Features Pulses
How many years are too many years?

Soybeans offer valuable benefits in a crop rotation. However, like any crop, if you grow soybeans too often, problems can start to emerge. What is the optimal rotational frequency of soybeans that will sustain strong yields, good economic returns and healthy soils?

Agronomist Yvonne Lawley investigated that question in an eight-year rotational study in central Manitoba.

The first four years

“When this study started in 2014, soybeans were rapidly expanding in Manitoba rotations. That was being driven in part by important breeding work and having early-season soybeans available,” explains Lawley, an assistant professor in the Plant Science Department at the University of Manitoba (U of M). “But it was also driven by market and weather conditions. It was very profitable to grow soybeans. And Manitoba was in a wet cycle and soybeans were doing really well relative to other crops.

“When you have a very profitable crop, you grow a lot of it. But how much soybean in a rotation should you be aiming for? What are some of the consequences and trade-offs of having a really soybean-heavy rotation?”

Lawley’s selection of rotation treatments for the study was based on several considerations. “One consideration was the existing research on optimizing soybean rotations in the U.S. Midwest and Ontario, which often focused on optimizing soybean and corn rotations. So, we included a corn-soybean rotation to build on that existing knowledge.” 

In addition, Lawley chose other rotations that aligned with the choices Manitoba farmers were making at the time. “We selected a canola-soybean rotation because these were Manitoba’s two highest income-earning crops, even though it’s uncommon to have a rotation with only broadleaf crops and no cereals. We also took a more diversified approach, alternating between cereal and broadleaf crops with a wheat-canola-corn-soybean rotation,” she explains.

“We compared these three rotations to continuous soybean. At that time in Manitoba, some farmers were growing soybean after soybean, and we knew there could be challenges with that.”

The first four-year phase of the study ran from 2014 to 2017. The rotations were sequenced so that in the study’s fourth year, all rotations involved soybeans.

Interestingly, there were no significant differences in soybean yields between the rotations in 2017.

Lawley suspects this was likely because soybean production in Manitoba was still in a “honeymoon phase.” Since soybeans were relatively new to the province, there wasn’t substantial disease and insect pest pressure. This possibility aligns with the fact that Manitoba farmers were still achieving reasonable soybean yields despite frequent planting.

Phase 2: 2018 to 2021
In 2017, with soybean acres at an all-time high in Manitoba, many farmers were continuing to plant soybeans in tight rotations. Lawley was particularly interested in what would happen, not only to soybean yields but also to soil health if the study’s rotations, especially the continuous soybean treatment, were extended.

So, in 2018, she initiated a second four-year cycle of the study, with funding from Manitoba Pulse and Soybean Growers, Western Grains Research Foundation and Manitoba Agriculture through the Canadian Agricultural Partnership.

“It was a great opportunity to look at that long-run soil health question,” she notes. “Soybean is a low-residue crop. If we grow soybeans really frequently and we’re not returning much residue to the soil, how will that affect soil health?”

In an innovative approach, Lawley used this extended study as a field lab for interdisciplinary research on rotational effects. From the final year of phase 1 to the end of phase 2, Lawley collaborated with her U of M colleagues: plant pathologist Fouad Daayf on soybean disease and microbiologist Ivan Oresnik on Bradyrhizobium japonicum, soybean’s nitrogen-fixing bacterial partner. 

“We can learn so much from long-term rotation studies,” she says. “It’s exciting to have funders willing to invest in these studies, and it is exciting to work in a multidisciplinary environment and train graduate students in a multidisciplinary group. It was a really fun and stimulating environment.”

Study details
The second phase of the study marked a shift in Manitoba’s growing conditions, Lawley says. “The first four years were wetter, but in the last four years, we transitioned into a drier cycle. It was especially dry in 2021, our final field season, with soybeans growing side by side in each of our treatments.”

In phase 2, the fieldwork continued at the University of Manitoba’s Carman Research Station and Richardson International’s Kelburn Farm with the same four rotations: corn-soybean-corn-soybean; canola-soybean-canola-soybean; wheat-canola-corn-soybean; and continuous soybean.

Lawley’s team continued to monitor various factors in the rotations, including crop yields, biomass amounts, carbon inputs from crop residues, nutrient cycling and levels of soil nutrients like nitrogen, phosphorus, potassium and sulfur.

They expanded their evaluation of soil health, using various tests to assess different soil carbon pools, including total organic carbon and active carbon, a newer test. Lawley explains, “Plant residues can be at various stages of decomposition. Some soil carbon is challenging for microbes to decompose, while active carbon is the pool of carbon readily available for microbial activity or mineralization, releasing nutrients from it.”

They also examined soil enzymes, a recent innovation for assessing soil health. “Bacteria use enzymes to break down plant residues, allowing them to access the nutrients from those residues. This is how bacteria contribute to nutrient cycling,” she explains. “These enzymes can be extracted from the soil to determine their concentration. Since different enzymes are involved in various reactions to break down plant residues, we can select specific enzymes to target different bacterial functions in soils.”

Lawley focused on enzymes related to carbon cycling because soybeans, being a low-residue crop, return relatively low amounts of carbon to the soil. The study primarily focused on β-glucosidase, an enzyme involved in carbon cycling, and β-glucosaminidase, an indicator of both carbon cycling and nitrogen mineralization. She adds, “I think our study is among the first to look at these two enzymes in our region, where we have young soils with higher soil organic matter.” 

 Yield and disease findings
In the second phase, Lawley and her team still found very few differences in soybean yields between the different rotations. “It was only at the end of that second four-year cycle where we observed a yield reduction in continuous soybeans,” she says.

“We had numerically lower yields in the continuous soybean treatment at both sites but statistically lower yields only at our Carman site where some disease issues were driving the poor soybean performance.” 

Daayf’s post-doctoral researcher, Ahmed Abdelmagid, who is now a soybean pathologist with Agriculture and Agri-Food Canada, monitored the plots for soybean root and foliar diseases. Root rots, especially those caused by Fusarium species, were the most significant disease issue and were particularly severe in continuous soybean.

Also in the continuous soybean plots, Abdelmagid found two pathogens that had never been reported on soybeans in Western Canada: Fusarium cerealis, causing root rot, and Diaporthe caulivora, causing northern stem canker.

“Despite those disease problems, the continuous soybean treatment wasn’t a meltdown – it still had reasonable yields at the end of the eight years,” Lawley notes. “That is consistent with what we are seeing in Manitoba: we’re starting to have some pest problems, but we still can handle a fair amount of intensity of soybeans in rotation because we’re still in the process of building up these pest problems.”

Rotation economics results
To assess the economics of the rotations, Lawley’s team compared grain prices and input costs using provincial averages and analyzed the profitability of the different rotations. “Due to the limited differences in soybean yields, input costs were the primary drivers of profitability for these rotations,” she explains.

“This is consistent with what we are seeing in farmers’ rotations especially in Manitoba. The low input costs of soybeans make it an attractive crop option as long as yields and prices are good. For the rotations with higher input crops, like corn and canola, input costs are driving profitability.

“The continuous soybean treatment was very profitable because we got reasonable yields through all the years of the rotation, despite the yield drag in the final year. Right now in Manitoba, farmers are growing other crops in place of some of the soybeans in their rotations, but that is driven by lower soybean yields due to drier conditions, not because of its fit within the rotation itself.”

Soil health findings
The different rotation treatments showed substantial differences in biomass and carbon returns, with continuous soybean having the lowest levels for both measures.

Although the total carbon content did not change significantly for any of the rotations, active carbon levels did differ. “We observed a reduction in active carbon in the continuous soybean treatment compared to all other rotations,” she says.

“Another indicator that did move was the soil enzymes, particularly β-glucosidase, the carbon cycling enzyme. Where a higher residue crop, corn or wheat, preceded soybean, we saw increased activity of β-glucosidase; the bacteria were busy breaking down those residues and this was reflected in increased enzyme activity.” β-glucosidase activity was lowest for the continuous soybean treatment. 

Lawley notes, “To build populations of nutrient-cycling bacteria in the soil, we need to provide crop residues – a food source for these microbes – in our rotations. Since soybeans are a low-residue crop, it’s essential to consider how to address this in your soybean rotations.” As these findings show, one option is to include high-residue cereals in the rotations.

Yvonne Lawley has been studying how the frequency of soybeans in Manitoba crop rotations impacts crop yields and soil health.

Soil nutrient levels
“In our continuous soybean treatment, we did not apply phosphorus fertilizer. Not surprisingly, we observed a decline in soil test phosphorus levels in that treatment,” Lawley notes.

She explains that this finding aligns with research conducted by U of M soil scientist Don Flaten regarding soybean fertilization with phosphorus. “That research shows soybeans respond much more to residual phosphorus in the soil than to phosphorus fertilizer with the crop. So you need to think about where in your rotation you are going to fertilize with phosphorus to meet the needs of the soybean crop.”

Annual soil tests also revealed that, although soybeans meet their nitrogen requirements through nitrogen fixation, they do not provide a nitrogen boost for the following crop.

The carryover nitrogen findings also indicate a useful crop sequencing strategy for soybeans. Lawley says, “If soil pools of nitrogen are present, then soybeans will use that nitrogen rather than fixing their own nitrogen. That means we could place soybeans after a crop that uses a lot of nitrogen. If we let that preceding crop drawdown the soil nitrogen levels, then the soybeans will work harder for us and fix more nitrogen.” That approach would help reduce nitrogen fertilizer use in the rotation, lowering input costs and potentially minimizing greenhouse gas emissions. 

Bradyrhizobia results
Oresnik’s team quantified the bradyrhizobia populations in the different rotations from 2017 to 2021. Their findings indicate that bradyrhizobia can survive the winter, but the population gradually declines over time, mainly depending on the number of years since soybeans were last grown in the soil. The population was highest in continuous soybean and lowest in the canola-soybean rotation, but these differences did not significantly affect soybean yields.

To investigate the impact of flooding on bradyrhizobia populations, Lawley and Oresnik conducted a controlled environment experiment using soils collected from the different rotations. They compared the effects of various flooding durations (one, three or six weeks) under different temperatures (five or 25°C).

“The really cool thing about this experiment’s results was that bradyrhizobia are pretty tough and resilient, and they can survive flooding under both warm and cold conditions,” says Lawley. These findings suggest that recent flooding might not be a critical factor for soybean growers when making decisions about soybean inoculation in Manitoba fields with a history of soybean production. 

Key takeaways
“Soybeans are a great crop to have in a rotation, and they can be a very profitable crop. But when considering the frequency of soybean in a rotation, some long-run trade-offs need to be weighed against the short-run economics of returns each year. Those trade-offs could include buildup of disease pressure and the need to manage soil health,” says Lawley.

“The results of this study mirror what we have seen with soybeans in Manitoba – that we still do have some honeymoon left, but that things are changing. For instance, we will likely have more soybean disease problems in the future. Are they threatening yields right now? Probably not in every field. But those small yield drags are probably around the corner. Problems like soybean cyst nematode are moving into Manitoba and will influence how frequently soybeans should be grown in a rotation.”

Focusing on soil health, Lawley points out that growers have several tools at their disposal to compensate for soybean’s low crop residues. Studies by Lawley and her lab demonstrate that adding higher-residue crops to the rotation, practicing reduced tillage and direct seeding after soybeans, and incorporating cover crops in the rotation can effectively maintain soil health. These measures increase carbon inputs, nourish soil microbes and reduce soil erosion.

“Interestingly, now that over eight years have passed since we started our soybean rotation study, the rotation that has emerged as very common in Manitoba is soybean-canola-wheat. That three-year rotation balances the need for high carbon crops like wheat or corn in a rotation with broadleaf crops like soybeans and canola.”

Lawley concludes, “Our eight-year study addressed the information needs we had at the time. Now is a good time to take stock of where we’re at and explore the emerging questions about Manitoba rotations.”