IMPACT OF CROP-ROTATION AND TILLAGE SYSTEM ON HETERODERA-GLYCINES POPULATION-DENSITY AND SOYBEAN YIELD

The long-term effects of no-till planting practices and rotation on th e population dynamics of the soybean cyst nematode (Heterodera glycine s) and soybean yield were investigated in field experiments over a per iod of 8 yr. The experiment was a 2 X 4 factorial, comparing no-till v s. conventional tillage practices in four cropping patterns (continuou s 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 sys tem). 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 cr ops 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 yiel d early in the study, but yields of no-till soybean were lower (P = 0. 01) than conventionally tilled soybean after several years because wee d pressure was greater in no-till plots. Population densities of H. gl ycines 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 uni dentified biological control agents or excessive moisture in certain y ears. H. glycines population densities declined in a predictable manne r when a nonhost was planted.