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.

Fall panicum is a problematic weed in cropping systems including rice in southern Florida. There is limited information on growth and reproductive ability of fall panicum in water-stressed environments. The objective of this study was to determine the effect of 12.5%, 25%, 50%, 75%, and 100% pot soil water content (SWC) levels on fall panicum growth and panicle branch production under greenhouse conditions. Fall panicum height, number of leaves, and tillers decreased over time as SWC decreased. Fall panicum height decreased by 65% and 50% at 12.5% and 25% SWC, respectively, relative to height achieved at 100% SWC. Plants at 50% to 100% SWC were able to achieve 50% tiller production within 31 to 43 d compared with 28 d at 25% SWC. The 50% tiller production was not reached at 12.5% SWC during the duration of the study. Fall panicum shoot and root biomass, total leaf area, and number of panicle branches per plant at 56 d after SWC treatment initiation decreased as SWC decreased. Fall panicum biomass decreased 83% to 85% and 66% to 68% at 12.5% and 25% SWC, respectively, relative to 100% SWC. Leaf area declined 79% and 65% at 12.5% and 25% SWC levels, respectively, compared to the 100% SWC. Fall panicum was able to produce panicles at all SWC levels, although the plant produced significantly fewer panicle branches as SWC decreased. Plants at 12.5% and 25% SWC produced 82% and 59% fewer panicle branches, respectively, compared with plants at 100% SWC. This study shows that SWC influences the growth and reproductive capacity of fall panicum. Although fall panicum did not reach its full growth potential at low SWC levels, it was able to survive and develop panicles, showing its ability to adapt and reproduce under dry conditions.

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.

Sugarcane growers in Florida have been reporting reduced control of fall panicum with asulam, the main herbicide used for POST grass control. Therefore, outside container experiments were conducted to determine the response of four fall panicum populations from Florida to asulam applied alone and to evaluate whether tank-mix combination with trifloxysulfuron enhances control. Asulam was applied at 230 to 7,400 g ai ha−1, corresponding to 1/16 to 2X the maximum labeled rate for a single application in sugarcane, with or without combination with trifloxysulfuron at 16 g ai ha−1. Three fall panicum populations were collected from fields in which reduced control had been reported, while one population was from a field not used for sugarcane production but adjacent to a sugarcane field. The potency of asulam based on ED50 values (the rate required to cause 50% dry weight reduction at 28 d after treatment) ranged from 2,249 to 5,412 g ha−1 for tolerant populations with reported reduced fall panicum control compared with 1,808 g ha−1 for the susceptible population from the field not used for sugarcane production, showing that the latter was most sensitive to asulam. Addition of trifloxysulfuron to asulam increased potency on fall panicum by 5- to 15-fold, indicating that the tank mix enhanced dry weight reduction for all populations. The probability of fall panicum survival (regrowth after aboveground biomass harvesting) at the labeled rate of asulam ranged from 2% to 47% compared with 0% to 6% when trifloxysulfuron was added to the tank mix. Our results show differential response of fall panicum populations in Florida to asulam, which can be overcome by tank mixing with trifloxysulfuron even for populations that are difficult to control in sugarcane, but no evolution of resistance to asulam.

Glyphosate-resistant (GR) Palmer amaranth has become a serious pest in parts of the Cotton Belt. Some GR cotton cultivars also contain the WideStrike™ insect resistance trait, which confers tolerance to glufosinate. Use of glufosinate-based management systems in such cultivars could be an option for managing GR Palmer amaranth. The objective of this study was to evaluate crop tolerance and weed control with glyphosate-based and glufosinate-based systems in PHY 485 WRF cotton. The North Carolina field experiment compared glyphosate and glufosinate alone and in mixtures applied twice before four- to six-leaf cotton. Additional treatments included glyphosate and glufosinate mixed with S-metolachlor or pyrithiobac applied to one- to two-leaf cotton followed by glyphosate or glufosinate alone on four- to six-leaf cotton. All treatments received a residual lay-by application. Excellent weed control was observed from all treatments on most weed species. Glyphosate was more effective than glufosinate on glyphosate-susceptible (GS) Palmer amaranth and annual grasses, while glufosinate was more effective on GR Palmer amaranth. Annual grass and GS Palmer amaranth control by glyphosate plus glufosinate was often less than control by glyphosate alone but similar to or greater than control by glufosinate alone, while mixtures were more effective than either herbicide alone on GR Palmer amaranth. Glufosinate caused minor and transient injury to the crop, but no differences in cotton yield or fiber quality were noted. This research demonstrates glufosinate can be applied early in the season to PHY 485 WRF cotton without concern for significant adverse effects on the crop. Although glufosinate is often less effective than glyphosate on GS Palmer amaranth, GR Palmer amaranth can be controlled with well-timed applications of glufosinate. Use of glufosinate in cultivars with the WideStrike trait could fill a significant void in current weed management programs for GR Palmer amaranth in cotton.