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Last call for continuous soybean crops

Continuous soys are tough on the soil, tough on yields and tough on the pocketbook. So why do we still plant them?

Many years ago, Ross Daily, the television host of “This Business of Farming” spoke at a county-level federation of agriculture meeting, and in the course of his speech congratulated growers on their six-year rotations. The audience was frankly bewildered until Daily offered an explanation to clear the confusion.

“You know what I’m talking about: soybeans-soybeans-soybeans-soybeans-soybeans, and then in the sixth year, you plant winter wheat,” said Daily, counting off the years on the fingers of his hand. “But during the winter, the wheat dies off, and you ask yourself, ‘What can I grow now? Oh yeah! — Soybeans!”

Following his presentation, the federation president thanked Daily for his sarcasm-laced lesson about continuous soybeans. When Daily suggested he had maybe exaggerated a little, the federation president replied bluntly, “Not as much as you’d like to think!”

It’s a reflection of what is now a bygone practice, particularly where convenience has been replaced with knowledge and new research into agronomic stewardship. But again, perhaps not as much as we’d like to think.

In 2013 and 2014, fall planting conditions for winter wheat in Ontario were poor enough to propel farmers into a second or perhaps even third year of soybeans, even though most growers today are fully aware of the dangers of two or three years of soybeans.

Message bears repeating

Continuous soybeans are not a good idea, especially in the era of glyphosate-resistant weeds (even if that risk seems potentially offset by the coming availability of the new dicamba-resistant Xtend technology).

“Everybody knows that it’s a bad decision to grow continuous soybeans for the obvious reasons,” says Horst Bohner, provincial soybeans specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. “It’s hard on the soil, hard on the soil structure, on organic matter, it removes a lot of nutrients and there’s a whole package of diseases and insects that build with continuous soybean production. Everyone knows they’re playing with fire, but it’s the issue of what might be the lesser of two evils.”

Bohner likens it to speeding on a major highway. Driving at 30 km/h over the posted speed limit might be something you have to do short term, to pass, for instance. But the longer you do it, the greater the chance that something catastrophic will occur. The same is true with soybeans: the more consecutive years of soybeans, the higher the frequency and potential for several different management challenges, particularly pest or disease issues such as soybean cyst nematode or white mould.

Despite growth in eastern and parts of northern Ontario, soybean production is more likely to plateau in the next few years.
Despite growth in eastern and parts of northern Ontario, soybean production is more likely to plateau in the next few years. photo: Supplied

Yet continuous cropping is not a practice that Bohner believes growers do light-heartedly. Instead, it’s an option in a wet or hard-to-plant fall, or as a hedge against lower corn prices.

So while Bohner believes the prospect of using dicamba blended with glyphosate will put many growers into a better position to manage weed-resistance issues through the reintroduction of Group 2 chemistries, it won’t be enough to convince them to get back into continuous soybeans.

Besides, there’s also another shift he’s witnessing.

“I actually see a trend back to some conventional varieties by those growers who have resistance problems because they say that they need to use different chemistries anyway,” Bohner says. “And the cost of glyphosate-resistant seed is relatively high, so they’re saying, ‘Why not just save my input dollar on the seed if I have to spend it on the herbicide anyway?’”

Regardless of issues such as fertilizing soybeans or discussing the dangers (or virtues) of continuous soybeans, Bohner says soybean production really comes down to economics. How low are corn prices, and what’s the cost of nitrogen-based fertilizer? The main shift in soybean acres and its impact on continuous soybean management have largely come as a result of pricing and weather.

Site-specific issues

Also important is the local weed spectrum. For Chad Anderson, an innovation such as Roundup Ready Xtend technology has nothing to do with justifying continuous soybeans, it’s about having a solution to glyphosate-resistant Canada fleabane, which is particularly bad in his home county of Lambton. Anderson also refers to Lambton as the recordholder for the most verified cases of Group 2-resistant weed species in Ontario, which he believes creates a better understanding in the region for the dangers of relying too heavily on any technology.

“Growers who adopted tank-mix options in 2015 were quite satisfied with the results, so I think there is some confidence that management can help deal with resistance issues outside of new technologies,” says Anderson, an independent certified crop adviser (CCA) from Brigden. “But this comes at additional input costs which are worrying, given the current grain-pricing complex. It’ll come down to costs and performance: if the new technologies can deliver comparable or improved yields at costs equal to conventional options, producers tend to lean to convenience as long as yield doesn’t suffer and profit margins are comparable.”

The continuous soybean option has also been appealing in Lambton because its heavier clays are tough on corn, so it’s tempting to alternate soybeans with wheat. There is also little flexibility for growing spring cereals, and there isn’t sufficient livestock density in Lambton to make money growing alfalfa or forages. That said, Anderson notes that weed management options are much more flexible in corn than in soybeans, with a variety of different groups available to add as tank-mix partners with glyphosate. More importantly, these options are effective in controlling hard-to-control glypho-sate-restistant weeds.

“The big message I’ve heard from the likes of Mike Cowbrough (OMAFRA) and Dr. Peter Sikkema (University of Guelph’s Ridgetown Campus) is that in soybeans if you miss fleabane and giant ragweed in your soybean burn-down, you’re cooked,” says Anderson. “In corn, you can add several products which will control problem weeds, keeping the seed bank low, which will make corn even more important in the rotation than ever before.”

Changes in cropping practices

Through all of the discussion about soybeans and corn, and any long-term potential for fewer wheat acres in the rotation, Bohner echoes others on the need for longer rotations. Despite concerns about wet and tough planting in the fall, or whether wheat can be profitable, the work of Doug Young and Dr. Dave Hooker at University of Guelph’s Ridgetown Campus offers little room for debate about the value of maintaining wheat in the rotation.

“I actually believe that with all of this talk of cover crops in the rotation, the message we’re missing is that winter wheat is the best cover crop there is,” says Bohner. “To keep winter wheat in our rotation is far more important than any other cover crop you can come up with, because it’s in the ground longer and it has the benefits for the other crops.”

But there’s no doubt that crop production in Eastern Canada is in the midst of some interesting changes. In spite of one of the more open autumn planting seasons in 2015, the estimated number of winter wheat acres planted was well below one million (one estimate suggests a little over 900,000) in Ontario.

Source: Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)
Source: Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)

Thanks to the efforts of plant researchers and breeders, there’s also been a sustained northward migration for soybeans, particularly into central, eastern, and now the Near North region of Ontario and western Quebec. Growers in those regions are benefiting greatly from early-season maturities favoured in parts of Manitoba and Saskatchewan. In the Near North region of Temiskaming and New Liskeard, soybeans represent an option away from canola, where the swede midge has decimated canola production. And yet Bohner still maintains that 2.5 million acres of soybeans is the optimum production level for  soybeans in Ontario, if we want to keep a good crop rotation.

“Corn yields continue to outpace soybeans, so it’s hard for soybeans to compete with a good corn crop,” Bohner says, adding that many growers are disappointed with corn yields of ‘just’ 190 bu./ac. “For those growers who can grow a good corn crop, I believe corn is going to take more of the acreage than soybeans over the next few years. Corn will go up and soybeans will settle well below three million acres because of the frustration of not being able to get 55- or 65-bushel beans consistently, and the fact that wheat profitability can be questionable, driving more acres into corn.”  SG

Continuous cropping is an option growers take because of economics and weather more than as a regular practice, says OMAFRA’s Horst Bohner.

Despite growth in eastern and parts of northern Ontario, soybean production is more likely to plateau in the next few years.

Yield lag on continuous soybeans

For years, continuous soybeans have mainly been planted out of  short-term necessity, not because that’s what the farmer had planned. Soybeans are harder on soils than corn, they leave less residue than corn or wheat, thus impacting soil organic matter, and growers generally underestimate the crop’s nutrient use, thereby mining soils over several years. Add to that the potential for more pest and disease issues, and the practice is simply not conducive to long-term cropping success.

According to Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), research done in Wisconsin (among other locations), indicates an initial yield lag in the second and third year of soybeans. But then something interesting happens; the yield loss levels off. Second-year soybeans can lose 10 to 15 per cent off their yield potential. In third-year soybeans, a grower might lose an additional five to eight per cent, so 15 to 23 per cent in total.

But after that, says Bohner, the yield loss levels off in the fourth, fifth and sixth years, and beyond. How much more of a yield hit a grower might take beyond that is harder to predict, and largely depends on SCN and white mould pressure.

What is known is that the longer a grower plants soybeans, the greater the risk of a widespread issue arising — and with a vengeance. White mould might spring up suddenly, or a weather-related challenge, such as drought stress.

“The overall message is that it does plateau for quite a few years, but you’re just losing a good chunk of yield,” says Bohner. “And there’s no technology or seed treatment or fungicide that cures that rotation effect. All of those things help the crop and there’s more of a benefit to those inputs if you have a lot of beans in the rotation, but they don’t solve the rotation effect and the yield loss.”

This article first appeared in the February 2016 issue of the Soybean Guide

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CG Production Editor

Ralph Pearce

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