Greenhouse studies were conducted to evaluate potential interactions among glyphosate mixtures with five acetolactate synthase (ALS)-inhibiting herbicides (chlorimuron, imazamox, imazaquin, MON 12,000, or pyrithiobac) for the control of purple nutsedge and sicklepod at two growth stages. Herbicides were tested alone at 0.5X and 1X rates (1X being suggested use rate for these herbicides) and in combination with glyphosate at 560 (0.5X) and 1,120 (1X) g ai/ha on 3-wk-old plants and at 1,120 g/ha on 6-wk-old plants. Glyphosate alone at 1,120 g/ha gave complete control of purple nutsedge and at least 78% control of sicklepod regardless of growth stage. In 3-wk-old purple nutsedge plants, three of the 20 herbicide combinations were antagonistic and 17 combinations were additive, whereas all five combinations were additive in 6-wk-old plants. In sicklepod, eight combinations were antagonistic and 12 combinations were additive in 3-wk-old plants, and all five combinations were antagonistic in 6-wk-old plants. In 3-wk-old plants, the glyphosate (0.5X) plus imazaquin (0.5X) combination resulted in highest antagonism in purple nutsedge control (79%), and the combination of glyphosate (0.5X) plus imazamox (0.5X) resulted in highest antagonism in sicklepod control (54%). These results indicate that mixing chlorimuron, imazamox, imazaquin, MON 12,000, or pyrithiobac with glyphosate does not increase glyphosate efficacy on purple nutsedge or sicklepod.

Greenhouse and laboratory experiments were conducted to study activity, rainfastness, absorption, and translocation of glyphosate with and without a nonionic organosilicone surfactant in purple nutsedge. Purple nutsedge responded differently to glyphosate depending on growth stage. Glyphosate at 2.24 kg ai/ha in 17-d-old and at 4.48 kg/ha in 10-wk-old plants controlled purple nutsedge at least 96%. Regrowth of plants and tuber resprouting were greatly reduced in these treatments. Organosilicone surfactant did not increase efficacy of glyphosate. A simulated rainfall of 2.5 cm (7.5 cm/h intensity) at 1 and 24 h after glyphosate application reduced efficacy by one-half and one-third, respectively, compared with no simulated rainfall. A rain-free period of 72 h prevented loss of glyphosate activity. Absorption of 14C-glyphosate increased from 2.8% at 1 h after application to 21.4% at 168 h after application and translocation increased from 0.43% at 1 h after application to 5.18% at 168 h after application. Organosilicone surfactant did not affect absorption and translocation of glyphosate in purple nutsedge.

Tropical signalgrass is one of the most serious weed problems in the St. Augustinegrass sod production in Florida, and its presence increases production costs and lowers turfgrass quality. The objectives of our research were to: (1) evaluate herbicides preemergence and postemergence for control of tropical signalgrass and (2) compare control of tropical signalgrass and other problem weeds (torpedograss, blanket crabgrass, and India crabgrass) with postemergence herbicides. In preemergence herbicide field trials, only benefin + oryzalin, imazapic, imazapic + 2,4-D, and oryzalin provided ≥75% tropical signalgrass control 8 wk after application (WAA). By 11 WAA, only benefin + oryzalin and imazapic + 2,4-D provided ≥75% tropical signalgrass control. In greenhouse experiments, eight herbicide treatments were applied postemergence to tropical signalgrass seedlings at the two-, four-, six-, and eight-leaf stages. Asulam and CGA 362622 provided ≥89% tropical signalgrass control at all application timings. Imazaquin controlled tropical signalgrass ≥98% when applied before the eight-leaf stage. However, in field trials with mature tropical signalgrass (>20 cm stolons), none of the 20 herbicide treatments applied postemergence provided acceptable control.