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.

Nomenclature: Dicamba; glyphosate

Precision means being exact and accurate and is an important management component for cropping systems. However, precision does not mean integration, which encompasses spatial and temporal dimensions and is a necessary practice rivaling precision. True IWM merges precision and integration by incorporating advanced technology that allows for greater flexibility of inputs and enhanced responsiveness to field conditions. Examples of this approach are non-existent due to a lack of suitable technological tools and a need for a paradigm shift. Herein a potential model startup company is offered as a guide to advance beyond precision weed control to true integration. The critical components of such a company include grower connections, investor support, proven and reliable technology, and adaptability and innovation in the agricultural technology market. The company with the vision and incentive to make true IWM a reality will be the first to more fully integrate available tools using technology, thus helping many growers overcome ongoing challenges associated with resistance, soil erosion, drift, and weed seedbanks.

Obtaining spatially explicit, cost-effective, and management-relevant data on invasive plant distributions across large natural areas presents considerable challenges. This is especially true when multiple monitoring objectives exist, because the utility of different monitoring methodologies varies with scale, logistical considerations, and information needs. The Florida Everglades is a vast wetland landscape with widespread invasive plant infestations and multiple management jurisdictions. A multi-agency team Working Group conducted a workshop in 2013 to explore opportunities to enhance the performance of a regional weed control program. Among the most important developments occurring at this meeting was the recognition that relevant management questions are scale-dependent. This led the team to define multiple monitoring objectives. Essential for conveying the success of the weed management program is quantifying large-scale patterns of change, as are quantifying finescale patterns informing control activities, defining mechanisms of spread, recognizing accelerating rates of spread, and detecting patterns of occupancy immediately before management intervention. The group's deliberation resulted in the emergence of a multiscale monitoring program utilizing several distinct monitoring protocols, including systematic landscape-level reconnaissance, a sample-based spatially stratified monitoring system, detailed inventories in planned treatment areas, and a set of methods focused solely on early detection and rapid response. Here we provide an overview of the Everglades multiscale invasive plant monitoring program, highlight benefits and challenges of each program component, and discuss how this program has improved regional invasive plant management.

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.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; pigweeds, Amaranthus spp.; redroot pigweed, Amaranthus retroflexus L. AMARE; tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer AMATU

Glyphosate-resistant (GR) kochia is an increasing management concern in major cropping systems of the northwestern US. In 2014, we investigated four putative GR kochia accessions (designated as ALA, VAL, WIL, DB) collected from sugar beet fields in eastern Oregon and southwestern Idaho to characterize the level of evolved glyphosate resistance and determine the relationship between the 5-enol-pyruvylshikimate-3-phospate synthase (EPSPS) gene copy number and level of glyphosate resistance. The EPSPS gene copy number was used as a molecular marker to detect GR kochia in subsequent surveys in 2015 and 2016. Based on LD₅₀ values from a whole-plant dose-response study, the four putative GR kochia populations were 2.0- to 9.6-fold more resistant to glyphosate than the glyphosate-susceptible (GS) accession. In an in vivo leafdisk shikimate assay, leaf disks of GS kochia plants treated with 100-µM glyphosate accumulated 2.4- to 4.0-fold higher amounts of shikimate than the GR plants. The four GR accessions had 2.7 to 9.1 relative EPSPS gene copies compared with the GS accession (<1 EPSPS gene copies), and there was a linear relationship between EPSPS gene copy number and glyphosate resistance level (LD₅₀ values). The 2015 and 2016 GR kochia survey results indicated that about half of the collected populations from sugar beet fields in eastern Oregon had developed resistance to glyphosate whereas only one population from the Idaho collection was confirmed glyphosate resistant. This is the first confirmation of GR kochia in sugar beet fields in eastern Oregon and southwestern Idaho. Diversified weed control programs will be required to prevent further development and spread of GR kochia in sugar beet-based rotations in this region.

Nomenclature: Glyphosate; kochia, Kochia scoparia (L.) Schrad.; sugar beet, Beta vulgaris L.

Herbicide-resistant Echinochloa spp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterize Echinochloa resistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was the Echinochloa Herbicide Resistance Confirmation Survey conducted annually since 2006; the other was the Echinochloa Herbicide Resistance Demographics Survey conducted since 2010. The Resistance Confirmation Survey showed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil quinclorac (12%). The parallel Resistance Demographics Survey tested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance among Echinochloa populations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.

Nomenclature: Cyhalofop; imazethapyr; quinclorac; propanil; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; junglerice, Echinochloa colona (L.) Link; rough barnyardgrass, Echinochloa muricata (Beauv.) Fern; rice, Oryza sativa L.

A study was conducted in three locations in Louisiana to evaluate interactions of imazethapyr at 0 and 70 g ai ha^-1 mixed with propanil at 0, 1,120, 2,240, 3,360, and 4,480 g ha^-1 for the control of red rice, barnyardgrass, and hemp sesbania. According to Blouin's modified Colby's, a synergistic response occurred for red rice treated with imazethapyr mixed with propanil at 4,480 g ha^-1 for all evaluations. Observed control was 93% to 95% compared with expected control of 81% to 87%. An antagonistic response occurred for barnyardgrass control with imazethapyr mixed with propanil at 1,120 g ha^-1 at 35 and 49 d after treatment (DAT), with control of 75% and 64%, respectively, compared with expected control of 89% and 78%. However, a neutral response occurred for barnyardgrass control when treated with all other imazethapyr plus propanil combinations. An antagonistic interaction occurred for hemp sesbania when treated with imazethapyr plus propanil at 3,360 and 4,480 g ha^-1 at 21 DAT with an observed control of 89% compared with an expected control of 96%; however, a neutral response occurred at all other evaluation dates. An increase in rice yield was observed with an imazethapyr plus propanil at 4,480 g ha^-1 mixture compared with a single application of imazethapyr or propanil at any rate evaluated.

Nomenclature: Imazethapyr; propanil; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; hemp sesbania, Sesbania herbacea (P. Mill.) McVaugh; red rice/rice, Oryza sativa L.

Glyphosate application to the rapid-response (RR) biotype of glyphosate-resistant (GR) giant ragweed ensues in loss of foliage via rapid tissue death, thereby reducing glyphosate translocation. Experiments were performed to determine if this GR response, in contrast to a non-rapid response (NRR) GR biotype, results in antagonism of the selective herbicides atrazine, cloransulam, dicamba, lactofen, and topramezone. Application of glyphosate at 1,680 g ae ha^-1 in the greenhouse resulted in antagonism between all five selective herbicides for the RR biotype, whereas glyphosate applied at 420 g ha^-1 was antagonistic only for cloransulam. Application of selective herbicides 2 d prior to glyphosate treatment avoided the antagonism observed in the RR biotype. In the field, glyphosate mixtures with dicamba and topramezone were antagonistic on the RR biotype across both 2015 and 2016 field seasons. Thus, the RR effectively reduces glyphosate efficacy but also has potential to diminish the activity of glyphosate mixtures with selective herbicides, and the degree of antagonism between these mixtures escalates at increasing glyphosate rates.

Nomenclature: Atrazine; cloransulam; dicamba; glyphosate; lactofen; topramezone; giant ragweed, Ambrosia trifida L. AMBTR

With the recent confirmation of protoporphyrinogen oxidase (PPO)-resistant Palmer amaranth in the US South, concern is increasing about the sustainability of weed management in cotton production systems. Cover crops can help to alleviate this problem, as they can suppress weed emergence via allelochemicals and/or a physical residue barrier. Field experiments were conducted in 2014 and 2015 at the Arkansas Agricultural Research and Extension Center to evaluate various cover crops for suppressing weed emergence and protecting cotton yield. In both years, cereal rye and wheat had the highest biomass production, whereas the amount of biomass present in spring did not differ among the remaining cover crops. All cover crops initially diminished Palmer amaranth emergence. However, cereal rye provided the greatest suppression, with 83% less emergence than in no cover crop plots. Physical suppression of Palmer amaranth and other weeds with cereal residues is probably the greatest contributor to reducing weed emergence. Seed cotton yield in the legume and rapeseed cover crop plots were similar when compared with the no cover crop treatment. The seed cotton yield collected from cereal cover crop plots was lower than from other treatments due to decreased cotton stand.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats.; Austrian winterpea, Lathyrus hirsutus L.; cereal rye, Secale cereale L.; cotton, Gossypium hirsutum L.; crimson clover, Trifolium incarnatum L., hairy vetch, Vicia villosa Roth; oats, Avena sativa L.; rapeseed, Brassica napus L.; wheat, Triticum aestivum L.

Control of noxious weeds such as cogongrass depend heavily on chemical treatment, but success is limited unless integrated with other practices. Utilization of cover crops in the system is ideal to avoid the use of excess herbicide and replace vegetation that will resist cogongrass reinvasion. Greenhouse studies were conducted from 2013 through 2015 at Mississippi State University with the objective to evaluate ‘AG4934’ RR/STS soybean, Korean lespedeza, crimson clover and ‘Durana’ white clover tolerance to soil-applied imazapyr at selected rates and various planting times after application. Plastic containers filled with a mixture of 2:1 sand:topsoil were treated with imazapyr at 0, 70, 140 and 280 g ae ha^-1. Legume species were planted 0, 1, 3 and 6 months after treatment (MAT). The factorial experimental design included legume species, imazapyr rate and planting time. At 6 weeks after each planting, the number of seedlings, average plant height and shoot biomass were measured. Statistical analysis revealed the imazapyr rate x planting time interaction was significant with respect to number of emerged seedlings, average height and shoot biomass per plant for each species. It was observed that the legumes planted at 0 MAT of imazapyr at 70 g ae ha^-1 or higher reduced emerged seedlings, average height and biomass production. In general, seeds planted 1 MAT or later in combination with these same herbicide rates, showed less growth reductions than treatments seeded 0 MAT. In conclusion, sites treated with imazapyr rates from 70 to 280 g ae ha^-1 for weed control, should not be seeded with legume ground covers less than 1 month after treatment to reduce emergence failure, plant height and biomass production.

Nomenclature: Imazapyr; cogongrass, Imperata cylindrica (L.) Beauv.; crimson clover, Trifolium incarnatum L.; Korean lespedeza, Kummerowia stipulacea (Maxim.) Makino; soybean, Glycine max (L.) Merr.; white clover, Trifolium repens L.

Drift reduction technologies aim to eliminate the smaller droplets that occur with some sprays because these small droplets can move off-target in the wind. Commonly used drift reduction technologies such as air-induction nozzles and spray additives impact on reducing off-target movement is well documented, however, the impact on herbicide penetration into an established crop canopy is not well known. This experiment evaluated the canopy penetration and efficacy of glyphosate treatments applied using four nozzle types (XR11005, AIXR11005, AITTJ11005, and TTI11005), two carrier volume rates (94 and 187 L ha^-1), and glyphosate applications with and without a commercial drift reducing adjuvant. Applications were made to corn and soybean fields using glyphosate applied at 1.26 kg ae ha^-1 with liquid ammonium sulfate at 5% v/v. A rhodamine dye was added (0.025% v/v) to the spray tank of each mixture as a tracer. Mylar^™ cards were placed in the field above the canopy, in the middle canopy, and on the ground for corn and above and below canopy for soybean. Five cards were at each position in the canopy arranged across the crop row. The addition of a drift reducing adjuvant did not impact canopy penetration. Doubling the carrier volume increased the amount of penetration proportionally and as such the percent reduction was not different. The TTI11005 nozzle had the greatest amount of spray penetration (28%) in the soybean canopies and the XR nozzle had the greatest amount (50%) in the corn canopies. Deposition across the row, beginning in-between the row crop and ending in the row of the crop was 44, 18, and 8% for soybean and 59, 50, and 36% for corn. For both crops, more than half of the herbicide application was captured in the crop canopy. Proper nozzle selection for canopy type can increase herbicide penetration and increasing the carrier volume will increase penetration proportionally.

Nomenclature: Glyphosate; corn, Zea mays L; soybean, Glycine max (L.) Merr

Strawberries, an important Florida crop, are grown on raised beds covered with plastic mulch. The plastic mulch provides good control of many weeds, but some problem species can emerge from the transplant hole during crop establishment. POST herbicide options for broadleaf weed control within the strawberry bed is limited to clopyralid, which only provides suppression. Strawberry canopy shielding may be responsible for the observed incomplete control with clopyralid application for problematic broadleaf weed species such as black medic and Carolina geranium. Two field experiments were established on mature strawberries to evaluate spray penetration through the canopy. The first examined spray penetration through the canopy of multiple strawberry cultivars at various distances from the crown. The second examined the effects of application volumes and nozzle selection on spray penetration. Cultivar selection had no effect on spray penetration through the canopy. In the first study, when applying at 281 L ha^-1, the area around the planting hole (0 to 5 cm from the crown) had 8% coverage below the canopy while the area below the canopy edge (10 to 15 cm from the crown) had 27% coverage. In the second study, increasing the application volume from 187 to 375 L ha^-1 increased coverage by 81%. Increasing the application volume from 375 to 740 L ha^-1 increased coverage 33% with maximal coverage of 53% at 740 L ha^-1. Nozzle type (standard even flat spray tip, Drift Guard, or TwinJet nozzles) did not affect coverage or deposition volume below the canopy. Overall, mature strawberry canopies demonstrated similar spray droplet penetration across cultivars with increased penetration with increased distance from the crown. Penetration increased with increasing application volume, but the nozzle types used in this experiment did not affect penetration. Additional research is needed to better define the effect of application volume on herbicide efficacy.

Nomenclature: Strawberry, Fragaria × ananassa (Weston) Duchesne ex Rozier

A field study was conducted in 2014 and 2015 in an established 5-yr old commercial blackberry planting to determine the effect of vegetation-free strip width (VFSW) on ‘Navaho’ blackberry vegetative growth, yield and fruit quality parameters, identify the optimum VFSW for blackberry plantings in the southeastern USA, and provide practical groundcover management recommendations that can increase the productivity of blackberry plantings. In Fall 2013, tall fescue was seeded in-row and allowed to establish. In Spring 2014, VFSW treatments (0, 0.6, 0.9, 1.2, and 1.8 m) were established in a randomized complete block statistical design with four replications. Blackberry growth measurements included primocane and floricane number, cane diam, individual fruit weight and yield. Fruit quality measurements included, soluble solids concentration (SSC), titratable acidity (TA) and pH. Primocane number increased with increasing VFSW in both years. Floricane number increased with increasing VFSW in 2014. Primocane diam decreased with increasing VFSW in 2014 but had a quadratic response in 2015. Berry weight and cumulative yield increased with increasing VFSW in both years. The only berry quality component affected by VFSW was pH, which decreased as VFSW increased. Results indicate that widening the VFSW in blackberry from the current recommendation of 1.2 m to 1.8 m could provide growers a means to increase plant growth, berry weight, and cumulative yield blackberry of a planting.

Nomenclature: Blackberry, Rubus L.; tall fescue, Lolium arundinaceum (Shreb.) S.J. Darbyshire FESAR

Ammonium nonanoate is registered for weed control in certified organic cropping systems and may be useful to control cool-season weeds in organic Vidalia^® sweet onion production. Ammonium nonanoate combined with tine-weeder cultivation was evaluated for weed control in organic onion in Georgia. There were no statistical interactions between main effects of herbicides and cultivation with a tine weeder for cool-season weed control and onion yield, indicating that ammonium nonanoate does not improve weed control compared with cultivation. Ammonium nonanoate at 4% and 6% did not adequately control weeds and onion yields were reduced. Ammonium nonanoate at 8% and 10% controlled cutleaf evening-primrose and lesser swinecress equal to the standard of d-limonene (14%), but the degree of control did not consistently protect onion yields from losses due to weeds. These results are in agreement with previous studies using clove oil and pelargonic acid. There is no advantage to using ammonium nonanoate for cool-season weed control in organic Vidalia^® sweet onion production.

Nomenclature: Ammonium nonanoate; clove oil; d-limonene; pelargonic acid; cutleaf evening-primrose, Oenothera laciniata Hill; lesser swinecress, Coronopus didymus (L.) Sm.; dry-bulb onion, Allium cepa L.

Palmer amaranth resistance to protoporphyrinogen oxidase (PPO)-inhibiting herbicides has become an increasing problem to producers throughout the southeast region of the United States. Traditionally, these herbicides can be used as foliar-applied and soil-applied in glyphosate resistant (GR) cropping systems to control GR Palmer amaranth. Heavy reliance on PPO herbicides has contributed to the increased selection for PPO inhibitor-resistant (PPO-R) Palmer amaranth biotypes. Dose response greenhouse research was conducted to evaluate the efficacy of soil-applied flumioxazin, fomesafen, saflufenacil and sulfentrazone on a known susceptible (S) and resistant (R) Palmer amaranth biotype. Both R and S populations reached maximum germination at 14 d after treatment (DAT). The data from this study suggests complete control (100%) was achieved for the S biotype at 35 d after treatment (DAT) with all herbicides. The R biotype showed difference among herbicide treatments with flumioxazin and saflufenacil having similar responses in control and fomesafen and sulfentrazone resulting in less control of the R Palmer amaranth biotypes. The calculated relative resistance factor ranged from 3.5 to 6.0, and averaged 5X for the four herbicides. This research indicated that the PPO-R population was still responsive to all tested herbicides, but a low level of resistance was present.

Nomenclature: Flumioxazin; fomesafen; saflufenacil; sulfentrazone; Palmer amaranth, Amaranthus palmeri S. Wats