AMES, Iowa - Recent research at Iowa State University greenhouses confirms that herbicide injury causes little loss of yield in soybeans. The study is a follow-up to a three-year study conducted in Midwestern fields.

"Herbicides are not consistently causing yield reductions," said Mike Owen, professor of agronomy. "The benefits of weed control far outweigh the risks of crop injury, period."

The research in both the field and the greenhouse has focused on interactions among a variety of factors in soybean fields, such as herbicides, diseases, variety selection, soil texture and soil pH. In the field, those factors produce a dizzying array of stresses that may or may not cause damage to soybeans.

The greenhouse experiments are critical, Owen said, to isolating the factors and understanding the interactions among them. "We use the greenhouse to assess the relative importance of possible management decisions," he said.

"Growers can change their management routines, and all their decisions will have economic consequences," Owen said. "The greenhouse research gives us the ability to tell producers that when they're juggling three balls, or factors, this is the one they should pay attention to."

In the first year of a three-year project, what Owen has learned from isolating the interactions is that when crop damage does occur and soybean cyst nematode (SCN) is involved, the disease usually is the primary culprit.

Soybean growers should use caution when applying herbicides in SCN-infested fields, Owen said. "Growers should use a herbicide that's less likely to cause injury, all the while balancing the risk of damage against weed control," he said.

Though there is evidence of an interaction between soil texture and SCN that may affect plant growth, the primary factor still comes back to SCN management. "The results from soybeans grown in different soil textures weren't sufficient to suggest that growers avoid planting soybeans in a particular soil texture, nor was there evidence to suggest that growers would benefit from avoiding a certain texture due to SCN infestation," Owen said.

His research also has shown that growers risk little in yields from planting soybeans in high pH soil. Other studies have suggested that soil pH may help predict the presence of yield-reducing populations of SCN or brown stem rot.

In continuing studies, Owen is examining the interaction between SCN and brown stem rot, the effect of deep tillage and the importance of adequate moisture on SCN. The studies will be conducted in both the field and greenhouse.

In the meantime, controlling weeds is a must. "If it's a dry year and a grower has SCN and weeds, he needs to make better decisions and make them quicker," he said.

Owen's research is part of the Yields Project; a multi-state research program funded by Iowa and Illinois checkoff dollars through the Soybean Research and Development Council (SRDC). The Yields Project is one of the largest soybean research projects ever undertaken to understand factors that limit soybean yields. The project involves scientists at Iowa State, the University of Illinois, the University of Wisconsin and Southern Illinois University.