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Second-year soybeans: Dethroning King Corn?Second-year soybeans: Dethroning King Corn?

Soybean Source: More farmers are considering planting beans after beans, which might make more money than a corn-bean rotation this year.

Clarke McGrath

January 24, 2018

6 Min Read
Second-year soybeans: Dethroning King Corn?
BE CAREFUL: Before planting beans after beans, know the soybean cyst nematode situation in your fields. Also, be prepared for diseases and pests, and scout fields often.

If I used a chair at the office instead of a standing desk, I might have fallen out of it when I read the headline, “USDA predicts King Soybean by 2019.”

Having gotten a lot more questions this winter about managing second-year soybeans, it made sense that there may be quite a few more soybean acres in Iowa this year. But taking corn’s crown away the year after? I don’t know enough about global markets to argue, but taking the acreage crown is one thing; keeping it through good performance is another.

Whether it ends up being King Corn or King Soybean, I just want whatever will be the best for our farmers and their operations. So, if we are going to have some second-year beans, there are agronomic issues we’ll definitely want to talk about to increase the odds of success.

Rotation effect on yield?
We are pretty familiar with the rotation effect with corn on corn compared to corn on soybeans. While beans on beans aren’t nearly as common (yet), there has been a surprising amount of research done on how different crop rotations impact bean yields.

I encourage you to peruse the research reports for a more complete picture, but a rough interpretation of work done across the Midwest matches up pretty well with what growers have experienced with well-managed soybeans.

According to the literature, the best soybean yields occur after several years of corn. Taking a look at yields for second-year beans compared to first-year beans (after corn) — while results were a little more variable across years and locations — overall the yields were about equal. Third-year beans and beyond was where things started to slip. The studies showed increased potential for a yield hit — somewhere in the neighborhood of 5% to 15%.

What about weed control?
Since space is limited here, we can’t dive too far into the importance of the word “rotation” in terms of weed management programs. But it is an important concept we’ll come back to after we hit on a few key points growers have regularly brought up in discussions. While these points are right on target for corn on corn and rotated acres as well, they are more critical for beans on beans. Several times I’ve seen fields of first-year soybeans that had pretty decent weed control get put back into beans a second year, with very different results. When last year’s “decent” control leaves more weed pressure than expected in the second bean crop, and you have limited tools to clean up escapes, it can become a jungle.

With that in mind, growers tell me that when they prioritize fields for beans on beans, they want the first-year bean field to have been essentially spotless. To aim for the best weed control:

• Optimize the timing (typically applied close to planting) and rates (full-labeled rates in many cases) of early-season residual herbicides. Then consider applying an additional residual herbicide when you make your early-postemerge trip.

• Scout your bean-on-bean fields weekly. Besides catching weeds early, there are plenty of other things to scout for anyway.

• Use multiple herbicide groups that are effective against your weeds and rotate groups over time.

• To add to that last point, take a look at the herbicide resistance traits available now or expected to be approved in the near future. Having the ability to use products from Groups 4 (dicamba, 2,4-D), 10 (Liberty/glufosinate) and potentially 27 (HPPDs) in both corn and soybeans should make it easier to diversify and rotate herbicide groups.

The flip side is having them available for both corn and beans could speed up development of weed resistance. Rotating crops won’t do us a lot of good if we don’t rotate the herbicide groups as well.

Pest control considerations
A bit of good news, as far as insect pressure goes, is that first- and second-year beans tend to have a pretty similar risk of insects. If there happens to be some increased issues, you’ll likely be on top of them anyway since we’ll have you scouting on a regular basis watching for other issues like weeds and diseases.

Frogeye and brown spot are a couple of foliar diseases that come to mind as having an elevated risk in beans on beans. If these diseases are caught early, foliar fungicides are effective on them.

Diseases are probably the biggest potential challenge for beans on beans. You are right on the money. The first step is identifying good candidate fields for beans on beans — if feasible, fields with lower disease pressures. Matching soybean varieties with strong ratings against the diseases you may face on those acres is critical. We can up our odds with seed treatments for early-season diseases like phytophthora, rhizoctonia, fusarium and pythium.

If we start talking about fields that have a significant history of soybean death syndrome or white mold, I agree with you, that’s a challenging place for first-year beans, let alone beans on beans. A history of “manageable” SDS or white mold may not be a deal-breaker if the beans won’t face many other significant challenges in the field. But in combination with other agronomic issues that would challenge your soybeans, consider moving those fields way down on the list if you have other options. A more optimistic perspective, on SDS anyway, is that improvements in genetic resistance, plus a pretty solid seed treatment, give us a fighting chance.

Soybean cyst nematode threat
Another factor that could be a deal-breaker is if you have fields with soybean cyst nematodes. Let’s just say you’ll be making their day (or year) with beans on beans. The performance of our most common source of resistance (PI 88788) continues to erode, and there aren’t many other options. A second source of resistance (Peking) is out there, but at last check it only comprised about 3% of all SCN-resistant soybean varieties.

Growers tell it to me this way: If SCN resistance in your first-year beans was subpar, what went on in that field that may have been under the radar? There could have been increased SCN feeding on soybean roots, small but significant yield losses, and increased reproduction (higher SCN pressure for the next bean crop).

They go on to point out that if they put beans back into that same field again — with almost no chance of finding seed with another source of resistance — they just don’t do it if they can help it. True, we have SCN seed treatments available, but experts indicate these seed treatments tend to work best on low to moderate populations. If there is potential for a lot of damage and high SCN populations, beans on beans in an infested field might be a prime example of “That’s asking a lot of a seed treatment.”

We’ll wrap up with a couple other suggestions I hear from growers as they look at planting second-year beans. Double-check fertility levels. For a quick example, let’s compare 180-bushel corn with 60-bushel beans. Per acre the beans may use less P, but they’ll take around 32 more pounds of K. The other point often mentioned is to consider slope of the field and the reduction in crop residue with beans on beans when choosing fields.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU.

About the Author(s)

Clarke McGrath

Clarke McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU.

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