Impact of Crop Rotation and Tillage System onHeterodera glycinesPopulation Density and Soybean Yield

The long-term effects of no-till planting practices and rotation on the population dynamics of the soybean cyst nematode (Heterodera glycines) and soybean yield were investigated in field experiments over a period of 8 yr. The experiment was a 2 X 4 factorial, comparing no-till vs. conventional tillage practices in four cropping patterns (continuous soybean, a 1-yr rotation of corn and soybean, a rotation of 2 yr of corn followed by soybean, and a corn-wheat/soybean double-cropping system). Treatments were arranged so that each combination occurred every year after 1986. Soybean after 1 yr of corn had higher yields (P = 0.0001) than soybean after soybean. Two years of corn between soybean crops resulted in soybean yields higher than those after 1 yr of corn in only 2 out of 6 yr. The yields of soybean in the corn, wheat/soybean double-cropping system, however, were generally similar to monoculture soybean. No-till practices had positive or no effects on soybean yield early in the study, but yields of no-till soybean were lower (P = 0.01) than conventionally tilled soybean after several years because weed pressure was greater in no-till plots. Population densities of H. glycines were greater (P < 0.10) in conventionally tilled plots than in no-till plots in 1988 and 1990-1992. Numbers of H. glycines fluctuated in an unpredictable manner from year to year, possibly because of unidentified biological control agents or excessive moisture in certain years. H. glycines population densities declined in a predictable manner when a nonhost was planted