Features Agronomy Soybeans
Using a shorter rotation and no-till can worsen some diseases

Attention growers: using no-till and a shorter rotation with soybeans can put the crop at greater risk for many detrimental outcomes. However, with proper awareness and some changes in crop management practices, the risks can be adequately managed.

There are no soybean varieties with resistance to Rhizoctonia or Fusarium. Photo courtesy of Albert Tenuta, OMAFRA. Advertisement

Short rotations have caused soybean acreage in Ontario to skyrocket in the past three decades. During that time frame, soybeans have been planted every other year or two out of three years, to take advantage of prices being paid for both glyphosate-tolerant and identity preserved (IP) soybeans. As a result, growers have pushed production from 22.3 million bushels in 1981 to 96.3 million in 2009.

“Global demand for soybeans has increased dramatically, but we’re probably close to maximum acreage in this province,” notes Crosby Devitt, manager of research and innovation for the Grain Farmers of Ontario. In addition to good prices, Devitt also attributes expanded acreage to new varieties adapted to larger areas of Ontario, especially the lower heat unit areas.

Despite the profit potential however, the risks of using a short rotation are numerous. “This practice causes P, N and especially K to decrease more than growers generally believe,” says Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. “Growing soybeans more often also decreases organic matter and soil stability and increases the potential for damage from insects and disease.”

Bohner estimates that in 2009, only about half the potassium removed by the provincial soybean crop was replaced through fertilizer and manure. “About 40 pounds of P and 70 pounds of K are removed by a 50 bushel per acre crop,” he notes. “We generally think of soybeans as a low-input crop, but it’s not. It removes a lot of nutrients, and only a small amount of soybean crop residue is put back into the soil.”

Bohner adds that in the long term, “When you remove wheat or other crops from your rotation, through the rotation effect (a combination of many interacting factors) you are hurting your soybean crop more than you think. Research results from long-term rotation trials conducted at the University of Guelph’s Ridgetown Campus have shown that removing wheat from the rotation will reduce soybean yields by four bushels per acre.” 

Some growers in the southern end of Lambton County are using a rotation of back-to-back soybeans with wheat in the third year, notes John Waters, certified crop advisor at Lakeside Grain and Feed Ltd. in Forest, Ontario. “The heavy clay soil there isn’t suitable for corn,” he says, “but back-to-back isn’t helping with disease control and other factors.”

However, soybean yields have also been inconsistent in surrounding areas where a three-year rotation of corn, wheat and soybean is used, something Waters attributes to several factors. “Better varieties would really help,” he says.

Dave Hendrick, president of Hendrick Seeds in Inkerman Ontario, agrees. “Four years ago, we decided to create our own soybean research program,” he states. “We are collaborating with researchers and also encourage growers and buyers to participate in the selection of new varieties that produce better yields and have better disease resistance and food traits.” 

There are, for example, no resistant soybean varieties on the market for either Fusarium or Rhizoctonia, notes OMAFRA field crops pathologist Albert Tenuta. “Fusarium, Rhizoctonia and many of the best soybean soil pathogens occur on most soil types and infect multiple hosts,” says Tenuta. “Rhizoctonia root rot affects multiple crops, while there are more than five species of Fusarium that affect soybean, and many affect corn and wheat as well.”

While rotation generally helps a great deal with disease control, Tenuta warns growers that lengthening rotation will not have a significant effect on managing some soybean diseases such as soybean cyst nematode and Phytophthora root rot. In addition, he observes that disease and pest management will begin to get more difficult if the expected effects of climate change become more pronounced. Asian soybean rust, which arrived in the US in 2004 and was detected in Ontario in 2007, plus other diseases such as frogeye leaf spot and charcoal rot, may become more common in the future.

No-till, no problem
For growers with soybeans in a shorter rotation in combination with no-till, their crops are not just at greater risk of disease; soil compaction could also be a problem.

Growers with soybeans in a shorter rotation in no-till are not only at greater risk for disease, but also for soil compaction.

Approximately two-thirds of the soybean crop in Ontario is grown under no-till systems or with reduced tillage systems. “No-till has had a very big impact on the incidence of soybean diseases in the province,” Tenuta observes. “It provides a good starting point for many stem, root and foliar diseases.”

Soil stability and organic matter are also lost when a short rotation with lots of soybeans is used, says Bohner. Because of the root structure of the crop, soybeans make the soil “tighter” and more prone to compaction due to a reduction in the stability of aggregates.

Fortunately, “From a disease-control perspective, a rotation of one in three years still provides good benefits in terms of reducing risk, if you are on top of things,” says Tenuta. “You can’t just rotate your crop and walk away from your field.”

Plant only when conditions are fit in terms of soil moisture, says Dave Townsend, crop manager at Syngenta Seeds Canada. “Minimum tillage,” he also advises, “can help reduce soil pathogen inoculum on trash, and warms and aerates the soil for early planting.”

In terms of yield, OMAFRA studies show that tillage with a one-pass coulter unit provides a two bushels-per-acre benefit over straight no-till in soybeans. Coulters used at the time of planting were also found to provide a marginal benefit if run at a depth of nine centimetres (3.5 inches), but coulters operated at a depth of 3.8 centimetres (1.5 inches) offered no yield gain.“Tillage is a tool we’ve forgotten about in terms of management of many diseases,” observes Tenuta. “If it doesn’t increase erosion or harm your soil, tillage can do a lot to help manage disease risks.”