Horsenettle was described quantitatively in two grazed bermudagrass pastures for two years. Six times from April through October, plants were counted, phenologically documented, and tops were harvested for estimation of above-ground phytomass. Roots were sampled for carbohydrate, N fraction, and macronutrient analyses. Top growth, flowering, fruiting, and chemical composition were similar between sites within years, but differed between years. Starch and total non-structural carbohydrates in roots increased following reproductive periods both years. Root concentrations of protein and non-protein N increased and concentrations of P decreased in August and September in one year only. Analysis of the relationship between seasonal trends in carbohydrate fluctuation and pasture management practices suggests that systemic herbicide application in mid-summer may lead to more effective control than application in spring.

The potential diversity in tolerance of eight commercial sugarcane cultivars to metribuzin, terbacil, and hexazinone was investigated. Herbicides were applied preemergence in the fall after planting and postemergence to the crop in the spring each year over the 3-yr crop cycle. Reductions in shoot counts and heights were not observed with any of the herbicide treatments in the fall following planting. Cultivar differences in growth and yield responses were obtained, particularly in the plant cane year of the 3-yr crop cycle. Metribuzin was the least phytotoxic and hexazinone the most phytotoxic. Reductions in sugar yields (kg/ha) were attributed to reductions in stalk populations and, to a lesser extent, to reductions in stalk height and sugar (sucrose) content. Cultivars sensitive to hexazinone either were equally sensitive to terbacil or were less sensitive to terbacil. Terminal fluorescence (FT)3 measurements indicated that photosynthesis following postemergence applications of terbacil and hexazinone was no longer inhibited 5 weeks after treatment in the tolerant cultivars, CP 70–321 and CP 74–383, but continued to be reduced in the cultivars showing sensitivity, CP 65–357 and CP 48–103. The order of cultivar tolerance to hexazinone and terbacil from least tolerant to most tolerant is CP 48–103 < CP 65–357 ≤ CP 72–370 < CP 70–321 = CP 70–330 < CP 72–356 = CP 73–351 = CP 74–383.

Greenhouse studies were conducted to determine host status of weed species for Rhizoctonia solani AG-1, which causes Rhizoctonia foliar blight of soybean. Weed species were barnyardgrass, broadleaf signalgrass, common cocklebur, entireleaf morningglory, hemp sesbania, itchgrass, johnsongrass, large crabgrass, northern jointvetch, prickly sida, purple nutsedge, redweed, sicklepod, and smooth pigweed. Seedling weeds were inoculated with suspensions containing intraspecific group IA and IB isolates of the fungus. In the first study, sclerotia of IA were recovered from tissue of all weeds except smooth pigweed, and mycelia of IA were recovered from all except smooth pigweed and redweed. In that study, neither microsclerotia nor mycelia of IB were recovered from sicklepod, barnyardgrass, or large crabgrass, and only microsclerotia were recovered from itchgrass and purple nutsedge. In the second study, sclerotia of IA, microsclerotia of IB, and mycelia of each isolate were recovered from all weed species. In other studies, R. solani spread from at least six of seven weed species to a noninfected soybean plant growing in close proximity. These studies emphasize the importance of weed control, not only for reducing plant competition and increasing yield, but also for the potential impact on development of RFB.

Greenhouse and field experiments were conducted at Athens, GA, during 1984 and 1985 to evaluate the interference of several soybean cultivars with sicklepod. The 12 soybean cultivats evaluated in the field were commonly used from 1907 to the present. No correlation of soybean cultivar maturity group or date of introduction with interference on sicklepod was observed. ‘Essex’ and ‘Bragg’ soybean reduced sicklepod dry weight in the field; however, only Essex was unaffected by sicklepod. In the greenhouse experiments, root and shoot growth of sicklepod was less than the cultivars ‘CNS', ‘Peking’, and ‘Forrest’. Sicklepod produced the least above- and below-ground growth while ‘CNS' produced the most of the three cultivars. When grown together, the combined total root growth of each cultivar plus sicklepod was less than calculated expected values. The genetic development of soybean cultivars with improved tolerance to sicklepod infestations possibly could reduce weed control inputs.

We tested the effects of seeding date and weed control during switchgrass establishment in a field experiment that was conducted in central Pennsylvania in 2007 and repeated in 2008. Switchgrass was no-till seeded in early May, late May, and mid-June, and three postemergence weed management treatments were evaluated, including Mow (only a single mowing), Broadleaf (2,4-D + dicamba), and Broad Spectrum (2,4-D + dicamba + atrazine + quinclorac). Switchgrass density increased at later seeding dates, except in 2008, when the middle seeding date had the lowest density. In both years, weed biomass in late summer was lowest in the last seeding date of the Broad Spectrum treatment. In contrast, switchgrass biomass in late summer was greatest in the first seeding date of the Broad Spectrum treatment in both years. In the year after establishment (production year), plots were split to test the effects of supplemental weed control, composed of metsulfuron + 2,4-D applied in May, on total aboveground yield. Supplemental control in the production year increased total aboveground yield in the Mow treatment only, indicating that effective weed control during the establishment year might reduce the need for weed control in the following year. Although maximum aboveground yield was achieved when switchgrass was seeded in May and herbicides were used, results from our experiment suggest that seeding switchgrass at a relatively high seeding rate in June in our study region and mowing annual weeds to reduce competition and prevent seed production could be an effective strategy if minimizing herbicide use is a priority.

With increasing incidence of glyphosate-resistant weeds worldwide, greater farmer awareness of the importance of glyphosate stewardship and proactive glyphosate-resistance management is needed. A Web-based decision-support tool (http://www.weedtool.com) comprising 10 questions has been developed primarily for farmers in western Canada to assess the relative risk of selection for glyphosate-resistant weeds on a field-by-field basis. We describe the rationale for the questions and how a response to a particular question influences the risk rating. Practices with the greatest risk weighting in western Canadian cropping systems are lack of crop-rotation diversity (growing mainly oilseeds) and a high frequency of glyphosate-resistant crops in the rotation. Three case scenarios are outlined—low, moderate, and high risk of glyphosate-resistance evolution. Based on the overall risk rating, three best-management practices are recommended to reduce the risk of glyphosate resistance in weeds.

The anticipated release of EnlistTM cotton, corn, and soybean cultivars likely will increase the use of 2,4-D, raising concerns over potential injury to susceptible cotton. An experiment was conducted at 12 locations over 2013 and 2014 to determine the impact of 2,4-D at rates simulating drift (2 g ae ha−1) and tank contamination (40 g ae ha−1) on cotton during six different growth stages. Growth stages at application included four leaf (4-lf), nine leaf (9-lf), first bloom (FB), FB + 2 wk, FB + 4 wk, and FB + 6 wk. Locations were grouped according to percent yield loss compared to the nontreated check (NTC), with group I having the least yield loss and group III having the most. Epinasty from 2,4-D was more pronounced with applications during vegetative growth stages. Importantly, yield loss did not correlate with visual symptomology, but more closely followed effects on boll number. The contamination rate at 9-lf, FB, or FB + 2 wk had the greatest effect across locations, reducing the number of bolls per plant when compared to the NTC, with no effect when applied at FB + 4 wk or later. A reduction of boll number was not detectable with the drift rate except in group III when applied at the FB stage. Yield was influenced by 2,4-D rate and stage of cotton growth. Over all locations, loss in yield of greater than 20% occurred at 5 of 12 locations when the drift rate was applied between 4-lf and FB + 2 wk (highest impact at FB). For the contamination rate, yield loss was observed at all 12 locations; averaged over these locations yield loss ranged from 7 to 66% across all growth stages. Results suggest the greatest yield impact from 2,4-D occurs between 9-lf and FB + 2 wk, and the level of impact is influenced by 2,4-D rate, crop growth stage, and environmental conditions.

Sustainable weed management strategies are needed for organic orchard systems. A study was conducted in an almond orchard in Fresno, CA from 2009 to 2011. Treatment comparisons included steam, flame, and broad applications of either lemongrass oil or D-limonene. An untreated control was also included. The experimental design was a randomized complete block with four replications. Weekly evaluations on percent weed control were taken and weed biomass was sampled 4 to 8 wk after treatment (WAT). Weed control and biomass differed between seasons but, in general, steam and flame provided as much as 95% control 1 WAT. However, the effects lasted only 3 to 4 wk as new weeds emerged or the treated weeds overcame the suppressive effects of the thermal treatments. Weed biomass was 95% lower in the steam- and flame-treated plots compared with the untreated plots in summer. Both steam and flame were more effective on certain erect-growing broad-leaved weed species than on prostrate-growing weeds and grasses. Lemongrass oil provided very little weed control. However, D-limonene provided up to 95% weed control 1 WAT and in one experiment 53% control was observed up to 5 WAT. This herbicide also resulted in lower weed biomass than the untreated and the thermal-treated plots. Monthly applications of steam or flame or applications of D-limonene every 5 to 6 wk may have to be made to adequately suppress weeds in organic almond orchards. Cost estimates of propane use were $41 to 56 ha−1 and $26 ha−1 for the steam and flame treatments, respectively. The cost of D-limonene was estimated as $275 ha−1. To optimize weed control and costs, these tools may need to be used in combination rather than by themselves.

Weed control in organic peanut is difficult and lack of residual weed control complicates weed management efforts. Weed management systems using corn gluten meal in combination with clove oil and sweep cultivation were evaluated in a series of irrigated field trials. Corn gluten meal applied in a 30 cm band over the row at PRE, sequentially at PRE+2 wk after emergence, and PRE+2wk+4wk did not adequately control annual grasses and smallflower morningglory. Similarly, a banded application of clove oil applied POST did not adequately control weeds. The only treatment that improved overall weed control was sweep cultivation. Peanut yields were not measured in 2006 due to heavy baseline weed densities and overall poor weed control. Peanut yields were measured in 2007 and were not affected by any weed control treatment due to poor efficacy. While sweep cultivation improved weed control, weeds were controlled only in the row middles and surviving weeds in-row reduced peanut yield. Even when used in combination with sweep cultivation, corn gluten meal and clove oil were ineffective and offer little potential in a weed management system for organic peanut production.

Tolpyralate is a new 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibiting herbicide for POST weed management in corn; however, there is limited information regarding its efficacy. Six field studies were conducted in Ontario, Canada, over 3 yr (2015 to 2017) to determine the biologically effective dose of tolpyralate for the control of eight annual weed species. Tolpyralate was applied POST at six doses from 3.75 to 120 g ai ha−1 and tank mixed at a 1:33.3 ratio with atrazine at six doses from 125 to 4,000 g ha−1. Regression analysis was performed to determine the effective dose (ED) of tolpyralate, and tolpyralate+atrazine, required to achieve 50%, 80%, or 90% control of eight weed species at 1, 2, 4, and 8 wk after application (WAA). The ED of tolpyralate for 90% control (ED90) of velvetleaf, common lambsquarters, common ragweed, redroot pigweed or Powell amaranth, and green foxtail at 8 WAA was ≤15.5 g ha−1; however, tolpyralate alone did not provide 90% control of wild mustard, barnyardgrass, or ladysthumb at 8 WAA at any dose evaluated in this study. In contrast, the ED90 for all species in this study with tolpyralate+atrazine was ≤13.1+436 g ha−1, indicating that tolpyralate+atrazine can be highly efficacious at low field doses.

Organizer's comments: Systems inquiry represents an array of research-based problem-solving approaches. European experience with the pesticide debate suggests several simple messages contained in this paper. As weed scientists, we have everything to gain by considering a global sphere of views, practices, logic, experience, and message. Read this paper for the essence, the message, the systems thinking, the research framework, and the interface between society and weed science. Read it because science and society must communicate.

Field studies were conducted in 1995 and 1996 to investigate postemergence (POST) applications of rimsulfuron (12 g ai/ha) plus thifensulfuron-methyl (6 g ai/ha) in tank-mixtures with various acetolactate synthase (ALS)- and non–ALS-inhibitor herbicides for weed control in corn. Rimsulfuron plus thifensulfuron-methyl controlled giant foxtail and common lambsquarters at least 95% but did not control common ragweed. Rimsulfuron plus thifensulfuron-methyl tank-mixed with 20 g ai/ha primisulfuron-methyl, 17 g ai/ha CGA-152005 plus 18 g ai/ha primisulfuron, 18 or 36 g ai/ha halosulfuron-methyl, 18 g ai/ha nicosulfuron, or 280 g ai/ha dicamba controlled giant foxtail at least 89%, common lambsquarters at least 96% and, with the exception of the nicosulfuron combination, controlled common ragweed at least 88%. Rimsulfuron plus thifensulfuron-methyl tank-mixed with flumetsulam (26 g ai/ha) plus clopyralid (69 g ai/ha) plus 2,4-D (140 g ai/ha), atrazine (560 g ai/ha), 2,4-D (280 g/ha), or dicamba (308 g/ha) plus atrazine (588 g/ha) reduced the control of giant foxtail to less than 78% 26 d after treatment (DAT). Corn injury was less than 12% from rimsulfuron plus thifensulfuron-methyl and from mixtures of rimsulfuron plus thifensulfuron-methyl with other herbicides except when rimsulfuron plus thifensulfuron-methyl was mixed with flumetsulam plus clopyralid plus 2,4-D. This combination injured corn 26%. In these studies the appropriate tank-mix partners for rimsulfuron plus thifensulfuron-methyl were primisulfuron, CGA-152005 plus primisulfuron, and halosufluron-methyl.

Field studies were conducted in 1987, 1988, and 1989 to evaluate the effects of freshly applied residual levels (simulated residue carryover) of imazaquin and norflurazon on the growth and yield of corn, cotton, grain sorghum, rice, and wheat. Studies ware conducted on five soils, and concentrations of herbicides were determined at crop planting. Crop injury ratings were recorded 4 wk after emergence, and yields were measured at the end of each growing season. In simulated residue carryover studies, corn and grain sorghum injury occurred and yields were reduced when norflurazon concentrations were 450 ng g-1 soil or greater. Rice yields were reduced at norflurazon concentrations of 710 ng g-1. Corn, cotton, and wheat heights and yields were reduced when imazaquin concentrations were 13 ng g-1, 13 ng g-1, and 45 ng g-1 soil, respectively for crops. Comparisons between simulated carryover and actual carryover were made for corn and cotton grown in imazaquin treated fields as well as for corn grown in a norflurazon treated field.

A johnsongrass population from a cotton field in northern Greece along with a population from the university farm (“Control”) were evaluated for resistance to the herbicide quizalofop; cross-resistance to cycloxydim, propaquizafop, and fluazifop (acetyl coenzyme A [CoA] carboxylase [ACCase]-inhibiting herbicides), and multiple resistance to nicosulfuron (acetolactate synthase [ALS]-inhibiting herbicides). In greenhouse experiments, the application of four times the recommended rates of quizalofop and propaquizafop to suspected resistant rhizomatous plants resulted in 4 and 5% growth reduction, respectively. However, the growth of suspected resistant seedlings was reduced by 54 and 28% after the application of two times the recommended rate of the same herbicides. In contrast, the application of quizalofop and propaquizafop at recommended rates on rhizomatous plants and seedlings of the Control population reduced their growth by 97 to 100%. Also, the growth reduction of both populations by the application of cycloxydim, fluazifop, and nicosulfuron at recommended rates ranged from 93 to 100%. In the field experiment, quizalofop and propaquizafop applied at four times the recommended rate reduced growth of the suspected resistant population by 9 and 18%, respectively, whereas the recommended rate of fluazifop gave a 94% growth reduction of this weed. The herbicide rate required for 50% growth reduction (GR50 ) values for rhizomatous plants of the suspected resistant population were 0.90 and 2.465 kg ai/ha for quizalofop and propaquizafop, respectively, whereas the corresponding GR50 values for the seedlings were 0.074 and 0.185 kg ai/ha. These results indicate that a johnsongrass population developed cross-resistance to quizalofop and propaquizafop, but did not evolve cross-resistance to cycloxydim and fluazifop or multiple resistance to nicosulfuron.

Experiments were conducted at two locations in North Carolina from 1999 to 2000 to evaluate flumioxazin preplant (PP) for weed management in strip-tillage cotton planted in winter-wheat cover. Flumioxazin PP was evaluated at two rates alone and in mixture with two commonly used PP herbicides and one experimental PP herbicide. Flumioxazin PP at 71 or 105 g ai/ha tank mixed with the isopropylamine salt of glyphosate at 1.12 kg ai/ha, paraquat at 1.05 kg ai/ha, or the trimethylsulfonium salt of glyphosate at 1.12 kg ai/ha controlled common chickweed, common lambsquarters, common ragweed, Palmer amaranth, and smooth pigweed ≥ 96% 29 to 43 d after treatment (DAT). Both glyphosate formulations and paraquat alone provided ≥ 91% control of common chickweed and henbit 29 to 43 DAT, but control of common lambsquarters, common ragweed, large crabgrass, Palmer amaranth, and smooth pigweed was ≤50%. Treatments including flumioxazin injured cotton (≤ 5%) at one location. In all comparisons within a location, cotton treated with flumioxazin PP at 71 or 105 g/ha in mixture with either glyphosate formulation or with paraquat provided equivalent or higher yields than did cotton not treated with flumioxazin PP.

A field study was conducted in 2015 and 2016 at the H. Rouse Caffey Rice Research Station (RRS) to evaluate antagonistic, synergistic, or neutral interactions of quizalofop when mixed with ALS-inhibiting herbicides labeled in rice production. Quizalofop was applied at 120 g ai ha−1. Mixture herbicides included penoxsulam at 40 g ai ha−1, penoxsulam+triclopyr at 352 g ai ha−1, halosulfuron at 53 g ai ha−1, bispyribac at 34 g ai ha−1, orthosulfamuron+halosulfuron at 94 g ai ha−1, orthosulfamuron+quinclorac at 491 g ai ha−1, imazosulfuron at 211 g ai ha−1, and bensulfuron at 43 g ai ha−1. All ALS herbicides mixed with quizalofop indicated antagonistic responses for red rice, CL-111, CLXL 745, or barnyardgrass control at either 14 or 28 days after treatment (DAT). At 28 DAT, quizalofop mixed with penoxsulam or bispyribac controlled barnyardgrass 34 to 38%, compared with an expected control of 97%. In addition, these same mixtures controlled red rice, CL-111, and CLXL-745 61 to 67% at 28 DAT compared with an expected control of 96 to 97%. A second application of quizalofop at 120 g ha−1 was applied at 28 DAT. At 42 DAT, neutral responses were indicated for all mixtures except with quizalofop mixed with penoxsulam containing products.

Annual bluegrass is a troublesome weed in turf management and there are currently limited POST herbicides labeled for use in seashore paspalum. Field and greenhouse experiments were conducted to evaluate seashore paspalum tolerance to pronamide and other herbicides for annual bluegrass control. In field experiments, turf injury never exceeded 7% from pronamide applied at dormancy, 50% green-up, or complete green-up of seashore paspalum in spring. Annual bluegrass control from pronamide was initially similar across timings and averaged 67, 90, and 98% control from 0.84, 1.68, and 3.36 kg ai ha−1, respectively, after 6 wk. In greenhouse experiments, the aforementioned pronamide rates caused less than 10% injury on seashore paspalum. Seashore paspalum injury in the greenhouse was excessive (> 20%) from atrazine, bispyribac-sodium, and trifloxysulfuron and moderate (7 to 20%) from foramsulfuron, rimsulfuron, and ethofumesate. Seashore paspalum seedhead count reductions by 4 wk after treatment (WAT) were good to excellent (87 to 98%) from atrazine, bispyribac-sodium, rimsulfuron, and trifloxysulfuron and poor (≤ 0%) from ethofumesate, foramsulfuron, and pronamide. By 4 WAT, seashore paspalum clippings were reduced 0 to 39% from pronamide, whereas atrazine, bispyribac-sodium, and trifloxysulfuron reduced clippings by 54 to 69% from the untreated and ethofumesate, foramsulfuron, and rimsulfuron reduced clippings by 27 to 39%.

Despite the abundance of common ragweed in crops and the potency of ragweed pollen as an allergen, pollen production in agricultural fields has hardly been evaluated. Our goal was to evaluate pollen and seed production of early- (i.e., plants missed by weed control) and late- (i.e., after weed control) emerging common ragweed growing in corn and soybean. Allocation and gender distribution were also evaluated. The experiment included 2 yr (2008, 2009), three competition treatments, two seeding/emergence dates, three densities, and four replicates. Competition treatments (main plots) included no crop or weeds (bare), corn, or soybean. Crops were glyphosate resistant. Subplots were seeded with common ragweed before or after glyphosate application at densities of 1 (4 m−2), 3 (12 m−2), or 6 (24 m−2) plants per plot. Ragweed plants were harvested in mid-October and measured (aboveground biomass, length of all male inflorescences, stem diameter, and seed production). Based on our estimates, mean (backtransformed from ln[x + 1]) pollen production values were: 6.25 (bare), 0.74 (corn), and 1.13 (soybean) × 108 pollen grains per ragweed. Biomass and diameter were good predictors of ragweed male and female fitness. Plant height was not correlated with maleness. In crops, ragweed gender distribution was shifted toward maleness. Estimations indicate early-emerging (June 18 to 23) ragweed produced three times more pollen than late (July 7 to 11) plants.

Extension workers are sensing pressure to use soils information and chemical characteristics data to guide farmers in selecting pesticides least prone to leach into groundwater. Our objective was to estimate differences in herbicide migration to groundwater under conditions typical for the Southeast Coastal Plain, and to consider how a farmer might be advised to use such knowledge in selecting herbicides. We used a simple computer code for microcomputers to predict persistence and migration of 17 herbicides through a hypothetical, coarse-textured soil typical of the Southeast Coastal Plain. Appropriate herbicides were selected for several common crop-weed problems, such as sicklepod in soybean and Palmer amaranth in corn. Groundwater was assumed to be 3.15 m below the soil surface. Herbicides selected covered a broad range of half-lives and organic carbon partition coefficients. Only after the first-order degradation rate constant was reduced by a factor of five did predicted soil water concentrations of several herbicides at the groundwater interface reach normal detection limits. Still, predicted concentrations were below the level established for health effects advisory purposes. Due to the large number of uncertainties and the inability to estimate practical benefits, we conclude that data relating to soil and herbicide characteristics cannot be used at this time to override cost effectiveness, efficacy, and other factors normally considered by farmers and Extension professionals in herbicides for weed control.

An experiment was conducted in 1998 and 1999 in southeastern Pennsylvania to examine the effect of tillage and soybean row spacing on burcucumber emergence and growth. A second experiment evaluated postemergence (POST) soybean herbicides on burcucumber control. In the tillage and row spacing study, a glyphosate-resistant soybean variety was planted in no-till and reduced-tillage systems in 38- and 76-cm row spacings. In the POST herbicide experiment, chlorimuron, glyphosate, CGA-277476, thifensulfuron, and several combinations of these herbicides were applied at two different POST application timings in 38-cm row soybean planted in a reduced-tillage system. In the tillage and row spacing study, burcucumber emergence was greatest starting in late May through mid-June and mostly ceased by early July, regardless of tillage system or row spacing. Although there was no difference in germination period in either tillage system, preplant tillage increased the number of emerged plants by 110% in 1997 and 70% in 1998 compared to the no-till system. Row spacing had no effect on burcucumber emergence or biomass production. In general, most POST herbicide programs controlled burcucumber, and there was no difference between early POST and mid-POST application timings. Chlorimuron at 13 g ai/ha, chlorimuron plus thifensulfuron, glyphosate, glyphosate plus chlorimuron, and glyphosate plus CGA-277476 provided 87% or greater control of burcucumber 12 wk after planting. These herbicides reduced burcucumber density and biomass by more than 56% in 1997 and 96% in 1998.