A study to evaluate the effect of application time of day (TOD) on the efficacy of five burndown herbicides was conducted in Alabama and Tennessee. Treatments of 2,4-D, dicamba, glufosinate, paraquat, and saflufenacil were applied at sunrise, midday, or sunset to a native population of horseweed and analyzed separately. Control of glyphosate-resistant (GR) horseweed with 2,4-D, dicamba, glufosinate, and saflufenacil was greatest from the midday application. Percentage of living horseweed counts for all of these herbicides followed a similar pattern. Control from paraquat was lowest at the midday timing and greatest from the sunset application with surviving horseweed plant populations reflecting those control ratings. Application TOD significantly affected all of the herbicides in this research. Applications of 2,4-D, dicamba, glufosinate, and saflufenacil are more efficacious when applied during the middle portion of the day, while paraquat is more efficacious when applied at sunset for maximum horseweed control.

The fate of field-applied herbicides, including losses in surface runoff with water and sediment, is highly dependent on herbicide properties. The two most important properties are soil adsorption and persistence. Adsorption affects the potential for a herbicide to be lost primarily with sediment, runoff water, or possibly leaching water. Solubility, often though not always inversely correlated with adsorption, is of secondary importance, although low solubility can limit transport with water. Persistence affects the time available to be lost in runoff. Studies have shown that for soil-applied herbicides; extraction into runoff water or movement with sediment takes place from a thin soil layer at the surface. In addition, for herbicides studied, there is little interaction between surface crop residue and applied herbicides, and washoff from the residue readily occurs with small amounts of rainfall. Runoff loss equals the volume of carrier (water or sediment) times the concentration in that carrier; therefore, practices that reduce either, or both, can reduce losses. Rate of application has been directly related to concentration and therefore loss. Reducing rate, such as by banding, soil incorporation, and avoidance of application to crop residue reduce losses. The choice of herbicide and herbicide formulation, in conjunction with application technology, as they affect properties, rate, and placement, play a large role in determining runoff loss. Runoff losses of herbicides that are strongly adsorbed and therefore transported mainly with sediment can be reduced by erosion control; runoff volume reduction can reduce losses with water of moderately to weakly adsorbed herbicides. Conservation tillage has potential to reduce both runoff and erosion. Timing of application relative to expected intense storms, both in the short and long term, can reduce the potential for runoff. If possible to determine thresholds, herbicide use could be avoided if weed infestation is below the economic effect level. Buffer or filter strips have the potential to reduce transport of herbicides lost from fields to surface water resources.

When herbicides are applied in mixture, and infestation by weeds is less than expected compared with when herbicides are applied alone, a synergistic effect is said to exist. The inverse response is described as being antagonistic. However, if the expected response is defined as a multiplicative, nonlinear function of the means for the herbicides when applied alone, then standard linear model methodology for tests of hypotheses does not apply directly. Consequently, nonlinear mixed-model methodology was explored using the nonlinear mixed-model procedure (PROC NLMIXED) of SAS System®. Generality of the methodology is illustrated using data from a randomized block design with repeated measures in time. Nonlinear mixed-model estimates and tests of synergistic and antagonistic effects were more sensitive in detecting significance, and PROC NLMIXED was a versatile tool for implementation.

Presented and illustrated is an easy-to-implement and flexible methodology for the analysis of synergistic and antagonistic effects when the effects are defined as nonlinear functions of means. The methodology augments standard mixed-model analyses with the Delta method for standard errors of nonlinear functions of means. Explained is why standard ANOVA methods that have been adopted in the literature are not recommended. To illustrate the methodology, the joint-action effects of fenoxaprop with companion herbicides in two-component mixtures for weed control in rice were evaluated. The companion herbicides were halosulfuron, bispyribac-sodium, bensulfuron, penoxsulam, carfentrazone, quinclorac, and imazethapyr. Weeds evaluated were barnyardgrass and broadleaf signalgrass. Experiments in Louisiana and Mississippi in 2009 revealed a preponderance of antagonistic effects. The analysis showed that mixtures with bispyribac-sodium, penoxsulam, quinclorac, and imazethapyr were generally the most antagonistic and provided the least control.

An area of influence method with biweekly destructive harvests 5 through 19 wk after planting (WAP) was used to monitor the reciprocal interference of common cocklebur and cotton in 1987 and 1988. Plant heights, leaf area, and leaf, stem, boll, fiber, and total plant biomass dry weights were measured in 15-cm increments away from common cocklebur or from a randomly selected cotton plant out to 105 cm. Data indicated that cotton less than 60 cm from common cocklebur was shorter, had less leaf area and lower leaf, stem, boll, and biomass dry weights than cotton beyond 60 cm from common cocklebur or cotton grown without common cocklebur interference. Differences in leaf area and biomass between cotton grown with and without common cocklebur interference were greater at and beyond 13 WAP sample dates than before 13 WAP. By 15 WAP, cotton leaf area and biomass were reduced an estimated 11% and 15%, respectively, averaged over the entire 105 cm of row. Cotton yield, harvested 27 WAP, was reduced an estimated 31% on plants grown with one common cocklebur plant per 2.1 m. Cotton yield was reduced on plants out to 99 cm from common cocklebur. Common cocklebur and cotton plants grown alone were taller, had greater leaf area, and greater leaf, stem, and biomass dry weight than those respective plants grown adjacent to cotton. Common cocklebur grown alone produced 67% more biomass than did cotton grown alone. Cotton plants grown adjacent to other cotton plants produced 89% and 96% less boll and fiber weight, respectively, than plants grown alone.

Field studies were conducted to evaluate weed control with combinations of glyphosate at 750 g ae/ha and the insecticides acephate (370 g ai/ha), dicrotophos (370 g ai/ha), dimethoate (220 g ai/ha), fipronil (56 g ai/ha), imidacloprid (53 g ai/ha), lambda-cyhalothrin (37 g ai/ha), oxamyl (280 g ai/ha), or endosulfan (420 g ai/ha) and insect control with coapplication of the herbicide with insecticides acephate, dicrotophos, dimethoate, and imidacloprid. Applying lambda-cyhalothrin or fipronil with glyphosate reduced control of hemp sesbania by 19 and 9 percentage points, respectively, compared with glyphosate alone. Acephate, dicrotophos, dimethoate, imidacloprid, lambda-cyhalothrin, oxamyl, and endosulfan did not affect hemp sesbania, pitted morningglory, prickly sida, and redweed control by glyphosate. Lambda-cyhalothrin and fipronil did not affect glyphosate control of weeds other than hemp sesbania. Addition of glyphosate to dicrotophos improved cotton aphid control 4 d after treatment compared with dicrotophos alone. Thrips control was improved with addition of glyphosate to imidacloprid. Insect control was not reduced by glyphosate regardless of insecticide.

Jointed goatgrass and downy brome continue to plague winter wheat producers in the western United States. Because there are no effective herbicides for in-crop control of these weeds, producers are seeking cultural practices that stimulate seed germination and deplete the soil seed bank. We determined the effect of limited tillage and crop canopy on seedling emergence of these grasses. One tillage operation with a sweep plow increased jointed goatgrass seedling emergence 74% in the first year but did not affect emergence in later years. Downy brome emergence was not affected by tillage. Jointed goatgrass seedlings emerged over 5 yr, whereas downy brome did not emerge after 3 yr. Seedling emergence of both species was two times greater in corn and barley than in proso millet. Producers will accrue more benefit for seedbank management with cultural strategies such as alternative rotations and competitive wheat canopies than with limited tillage using a sweep plow.

Sensor technologies are expedient tools for precision agriculture, aiming for yield protection while reducing operating costs. A portable sensor based on chlorophyll fluorescence imaging was used in greenhouse experiments to investigate the response of sugar beet and soybean cultivars to the application of herbicides. The sensor measured the maximum quantum efficacy yield in photosystem II (PS-II) (Fv/Fm). In sugar beet, the average Fv/Fm of 9 different cultivars 1 d after treatment of desmedipham plus phenmedipham plus ethofumesate plus lenacil was reduced by 56% compared to the nontreated control. In soybean, the application of metribuzin plus clomazone reduced Fv/Fm by 35% 9 d after application in 7 different cultivars. Sugar beets recovered within few days from herbicide stress while maximum quantum efficacy yield in PS-II of soybean cultivars was reduced up to 28 d. At the end of the experiment, approximately 30 d after treatment, biomass was reduced up to 77% in sugar beet and 92% in soybean. Chlorophyll fluorescence imaging is a useful diagnostic tool to quantify phytotoxicity of herbicides on crop cultivars directly after herbicide application, but does not correlate with biomass reduction.

A study was conducted at three locations in Saskatchewan, Canada, in 1996 and 1997 to determine if increasing the seeding rate of wheat, barley, and lentil by 50% would maintain weed control and crop yield when herbicides are applied at reduced rates or not at all. Three herbicide rates (½ of full, ¾ of full, and full recommended label rate), along with an untreated check, two crop seeding rates (normally recommended and 1.5 times normally recommended rates), and three crops were tested. Increasing seeding rate did not affect weed fresh weights, crop yield, and net return responses to herbicides applied at reduced rates or not at all when averaged across crops, years, and locations. Increased seeding rate, independent of the different herbicide applications, had infrequent and inconsistent effects among the crop by year by location combinations. More broadleaf and grass weed growth, less crop yield, and lower net returns generally occurred when herbicides were not applied or applied at reduced rates. These trends were especially prominent when herbicides were not applied to cereal crops at Saskatoon (40% yield reduction) and when herbicides were applied at ½ the full label rate rather than higher herbicide rates to wheat at the other two locations (16% yield reduction). In 1996, lentil yield and net returns did not respond to herbicide application and rate because of poor grass weed control across all herbicide rates. Lentil yield and net returns decreased by 11% (full vs. ¾), 22% (¾ vs. ½), and 46% (½ vs. none) when herbicides were applied at progressively lower rates in 1997. Reduced herbicide rates did not affect net returns for cereal crops, indicating that herbicide rates lower than the full label rate may be economically viable in certain crops.

Effects of soil pH and cation exchange capacity (CEC) on sunflower tolerance to sulfentrazone were investigated in a greenhouse study. Variables were soil pH (7.0, 7.3, 7.5, and 7.8), soil CEC (8.2, 13.7, 18.4, and 23.3 cmol/kg), and sulfentrazone rate (0, 105, 158, and 184 g ai/ha). Sulfentrazone-induced leaf chlorosis was affected by soil pH at 12 d after planting (DAP), but plants recovered, and earlier differences were not visible 9 d later. At 12 DAP, leaf chlorosis was 3 or 4% more severe in soils with pH 7.3 or higher compared with soils with pH 7.0 when averaged over both sulfentrazone rate and soil CEC. Leaf chlorosis resulting from sulfentrazone rates of 105, 158, and 184 g/ha was 17, 25, and 35% less at 23 cmol/kg than at 8.2 cmol/kg, respectively. Differences in chlorosis among sulfentrazone rates were greatest in soil with low CEC and lessened as soil CEC increased. Plants regained normal color over time, and newly emerging leaves were not affected. However, plant dry weights were reduced when sulfentrazone rate was ≥158 g/ha. Averaged over sulfentrazone rate and soil pH, sunflower dry weights were less when soil CEC was 8.2 compared with a CEC of 13.7 cmol/kg or higher, indicating a greater response at low CEC. Sunflower plant dry matter was not different in sulfentrazone-treated soil with a CEC above 13.7 cmol/kg. At the ranges tested, soil CEC had a considerably greater effect than did pH on sunflower tolerance to sulfentrazone.

An experiment was conducted near Scottsbluff, NE, to assess three techniques for establishing perennial grasses in pasture sites and to evaluate the effectiveness of five perennial grasses compared with herbicide or mowing for Canada thistle control. Perennial grass density 9 mo after seeding and perennial grass biomass 12 mo after seeding both followed the same trend, indicating that preplant rototilling improved perennial grass establishment. After 3 yr, Canada thistle control was greater than 90% in plots where perennial grasses had been established utilizing preplant rototilling, and competitive grasses were as effective as yearly applications of clopyralid at 0.55 kg/ha for controlling Canada thistle. Averaged across two studies conducted for 3 yr, hybrid wheatgrass, intermediate wheatgrass, Russian wildrye, tall fescue, and western wheatgrass provided 85, 74, 76, 78, and 66% Canada thistle control, respectively.

Aquatic plants provide many benefits to the environment, but must be managed when growth reaches nuisance levels or when invasive plant species are released. Management tactics include biological, chemical, cultural, mechanical, and physical tools. Each specific management technique has advantages and disadvantages. In addition, the implementation of these techniques can become complicated because of the multiple users, managers, and stakeholders that may be present on large bodies of water. As an example, hydrilla is the most economically damaging aquatic weed in the United States. It reproduces through fragmentation, turions, and occasionally seed and can colonize a wide variety of aquatic environments. The most common management tactics for hydrilla include biological, chemical, and mechanical tools. Triploid grass carp have been the primary biological control agent, whereas fluridone has been the only systemic herbicide used. Because of heavy utilization of fluridone, biotypes have developed resistance to this herbicide in Florida. Although several acetolactate synthase-inhibiting herbicides are in development, herbicides with additional modes of action are needed for resistance management. Other aquatic plant management needs include additional control tactics for algae and additional extension resources for public education.

Purple nutsedge management with herbicides (halosulfuron, imazaquin, MSMA, S-metolachlor, and sulfentrazone) and mowing was investigated in a bare ground homogenous purple nutsedge field site. Mowing at 5 cm increased control of purple nutsedge by 6% compared to not mowing. Sequential applications of halosulfuron, MSMA, and sulfentrazone provided at least 80% control of purple nutsedge shoots, whereas imazaquin controlled purple nutsedge shoots less than 65%. All herbicide treatments reduced purple nutsedge total and viable tuber densities at least 40%. S-metolachlor PRE reduced total and viable tuber densities 65 and 69%, respectively. Sequential applications of sulfentrazone or MSMA reduced total and viable tubers 80%. Early postemergence (EPOST) or EPOST followed by late-postemergence applications of halosulfuron and imazaquin reduced total and viable tuber densities 52 and 59%, respectively. Data indicate that S-metolachlor PRE and sequential applications of MSMA and sulfentrazone may be viable treatments for control of purple nutsedge shoots and tubers.

Field experiments in 2001 and 2002 at Stuttgart, AR, demonstrated cyhalofop applied POST controlled propanil-susceptible barnyardgrass, propanil-resistant barnyardgrass, and broadleaf signalgrass 67, 65, and 71%, respectively 28 days after treatment (DAT). Halosulfuron applied 1 d before cyhalofop or applied in a tank-mixture reduced broadleaf signalgrass and propanil-resistant or propanil-susceptible barnyardgrass by 35 to 59 percentage points. Triclopyr applied 5, 3, or 1 d before, or mixed with, cyhalofop reduced control of propanil-resistant and propanil-susceptible barnyardgrass, 16 to 41 percentage points, respectively. Triclopyr applied 1 d before cyhalofop or combined with cyhalofop reduced broadleaf signalgrass control 38 and 18 percentage points, respectively compared with cyhalofop alone. Propanil reduced cyhalofop grass control only when mixed with cyhalofop. Application timings, averaged across halosulfuron, propanil, and triclopyr, made 1 d before cyhalofop or tank-mixed with cyhalofop reduced grass control more compared with other application timings made before or after cyhalofop. Barnyardgrass absorption of 14C-cyhalofop alone or mixed with propanil was 86 and 75%, respectively, with 14C-cyhalofop translocation being reduced by 10-fold when propanil was mixed with cyhalofop. Reductions in cyhalofop translocation in combination with propanil may have partially explained the reductions in grass control observed in the field.

A study was conducted in 1999 to evaluate weed control and imidazolinone-tolerant (IMI-tolerant) rice response to 140 g ai/ha imazethapyr at different application timings and to flood depths of 5, 10, and 20 cm at the Rice Research Station near Crowley, LA, and at the Northeast Research Station near St. Joseph, LA. Barnyardgrass control, rice injury, and rice yield were not influenced by flood depth; therefore, data were averaged over flood depths. Season-long barnyardgrass control was greater than 80% at both locations with imazethapyr applied preplant incorporated (PPI), early postemergence (EPOST), and mid-postemergence (MPOST). IMI-tolerant rice injury at 7 d after late postemergence (LPOST) treatment (DAT) was less than 5% for all imazethapyr treatments at Crowley and 1 to 8% for PPI, preemergence (PRE), delayed preemergence (DPRE), and EPOST timings at St. Joseph. However, IMI-tolerant rice injury was 5% or less at 42 DAT for all treatments at Crowley and St. Joseph. IMI-tolerant rice yield at Crowley was 2,780 to 3,000 kg/ha following applications of imazethapyr at PPI, PRE, DPRE, and EPOST, and these yields were higher than the yields of the MPOST and LPOST treatments. At St. Joseph, IMI-tolerant rice yields were equal for imazethapyr applied PPI, DPRE, and EPOST treatments compared with Crowley.

There are limited options for controlling large crabgrass in spring-seeded cool-season turf. The objective of this experiment was to evaluate the safety and effectiveness of dithiopyr and quinclorac to control large crabgrass in spring-seeded Kentucky bluegrass and perennial ryegrass stands. Quinclorac at 0.84 kg/ha and dithiopyr at 0.56 kg/ha were applied separately at various times prior to seeding and after emergence of Kentucky bluegrass and perennial ryegrass. Quinclorac applied once at 2, 4, 8, or 12 weeks after emergence (WAE) or split-applied preplant incorporated (PPI) + 12 WAE will give season-long control of large crabgrass with no injury to spring-seeded Kentucky bluegrass or perennial ryegrass. Quinclorac applied PPI or 0 WAE is safe to use in spring seedings, but it will not provide season-long control of large crabgrass. Dithiopyr applied 2 or 4 WAE will control large crabgrass season-long while not injuring spring-seeded Kentucky bluegrass or perennial ryegrass. However, dithiopyr applied PPI or 0 WAE will severely limit germination of desirable turf, and dithiopyr applied 8 or 12 WAE will not control mature crabgrass.

Glyphosate-resistant spring wheat is being developed and may soon be available to growers. Field experiments were conducted to examine the effect of various rates and application timings of glyphosate on the tolerance of glyphosate-resistant spring wheat and the level of weed control attained with glyphosate in wheat. Glyphosate-resistant wheat exhibited excellent tolerance to glyphosate applied at the one- to two- or four- to five-leaf stages at rates up to 1,780 g ae/ha, representing a fourfold safety margin to the proposed field use rate. Glyphosate at 445 g/ha reduced shoot biomass of green foxtail, wild oat, redroot pigweed, and wild mustard > 95% in glyphosate-resistant wheat in all years, but wild buckwheat biomass was only reduced 67 to 75% at doses up to 890 g/ha. Glyphosate applied in-crop at 667 g/ha reduced quackgrass biomass by 97 to 99%. Comparison of glyphosate with other spring wheat herbicides indicated that glyphosate provided similar or better in-crop control of several weed species. Yield of weed-infested glyphosate-resistant spring wheat responded positively to glyphosate in all years.

A study was established to evaluate weed control and crop response of imidazolinone-tolerant (IMI-tolerant) rice in water-seeded culture with imazethapyr at 70, 105, and 140 g ai/ha at four different soil application timings with or without 70 g/ha imazethapyr postemergence (POST). Application timings included preplant incorporated (PPI), surface application prior to seeding (SURFACE), following seeding (SEED), and at pegging (PEG). The PEG treatments were applied when green leaf tissue had emerged from the seed and the root had begun to extend downward into the soil. Response of barnyardgrass, red rice, and rice injury was not affected by the addition of an imazethapyr POST application. Barnyardgrass control was above 90% at 28 d after treatment (DAT) for both years. At 42 DAT, barnyardgrass control decreased to 88% in 1998, compared with 94% control in 1999. Averaged over years, Indian jointvetch control ranged from 44 to 74% at 28 DAT and 41 to 71% at 42 DAT. Indian jointvetch control was inconsistent and lacked uniformity over all factors. Rice injury increased as application timing was delayed from PPI to PEG in 1998 at both rating dates. In 1999, injury was not observed with imazethapyr at 28 DAT, and 1% injury was observed at 42 DAT with all imazethapyr application timings.

Field studies evaluated the effects of crop, crop rotation, planting date, row spacing, no-till planting, and herbicides on wild-proso millet control. Benefin and EPTC reduced wild-proso millet biomass in new seedings of alfalfa, and the combination of herbicides and forage harvest prevented wild-proso millet seed production in that crop. Wild-proso millet seedling populations were reduced and corn yields increased when corn followed 1 to 4 yr of alfalfa. Apparent effectiveness of herbicides in corn increased after cropping with alfalfa. Germination of buried wild-proso millet seed decreased approximately 90% after 54 months which is equivalent to 4-yr prior cropping to alfalfa. Wild-proso millet control and field and sweet corn yields increased when the crops were planted late season (May 17) rather than early season (April 27). Planting sweet corn in rows spaced 76 cm rather than 108 cm apart also increased sweet corn yields. EPTC plus dichlormid (EPTC+) plus cyanazine always controlled wild-proso millet equal to or better than EPTC+ alone. When field corn was planted no-till into killed alfalfa sod, combinations of pendimethalin plus simazine and pendimethalin plus cyanazine provided the best wild-proso millet control and corn yields. But the following year when corn was no-till planted into corn stubble of the identical plots, wild-proso millet control from the same treatments tended to be lower and corn yields were reduced.

Field studies were conducted during the 2003 through 2005 growing seasons to evaluate postemergence herbicides and timing of application for horse purslane, smellmelon, and Palmer amaranth control in peanut. Acifluorfen provided inconsistent control of the three weed species. Bentazon failed to control Palmer amaranth, horse purslane, and smellmelon (< 65%), whereas 2,4-DB controlled horse purslane and smellmelon less than 70%. Diclosulam applied early postemergence (EPOST) controlled Palmer amaranth at least 77% but horse purslane control varied from 27 to 73% when applied EPOST and was less than 50% with the late postemergence (LPOST) application. Diclosulam controlled smellmelon less than 65%. Imazethapyr and imazapic controlled Palmer amaranth and smellmelon at least 70% but controlled horse purslane less than 65%. Lactofen controlled Palmer amaranth and horse purslane at least 93% with the EPOST application but had inconsistent control (40 to 100%) when applied LPOST. Smellmelon control with lactofen was also inconsistent. Removal of weed interference with all herbicides except bentazon provided peanut yields greater than the nontreated check.