The Risk of Beans on Beans (on Beans)

The soybean bubble today could burst like the sunflower bubble did in the early 1980s, when growers began to run rotations too short, resulting in pest problems.

By Tracy Sayler

Economics is driving some growers to continuous soybeans, and 2006 will likely see more beans on beans – or beans on beans on beans – than any other growing season in the Northern Plains.

However, the soybean bubble today could burst like the sunflower bubble did in the early 1980s, when growers began to run rotations too short, resulting in pest problems.

Soybean on wheat Four years soybeans

Research data points to improved N production by soybean in the second year, due to the presence of N-fixing bacteria in the soil profile. And research data suggest only a slight risk in yield drop-off and disease potential in the second year of beans, compared to the first year.

But North Dakota State University extension agronomist Duane Berglund advises no more than two years of continuous beans.  “It’s the third year of continuous beans and on where we risk running into all kinds of potential problems,” says Berglund.

Research indicates yield potential drops further in three or more years of continuous beans, compared to two years of continuous beans. The decline is about 4 to 5 bu/ac, a loss of about $20 to $25 per acre.  Berglund speculates that poor nodulation and weaker, diseased root systems in continuous beans are a key contributor to lower yields.  The yield effect can be hidden, if there is no check field to compare continuous beans to beans in a proper rotation with a grass crop like wheat or corn.

Berglund says the production risks of continuous beans are many:

• Reduced root structure and health
• High potential for root rot
• Increased potential for foliar and stem diseases
• Sclerotinia risk
• Soybean Cyst nematode risk
• Increased potential for soybean aphid problems
• Weed species shifts –continuous planting of  RR soybeans
• Reduced soil structure/tilth

Berglund is quick to dismiss the coffee shop stories that circulate about continuous beans grown further south without repercussions.  Not true, he says, and those who attempt it run into problems. Root rot, brown stem rot, stem canker, and sudden death syndrome are diseases more common further south that could spread north, with shorter soybean rotations. In some areas of the corn belt, soybean on soybean results in 35 to 55% or more yield reductions.

A seed treatment is a virtual prerequisite for continuous beans, to help provide protection against early season diseases, such as Fusarium , Pythium, and Rhizoctonia.  A broad-spectrum seed treatment will help, but residual effectiveness will wear down as the growing season progresses, says Berglund. Always plant a Phytophthora resistant soybean cultivar when going soybean on soybean since this disease also can build under continuous soybeans in wet years.

Inoculation to encourage root development, and planting the most disease-resistant bean type possible may help continuous soybeans, but Berglund says that ultimately, a proper crop rotation is still the best option, such as:

• Wheat – Soybean – Corn – Oilseed crop
• Dry Bean – Wheat – Soybean – Corn
• Wheat – Soybeans – Wheat – Soybean
• Wheat – Dry Beans – Corn – Soybean
• Wheat – Sugarbeet – Corn – Soybean
• Corn – Soybean – Wheat

Considerations when designing crop rotations:  yield enhancement, disease and insect management, weed management (volunteer crop, resistant weeds, and herbicide carryover) Soil/residue management, fertility, water use efficiency, long-term productivity, climatic limitations, production equipment, management skills, marketing, and of course, economics.

Excessive Residual N May Favor Iron Chlorosis
Evidence at the University of Minnesota suggests that too much residual N in the soil may be a condition favorable to iron chlorosis in soybeans, according to George Rehm, University of Minnesota extension fertility specialist. 

“That’s our theory.  We collected quite a bit of evidence last year that suggests this is the case, and it matches up with fundamental plant physiology work, so next summer we plan to go out and see if our theory is correct,” he says.

Give serious thought to rotating to a cereal crop in fields where iron chlorosis is or has been a problem, Rehm advises.

Research at NDSU on iron chlorosis in soybeans:  www.soilsci.ndsu.nodak.edu/yellowsoybeans .