Mangroves are a critical component of many coastal ecosystems in Florida. Woody species, including Brazilian peppertree (Schinus terebinthifolia Raddi), have invaded thousands of hectares of mangrove habitat. The difficulty associated with ground-based management of invasive plants in mangrove communities has warranted a need to identify selective herbicides that can be applied aerially. Recent work suggests that Florida mangrove species are extremely sensitive to synthetic auxin herbicides; however, other herbicides have yet to be tested for selectivity. Greenhouse studies in 2018 and 2019 evaluated broadcast foliar applications of the acetolactate synthase inhibitor imazamox and protoporphyrinogen oxidase inhibitor carfentrazone-ethyl, both as individual treatments and in combinations, for control of S. terebinthifolia and injury to four non-target mangrove species. Across all posttreatment sample dates and species tested, there were no significant interactions between imazamox applied at 0.28 or 0.56 kg ai ha−1 in combination with carfentrazone-ethyl applied at 0 or 0.1 kg ha−1. Main effects of imazamox applied at 0.56 kg ai ha−1 and carfentrazone-ethyl applied at 0.1 kg ha−1 resulted in 99% and 97% defoliation, respectively, of Schinus terebinthifolia at 180 DAT. However, S. terebinthifolia percent survival was 56% and 44% for the same treatments. Both herbicides severely injured all four mangroves by 90 DAT and resulted in 58% to 100% defoliation across species. At 180 DAT, significant increases in percent cambium kill were also observed for all four species. Across species, mangrove survival varied, but red mangrove (Rhizophora mangle L.) survival was reduced to 6% when imazamox was applied at 0.56 kg ha−1. These results indicate both imazamox and carfentrazone-ethyl exhibit activity on S. terebinthifolia but also injure all four mangroves enough to preclude their use as selective treatments.

Palmer amaranth is one of the most problematic weeds in cropping systems of North America, especially in midsouthern United States, because of its competitive ability and propensity to evolve resistance to several herbicide sites of action. Previously, we confirmed and characterized the first case of nontarget site resistance (NTSR) to fomesafen in a Palmer amaranth accession from Randolph County, AR (RCA). The primary basis of the present study was to evaluate the cross- and multiple-resistance profile of the RCA accession. The fomesafen dose-response assay in the presence of malathion revealed a lower level of RCA resistance when compared with fomesafen alone. The resistance index of the RCA accession, based on 50% biomass reduction, ranged from 63-fold (fomesafen alone) to 22-fold (malathion plus fomesafen), when compared with a 2007 susceptible, and 476-fold and 167-fold, respectively, relative to a 1986 susceptible check. The RCA accession was resistant to other protoporphyrinogen oxidase (PPO) inhibitors (i.e., flumioxazin, acifluorfen, saflufenacil) as well as the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor tembotrione and acetolactate synthase (ALS) inhibitor pyrithiobac sodium. Sequencing of the ALS gene revealed no point mutations, indicating that a target-site mechanism is not involved in conferring ALS-inhibitor resistance in the RCA accession. Of the three PPO-inhibiting herbicides tested in combination with the malathion, saflufenacil resulted in the greatest biomass reduction (80%; P < 0.05) and lowest survival rate (23%; P < 0.05) relative to nontreated plants. The application of cytochrome P450 or glutathione S-transferase inhibitors with fomesafen did not lead to any adverse effects on soybean, suggesting a possible role for these compounds for management of NTSR under field conditions. These results shed light on the relative unpredictability of NTSR in conferring herbicide cross- and multiple resistance in Palmer amaranth.

Reducing herbicide rates can reduce production costs and the potential for carryover injury in subsequent crops, but may also compromise residual weed control. When used at the highest recommended rates (0.35 kg ai/ha), fomesafen may cause carryover injury on sweet corn. One of the most valuable attributes of fomesafen use in snap and dry beans is control of common ragweed. Field experiments were conducted in 1999, 2000, and 2002 to investigate both the residual ragweed control from fomesafen at 0.09, 0.18, and 0.36 kg ai/ha and the potential for a second application to extend the duration of residual control, while reducing the total amount applied per season. Results were significantly affected by weather conditions. In a very wet year (2000), with the greatest ragweed pressure, a single application of fomesafen at 0.20 kg/ha was predicted to provide at least 90% control of ragweed throughout snap and dry bean seasons. In the driest year (1999), 0.09 kg ai/ha provided greater than 90% control for the same period. Regardless of whether a single or double application was made, the total amount of fomesafen required per season was less than the manufacturer's recommended rate. Compared with making a single application, two low-rate applications did not reduce the total fomesafen rate required per season. To help insure at least 90% control of ragweed throughout the snap and dry bean seasons a grower should apply 0.20 kg ai/ha early in the season. Lower rates may be used, but in wet seasons, a second application may be needed to prevent yield loss or interference of ragweed with mechanical harvesting.

Glyphosate-resistant (GR) horseweed management continues to be a challenge in no-till cotton systems in Tennessee and Mississippi. Field studies were conducted in 2009 and 2010 to evaluate saflufenacil in tank mixtures with glyphosate, glufosinate, or paraquat on GR horseweed prior to planting cotton. Saflufenacil and saflufenacil tank mixtures were applied 7 d before planting (DBP). Three broad spectrum herbicides were tank-mixed with saflufenacil at rates of 0, 6.3, 12.5, 25, and 50 g ai ha−1. Saflufenacil at 25 and 50 g ai ha−1 in tank mixture with all three broad-spectrum herbicides provided similar GR horseweed control when compared to the current standard of glyphosate + dicamba. Across all saflufenacil rates, lint cotton yield among the glyphosate, glufosinate, and paraquat tank mixture treatments did not differ from each other. Control of horseweed with 25 or 50 g ha−1 of saflufenacil across all tank mixtures also was not different from the standard of glyphosate + dicamba. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more cotton injury than glyphosate applied 7 d or more before planting. Saflufenacil applied alone at 25 g ha−1 provided lower control of GR horseweed than the standard, which translated to lower lint yield compared to the glyphosate + dicamba treatment or saflufenacil with each tank mixture partner. The 12.5 g ha−1 rate of saflufenacil tank mixed with either paraquat or glufosinate provided less horseweed control (< 85%) than if higher rates of saflufenacil were used (> 95%). However, lint cotton yield was not different between these treatments. This research suggests that saflufenacil at 25 g ha−1 is the most optimal rate for tank mixtures with glyphosate, glufosinate, or paraquat. It also reaffirms earlier research that the 25 g ha−1 saflufenacil rate safely can be applied inside the currently labeled 42-d waiting period between a saflufenacil application and cotton planting.

Glyphosate-resistant common waterhemp is a difficult-to-control annual broadleaf weed that has become a serious management challenge for growers in Nebraska and other states in the United States. The objectives of this study were to confirm glyphosate-resistant common waterhemp in Nebraska by quantifying level of resistance in a dose-response study, and to determine the sensitivity and efficacy of POST soybean herbicides for controlling suspected glyphosate-resistant common waterhemp biotypes. Seeds of suspected glyphosate-resistant common waterhemp biotypes were collected from seven eastern Nebraska counties. Greenhouse dose-response experiments were conducted to evaluate the response of common waterhemp biotypes to nine rates of glyphosate (0 to 16×). Common waterhemp biotypes were 3- to 39-fold resistant to glyphosate depending on the biotype being investigated and the susceptible biotype used for comparison. Results of the POST soybean herbicides efficacy experiment suggested that glyphosate-resistant biotypes, except a biotype from Pawnee County, had reduced sensitivity to acetolactate synthase (ALS)–inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Glufosinate and protoporphyrinogen oxidase (PPO)–inhibiting herbicides (acifluorfen, fluthiacet-methyl, fomesafen, and lactofen) provided ≥ 80% control of glyphosate-resistant common waterhemp at 21 d after treatment (DAT). This study confirmed the first occurrence of glyphosate-resistant common waterhemp in Nebraska, and also revealed reduced sensitivity to ALS-inhibiting herbicides in most of the biotypes tested in this study.