Continuous Soybeans in 2018? Help Farmer Focus on Seed Selection

With around 50 growing seasons under his belt, Paul Uphoff has been raising continuous soybeans for 16 of those years and hasn’t looked back. Despite challenges, continuous soybeans pay off for this north-central Illinois farmer and his family.

“We’re still coming out better on soybeans than we would on corn,” Uphoff says. “With genetics and defensive disease packages it’s even easier now.”

With weaker corn prices, more farmers are turning to soybeans to stay in the black this year—to the tune of about six million additional acres, according to USDA. If you are giving soybeans on soybeans a try this year or considering it for next season, be mindful of potential challenges and the role seed selection plays in protecting your crop.

The first factor to focus on is variety selection, says Shawn Conley, University of Wisconsin Extension soybean specialist. “Higher yielding varieties tend to not always have the best disease resistance traits. Look for resistance to sudden death syndrome or good tolerance to white mold, for example.”

Take time to consider the genetic and biotech traits your soybeans might need. Whether it’s enhanced disease tolerance, nematode defense or mixing up your herbicide options, you can’t focus your analysis on just one year. Instead, consider your field’s history and what could potentially be in the field in the future before selecting seed.

Be sure to consider diseases. Watch specifically for fusarium, rhizoctonia, pythium, phytophthora, frogeye leaf spot and white mold because they overwinter in soybean residue.

Know what pathogens are in your field before selecting seed. Frogeye leaf spot and white mold are especially threatening to continuous soybeans because by the time you see them, it’s often too late for treatment.

Soybeans susceptible to frogeye leaf spot, even when sprayed with a fungicide, can continue showing symptoms while tolerant soybeans halt the disease or slow its spread. If you employ no-till, those fields are at a higher risk of frogeye leaf spot.

White mold can wreak havoc on soybeans, robbing up to 50% of yield potential when infection is severe. The disease can also live on weeds, so weedy fields could be at a higher risk. Because white mold spreads quickly, a tolerant variety could help slow it enough to save yield.

Fusarium, rhizoctonia, pythium and phytophthora infect the plant when it’s a seedling. While resistant varieties could be helpful, one of your most powerful allies is seed treatment.

“Do not skip on seed treatments, especially if you’re considering cutting down on seeding rates,” Conley says. “Use the max rate seed treatment to avoid soil-borne pathogens that could cause stand issues.”

Seed treatment can also help protect soybeans from onset of sudden death syndrome, early season insect pests and nematodes. Look for a seed treatment with activity against these ailments to prolong health, because if a soybean starts out stunted or damaged, yield takes a hit from the start.

Don’t forget to select traits to manage weeds. One of the biggest challenges in continuous soybeans is weeds, especially resistant ones. Because you won’t be switching crops, you’ll likely be looking at a limited number of effective herbicides, which could lead to unintentional overuse.

“Make sure you understand there are a lot of names for similar chemicals,” says Bill Wiebold, University of Missouri Extension soybean specialist. “Have more than one site of action for each tank and more than one site of action each year.”

Many of the most challenging weeds are broadleaf, like soybeans, so you’ll need to find the herbicide tolerance that best fits your field and your checkbook. Currently, there are three soybean herbicide tolerances on the market: LibertyLink (glufosinate), Roundup Ready 2 (glyphosate) and Roundup Ready 2 Xtend (glyphosate and dicamba). Following approvals, Balance GT (HPPD and glyphosate) and Enlist (2,4-D and glyphosate) will join the group.

Researchers typically advise against continuous soybeans. “The only pro is economics this year,” Wiebold says. “Expect reduction in yield, increased disease potential and limited control options for weeds.”

In 20 years of field tests, Wiebold sees a 5% to 10% yield loss starting the first year of repeat soybeans. After that, the average levels out at 8% yield loss each year compared with soybeans in rotation with corn. Keep these estimates in mind when deciding whether or not to plant back-to-back soybean crops. 

Feed the Plant Before Seed Hits the Soil

Yes, soybeans fixate their own nitrogen, but that doesn’t mean you drop the seed without knowing nutrient levels. Phosphorus, potassium and many micronutrients are critical to plant development.

“We soil test every three years,” says Paul Uphoff, continuous soybean farmer from north-central Illinois. “We make sure we give the plants what they need and are experimenting with in-furrow fertilization now. So far, places where we put starter have better roots.”

The past two years have produced stellar soybean yields, and each bushel that leaves the field takes nutrients. One deficiency to keep an eye out for in continuous soybeans is potassium.

“An 80-bu. soybean crop, for example, is the equivalent of removing 100 lb. of P205,” says Shawn Conley, University of Wisconsin Extension soybean specialist. “Make sure you put that back—there are interactions with soil fertility and insect and disease complexes.”

When plants have low fertility they’re more susceptible to diseases and insects. For example, soybean aphids reproduce more rapidly when soybeans are low in potassium, Conley adds.

Watch for low potassium in fields that are sandy with low organic matter, wet or heavily compacted and those that have previously tested low in potassium. In addition to encouraging aphid reproduction, soybeans with low potassium can be stunted and have irregular yellow mottling around leaflet margins, dying leaf margins, ragged older leaves, purple seed stains and misshapen or wrinkled seeds.

“If soybeans are your most profitable crop you need to treat them like they are,” says Bill Wiebold, University of Missouri Extension soybean specialist.

Calculate Breakeven for Continuous Soybeans

Despite years of research, scientists can’t pin down the exact reason second-year soybeans yield less than those in rotation with corn. Researchers estimate a 5% to 10% yield loss right out of the gate in second-season soybeans. They recognize having the same host year after year can increase disease, insect and weed pressure—all of which could play a role in soybean yield loss. Take a critical look at soil type, propensity for disease and productivity of fields to estimate percent year-over-year yield loss.