Atrazine and S-metolachlor are the herbicides most relied on by growers to control weeds in sweet corn crops grown in the Everglades Agricultural Area (EAA) in southern Florida. Alternative weed management programs are needed. Field experiments were conducted in 2021 and 2022 to evaluate the efficacy of 1) pyroxasulfone (183 and 237 g ha−1) alone or as a premix with carfentrazone-ethyl (13 and 17 g ha−1) or fluthiacet-methyl (6 and 7 g ha−1), S-metolachlor (1,790 g ha−1) alone or in combination with atrazine (3,360 g ha−1) applied preemergence(PRE); 2) mesotrione (105 g ha−1), topramezone (25 g ha−1), and tembotrione (92 g ha−1) applied postemergence alone or in combination with atrazine (560 and 2,240 g ha−1) or bentazon (1,120 g ha−1); and 3) mechanical cultivation alone at the fourth and the fourth followed by the sixth leaf stages of sweet corn. PRE-applied herbicides did not provide acceptable control of fall panicum, common lambsquarters, or common purslane probably due to a lack of incorporation into the soil because of limited rainfall. POST-applied topramezone alone or in combination with atrazine or bentazon resulted in effective fall panicum control (>91%). Topramezone alone provided 83% and 88% control of common lambsquarters and common purslane, respectively, whereas atrazine added to topramezone resulted in >94% control of both weed species. Mesotrione and tembotrione plus atrazine provided excellent control (>93%) of both broadleaf weed species but poor fall panicum control (<72%). Mechanical cultivation alone did not effectively control any weeds. Overall, treatments that contained topramezone resulted in greater sweet corn yield. These results show that a combination of topramezone, mesotrione, and tembotrione with atrazine resulted in improved broadleaf weed control. Fall panicum control was improved only with the combination of topramezone with atrazine, showing that atrazine is an important mixture component of these herbicides to provide effective POST weed control in sweet corn on organic soils of the EAA.

Owing to the lack of effective POST herbicide options, producers typically rely on nicosulfuron as the main POST grass herbicide in sweet corn production systems. In 2019, a Wisconsin sweet corn producer reported fall panicum control escapes after spraying nicosulfuron. Seeds from mature plants were collected to (1) measure fall panicum response to acetolactate synthase (ALS)-inhibiting herbicides, (2) elucidate the resistance mechanism, and (3) evaluate its response to alternative POST herbicides. Greenhouse and laboratory investigations were conducted to assess fall panicum response to ALS-inhibiting herbicides and elucidate the resistance mechanism. Dose–response results showed that fall panicum was highly resistant to nicosulfuron with a resistance ratio of >12.9-fold (survived rates >254 g ai ha−1, or 8× the field label rate). Molecular and genetic studies indicated that there are multiple ALS gene copies in fall panicum and that resistance was due to a mutation in one copy, resulting in an Asp-376-Glu amino acid substitution. Additional greenhouse experiments indicate that clethodim (105 g ai ha−1), quizalofop-p-ethyl (70 g ae ha−1), glyphosate (864 g ae ha−1), and glufosinate (650 g ai ha−1) are effective POST options to manage the ALS-resistant fall panicum (>90.0% control and 96.8% biomass reduction) in rotational years. The 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides isoxaflutole (105 g ai ha−1), mesotrione (105 g ai ha−1), tembotrione (92 g ai ha−1), and tolpyralate (39 g ai ha−1) did not provide effective POST fall panicum control. Because these herbicides are commonly used for POST weed control in sweet corn, more investigations are required to evaluate combinations of HPPD-inhibiting herbicides with herbicides from other sites of action for POST fall panicum control. Herein we confirm the first case of herbicide resistance in fall panicum in the United States.

A single application of 0.56 or 0.84 kg ai/ha tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl) oxirane] plus 2.3 kg ai/ha of atrazine [6-chloro-N-ethyl-N′-(1-niethylethyl)-1,3,5-triazine-2,4-diamine] at the one-to three-leaf stage controlled large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] and fall panicum (Panicum dichotomiflorum Michx. #PANDI) 96% or more. A sequential application of 1.1 kg/ha of atrazine, in addition to the above single application, was required for control of the two grass species at the four-to six-leaf stage. Application of tridiphane plus atrazine to large crabgrass and fall panicum beyond the six-leaf stage was ineffective. Tridiphane at 0.75 kg/ha, applied alone, reduced the shoot dry weight of large crabgrass by 71%. The response of tridiphane and atrazine combinations at the four- to five-leaf stage of large crabgrass was additive.

A herbicide safener, CGA-43089 [α-(cyanomethoximino)-benzacetonitrile], has been discovered which will allow the safe use of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethylacetamide] in grain sorghum [Sorghum bicolor (L.) Moench]. Experiments revealed that sorghum is protected from injury by metolachlor when it is applied at rates as high as 4 kg/ha. Better protection was achieved when sorghum seed were treated directly with the safener than when it was mixed with the herbicide. A CGA-43089 rate of 1.25 to 1.5 g per kg of sorghum seed was adequate for protection. Sorghum tolerance to metolachlor was maintained with CGA-43089 when metolachlor was combined with several s-triazine herbicides. Metolachlor effectively controlled several annual grasses with an increase in sorghum yield.

Factors accounting for the differential response of five quackgrass biotypes to primisulfuron were compared. Previous field and greenhouse experiments had identified a biotype from Montpelier, OH, as being less susceptible to primisulfuron than biotypes from Amherst, MA; Mahnomen, MN; Fargo, ND; and State College, PA. Differences in shoot, root, and rhizome fresh weights and rhizome bud numbers did not account for the reduced susceptibility of the Montpelier, OH, biotype to primisulfuron. Spray retention was lower in biotypes from OH and MA compared to the others. Absorption of 14C following 14C-primisulfuron application was greater in the Amherst, MA, biotype compared to the others. Only 29% of absorbed 14C was present as primisulfuron in the Montpelier, OH, biotype versus 41 to 45% in the other biotypes 72 h after application. The amount of primisulfuron present in the Montpelier, OH, biotype 72 h after application was less than half the amount present in the other four biotypes due to reduced spray retention and faster metabolism. These factors combined explain lower susceptibility of the Montpelier, OH, biotype to primisulfuron.

Several plant growth regulators applied to established sod driveways in an apple orchard suppressed growth of the ground cover sufficiently to eliminate one to three mowings. MH at 4.5 or 6.7 kg ai/ha applied spring and fall reduced the growth of a single species sod cover crop, ‘Kentucky 31’ tall fescue, the year after treatment. MH at both rates also reduced the dandelion population growing in the mixed species orchard sod. Paclobutrazol or EPTC applied in the spring before or during initial grass growth reduced dry matter production in the fescue sod cover crop and the number of mowings compared to the mowed and non-mowed control plots.

Postemergence applications of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl) oxirane] and atrazine, and tridiphane and cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile} resulted in 90% or better control of giant foxtail (Setaria faberi Herrm. # SETFA) less than 2.5 cm in height. Delaying the application of tridiphane plus atrazine or atrazine alone reduced the control of giant foxtail in 1984, control of giant foxtail over 2.5 cm in height was significantly greater with a mixture of tridiphane and atrazine (0.4 and 1.7 kg ai/ha) than with atrazine alone (2.2 kg/ha) or tridiphane plus cyanazine (0.6 and 1.8 kg/ha). Two successive applications of tridiphane and atrazine combinations controlled giant foxtail 8 to 16 cm tall (four to five leaves). Cultivation following applications of tridiphane plus atrazine improved giant foxtail control but was less effective than a second application of atrazine or tridiphane plus atrazine. Giant foxtail control in greenhouse experiments was similar to control obtained in the field. Tridiphane plus atrazine controlled 70% of giant foxtail in the four- to five-leaf stage, while atrazine applied alone resulted in only 23% control, indicating a synergistic effect.

Field experiments were conducted from 1983 through 1985 to evaluate the influence of tridiphane at 0.3, 0.6, 1.1, and 2.2 kg ai/ha when tank mixed with propanil at 2.2 or 3.4 kg ai/ha on control of 3- to 4-leaf or 4- to 6-leaf barnyardgrass in rice. Tridiphane at 0.6 kg/ha or greater tank mixed with propanil at 2.2 or 3.4 kg/ha enhanced control of 3- to 4-leaf barnyardgrass compared to propanil applied alone at 3.4 kg/ha. Rates of tridiphane at 1.1 kg/ha or greater plus propanil at 3.4 kg/ha were necessary to control 4- to 6-leaf barnyardgrass. ‘Starbonnet’ rice injury was minimal for all treatments.

Glyphosate-resistant (GR) crops have been adopted rapidly since their commercial introduction, and with the increase in commercially available crops resistant to glyphosate, continuous use of the same herbicide mode of action is now possible in some crop rotations. A 6-yr study was initiated to investigate the effects of conventional herbicides compared with continuous use of glyphosate in GR or Roundup Ready corn and GR soybean in a corn–soybean and a corn–soybean–winter wheat rotation. Individual experiments were fully phased and established at three locations under conventional tillage (CT) and at two locations under no-tillage (NT). Results indicated that midseason weed ground cover was lower when weeds were controlled with glyphosate; however, in most cases, this did not result in improved corn or soybean yields. Within locations, species richness, which strongly influenced other diversity indicators, was most affected by the herbicide treatments. Including winter wheat in the crop rotation had little effect on corn and soybean weed ground cover, density, and community structure and only affected soybean yield. Moreover, no effects of herbicide system used in previous corn and soybean were observed in winter wheat, with the exception of species diversity in NT, where species diversity tended to be greater when weeds in previous corn and soybean were treated with conventional herbicides. After 6 yr, the effects of continuous use of GR crops in rotation were similar to those reported in previous studies; however, continued monitoring and longer-term investigations of these systems are necessary to detect the early stages of development of herbicide-resistant biotypes.