Abstract: Field research was conducted at three locations in Iowa to determine the effectiveness of glyphosate for velvetleaf control in glyphosate-resistant soybean. Velvetleaf survival ranged from 46 to 81% when treated with 420 g ae/ha glyphosate, whereas survival ranged from 13 to 37% at the 840 g/ha rate. At two locations, velvetleaf survival was greater when treated at the 12- to 17-cm height compared with application to either 8- to 10- or 20- to 30-cm-tall plants. Growth of plants surviving glyphosate application was greatly reduced, with the high glyphosate rate reducing velvetleaf aboveground biomass by over 90% compared to untreated plants. A linear relationship between biomass and capsule number was observed for both control plants and plants surviving glyphosate, both having slopes of approximately one. Velvetleaf surviving glyphosate treatment should have minimal impact on soybean yield due to reduced growth; however, surviving plants may replenish the seed bank, leading to future management problems.
Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr. #3 GLYMA; velvetleaf, Abutilon theophrasti Medik. # ABUTH.
Additional index word: Reproductive fitness.
