Be Careful Planting Soybeans on Soybeans 

High soybean prices and loan support have some farmers considering planting a second or even third year of soybeans in some fields. This 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 steam rot over-winters in the soil and can live as saphrophyte 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. 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. Soybean cyst  nematodes were found in North Dakota in Richland county for the first time in 2003.

 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. There are also a number of soybean cultivars that are resistant or tolerant to (certain races of)  phytophthora root rot.

Weed species shifts or the increase of a particular weed species in a field, because it escapes by herbicide tolerance or time of emergence, have been known to occur. By planting the second or third year of soybeans and applying the same herbicide or cultural practices, you are helping speed the selection process. 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.

University Extension staff suggest 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 go with wider rows (30 inches) to lessen the potential of white mold problems. Planting shorter season varieties and delayed planting may in some years also help reduce white mold pressure. Producers should use a different weed control program than last year if certain weeds are becoming a major problem. Rotating chemicals families and modes of action of herbicides is also suggested.

Condition and test any saved seed you are planting.  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.