Every year in Ontario, about 20% of soybeans are planted on acres where growers harvested soybeans the previous fall.
This practice of planting ‘beans on beans’ creates significant challenges for growers seeking higher soybean yields. It’s not a recommended practice and mountains of yield data tells us why.
“Our goal is to help growers grow the best soybean crops,” says Eric Richter, agronomic services manager. “But when we plant continuous soybeans we don’t take advantage of the benefits of crop rotation, we compromise soil health, and we increase the chances that disease, insects and weeds will have a negative impact on the crop yield.”
Richter notes there is really only one exception to this rule: growers often see a small yield bump when they plant soybeans for a second year on virgin soybean ground. Other than that scenario, Richter recommends growers avoid the practice, but he understands why many growers make the decision to continuously plant the popular oilseed.
Why growers plant continuous soybeans
When winter wheat suffers winterkill, growers will often default back to soybeans. In this case, many growers justify the move by considering the abandoned wheat crop as a break crop that’s included in their rotation, but does the killed wheat crop really count? “Unless it’s a full-season crop it’s not part of the rotation and they are really planting beans on beans,” says Richter.
Market dynamics can also play a part in a grower’s decision to plant continuous beans, notes Richter. “The crop certainly requires fewer inputs and many growers feel there’s higher profit potential so they opt for the short-term profit versus the longer-term yield payback on crop rotation and soil health.”
For growers who gravitate to the beans-on-beans option, he offers several agronomic tips to minimize the negative impacts and optimize yield. The first thing to consider is your soybean variety and genetics.
Choose defensive varieties
“Choosing varieties with exceptional defensive characteristics is extremely important because you are planting into an environment that has higher disease, insect and weed pressure,” says Richter. “White mould, pod and stem blight, phytophthora root rot, sudden death syndrome, brown stem rot and septoria are all a concern when you plant beans on beans. There’s also an increased risk from pests like soybean cyst nematode (SCN).”
Richter encourages growers to plant certified seed with a seed treatment like Cruiser Maxx Vibrance Beans, which delivers both disease and insect control. “With the proliferation of SCN we really need to take some extra precaution, so if possible we should also be treating seed with a biological seed treatment like Clariva pn that can help suppress the cyst populations.”
Soil fertility is another key consideration. In the beans-on-beans scenario, potassium is the one nutrient that growers really have to watch. “Potassium plays a vital role in the plant's natural ability to fight off disease and pests, notes Richter. Aphids, for example, can be a real menace in fields that are low in available potassium. In areas where aphids are present, these fields get hit first.
Adjusting seeding rates
What about seeding rate? That’s a fine balance, says Richter. On one hand, growers may consider increasing the rate by 10% to offset greater seed mortality that could result from higher disease levels, but upping the rate could also lead to excessive vegetative growth, which can create ideal conditions for other diseases such as white mould.
Frequent in-crop scouting is also important for managing beans on beans. “You really need to watch these fields for insects like bean leaf beetle and stinkbug – two common insects that can have a major impact when the previous crop was soybeans,” says Richter. He notes that growers would be wise to consider an aggressive in-crop fungicide strategy. “This goes back to the fact that our disease incidence and pressure tends to be higher in beans on beans. A two-spray program is highly recommended in this situation – one at R1 and the second at R2.5.”
Richter also reminds growers not to forget about weed management. With the challenges the crop faces in this environment it’s critical to have a good weed management program focusing on pre-emergence with multiple modes of effective action.
Avoid the cascade effect
Richter says long-time Syngenta soybean breeder Don McClure summed up the beans-on-beans scenario best. “Don always talked about the cascade effect. The soybean can manage one stress. When you have two it’s obvious the plant is under stress. When you get a third, it just shuts right down.”
There’s one last consideration for IP soybean growers. “If you plant Roundup Ready – a GM variety – stick with that technology the following year. If you go in there with a non-GM IP variety after planting Roundup Ready, you could see some contamination with volunteer soybeans from the previous year,” says Richter. “Thankfully, it’s at a very low frequency, but we've seen a number of situations documented where this does occur.