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The U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS) has been a leader in weed science research covering topics ranging from the development and use of integrated weed management (IWM) tactics to basic mechanistic studies, including biotic resistance of desirable plant communities and herbicide resistance. ARS weed scientists have worked in agricultural and natural ecosystems, including agronomic and horticultural crops, pastures, forests, wild lands, aquatic habitats, wetlands, and riparian areas. Through strong partnerships with academia, state agencies, private industry, and numerous federal programs, ARS weed scientists have made contributions to discoveries in the newest fields of robotics and genetics, as well as the traditional and fundamental subjects of weed–crop competition and physiology and integration of weed control tactics and practices. Weed science at ARS is often overshadowed by other research topics; thus, few are aware of the long history of ARS weed science and its important contributions. This review is the result of a symposium held at the Weed Science Society of America’s 62nd Annual Meeting in 2022 that included 10 separate presentations in a virtual Weed Science Webinar Series. The overarching themes of management tactics (IWM, biological control, and automation), basic mechanisms (competition, invasive plant genetics, and herbicide resistance), and ecosystem impacts (invasive plant spread, climate change, conservation, and restoration) represent core ARS weed science research that is dynamic and efficacious and has been a significant component of the agency’s national and international efforts. This review highlights current studies and future directions that exemplify the science and collaborative relationships both within and outside ARS. Given the constraints of weeds and invasive plants on all aspects of food, feed, and fiber systems, there is an acknowledged need to face new challenges, including agriculture and natural resources sustainability, economic resilience and reliability, and societal health and well-being.
In the southeastern United States, Amaranthus, or pigweed species, have become troublesome weeds in agricultural systems. To implement management strategies for the control of these species, agriculturalists need information on areas affected by pigweeds. Geographic information systems (GIS) afford users the ability to evaluate agricultural issues at local, county, state, national, and global levels. Also, they allow users to combine different layers of geographic information to help them develop strategic plans to solve problems. Furthermore, there is a growing interest in testing free and open-source GIS software for weed surveys. In this study, the free and open-source software QGIS was used to develop a geographic information database showing the distribution of pigweeds at the county level in the southeastern United States. The maps focused on the following pigweeds: Palmer amaranth, redroot pigweed, and tall waterhemp. Cultivated areas and glyphosate-resistant (GR) pigweed data were added to the GIS database. Database queries were used to demonstrate applications of the GIS for precision agriculture applications at the county level, such as tallying the number of counties affected by the pigweeds, identifying counties reporting GR pigweed, and identifying cultivated areas located in counties with GR pigweeds. This research demonstrated that free and open-source software such as QGIS has strong potential as a decision support tool, with implications for precision weed management at the county scale.
Precision weed management, an application of precision agriculture, accounts for within-field variability of weed infestation and herbicide damage. Unmanned aerial vehicles (UAVs) provide a unique platform for remote sensing of field crops. They are more efficient and flexible than manned agricultural airplanes in acquiring high-resolution images at low altitudes and low speeds. UAVs are more universal than agricultural aircraft, because the latter are used only in specific regions. We have developed and used UAV systems for red–green–blue digital and color–infrared imaging over crop fields to identify weed species, determine crop injury from dicamba at different doses, and detect naturally grown glyphosate-resistant weeds. This article presents remote sensing technologies for weed management and focuses on development and application of UAV-based low-altitude remote sensing technology for precision weed management. In particular, this article futher discusses the potential application of UAV-based plant-sensing systems for mapping the distributions of glyphosate-resistant and glyphosate-susceptible weeds in crop fields.
Bentazon degradation in soil typically proceeds with development of bound residue. Low sorption of bentazon suggests that this residue consists of degradation products; however, there is little data on the sorption behavior of these products. This study was undertaken to determine the sorption of bentazon and the degradation products 2-amino-N-isopropyl benzamide, 2-aminobenzoic acid, and N-methyl bentazon in Dundee silt loam and Sharkey clay, two common agricultural soils of the Mississippi Delta. Greater sorption of bentazon and degradation products in the Sharkey soil was related to finer texture and higher organic C content. Isotherms were nonlinear, with sorption increasing in the order bentazon ≪ 2-amino-N-isopropyl benzamide < N- methyl bentazon. In general, methanol extraction indicated reversible sorption of these compounds. Therefore, it is unlikely that sorption of either 2-amino-N-isopropyl benzamide or N-methyl bentazon contributes to bound residue. In contrast, essentially all of the 14C sorbed from radiolabeled 2-aminobenzoic acid solution remained bound after methanol extraction. However, due to degradation of 2-aminobenzoic acid, it was not possible to conclude the extent to which this compound contributed to bound 14C. Sorption of 14C from 2-aminobenzoic acid exceeded that of N-methyl bentazon, and at low initial concentrations (≤ 20 μM) was nearly 1000-fold greater than bentazon sorption.
Relationships between soil sorption normalized to organic carbon (Koc) and molecular properties of 71 herbicides were examined. The Koc values were obtained from the literature. Various molecular properties were calculated by quantum mechanical methods using molecular modeling software. The quantitative structure activity relationship (QSAR) models based on four molecular properties, van der Waals volume (VDWv), molecular polarizability (α), dipole moment (μ), and energy of highest occupied molecular orbital (eHOMO), together accounted for 70% of the variation in Koc. Herbicides were broadly divided into six families based on structural similarities, and separate equations were established for each group. The three descriptors, VDWv, α, and μ, along with either energy of lowest unoccupied molecular orbital (eLUMO), or electrophilic superdelocalizability (SE), or eHOMO appeared to be determinants and accounted for 82 to 99% of the variation in Koc. Applicability of these models was tested for one herbicide analogue and 10 metabolites. The QSAR models appear to be specific to structurally similar chemicals. The QSAR models could be developed to predict Koc of structurally similar compounds even before they are synthesized or for some of the metabolites of existing herbicides. Models of this type can also be developed to create priority lists for testing, so that time, money, and efforts can be focused on the potentially most hazardous chemicals.
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.
Sulfentrazone sorption kinetics, desorption, and mineralization were evaluated in surface 7.5 cm of soils collected from long-term conventional-till (CT) and no-till (NT) plots. The soils used were Miami silt loam and Drummer silty clay loam from Illinois and Dundee silt loam from Mississippi. Sulfentrazone sorption kinetics in Dundee silt loam CT and NT soils were adequately described by a simple two-site equilibrium/kinetic model. Rapid initial sorption (within 1 h) was followed by a slower sorption and equilibrium, largely achieved by 72 h of shaking, with a negligible increase in sorption thereafter. The sorption Kf ranged from 1.02 to 3.44 among the six CT and NT soils. The Kf values were greater for NT compared to their respective CT soils. Overall, Kf values were higher in Drummer silty clay loam followed by Dundee silt loam and Miami silt loam soil. The N values were less than unity in all soils indicating nonlinear sorption. Sulfentrazone desorption was hysteretic with a very low rate of desorption. The total amount desorbed in four desorptions ranged from 58 to 72% of that sorbed. Less than 2.1% of applied 14C-sulfentrazone was mineralized to 14CO2 in Dundee silt loam CT and NT soils during a 77–d incubation. Relatively low mineralization of sulfentrazone suggests poor adaptability of native microbial populations that have not been exposed to this herbicide. Higher sorption and lower desorption of sulfentrazone in NT soils compared to CT soils suggest that NT systems (which tend to increase plant residues) may prolong sulfentrazone residence time in soil.
The activity of soil-applied chlorimuron in yellow and purple nutsedge was studied in greenhouse and laboratory experiments. Soil-applied chlorimuron decreased tuber sprouting by 80% in yellow nutsedge and by 30% in purple nutsedge at 60 g ai/ha. Chlorimuron decreased shoot emergence by 53 to 83% and shoot growth by 85 to 99% in both species at rates as low as 10 g/ha. Previous exposure of tubers to chlorimuron-treated soil reduced tuber resprouting by 20 to 25% in herbicide-free soil at 60 g/ha in both species. There was no rate response in shoot emergence from tubers previously exposed to chlorimuron, but shoot dry weight decreased by 60 to 81% in both species at 60 g/ha. At 12 h after application, 47% of the total 14C applied to the shoot in yellow nutsedge and 32% of that applied in purple nutsedge were absorbed. However, less than 1% of the total 14C applied was translocated out of the shoot and into the roots and tuber in either species. In both species, 1.3% of the 14C applied to the roots and tuber was absorbed and 0.1% was translocated out of the roots and tuber into the shoot at 12 h after application. The pattern of root- and tuber-absorbed 14C distribution indicated that the 14C absorbed by the tuber remained in the tuber and that absorbed by the roots was translocated to the shoots.
Greenhouse studies were conducted to investigate the effects of adjuvant and rainfall on bentazon spray retention, efficacy, and foliar washoff in hemp sesbania, sicklepod, smooth pigweed, and velvetleaf. Bentazon was applied at 0.28 to 2.24 kg ai/ha with Agri-Dex, a crop oil concentrate (COC) or Kinetic, an organiosilicone-nonionic surfactant blend (OSB) when weeds were at the three- to five-leaf stage. Plants were subjected to 2.5 cm simulated rainfall for 20 min at 1 and 24 h after application of bentazon. Shoot fresh weight reduction assessed 2 wk after treatment was similar with either adjuvant on velvetleaf and smooth pigweed. OSB enhanced bentazon efficacy in hemp sesbania and sicklepod as compared to COC. Rainfall at 1 h after application generally reduced bentazon activity in all weeds. OSB maintained bentazon activity in hemp sesbania when subjected to rainfall at 1 h after application as compared to COC. Overall, bentazon spray retention on plants was 9 to 550% higher with OSB as compared to COC among the species at 1 h after application. Amount of bentazon residue washed off from the foliage by rainfall within a weed species was relatively similar for both adjuvants except in smooth pigweed and ranged from 39 to 98% among the four weed species at 1 h after application. OSB exhibited specificity for certain weed species and the potential to minimize bentazon spray reaching the soil by increasing deposition.
Effects of environmental factors on germination and emergence of hairy beggarticks were examined in laboratory and greenhouse studies. Optimum temperature range for germination of hairy beggarticks was 25/20 to 35/30 C (day/night, 12/12 h). Germination decreased above or below this range. Temperatures below 15/10 C and above 45/40 C were unfavorable for germination. Hairy beggarticks seed can germinate under both a 12-h photoperiod and a 24-h dark regime. Seed germinated 78 to 90% in buffer solutions of pH 4 to 9. Radicle growth was more sensitive to extreme pH than germination. Osmotic stress up to −0.1 MPa had little effect on germination, but less than 3% of the seed germinated at an osmotic stress of −0.75 MPa. Hairy beggarticks seed (13%) germinated at NaCl concentration of 100 mM but failed to germinate at 200 mM NaCl. Maximum emergence occurred when seed were planted less than 1 cm deep. No seedlings emerged when planted 10 cm deep. Flooding even for a day following planting decreased emergence to 25% compared to no flooding (56%). Seedling emergence decreased sharply with a further increase in duration of flooding, and no seedlings emerged when flooding was maintained up to 28 d after planting.
Greenhouse studies were conducted to evaluate the effectiveness of Kinetic, a silicone adjuvant that appeared to increase the efficacy and rainfastness of the isopropylamine salt of glyphosate on velvetleaf, sicklepod, barnyardgrass, guineagrass, yellow foxtail, and yellow nutsedge. Simulated rainfall of 1.3 cm in 5 min at 15 and 60 min after herbicide treatment reduced glyphosate efficacy on all weeds. Kinetic enhanced glyphosate efficacy when yellow nutsedge and guineagrass were subjected to post-spray rainfall at 60 min. On velvetleaf, sicklepod, and yellow foxtail, Kinetic improved glyphosate efficacy when the critical rain-free period was reduced to 15 min. Kinetic failed to provide rainfastness for barnyardgrass, thereby indicating a specificity of Kinetic for certain weed species.
Greenhouse and laboratory experiments were conducted to study the activity of foliar-applied chlorimuron in yellow and purple nutsedge. Foliar-applied chlorimuron caused injury to both yellow and purple nutsedge at rates as low as 5 g ai/ha. Visible injury increased as rates increased from 5 to 20 g/ha at all weekly evaluation dates. At 28 days after application, there was 84% control of yellow and 100% control of purple nutsedge from 20 g/ha of chlorimuron. In both species, all rates of chlorimuron reduced shoot dry weight, inhibited secondary shoot production, and inhibited resprouting of parent tubers attached to treated plants. Over 92% of the applied label was recovered, when 15 μl of 3.46 mM 14C-chlorimuron solution containing 0.18 μCi was applied to a 1 cm2 area in the middle of the fourth fully expanded leaf. Over 12% of the total 14C applied was absorbed, with over 15% of that being translocated within 1 day after application in both species. More than 76% of the absorbed 14C in yellow nutsedge and 72% in purple nutsedge remained in the treated area. In both species, basipetal transport was limited. Analysis of plant tissue extracts by thin-layer chromatography indicated slow degradation of chlorimuron in both species. Susceptibility of yellow and purple nutsedge to chlorimuron appears to be due to the rapid absorption and translocation rates in relationship to the slow degradation rate of the active parent compound.
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.
The hydrolytic and dealkylation products of cyanazine have been detected in soils, but the sorption of these products in soil has not been well studied. We examined sorption characteristics of five cyanazine degradation products in relation to cyanazine in Norfolk loamy sand, Tunica silty clay, and Dundee silt loam soils. Sorption was determined using a batch equilibrium method. Air-dried soil (3 g) was shaken in 6 ml of solution containing cyanazine or one of its degradation products for 48 h at 4 C. Five concentrations (2.04 to 54.67 μmol L−1) of each chemical were evaluated. The cyanazine Freundlich coefficient (Kf) ranged from 0.64 in Norfolk soil to 4.75 in Dundee no-tillage (NT) soil, and was higher in Dundee NT than in Dundee conventional-tillage (CT) soil. The Freundlich exponent (N) values for cyanazine were less than 0.85 in all soils, indicating nonlinearity of the sorption isotherm. In general, cyanazine sorption among the soils increased in the order of Norfolk << Dundee CT < Tunica < Dundee NT. Cyanazine sorption among the soils was correlated with fine texture and higher organic carbon content. Sorption of cyanazine degradation products was less than cyanazine sorption in all soils. Isotherms were nonlinear, with sorption decreasing in the order of cyanazine > desmethylpropanenitrile cyanazine > hydroxyacid cyanazine > desethyl cyanazine > cyanazine amide >> chloroacid cyanazine. The Kf for chloroacid cyanazine ranged from 0.21 in Norfolk soil to 0.42 in Dundee NT soil. Sorption patterns of five degradation products among the soils were generally similar to that of cyanazine. Our data indicate that under field conditions, cyanazine degradation products (especially cyanazine amide and chloroacid cyanazine) are more likely to remain in the aqueous phase and thus have a greater potential to move with water compared to cyanazine.
Field studies were conducted in 1982 to 1984 to determine the effects of common lambsquarters on growth, yield, and nutrient concentration of transplanted tomato. Common lambsquarters densities ranged from 16 to 64 plants/m tomato row and fresh weight ranged from 26 360 kg/ha at 16 plants/m to 46 000 kg/ha at 64 plants/m row. Common lambsquarters did not affect tomato shoot dry weight at the vegetative stage but decreased the weight at the early fruit stage. Season-long interference of common lambsquarters reduced marketable tomato fruit number and also, marketable fruit weight ranging from 17% at 16 plants/m to 36% to 64 plants/m row. Concentrations of N in tomato leaves were unaltered at vegetative and flowering stages but decreased regardless of common lambsquarters density at early fruit and harvest stages. Weed density did not alter concentrations of P, K, and Ca in tomato leaves.
Field studies were conducted to determine the effects of various barnyardgrass populations on growth, yield, and nutrient concentration of transplanted “Jetstar’ tomato. Barnyardgrass densities at 16, 32, and 64 plants/m tomato row were tested in 1982 and 1983. Barnyardgrass shoot fresh weights/unit area increased as density increased. Fresh weight of barnyardgrass shoots ranged from 17 100 kg/ha at 16 plants/m of row to 35 500 kg/ha at 64 plants/m of row. At the vegetative stage, tomato shoot dry weight was unaffected by barnyardgrass. As crop growth progressed, tomato shoot dry weight decreased at all barnyardgrass densities. Season-long interference of barnyardgrass reduced marketable tomato fruit number and fruit weight at all densities compared to weed-free plots. Reductions in marketable fruit weight ranged from 26% to 16 plants/m row to 84% at 64 plants/m row. In 1982, concentrations of N, P, K, Ca, and Mg in tomato shoots were unaffected by season-long interference of barnyardgrass at all densities. However, in 1983, concentrations of N and K decreased and concentration of P increased in tomato leaves as the density of barnyardgrass increased. Concentrations of Ca and Mg in tomato leaves were unaltered by barnyardgrass density.
Feasibility of supercritical CO2 fluid extraction of imazaquin from spiked soil (3.21 μmol kg-1) as an alternative to a conventional extraction method was investigated. The supercritical fluid extraction method involved single-step extraction of herbicide from soil with no further sample cleanup procedures. Extraction parameters were optimized for maximum herbicide recovery. Adding water as a modifier to air-dried soil significantly improved herbicide recovery. Extracting a 1-g soil sample with supercritical CO2 at 0.80 g ml-1 density and 3 ml min-1 flow rate, 80 C extraction temperature, 6 min static extraction followed by 25 min dynamic extraction, and analyte trap temperature of 40 C was optimum for maximum herbicide recovery. When optimum supercritical fluid extraction conditions were used, imazaquin recovery from three texturally different soils ranged from 55 to 64%, which was comparable to a conventional extraction method (63%). The supercritical fluid extraction method consumed 4 ml methanol and 75 ml supercritical CO2 and took approximately 1 h for sample extraction.
A 2-yr field study was conducted to evaluate the efficacy of ICIA-0051 for the control of annual grass and broadleaf weed species in conventional tillage corn. Treatments consisted of postemergence applications of ICIA-0051 alone and in combination with cyanazine or atrazine. ICIA-0051 at 0.6 kg ha-1 alone and in combination with cyanazine or atrazine at 1.1 kg ha-1 controlled large crabgrass, yellow foxtail, fall panicum, common lambsquarters, and redroot pigweed effectively (over 90%). The addition of cyanazine or atrazine at 1.1 kg ha-1 to the lowest rate (0.3 kg ha-1) of ICIA-0051 also improved large crabgrass, yellow foxtail, and fall panicum control by 9, 7, and 26%, respectively. None of the treatments of ICIA-0051 except the highest rate (1.1 kg ha-1) reduced either silage or grain yields of corn.
The influence of environmental factors on germination and emergence of horseweed was examined in growth chamber experiments. Germination was highest (61%) under 24/20 C day/night temperature under light. Horseweed seed germination was observed under both light (13 h photoperiod) and complete darkness (24 h), but germination under continuous darkness was only 0 to 15% compared with 0 to 61% under light. All other experiments were conducted under 24/20 C and 13-h light conditions. Germination was 19 to 36% over a pH range from 4 to 10, with a trend toward higher germination under neutral-to-alkaline conditions. Horseweed germination was > 20% at < 40 mM NaCl concentration and lowest (4%) at 160 mM NaCl. These data suggest that even at high soil salinity conditions, horseweed can germinate. Germination of horseweed decreased from 25% to 2% as osmotic potential increased from 0 (distilled water) to −0.8 MPa, indicating that germination can still occur under moderate water stress conditions. Horseweed seedling emergence was at its maximum on the soil surface, and no seedlings emerged from seeds placed at a depth of 0.5 cm or higher.