Weed control in white beans is currently limited by the small number of registered herbicides. The tolerance of two white bean cultivars, ‘AC Compass’ and ‘OAC Thunder’, to various postemergence (POST) herbicides at the maximum use rate and twice the maximum use rate for soybean or corn was evaluated at two Ontario locations in 2001 and 2002. Generally, the two cultivars did not differ in their response to the POST herbicides. POST applications of imazamox plus fomesafen, imazamox plus bentazon, and cloransulam-methyl decreased plant height, shoot dry weight, and yield by as much as 29, 41, and 55%, respectively, and increased seed moisture content up to 3.9%. POST applications of thifensulfuron, chlorimuron, and bromoxynil decreased plant height as much as 57%, shoot dry weight by up to 71%, yield as much as 93% and increased seed moisture content up to 15.5%. Based on these results, AC Compass and OAC Thunder white beans do not possess sufficient tolerance to support the registration of imazamox plus bentazon, imazamox plus fomesafen, cloransulam-methyl, thifensulfuron, chlorimuron, and bromoxynil.

In 1985, the Swedish government mandated a 50% reduction in agricultural pesticide use by 1990. The reference point was based on the average of total pesticide sales (kg ai) between 1981 and 1985. The goal was to halve the risks to users and to the environment. A two-pronged approach dealing with risk and with the actual reduction in agricultural pesticide use was developed. By 1991, 215 ‘unsafe’ or less effective pesticide products were withdrawn by producers or cancelled by the National Chemical Inspectorate, leaving only 322 registered. Additionally, the government was able to report a 47% reduction in agricultural pesticide use where 64, 54, and 2% reductions occurred in insecticides, herbicides, and fungicides, respectively. However, the treated area did not decrease. Pesticide reduction was achieved largely in cereal grain production by switching from high-dose to low-dose herbicides, reducing rates of herbicides, removal of inactive isomers from racemic mixtures, cancellation of TCA for quackgrass control, increased set-aside, and improved sprayer precision. During the same 5-yr period, Scotland achieved an equivalent reduction without government intervention, indicating that the reductions were largely due to changing herbicide technology. Determining the need for similar programs must take into account: 1. initial use rates (in 1987, average total pesticide use rates in the U.S. and in Sweden were 1.8 and 1.5 kg ai/ha, respectively, and following the mandate period, Swedish use has decreased to 0.8 kg ai/ha); 2. measurement criteria; and 3. the long-term effect of reducing herbicide rates on replenishing the soil weed seed bank reserves.

Imazethapyr applied to the soil at 0, 35, 53, 70, 87, 105, and 140 g ai/ha followed by (fb) early-postemergence (EPOST) or late-postemergence (LPOST) applications at 140, 105, 87, 70, 53, 35, and 0 g ai/ha, respectively, was evaluated for weed control, crop tolerance, and grain yield in imidazolinone-tolerant rice. In drill- and water-seeded rice, imazethapyr effectively controlled barnyardgrass 91 to 97% with a soil fb EPOST or LPOST application except when applied at 105 fb 35 g/ha EPOST at 49 d after late-postemergence treatment. Rice yields were 5,920 and 3,920 kg/ ha in the drill-seeded study with a single application of imazethapyr at 140 g/ha EPOST in 2000 and 2001, respectively. These yields were greater than or equal to those from treatments with two imazethapyr applications.

Threshold weed management methods have recently been elaborated to consider effects of threshold management on weed population dynamics. Such economic optimum thresholds are calculated using population-dynamics models which require detailed information about weed demography, including seed production (as affected by events between germination and seed dispersal), seed dispersal, and seed survival and movement in soil. Factors affecting any of these aspects of demography appear likely to modulate the growth rate of a sub-threshold population and therefore to influence the economic optimum threshold value. To test this conjecture and evaluate weed threshold management, including associated risk, improved understanding is particularly needed of weed seed dispersal, seedbank processes, and unpredictable demographic variation.

Kochia control in continuous corn became increasingly difficult in experimental plots where isoxaflutole was used PRE for 8 yr. Studies were conducted to determine if poor kochia control resulted from an escape mechanism based on different germination rates or from a difference in sensitivity to isoxaflutole. Germination at constant temperatures showed that the kochia population in the experimental plot had greater seed dormancy compared with populations growing in adjacent fields. Germination at 25 C for seeds collected from the isoxaflutole-treated area was near 20% after 20 d, whereas germination for the other populations was above 80%. The optimal temperatures to release seed dormancy for seeds from the experimental plot were alternating 35/25 C day/night temperatures. The kochia biotype that predominated where isoxaflutole was applied PRE had elevated levels of seed dormancy and required higher alternating temperatures to release dormancy than untreated control kochia. These characteristics were unique and not found in populations never exposed to isoxaflutole. Chlorophyll content was measured to determine if differences in sensitivity to isoxaflutole existed among biotypes. Absorption at 660 nm by photosynthetic pigments was similar among the biotypes at increasing herbicide rates, indicating no differences in sensitivity to isoxaflutole among populations. Reduced kochia control in the experimental plot was due to delayed seed germination, which allowed isoxaflutole to degrade before seeds germinated. The rapid herbicide dissipation from soil can be attributed in part to coarse soils, soil moisture, and the low isoxaflutole rate.

Base temperature (Tb) for leaf appearance and the thermal time interval between appearance of successive leaves (phyllochron) were determined for six common weed species from a series of growth chamber experiments. Mean leaf appearance Tb values for giant ragweed, velvetleaf, redroot pigweed, large crabgrass, woolly cupgrass, and wild-proso millet were 1.3, 8.0, 8.5, 4.5, 2.2, and 5.1 C, respectively, and mean phyllochron values were 37.2, 34.4, 17.3, 42.2, 65.2, and 34.2 growing degree-days per leaf, respectively. Phyllochron values increased slightly with mean temperature for each weed species. To our knowledge, these are the first reported leaf appearance Tb and phyllochron values for giant ragweed, woolly cupgrass, and wild-proso millet. The results confirm leaf appearance Tb and phyllochron values reported previously for velvetleaf and redroot pigweed. However, large crabgrass leaf appearance Tb and phyllochron values in our study varied somewhat from those reported previously. The results provide information needed for parameterization of plant growth models that predict leaf development based on thermal time.

A method of calculating the cost to farmers of herbicide resistance in weeds is described along with examples to demonstrate the inherent difficulties and benefits. The main problem is defining cropping systems/practices and herbicide strategies that prevent the development of resistance. The benefits include focusing advisory and research efforts and helping to persuade farmers to adopt practices that prevent the development of resistance.

Field experiments were conducted in 1986 to 1990 to determine the effect of different rates and application times of BAS 514 and dithiopyr alone and in combination on control of large crabgrass and dandelion in Kentucky bluegrass, tall fescue, and perennial ryegrass. Herbicides evaluated were applied PRE, early POST, and late POST. Results indicated that both herbicides provided excellent PRE control in all years. BAS 514 rates ranged from 0.84 to 0.56 kg ai ha-1. Rates of dithiopyr ranged from 0.43 to 0.56 kg ai ha-1. In addition to crabgrass control, both herbicides controlled dandelion, BAS 514 exhibiting slightly better activity than dithiopyr. BAS 514 at 0.84 to 1.12 kg ai ha-1 and dithiopyr at 0.26 to 0.56 kg ai ha-1 resulted in excellent early POST crabgrass control when applied at the one- to three-leaf stage of crabgrass. BAS 514 and dithiopyr applied late POST (2 to 3 tillers of crabgrass) resulted in 63 to 85% control of crabgrass and 29 to 85% control of dandelion at 10 wk after application. No weed control enhancement was evident from tank mixing the two herbicides. Excellent turf safety was demonstrated by both herbicides on all three turf species.

Research is needed to develop more comprehensive integrated weed management systems that would facilitate greater adoption by farmers. A field study was conducted to determine the combined effects of seed date (April or May), seed rate (recommended or 150% of recommended), fertilizer timing (fall or spring applied), and in-crop herbicide rate (50 or 100% of recommended) on weed growth and crop yield. This factorial set of treatments was applied in four consecutive years within a barley-field pea–barley-field pea rotation in a zero-till production system. Both barley and field pea phases of the rotation were grown each year to account for variable environmental conditions over years. Weed biomass was often lower with May than with April seeding because of more weeds being controlled with preplant glyphosate. However, despite fewer weeds being present with May seeding, barley yield was only greater in 1 of 4 yr and field pea yield was actually lower with May than with April seeding in 3 of 4 yr, indicating that optimum seed date is highly dependent on crop species and environmental conditions. Higher crop seed rates reduced weed biomass and increased crop yield in 2 of 4 yr in each of barley and field pea. Fertilizer timing had little effect on weed competition in barley, but spring- compared with fall-applied fertilizer reduced weed biomass and increased field pea yield in 2 of 4 yr. In-crop herbicides applied at 50% compared with 100% rates sometimes resulted in greater weed biomass and lower crop yields with recommended crop seed rates, but few differences were noted at high crop seed rates. Indeed, the weed seed bank at the conclusion of the 4-yr study was not greater with the 50% compared with 100% herbicide rate when high crop seed rates were used. This study demonstrates the combined merits of early seeding (April), higher crop seed rates, and spring-applied fertilizer in conjunction with timely but limited herbicide use to manage weeds and maintain high yields in rotations containing barley and field pea.

Leafy spurge is a deep-rooted perennial weed that displaces native rangeland vegetation and replaces forage for cattle and other forages used by vertebrate herbivores. Strategic planning to control this weed requires monitoring its distribution and spread. Classical monitoring techniques, which often involve extensive ground survey efforts, can be aided by the synoptic nature of remotely sensed imagery. This research addresses the use of Space Imaging's 4-m multispectral Ikonos imagery for the survey and detection of leafy spurge infestations. Survey data were collected at a site in western North Dakota and used to produce supervised classifications of leafy surge infestations with Ikonos imagery. Multiple image dates per year were combined with each other to assess the added accuracy afforded by multitemporal imagery. Finally, individual patches of leafy spurge were analyzed to determine the minimum patch size and percent cover that were detectable with supervised classification of Ikonos imagery. Under some circumstances, the imagery was effective at detecting leafy spurge, but in areas with a higher forb component, the classification was not as effective. Multidate imagery provided increased accuracy, but improvements were not consistently significant. Leafy spurge infestations of <30% cover and 200 m2 were not reliably detected.

Observations of the effects of living organisms on weeds dates from 1795 when an insect, Dactylopius ceylonicus, was introduced for drooping pricklypear (Opuntia vulgaris Miller) control over a vast area. Since that time, biological control of weeds employed mainly the classical strategy of introducing natural enemies from areas of co-evolution. Self-perpetuation and dissemination of these introduced enemies was essential to suppress successfully the weed below economic levels. This classical tactic is suited particularly for weeds that are distributed widely in less intensively cropped or noncropped areas. Guidelines to introduce foreign organisms for biological control of weeds in the United States have been established.

An experiment was conducted to determine the utility of multispectral imagery for identifying soybean, bare soil, and six weed species commonly found in Mississippi. Weed species evaluated were hemp sesbania, palmleaf morningglory, pitted morningglory, prickly sida, sicklepod, and smallflower morningglory. Multispectral imagery was analyzed using supervised classification techniques based upon 2-class, 3-class, and 8-class systems. The 2-class system was designed to differentiate bare soil and vegetation. The 3-class system was used to differentiate bare soil, soybean, and weed species. Finally, the 8-class system was designed to differentiate bare soil, soybean, and all weed species independently. Soybean classification accuracies classified as vegetation for the 2-class system were greater than 95%, and bare soil classification accuracies were greater than 90%. In the 3-class system, soybean classification accuracies were 70% or greater. Classification of soybean decreased slightly in the 3-class system when compared to the 2-class system because of the 3-class system separating soybean plots from the weed plots, which was not done in the 2-class system. Weed classification accuracies increased as weed density or weeks after emergence (WAE) increased. The greatest weed classification accuracies were obtained once weed species were allowed to grow for 10 wk. Palmleaf morningglory and pitted morningglory classification accuracies were greater than 90% for 10 WAE using the 3-class system. Palmleaf morningglory and pitted morningglory at the highest densities of 6 plants/m2 produced the highest classification accuracies for the 8-class system once allowed to grow for 10 wk. All other weed species generally produced classification accuracies less than 50%, regardless of planting density. Thus, multispectral imagery has the potential for weed detection, especially when being used in a management system when individual weed species differentiation is not essential, as in the 2-class or 3-class system. However, weed detection was not obtained until 8 to 10 WAE, which is unacceptable in production agriculture. Therefore, more refined imagery acquisition with higher spatial and/or spectral resolution and more sophisticated analyses need to be further explored for this technology to be used early-season when it would be most valuable.

Tropical soda apple (SOLVI) is an introduced, perennial broadleaf plant that has invaded Florida agricultural land and natural ecosystems and has spread to other states (Georgia, Alabama, South Carolina, Louisiana, North Carolina, Tennessee, and Pennsylvania). Tropical soda apple was first collected in Florida in 1988. In 1990, SOLVI-infested land was approximately 10,000 ha, in 1993, 162,000 ha, and in 1995, approximately 0.5 million ha were reported infested in Florida. Rapid spread of this invasive plant has occurred from seeds transported in cattle, hay, sod, grass seeds, water, and wildlife (deer, feral hogs, birds). Animals will not eat the foliage but will consume the fruits and spread the seeds in their feces. This exotic weed is an indeterminate plant with seed production averaging 50,000/plant/yr, seed germination of 70–90%, and seed longevity in soil of up to 1 yr. Integrated weed management strategies include prevention (avoidance of contaminated hay or grass seed, control of movement of cattle), control (mechanical, chemical), and monitoring. Seed production must be prevented and landowners should adopt a zero tolerance toward SOLVI. During the time (1990–1995) research (biology, ecology, control) was being conducted to control SOLVI from Florida, this weed had infested the entire state and escaped into other states. Risk assessment criteria for new plant introductions must be developed and implemented to prevent future biological pollution.

Studies were conducted from 1993 through 1995 to determine broomrape control from imazethapyr applied to pea seeds. Soaking seeds for 1 to 10 min in 0.0001% to 1% imazethapyr (Pursuit-10) solutions or coating seeds with imazethapyr at 0.025 to 0.30 mg/g (herbicide/seed, equivalent to 5 to 60 g/ha for a seeding rate of 200 kg/ha) did not affect seed germination or crop emergence and growth. Broomrape control with imazethapyr applied by coating seed at 0.1 to 0.2 mg/g (herbicide/seed, 20 to 40 g/ha) or by soaking seeds for 5 min in 0.1% to 1% imazethapyr solution was from 75 to 80% and 50 to 70%, respectively. Pea biomass and seed yield increased, compared to broomrape-infested nontreated control, when pea seeds were treated with imazethapyr and imazapyr at 5 g/ha was applied late postemergence, as a result of the excellent broomrape control.

Antagonism was evaluated among four grass and seven broadleaf herbicides applied postemergence for seedling and rhizome johnsongrass control in soybeans. Imazaquin and chlorimuron were most antagonistic to grass herbicides. Haloxyfop and quizalofop were least affected by the addition of broadleaf herbicides. Johnsongrass control was reduced 11 to 46% when grass herbicides were applied with broadleaf herbicides and were evaluated 3 and 12 weeks after treatment (WAT).

Over two years, acetolactate synthase (ALS)-inhibiting herbicides and herbicides with different mechanisms of action were tested individually and in combination for control of ALS-resistant Palmer amaranth in soybean. As expected, ALS-inhibiting herbicides did not control the resistant Palmer amaranth. Lactofen at 210 g/ha and acifluorfen at 560 g/ha gave the best postemergence control. Tank mixes of lactofen with either imazethapyr or chlorimuron plus thifensulfuron did not significantly increase control over lactofen alone. Sequential treatment with a soil-applied herbicide, either SAN 582 or pendimethalin, followed by lactofen postemergence, controlled weeds best (greater than 85%). The extent of cross-resistance of Palmer amaranth to ALS-inhibiting herbicides was determined in the greenhouse. Sixteen POST ALS-inhibiting herbicides were sprayed at ½, 1, 2, 4, and 8X field use rates on resistant and susceptible biotypes. All ALS-inhibiting herbicides at all rates controlled the susceptible biotype. The resistant biotype was cross-resistant to all ALS-inhibiting herbicides. Metsulfuron and imazapyr at 8X rates were the only treatments that provided 80% or greater control of the ALS-resistant Palmer amaranth. Chlorsulfuron and tribenuron were the only other herbicides that gave partial control at the highest rate. Control with these four herbicides decreased as the rate was decreased. Experimental results suggest that cultural practices and herbicides with different mechanisms of action will be required to manage this ALS-resistant Palmer amaranth biotype.

Under greenhouse conditions, annual bluegrass (Poa annua L.), common groundsel (Senecio vulgaris L.), shepherd's purse [Capsella bursa-pastoris (L.) Medicus], small nettle (Urtica urens L.), canola (Brassica napus L. ssp. napus), and pea (Pisum sativa L.) differed in sensitivity to ultraviolet (UV) radiation. Of the weed species, annual bluegrass was the least sensitive; whereas, among the crop species, canola was about sevenfold more sensitive than was pea. The sensitivity of a species to UV radiation was highly dependent upon its stage of development. The study indicates some potential for using UV radiation to control weeds, but the method needs further investigation to unravel the selectivity of the methods and potential health hazards.

Pyroxasulfone dissipation and mobility in the soil was evaluated and compared to S-metolachlor in 2009 and 2010 at two field sites in northern Colorado, on a Nunn fine clay loam, and Olney fine sandy loam soil. Pyroxasulfone dissipation half-life (DT50) values varied from 47 to 134 d, and those of S-metolachlor ranged from 39 to 63 d. Between years, herbicide DT50 values were similar under the Nunn fine clay loam soil. Under the Olney fine sandy loam soil, dissipation in 2009 was minimal under dry soil conditions. In 2010, under the Olney fine sandy loam soil, S-metolachlor and pyroxasulfone had half-lives of 39 and 47 d, respectively, but dissipation rates appeared to be influenced by movement of herbicides below 30 cm. Herbicide mobility was dependent on site-year conditions, in all site-years pyroxasulfone moved further downward in the soil profile compared to S-metolachlor.

Sprayer applicator-controlled variables, such as nozzle selection and spray volume, will become increasingly important for making labeled POST applications of dicamba in next-generation cropping systems. A field experiment was conducted in 2013 and 2014 at the Northeast Research and Extension Center in Keiser, AR. Tank mixtures of Engenia (a new form of dicamba), glyphosate, glufosinate, and S-metolachlor were applied with TeeJet AIXR, AITTJ60, and TTI nozzles. Two nozzle sizes, 11003 and 11006, were used to vary spray volume from 94 L ha−1 to 187 L ha−1, respectively. For barnyardgrass, a significant decrease in control was observed when spray volume was reduced for glyphosate + dicamba in 2013. In 2014, an overall decrease in control was observed for the TTI nozzle when spray volume was reduced to 94 L ha−1, averaged across all herbicide treatments. The addition of the product S-metolachlor to glyphosate + glufosinate + dicamba significantly reduced the droplet spectra for all nozzle types. For example, adding S-metolachlor into the tank-mix decreased the volume median diameter (Dv50) for the TTI nozzle at 187 L ha−1 spray volume from 789 μm to 570 μm. The results from this research demonstrate that using low spray volume and coarser nozzles could reduce efficacy of the herbicides on the weed species evaluated. Nozzle selection and spray volume have key roles in maximizing efficacy of POST applications in dicamba-resistant crops. Additionally, evaluating droplet spectra of potential dicamba-containing tank-mixtures is critical for producing the desired droplet size to minimize off-target movement.

Five studies were conducted at Clayton, Rocky Mount, and Lewiston-Woodville, NC, in 2001 and 2002, to evaluate weed management, crop tolerance, and yield in strip- and conventional-tillage glyphosate-resistant cotton. Cotton was treated with two glyphosate formulations; glyphosate-IP (isopropylamine salt) or glyphosate-TM (trimethylsulfonium salt), early postemergence (EPOST) alone or in a mixture with S-metolachlor. Early season cotton injury was minimal (3%) with either glyphosate formulation alone or in mixture with S-metolachlor. Weed control and cotton yields were similar for both glyphosate formulations. The addition of S-metolachlor to either glyphosate formulation increased control of broadleaf signalgrass, goosegrass, large crabgrass, and yellow foxtail 14 to 43 percentage points compared with control by glyphosate alone. S-metolachlor was not beneficial for late-season control of entireleaf morningglory, jimsonweed, pitted morningglory, or yellow nutsedge. The addition of S-metolachlor to either glyphosate formulation increased control of common lambsquarters, common ragweed, Palmer amaranth, smooth pigweed, and velvetleaf 6 to 46 percentage points. The addition of a late postemergence-directed (LAYBY) treatment of prometryn plus MSMA increased control to greater than 95% for all weed species regardless of EPOST treatment, and control was similar with or without S-metolachlor EPOST. Cotton lint yield was increased 220 kg/ha with the addition of S-metolachlor to either glyphosate formulation compared with yield from glyphosate alone. The addition of the LAYBY treatment increased yields 250 and 380 kg/ha for glyphosate plus S-metolachlor and glyphosate systems, respectively. S-metolachlor residual activity allowed for an extended window for more effective LAYBY application to smaller weed seedlings instead of weeds that were possibly larger and harder to control.