The effects of microtopographic position on soil microenvironment and weed populations in ridge-tilled soybean were evaluated on three farms in Iowa in 1989 and 1990. In both years, over all weed species (primarily giant foxtail, green foxtail, yellow foxtail, redroot pigweed, and Pennsylvania smartweed), seedling emergence was highest in late May and early June, with few seedlings emerging after mid-June. Weed populations were highest in May and early June, after which rotary hoeing and cultivation reduced weed numbers in all plots. Microtopographic position (row, shoulder, and furrow) had a large effect on soil microenvironment and weed populations. Furrows were the wettest position through most of the growing season. Rows were the warmest position early in the season and the coolest position late in the season. Cumulative weed emergence early in the season was closely related to growing degree days, which accumulated faster in the row position than the furrow position. Following rotary hoeing and cultivation, the row position had significantly more total weeds than the shoulder and furrow positions on all farms in August of both years.
Nomenclature: Giant foxtail, Setaria faberi Herrm. #3 SETFA; green foxtail, Setaria viridis (L.) Beauv. # SETVI; Pennsylvania smartweed, Polygonum pensylvanicum # POLPY; redroot pigweed, Amaranthus retroflexus L. # AMARE; yellow foxtail, Setaria glauca (L.) Beauv # SETLU; soybean, Glycine max (L.) Merr.
Additional index words: Conservation tillage, mechanical weed control, seedling emergence, weed density, weed population dynamics.
Abbreviations: GDD, growing degree days.
