Relative vaporization losses of five C14 ring-labeled 2-chloro-4,6-alkylamino-s-triazines were studied at 25 and 60 C. Herbicide solutions in 95% ethanol were deposited on metal planchets, dried and counted at intervals. The herbicides, ranked in order of decreasing volatility, were: trietazine (2-chloro-4-diethylamino-6-ethylamino-s-triazine), ipazine (2-chloro-4-diethylamino-6-isopropylamino-s-triazine), atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), propazine (2-chloro-4,6-bis(isopropylamino)-s-triazine) and simazine (2-chloro-4,6-bis(ethylamino)-s-triazine). Prometone (2-methoxy-4,6-bis (isopropylamino)-s-triazine) and the 2-hydroxy analogues of simazine and propazine, dissolved in water at pH 2–3 (HC1) before plating, showed no detectible loss in 25 hours at 60 C. Foliar penetration and acute toxicity following drop application of the five 2-chloro-triazines and prometone to corn, soybean, tomato, cotton, oat and sorghum appeared to be correlated with their water solubilities. A surfactant tended to enhance activity. Important differences were noted among species, although apparently not related to inherent herbicidal tolerance or susceptibility. Tomato was most sensitive to acute toxicity; soybean cotyledons were least permeable. Basipetal translocation was negligible under all conditions (e.g. 6 herbicides, 6 species, 2 solvents, with or without a surfactant). The 2-hydroxy analogues tended to translocate downward but appeared restricted by acute, acid (pH 2–3) destruction of the transport mechanism. Acropetal (apoplastic) movement of all compounds occurred readily with transpiration, reflecting the vasculature of the species. All 2-chlorotriazines and prometone (but not the two 2-hydroxy compounds) showed pronounced accumulation of C14 in the lysigenous glands of cotton. Root uptake and distribution of C14 within the plant body occurred generally for all compounds and species.

The impact of seed production by subthreshold weed populations on future weed problems has impeded the adoption of integrated pest-management principles for weed management. Studies were conducted in fields with no velvetleaf history to determine how seedbanks and seedling populations change following seed production 1 yr or 5 consecutive yr in plow-disk and no-tillage corn. Cumulative seed production by 0.19 velvetleaf plants m−2 increased in a linear fashion from 1989 to 1994, with annual additions averaging from 330 seeds m−2 for velvetleaf in corn to 2,500 seeds m−2 for velvetleaf without competition from corn. Five-year cumulative seed production was 1,480 seeds m−2 in plow-disk and 1,810 seeds m−2 in no-till corn. In no-till corn, 42 velvetleaf seedlings m−2 emerged the 1st year after the 1989 seed rain, but only 35 seedlings m−2 emerged over the next 4 yr. In plow-disk plots, annual emergence averaged 12 seedlings m−2. Five years after the 1989 seed rain, the proportion of seeds lost to emergence was about 20% in both tillage treatments. Where velvetleaf seeds were allowed to return to the soil every year, cumulative seedling emergence was lower in plow-disk than in no-till corn, with total emergence of 70 and 360 seedlings m−2, respectively, after 5 yr. Seedbank numbers ranged from 10 seeds m−2 5 yr after a single seed rain (290 seeds m−2) by velvetleaf in plow-disk corn to 1,020 seeds m−2 following 5 consecutive yr of seed rain where 12,580 seeds m−2 were returned without corn competition in no-till. Seedbank samples in the fall of the 5th year had 69 to 98% fewer seeds than were accounted for by cumulative seed rain and seedling emergence, with greater apparent seed losses in plow-disk corn than in no-till corn. Over 90% velvetleaf control would be required annually to maintain subthreshold populations for 5 yr following a single seed rain. By comparison, over 95% control would be required annually to maintain subthreshold populations where velvetleaf seed return is permitted each year.

Aminopyralid is a new auxinic herbicide that provides Canada thistle control at lower use rates than clopyralid. Studies were conducted to determine if differences in absorption, translocation, or metabolism account for aminopyralid's greater biological activity. Radiolabeled aminopyralid and clopyralid were applied to individual leaves of rosette-stage Canada thistle plants. Nonionic surfactant was used for the absorption studies because it provided higher aminopyralid absorption than methylated seed oil or crop oil concentrate. Clopyralid was absorbed very rapidly, reaching 72% 24 h after treatment (HAT) and remaining near or above 80% during a 192-h time course. During the same time period, aminopyralid absorption increased from 34 to 60%. Clopyralid translocation out of the treated leaf was significantly higher than aminopyralid, 39% compared with 17%, respectively, 192 HAT. More of applied clopyralid translocated to aboveground tissue 192 HAT (27%) than to roots (12%), whereas aminopyralid translocation was similar in aboveground tissue (10%) and roots (7%) 192 HAT. Neither aminopyralid nor clopyralid was metabolized 192 HAT. Although aminopyralid is effective at lower use rates than clopyralid, clopyralid absorption and translocation were higher in Canada thistle. These results suggest that aminopyralid's chemical structure may provide for greater biological activity at the target site than clopyralid.

Significantly higher rates of butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] were required to produce the same level of weed control as trifluralin [2,6-dinitro -N,N-dipropyl-4-(trifluoromethyl) benzenamine] when applied to soybeans [Glycine max (L.) Merr.] on seven different soils in the field. Higher rates of butralin were also required to control barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCG] in growth chamber studies. No differences in the extent of soil adsorption of trifluralin and butralin were apparent; therefore, differences in efficacy could not be attributed to differences in soil adsorption. Herbicide rates required for 80% weed control and Freundlich K-values (adsorption capacity indices) were mostly highly correlated with soil organic-matter content and soil surface area as measured by benzyl ethyl ether (BEE) and ethylene glycol monoethyl ether (EGME) on nine soils. Analysis of the organic-matter content of the nine soils by 10 soil testing laboratories resulted in highly significant differences among laboratories.

The extensive use of propanil for weed control in Oryza sativa over the past 30 yr has led to the development of propanil-resistant Echinochloa crus-galli. This problem was first identified in one Arkansas county in 1990. Since that time, 171 resistant populations have been identified in 20 counties. Random amplified polymorphic DNA (RAPD) analysis was used to assess genetic variation among populations of propanil-resistant and -susceptible E. crus-galli in Arkansas and to elucidate the origin of resistance and its dispersal. Sixteen E. crus-galli populations from seven Arkansas counties and one population from Mississippi were analyzed using 13 primers, which amplified a total of 159 repeatable fragments. Genetic distance was calculated among the populations, and the populations were clustered. The analysis revealed two distinct clusters with an average between-cluster genetic distance of 0.436 and average within-cluster distances of 0.012 and 0.029. These clusters may represent different Echinochloa species. Each cluster contained at least one resistant population and populations from different regions of the state. In addition, several resistant populations from different regions of the state were genetically nearly identical. Thus, it is likely that resistant biotypes have spread in Arkansas by seed dispersal and by independent mutation events. This suggests that there are multiple origins of the resistance, and simple control of weed seed dispersal will not stop the spread of resistance.

Glasshouse and growth chamber experiments were conducted to determine chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} phytotoxicity to kochia [Kochia scoparia (L.) Schrad. ♯ KCHSC] and green foxtail [Setaria viridis (L.) Beauv. ♯ SETVI] as influenced by temperature and humidity for 1 week after treatment, and by soil moisture and nitrogen. Chlorsulfuron was more phytotoxic to both kochia and green foxtail at 95 to 100% than at 45 to 50% relative humidity for 1 week after treatment whether at 10, 20, or 30 C. Chlorsulfuron phytotoxicity was similar with all posttreatment temperatures at each humidity, except that phytotoxicity was lower at 30 C than at 10 or 20 C at 90 to 100% humidity. Surfactant added to chlorsulfuron exhanced control of kochia more than that of green foxtail and overcame temperature and humidity effects on chlorsulfuron phytotoxicity. A simulated rainfall of 2 mm after chlorsulfuron application reduced toxicity of chlorsulfuron to green foxtail more than to kochia. Chlorsulfuron was more phytotoxic to green foxtail growing with a high than a low soil nitrogen level. High soil moisture following chlorsulfuron application enhanced phytotoxicity to kochia and green foxtail compared to high soil moisture before treatment.

Under field conditions, more photoassimilate moved to roots of Canada thistle at the bolt than at the bud, flower, or postflower stages. Similarly, greater photoassimilate accumulated in roots of Canada thistle in the greenhouse at the rosette and bolt than at the flower bud stage. Growth chamber experiments indicated that environmental conditions typical of fall, and possibly early spring, favored photoassimilate movement to the root and superseded growth stage control of assimilate partitioning. Allocation of assimilate within the root was strongly influenced by growth stage, with most assimilate being utilized for growth at the rosette stage and for fructan reserves in bolt and flower bud stages.

The distribution pattern of MCPA ([(4-chloro-o-tolyl)oxy] acetic acid) on leaf surfaces of three species was studied using the cathodoluminescence detection mode of a scanning electron microscope. On low-wax-content sugarbeet (Beta vulgaris L.) leaves MCPA concentrated in the depressions over the anticlinal cell walls when applied at high volumes (748 and 374 L/ha). At low volumes (23 L/ha), numerous small deposits of MCPA were randomly distributed over both anticlinal and periclinal walls. These distinct patterns were independent of herbicide concentration. Regardless of spray volumes, MCPA remaining on the waxy leaf surfaces of cabbage (Brassica oleracea L.) coalesced into small thick deposits. Large spray drops from high application volumes shattered on impact with the stellate hairs of turkey mullein (Eremocarpus setigerus Benth.) resulting in some MCPA reaching the leaf surface. Spray drops from low application volumes did not shatter but lodged on the hairs with very little reaching the leaf surface.

Studies were conducted to evaluate density-dependent effects of common ragweed on weed growth and peanut growth and yield. Common ragweed height was not affected by weed density and peanut canopy diameter. Weed height exceeded peanut height throughout the growing season, indicating that competition for light occurred between the two species. Common ragweed aboveground dry biomass per plant decreased as weed density increased, but total weed dry biomass per meter of crop row increased with weed density. The rectangular hyperbola model described the effect of weed density on percent peanut yield loss. With the asymptote constrained to 100% maximum yield loss, the I coefficient (yield loss per unit density as density approaches zero) was 68.3 ± 12.2%. Common ragweed did not influence the occurrence of tomato spotted wilt virus, early leaf spot (Cercospora arachidicola), southern stem rot (Sclerotium rolfsii), and Cylindrocladium black rot (Cylindrocladium crotalariae). However, as common ragweed density increased, the incidence of late leaf spot (Cercosporidium personatum) increased. Results indicate that common ragweed is one of the more competitive weeds in peanut and a potential economic threat to peanut growers.

The resistance of corn to the herbicides, 2-chloro-4, 6-bis(ethylamino)-s-triazine [simazine] and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), may be related to stalk rot and corn borer resistance. Of five inbred lines of corn studied, three with known resistance to stalk rot and corn borer were found to be much more resistant to simazine and atrazine than were two inbreds which were susceptible to stalk rot and corn borer.

Johnson grass (Sorghum halepense L., Pers.) is a native of the Mediterranean coastal countries and was introduced into the United States about 1830 by Governor Means of South Carolina (5). About ten years later some seed was planted in fertile river bottom soil near Selma, Alabama, by Colonel William Johnson. As Colonel Johnson was the first person to cultivate the grass extensively in the United States, it was named for him. Since that time, the grass has been intentionally cultivated and unintentionally disseminated in many areas of this country. A recent distribution map by Hitchcock (3) shows that the plant may be found in all but about a dozen northern states.

The overall degradation of chlorimuron was very similar at −0.1 and −1.5 MPa and slightly less in air-dry soil. Degradation rates increased with increasing temperature. The primary 14C-labeled compounds observed in moist-soil extracts were desmethyl chlorimuron and saccharin, while the primary 14C-labeled compound observed in air-dry soil extracts was saccharin. Saccharin is formed quantitatively from ethyl 2-(aminosulfonyl)benzoate (phenylsulfonamide) during extraction and therefore represents phenylsulfonamide formed in the soil as a result of chemical hydrolysis of the sulfonylurea bridge. These degradation products suggest that chemical hydrolysis of the sulfonylurea bridge is the primary mode of degradation in air-dry soil, while microbial degradation and chemical hydrolysis both occur in moist soil. These laboratory results demonstrate that chlorimuron will degrade in air-dry soil at a temperature-dependent rate by chemical hydrolysis.

Field studies were conducted at the Main Experiment Station, Fayetteville, AR, in 1994 and 1995 to evaluate the effect of postemergence herbicides applied at late-season timings on sicklepod seed production and viability. AC 263,222, dicamba, glyphosate, glufosinate, and paraquat were applied at five rates (zero; labeled rate; one-half, one-fourth, and one-eighth the labeled rate) to three sicklepod growth stages (bud formation, flowering to 9-cm pods, and 15- to 30-cm pods). The greatest reduction in seed production occurred when herbicides were applied at the flowering to 9-cm pod growth stage. All herbicides applied at one-half the labeled rate to sicklepod at bud formation and at the flowering to 9-cm pod growth stage reduced seed production greater than 80%, except glufosinate. Regardless of herbicide, rate, or application timing, viability of remaining seed was ≥ 90% with all treatments.

This study was conducted to ascertain movement potential of imazethapyr resistance and to measure the relative growth and productivity of imazethapyr-resistant (IR) and imazethapyr-susceptible (IS) biotypes of common sunflower under noncompetitive and competitive conditions. Susceptible biotypes of common sunflower were planted in the field in concentric circles at distances of 5.5, 8.0, 15.0, and 30.0 m around a center of densely planted IR biotypes in four locations in northeast Kansas in 1998 and 1999. Pollen movement was analyzed by sampling the IS progeny for the presence of imazethapyr resistance. The distance in which resistance is first detected from the IR pollen source, first unnatural resistant distance (FURD), ranged from 12.1 to 15.5 m. Wind direction was highly correlated with FURD; the north sections had larger FURD. Greenhouse studies were conducted to study growth of IR and IS biotypes under noncompetitive and competitive conditions. Under noncompetitive conditions, leaf area and dry weight were slightly greater for the IR than the IS biotype at early growth stages, but photosynthesis and height were similar. Under competitive conditions, photosynthesis, leaf area, height, and dry weight of IR and IS biotypes were similar. As a result, IR–IR and IS–IS intracompetition equaled IR–IS intercompetition. Gene flow from IR to IS biotypes occurred with movement up to 15.5 m. The lack of differences between growth of the IR and IS biotype at late growth stages in noncompetitive conditions and similar growth of IR and IS biotypes under competitive conditions indicated no competitive advantage from imazethapyr resistance.

Weed maps are typically produced from data sampled at discrete intervals on a regular grid. Errors are expected to occur as data are sampled at increasingly coarse scales. To demonstrate the potential effect of sampling strategy on the quality of weed maps, we analyzed a data set comprising the counts of capeweed in 225,000 quadrats completely covering a 0.9-ha area. The data were subsampled at different grid spacings, quadrat sizes, and starting points and were then used to produce maps by kriging. Spacings of 10 m were found to overestimate the geostatistical range by 100% and missed details apparently resulting from the spraying equipment. Some evidence was found supporting the rule of thumb that surveys should be conducted at a spacing of about half the scale of interest. Quadrat size had less effect than spacing on the map quality. At wider spacings the starting position of the sample grid had a considerable effect on the qualities of the maps but not on the estimated geostatistical range. Continued use of arbitrary survey designs is likely to miss the information of interest to biologists and may possibly produce maps inappropriate to spray application technology.

Brassicaceae cover crops are gaining attention as potential biofumigants for soil pest suppression because of their ability to release biologically active isothiocyanates (ITCs) and other compounds from hydrolysis of glucosinolates (GSLs). However, biofumigation potential of a Brassicaceae is related to its GSL and ITC profile and GSL to ITC conversion efficiency. Field and laboratory experiments were conducted to evaluate the biofumigation potential of seven Brassicaceae cover crops for weed control in plasticulture tomato and bell pepper. GSL concentration and composition varied among cover crops and between roots and shoots of each cover crop. Similar GSLs were produced in both years by roots or shoots of each cover crop, but GSL concentrations were variable between years. Total GSLs contributed to the soil by incorporation of Brassicaceae cover crop tissues were estimated between 47 to 452 nmol g−1 soil. Highest ITC concentration was detected in soil at 3 h after cover crop incorporation, and concentration decreased at later timings. GSL to ITC conversion efficiency ranged from 1 to 39%, with variation among cover crops and between years. No injury was observed in tomato and bell pepper transplanted 1 wk after cover crop incorporation, indicating the tolerance of tomato and pepper to ITCs released by the cover crops. Early-season yellow nutsedge control from Brassicaceae cover crops was ≤ 53% at 2 wk after transplanting and declined to ≤ 18% later in the season. This research demonstrates that Brassicaceae cover crops have marginal potential for early-season weed control and cannot be used as a weed control practice in commercial tomato and bell pepper production.

The response of ‘C-20’ navy bean and ‘Frankenmuth’ soft white winter wheat grown in rotation to clomazone, imazethapyr, bentazon, and acifluorfen was examined. Clomazone at 560 and 430 g ai ha−1 plus 800 g ai ha−1 pendimethalin and 2000 g ai ha−1 chloramben visibly injured navy bean in 1 of 2 yr. However, navy bean seed moisture at harvest and yield was not reduced compared to the weed-free control. PPI and PRE treatments of 70 g ai ha−1 imazethapyr did not injure navy bean or reduce yield. Imazethapyr applied POST at 70 g ha−1 plus nonionic surfactant visibly injured navy bean. The addition of urea ammonium nitrate to imazethapyr enhanced visible injury and seed moisture compared to nonionic surfactant alone in 1 of 2 yr. However, seed yield was not reduced. Seed moisture at harvest was greater following treatment with 430 g ai ha−1 acifluorfen plus nonionic surfactant or urea ammonium nitrate and 140 and 280 g ha−1 acifluorfen plus 840 g ai ha−1 bentazon in 1 of 2 yr compared to the weed-free control, but yield was not reduced. Wheat yield was reduced in 2 of 2 and 1 of 2 yr by 560 g ha−1 and 430 g ha−1 clomazone, respectively, plus pendimethalin plus chloramben compared to the weed-free control. Wheat yield was not reduced by imazethapyr, bentazon, or acifluorfen.

Competition of downy brome (Bromus tectorum L.) with seeded perennial grasses is an important factor in the lack of success of many seedings on sagebrush conversion areas in the Great Basin. The seriousness of downy brome competition was emphasized by Stewart and Hull. Their work in Idaho indicated repeated failure in seeding of crested wheatgrass (Agropyron cristatum [L.] Gaertn.) on areas where downy brome was not eliminated or reduced. Hull and Pechanec pointed out that many studies indicate that reduction of dense stands of downy brome is essential for the early establishment of seeded species. They also note that in some cases seeding of perennials has been successful without reduction of this weedy annual grass.

An overreliance on herbicides in several important grain- and cotton-producing regions of the world has led to the widespread evolution of herbicide-resistant weed populations. Of particular concern are weed populations that exhibit simultaneous resistance to multiple herbicides (MHR). Too often, herbicides are the only tool used for weed control. We use the term herbicide-only syndrome (HOS) for this quasi-addiction to herbicides. Growers and their advisers focus on herbicide technology, unaware of or ignoring basic evolutionary principles or the necessary diversity provided by other methods of weed control. Diversity in weed control practices disrupts resistance evolution. Significant challenges exist to implementing diversity, including how to address information so that producers choose to alter existing behaviors (HOS) and take calculated risks by attempting new and more complex strategies. Herbicide resistance management in the long term will require creativity in many sectors, including roles for growers, industry, researchers, consultants, retailers, and regulators. There can be creativity in herbicide registration and regulation, as exemplified by the recent U.S. Environmental Protection Agency program that encourages herbicide registrants to register products in minor crops. We propose one idea for a regulatory incentive to enable herbicide registrants in jurisdictions such as the United States to receive an extended data exclusivity period in exchange for not developing one new herbicide in multiple crops used together in rotation, or for implementing stewardship practices such as robust mixtures or limitations on application frequency. This incentive would provide a mechanism to register herbicides in ways that help to ensure herbicide longevity. Approaches based only on market or financial incentives have contributed to the current situation of widespread MHR. Our suggestion for regulatory creativity is one way to provide both financial and biological benefits to the registering company and to the overall stakeholder community by incentivizing good resistance management.

Palmer amaranth is a troublesome weed in cotton production. Yield losses of 65% have been reported from season-long Palmer amaranth competition with cotton. To determine whether water is a factor in this system, experiments were conduced in 2011, 2012, and 2013 in Citra, FL, and in Tifton, GA. In 2011, infrequent rainfall lead to drought stress. The presence of Palmer amaranth resulted in decreased soil relative water content up to 1 m in depth. Cotton stomatal conductance (gs) was reduced up to 1.8 m from a Palmer amaranth plant. In 2012 and 2013 higher than average rainfall resulted in excess water throughout the growing season. In this situation, no differences were found in soil relative water content or cotton gs as a function of proximity to Palmer amaranth. A positive linear trend was found in cotton photosynthesis and yield; each parameter increased as distance from Palmer amaranth increased. Even in these well-watered conditions, daily water use of Palmer amaranth was considerably higher than that of cotton, at 1.2 and 0.49 g H20 cm−2 d−1, respectively. Although Palmer amaranth removed more water from the soil profile, rainfall was adequate to replenish the profile in 2 of the 3 yr of this study. However, yield loss due to Palmer amaranth was still observed despite no change in gs, indicating other factors, such as competition for light or response to neighboring plants during development, are driving yield loss.