Greenhouse and field experiments were conducted to examine foliar absorption and activity of nicosulfuron and primisulfuron by quackgrass with various adjuvants. Foliar absorption of 14C-nicosulfuron and 14C-primisulfuron plus petroleum oil adjuvant (POA) was completed by 4 h after application. Absorption of nicosulfuron plus POA and primisulfuron plus POA increased from 11 and 2% of applied, respectively, to 51 and 12% with the addition of urea-ammonium nitrate liquid fertilizer (UAN). At least 83% of the absorbed 14C from either herbicide penetrated the leaf epicuticular waxes. Absorption of 14C-labeled herbicides was greatest with the following adjuvants: POA + UAN > nonionic surfactant (NIS) + UAN = methylated seed oil. The addition of UAN to either NIS or POA significantly increased 14C-herbicide uptake. In greenhouse studies, nicosulfuron and primisulfuron applied with POA plus UAN provided greater quackgrass control than with POA alone. Despite the differences in foliar uptake in the greenhouse, few differences were observed between these adjuvants in 1989 or 1990 field efficacy trials. Quackgrass control was reduced by the addition of atrazine to nicosulfuron plus POA in 1989 and to primisulfuron plus POA in 1990. Combinations with atrazine did not reduce 14C-nicosulfuron or 14C-primisulfuron uptake by quackgrass.

Quackgrass is a serious weed problem in the northern United States and southern Canada. Field experiments were conducted in 1990 and 1991 to examine the effect of herbicide rate, application timing, quackgrass growth stage, and cultivation on quackgrass control with nicosulfuron and primisulfuron in corn. Nicosulfuron (35 g ai/ha) and primisulfuron (40 g ai/ha) each provided 84% or greater control of two-leaf plants and 93% or greater control of four-leaf quackgrass plants. Sequential applications of both nicosulfuron and primisulfuron provided greater season-long control than single applications of the same rate on two-leaf but not four-leaf quackgrass. Cultivation 10 d after nicosulfuron or primisulfuron application often increased early-season control, but by corn harvest few differences were observed. Postemergence (POST) applications of nicosulfuron and primisulfuron provided at least 88% control, compared with 66 to 86% from an early preplant application of glyphosate. Corn yields were similar regardless of the quackgrass control program.