Soybean cultivar and planting date response to soil fumigation

Soybean [Glycine max (L.) Merr.] seed yield will be maximized by planting the last week ofApril or the first week ofMay in Iowa. Soybean cyst nematode (Heteroderaglycines Ichinohe; SCN) is present in the majority of Iowa fields and may influence cultivar responses to planting date. This study was conducted to determine whether specific cultivars need to be selected based on planting date. The effects of soil fumigation, planting date, and cultivar selection were evaluated at three sites in Iowa near De Witt, Nevada, and Whiting during 2005 and 2006. These sites varied in rainfall, soil type, and SCN population densities. Yields of the first two planting dates were increased an average of 10% by fumigation and yields of the SCN-susceptible cultivar NK-S32-G5 and the SCN-resistant cultivar P91M90 increased 11%. Cultivars with the P188788 source of SCN resistance produced greater yields than other sources of SCN resistance in the nonfurnigated treatment but yields were equal among all sources of SCN resistance in the fumigated treatment. Cultivars responded similarly to planting date, with maximum yields at early dates followed by a consistent decline as planting was delayed. The combination of SCN-resistant cultivars and early planting resulted in the greatest and most consistent yields across diverse production environments. Cultivar selection should focus on high-yielding, adapted cultivars with resistance to SCN when SCN is present in a field, and these cultivars should be selected for both early and late planting dates.