’t change the source of resistance, simply changing varieties can help reduce SCN damage as the resistance can come from a different portion of the genome.”

Focus On Fungicides And Fertility:

Using resistant varieties with a fungicide seed treatment is the ultimate way to combat SDS and many other soil-borne pathogens that increase under continuous soybean production. Pioneer testing has shown that soybeans treated with ILeVO seed treatment had a yield increase of 4.9 bushels per acre in high SCN environments and 4.5 bushels more in fields under moderate SDS pressure.

“Foliar fungicide application is another management tool that may become more necessary with continuous soybeans,” says Gumz. “The inoculum of diseases like white mold, frogeye leaf spot and others over-winter in soybean residue so they can be more likely to become problems. The best advice is to scout fields closely and be honest about needing a foliar fungicide application.”

Soil fertility also requires new thinking with consecutive soybean crops. “Growers typically apply ample fertilizer to the corn and then rely on the carryover to feed the subsequent soybean crop, but the revised rotation requires a new approach with a particular focus on potash requirements,” says Gumz.

Recent research shows that a 60 bushel per acre soybean crop uses about 60 pounds of P2O5 and actually removes 45 pounds of the nutrient from the field with the grain. The same situation used 160 pounds of K2O per acre and removed 70 pounds of the nutrient. This is 20 to 40 pounds per acre more K2O than is removed by a 150-bushel corn crop. Similarly, an 80-bushel soybean crop removed five pounds more K2O per acre than a 250-bushel corn crop.

“To ensure maximum yields in continuous soybean cropping situations it is very important to supply adequate fertility -- especially potash,” stresses Gumz.

Weed control challenges with continuous soybeans may also become more difficult because of the loss of the ample herbicide choices available in the corn portion of a corn/soybean rotation. “The best advice is to identify your problem weed species and use a fall burndown application if marestail is among them. Apply a residual herbicide at planting, layer an additional preemerge product when postemerge applications are made during the growing season and select herbicides so that multiple modes of action are included in the weed control program,” says Gumz.

Other tips for a successful soybean/soybean system include to increase the seeding rate slightly to offset the stand loss that can occur in continuous soybeans and to keep a close eye out for stem borer damage at harvest time. “The Dectes Stem Borer is becoming a problem in more areas and crop rotation is about the only way to combat it, so don’t plant continuous soybeans on fields where it is present,” she warns.

Plan For Success In Continuous Soybeans:

> Evaluate fields for potential weed, pest and disease problems

> Avoid fields with poorly drained soils

> Test to determine soybean cyst nematode population

> Use a seed treatment

> Increase planting rate

> Scout fields to evaluate need for foliar fungicides

> Provide adequate fertility, especially potash

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