May 15, 2003

 

Be Careful Planting Soybeans on Soybeans

Current high soybean prices and loan support have some farmers considering planting a second or even third year of soybeans in some fields according to Duane Berglund, North Dakota State University Extension Service agronomist. But diseases and weeds may steal any increased profit. "Continuous soybean planting may appear to be a good option after initial calculations but producers should examine the pros and cons very closely. Although production costs may be lower, there is a chance that yields may be lower as well and producers may be setting themselves up for long_term management headaches."

Organisms that attack soybeans such as root rots, white mold, brown stem rot and soybean cyst nematode are a major concern. Brown stem rot overwinters in the soil and can live on soybean residue. Other organisms that survive the winter on residue can cause bacterial blight, stem canker, pod and stem blight and brown spot.

"The first year of soybeans allows reproduction and buildup of disease inoculum and nematode populations," Berglund says. "The third or fourth year of soybeans planted into the elevated disease or nematode levels can bring about drastic results and low yields if conditions are right for disease buildup. A positive note is that soybean nematodes have not been found in North Dakota to date, but are found close by in South Dakota and Minnesota."

A piece of good news is that some root_rot_causing pathogens such as phytophthora, fusarium and pythium are already prevalent in the soil and a few more may not make a major difference, according to Berglund. There are also a number of soybean cultivars that are resistant or tolerant to certain races of phytophthora root rot.

Planting soybeans on the same field for numerous years can lead to weed species shifts or the increase of a particular weed species. The weeds develop herbicide tolerance escape treatment because they emerge after herbicides are applied each year. "By planting the second or third year of soybeans and applying the same herbicide or cultural practices, you are helping speed the selection process," Berglund says. "From a natural ecological perspective, your field wants to have weeds growing in it. The objective is to prevent a buildup of those weeds that are hard to control."

Berglund suggests maximizing the disease defensive characteristics in the varieties planted. Varieties with lower susceptibility to white mold should be selected. Producers planting soybeans on soybeans should use a wider row spacing (30 inches) to reduce the potential of white mold problems. Planting shorter_season varieties and delayed planting may also help reduce white mold pressure in some years. Producers should use a different weed control program than last year if certain weeds are becoming a major problem. Berglund suggests rotating herbicides from different chemical families or with different modes of action.

"Condition and test any saved seed you are planting," Berglund says. "A seed treatment may be appropriate depending on the disease. Producers should also remember that it is illegal to save and replant Roundup Ready soybeans."

Moldboard plowing may help control some fungi and bacteria that survive on residue. However, heavy tillage of soybean residue can destroy soil tilth and increase the potential for erosion.

Berglund suggests reducing or eliminating any nitrogen application and not inoculating seed unless there was poor nodule formation in last year’s crop. Phosphorous fertilization might be required depending on last year’s soil test and crop yield.

###

Source: Duane Berglund, (701) 231_8135, duane.berglund@ndsu.nodak.edu
Editor: Rich Mattern, (701) 231_6136, richard.mattern@ndsu.nodak.edu