Want to make $200 more per acre?
Greg Tylka was puzzled.
Actually, that’s putting it mildly. Tylka, an Iowa State University (ISU) Extension nematologist, coordinated a 2019 yield and soybean cyst nematode (SCN) trial in a southeastern Iowa field supported with funds from the Iowa Soybean Association. At planting, the field had moderate levels of eggs that could key an increase in SCN numbers during the season.
The trial compared yield and resistance levels of the following:
• 67 SCN-resistant varieties with the widely used PI 88788 resistance line . This resistance is used in approximately 95% of SCN-resistant soybeans.
• Two varieties with the less-common Peking-based SCN resistance.
• Three susceptible varieties with no SCN resistance.
Soybean yields from the PI 88788-based SCN-resistant varieties widely varied – from a low of around 41 bushels per acre to 58.7 bushels per acre. Average yields tallied 51.2 bushels per acre.
Not surprisingly, yields of susceptible varieties averaged 41.8 bushels per acre, although one yielded around 54 bushels per acre.
What stunned Tylka, though, were the 71.3- and 75.5-bushel-per-acre yields of the varieties with Peking-based resistance. The average of 72.4 bushels per acre dwarfed the 58.7 average of the varieties with PI 88788 resistance. At a $9 per bushel price, the 13.7 bushel per acre average yield difference made a $123.30-per-acre difference.
There’s more. The farmer who hosted the SCN experiment on his farm grew two different SCN-resistant PI 88788 soybean varieties in the remainder of the field. Those varieties averaged a yield of 50 bushels per acre.
Had he subbed one of those soybean varieties with one variety with Peking SCN resistance, the 22-bushel-per-acre difference would have magnified returns by around $200 per acre.
Why the Yield Increase?
"We’ve been doing these variety trials for 30 years,” says Tylka. “For the past two decades, we have always planted two to four Peking varieties.”
That’s because they were the only Peking varieties available. Nearly all SCN-resistant varieties are based on the PI 88788 breeding line. Meanwhile, Peking varieties are rare, although more firms are selling them.
“At first, Peking varieties were in the middle of the pack, from the highest yielding to the lowest,” says Tylka. “They have gotten progressively better relative to the PI 88788 varieties.”
Tylka reasons that’s due to two factors:
1. Seed companies are boosting yield potential of Peking varieties.
2. SCN is dragging down the yields of PI 88788-resistant soybean varieties.
Ever had waterhemp that resists glyphosate and other herbicides in your field? The same thing happens in SCN-infested fields where farmers continually plant soybeans steeped in PI 88788 resistance. Eventually, SCN resists the PI 88788 resistance.
“We really need to evolve the conversation into whether SCN is actually resistant to the resistance farmers are planting,” says Don Kyle, a Corteva Agriscience soybean breeder.
The ISU trial occurred in sandy soils, where SCN is particularly damaging. Still, it’s a vision of the future as more SCN populations resist PI 88788 resistance, says Tylka.
“If you get a hot, dry year in central Illinois or north-central Iowa, similar results will happen,” says Tylka. “In the short term, a simple fix is to start growing Peking soybeans where SCN is present. Farmers should ask seed companies for these varieties.”
Not So Easy
Still, it’s difficult for companies to redirect their breeding program to include a new source of resistance.
“What Dr. Tylka has discovered is real,” says Jim Schwartz, director of Practical Farm Research and Agronomy for Beck’s. “A challenge is to find Peking varieties that are more broadly adapted than just a field with cyst nematode.”
For example, there are areas where a malady like white mold can curtail yields akin to SCN.
The knock against varieties with Peking-based resistance has been yield drag.
“Typically, there has not been that yield punch (for Peking varieties) compared with PI 88788 varieties,” says Pat Duncanson, a Mapleton, Minnesota, farmer.
“Historically, Peking varieties have not yielded well enough to make the cut in our Elite trials system,” adds David Thompson, Stine national marketing and sales director.
That’s changing, though. “In the last couple of years, we have seen a handful of lines with Peking resistance that have made the cut and are now being commercialized,” he says.
In Corteva’s case, Kyle says its flagship Pioneer brand offers 25 varieties with Peking resistance – many that have a wide area of adaptation. Duncanson agrees with the industry trend. “We can get some Peking varieties in high-yielding genetics,” he says.
Farmers who want Peking-based varieties have to ask for them, though. “I have not had any trouble getting them in recent years,” says Ron Heck, a Perry, Iowa, farmer.
Still, Peking varieties are brand specific. “Not every brand has Peking varieties available,” says Duncanson.
Companies are launching other new sources of SCN resistance. In 2021, Syngenta plans to launch two new varieties with the PI 89772 source of SCN resistance, says Travis Kriegshauser, Syngenta strategic marketing manager for soybeans.
Where to Start
Before you begin any SCN management steps, though, sample your fields for nematodes.
“One blind spot for farmers is that without sampling, they don’t know exactly how many nematodes are out in their fields,” says Kyle.
Other tools besides alternative sources of resistance are seed treatments. Seed treatments that aim at sudden death syndrome (SDS) like Ilevo and Saltro also have activity on SCN. “They will slow the yield decline and prop up PI 88788 resistance,” says Tylka. “But they are not a substitute for Peking resistance.”
Seed treatment use on PI 88788-resistant soybeans while diversifying into Peking resistance and diversifying crop rotations with corn-after-corn and peas has helped Duncanson stave off SCN.
“We have been fighting this since the mid-1990s,” he says. “But we tested quite a few acres in 2018 and were happy to find our (egg) numbers were still low. By doing preventive steps, we can keep our (SCN) numbers low. We may never see a $200 difference due to planting Peking varieties, but we hope to never get into a situation where we have poor yields to begin with.”
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