NORTHERN "AG" EXPOSURE

by Mark Miller
Rolette County Extension Agent

Soybeans on Soybeans: What’s The Risk?

Low crop prices, loan rates and high production costs may have farmers considering planting a second year of soybeans in some fields. "This may appear to be a good option after the initial pencil pushing but should be examined more closely," says Duane Berglund, North Dakota State University Extension agronomist.

"Although production costs may be lower, there is a good chance that yields may be lower as well. Additionally, producers may be setting themselves up for long term management headaches."

Disease problems and weed specie shifts are the two major concerns with planting soybeans on soybeans Berglund says.It is not certain that a second year of soybeans will yield poorly, although this behavior does tempt fate according to Berglund. Organisms that parasitize soybeans are a major concern. Three problems that could occur are white mold, brown stem rot and soybean cyst nematode (SCN). At this point, SCN hasn’t been found in any North Dakota fields. "These diseases are well documented as greater problems in fields planted to soybeans from one year to the next," Berglund says. White mold and soybean cyst nematode persist in the soil. Brown stem rot overwinters and can also live as saphrophyte on soybean residue. A few other organisms that survive the winter on residue include: bacterial blight, bacterial pustule, anthracnose, stem canker, pod and stem blight, brown spot, and others.

The first year of soybeans allows for reproduction and buildup of disease inoculum and nematode populations. If soybeans are again planted on the same fields, it could mean they will be planted into fields with elevated disease or nematode levels. If we have a dry summer and somewhat dry conditions, disease incidence could be down. The higher the pathogen levels, however, the greater the potential for yield loss. "A piece of good news is that some root rot causing pathogens (Phytophthora, Fusarium, Pythium, and others) are already prevalent in the soil and a few more probably won't make a major difference," Berglund says.

Weed species shifts, or the increase of a particular weed species in the population of a field because it escapes by herbicide  tolerance or time of emergence, have been known to occur with  continued cropping. The increase of biennial wormwood, ALS resistant Kochia and waterhemp are examples. By planting the second year of soybeans and applying the same herbicide or cultural practices, growers are helping speed the selection process. "The objective is to prevent a buildup of those resistant weeds that are hard to control or to control the escape weeds which may occur in continuous Roundup Ready Soybean plantings," notes Berglund.

Berglund has some suggestions if you have no other choice. Choose varieties that are the most disease resistant. Also, varieties with lower susceptibility to white mold should be selected.

Try to use a different weed control program than in 2002. Use herbicides that are from different chemical families or have different modes of action. Condition, clean and test any saved seed if that is what you will be planting. A seed treatment may be appropriate, depending on the disease, if you suspect that the seed may be infected."Moldboard plowing may help control some, but not all, of the fungi and bacteria that survive on residue," Berglund says. "Unfortunately, heavy tillage of soybean residue may have a negative impact so spring moldboard plowing is sometimes not advisable. Soil tilth can be destroyed and increases erosion potential."

Reduce or eliminate any nitrogen application, and there is no need  to treat seed with inoculum if there was good nodule formation last year. Phosphorous levels may be low and added phosphorous, perhaps, is a must for higher yield potential.

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Updated April 19, 2002 Shelley Armstrong ,Webmaster