Greenhouse and laboratory studies were conducted to examine the effects of site of plant exposure to glyphosate spray on efficacy, absorption, and translocation in pitted morningglory. Absorption of 14C-glyphosate in four-leaf pitted morningglory gradually increased with time from 19% at 1 h after treatment (HAT) to 44% at 192 HAT. The amount of 14C translocated with time ranged from 0.4% at 1 HAT to 25% at 192 HAT. Vining 1-m tall plants were controlled 75 to 100% when the top-, middle-, bottom one-third, or entire plant was treated with 1.38 or 2.76 kg ha−1 glyphosate, with control affected more by glyphosate rate than plant section exposed to glyphosate spray. Absorption of 14C-glyphosate at 96 HAT was similar whether it was applied to the top-, middle-, bottom one-third, or entire plant of 1-m tall pitted morningglory. The amount of 14C translocated out of the treated area (5 to 6%) did not differ whether it was applied to top-, middle-, or bottom one-third plant section. Results indicate that absorption and translocation of 14C-glyphosate in pitted morningglory was rapid and increased with time. Treating any one-third section of pitted morningglory plants was as effective as entire plant exposure, and control with glyphosate is more affected by rate than the degree of plant exposure to glyphosate.

Studies were conducted to determine acifluorfen and lactofen absorption, translocation, and metabolism in protox-inhibiting herbicide-susceptible and -resistant common waterhemp. Acifluorfen and lactofen absorption was similar in both biotypes. Herbicide absorption was 12% in both susceptible and resistant common waterhemp 6 h after treatment (HAT). Absorption increased to 32 and 42% in susceptible and resistant plants, respectively, at 72 HAT. Translocation was similar in both biotypes for both herbicides. Herbicide translocation out of the treated leaf ranged between 5 and 15%. In a separate study, resistant common waterhemp plants were treated with acifluorfen or lactofen, alone or with tridiphane. Acifluorfen or lactofen injury to resistant common waterhemp was not altered with the addition of tridiphane. Treatments of 14C-acifluorfen or -lactofen on susceptible and resistant common waterhemp resulted in similar lactofen metabolism in both biotypes, but acifluorfen was not metabolized in either biotype within 24 HAT. This data indicate that differences in herbicide absorption, translocation, or metabolism are not the mechanism of common waterhemp resistance to protox-inhibiting herbicides.

This study reports evolved resistance to fenoxaprop-P in a population of sprangletop from a rice field in Thailand (BLC1). After eight applications of fenoxaprop-P, the herbicide appeared no longer effective. To confirm herbicide resistance in the BLC1 population, three experiments were conducted. First, glasshouse experiments revealed that the BLC1 population survived 600 g ai ha−1 of fenoxaprop-P without visual injury. Second, the BLC1 population was treated with fenoxaprop-P and other acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicides (quizalofop-P, cyhalofop-butyl, and profoxydim) under field conditions; BLC1 exhibited resistance to all of these herbicides. Third, seeds of susceptible SLC1 and resistant BLC1 were germinated on 0.6% (v/v) agar across a range of herbicide concentrations. The resistant BLC1 population exhibited 61-, 44-, 9- and 8-fold resistance to fenoxaprop-P, cyhalofop, quizalofop-P, and profoxydim, respectively, compared with a susceptible SLC1 population. At the enzyme level, ACCase from the resistant BLC1 exhibited 30, 24, 11, 4, and 5 times resistance to fenoxaprop, cyhalofop-butyl, haloxyfop, clethodim, and cycloxydim, respectively. The spectrum of resistance at the whole plant level correlated well with resistance at the ACCase level. Hence, the mechanism of resistance to ACCase-inhibiting herbicides in this biotype of sprangletop is a herbicide-resistant ACCase. The specific mutation(s) of the ACCase gene that endows resistance in this population remains to be investigated.

Characterizing the long-term effect of agricultural management systems on weed communities will aid in developing sustainable weed management practices. Weed seedbanks and aboveground biomass were measured within a corn–soybean–wheat crop sequence from 1990 through 2002 at Hickory Corners, MI. Four management systems were compared: conventional (CONV; full rates of N fertilizer and herbicides, moldboard tillage), no till (NT; same as CONV with no primary tillage), reduced input (RI; reduced rates of N fertilizer and herbicides, moldboard tillage, mechanical weed control, wheat underseeded with red clover), and organic (ORG; same as RI but no synthetic inputs). Multivariate ordinations of weed seedbanks showed a divergence of the CONV and NT systems from the RI and ORG systems. The CONV and NT seedbanks were dominated by grass species (mainly fall panicum and large crabgrass), whereas the RI and ORG systems were dominated by common lambsquarters and common chickweed. Within a single growing season, weed seedbanks in the RI and ORG systems were positively correlated with weed biomass whereas seedbanks in the CONV and NT system had little predictive value. Weed biomass from 1990 through 2002 showed a strong association of grass weed species with the corn phase of the CONV and NT system and common lambsquarters and redroot pigweed with the corn and soybean phases of the RI and ORG systems. Weed biomass diversity measures were negatively correlated with soybean yields in RI and ORG and wheat yields in NT, RI, and ORG. It is not clear whether crops were less competitive in the NT, RI, and ORG treatments, allowing new weed species to enter the plots, or whether less effective weed management in the NT, RI, and ORG treatments resulted in increased species richness, causing reduced crop yields. Mechanistic studies are needed to elucidate the relationship between weed community diversity and crop performance.

Developing ecological principles applicable to invasive plant management is central to implementing sustainable strategies. We tested portions of a potentially useful successional-based management framework to further our understanding of the relationship between disturbance and colonization during revegetation of invasive weed-dominated grasslands. We hypothesized (1) intermediate wheatgrass density and biomass would be greatest at highest seeding rates, (2) control and tillage procedures that increase the availability of safe sites would increase wheatgrass abundance, and (3) spotted knapweed density and biomass would be lowest in treatments with highest wheatgrass density and biomass. Treatments included three disturbance levels: (1) no disturbance, (2) application of glyphosate, and (3) fall tillage. Colonization treatments were seeding intermediate wheatgrass of 0, 500, 2,500, and 12,500 seeds m−2. Treatments were factorially applied in a randomized complete-block design with four replications at each of two sites located in Montana. Density and biomass of intermediate wheatgrass and spotted knapweed were sampled in 1997 and 2001. At both sites, seeding 2,500 or 12,500 seed m−2 increased wheatgrass density over that of the nonseeded control in 1997. The highest seeding rate produced almost three times as many wheatgrass plants as 2,500 seeds m−2 that year. By 2001, only the highest seeding rate produced wheatgrass densities greater than that of the nonseeded control at Bozeman. Seeding rates higher than 500 seeds m−2 yielded greater wheatgrass biomass than the nonseeded control with or without either tillage or glyphosate. At the highest seeding rate, tillage or glyphosate doubled intermediate wheatgrass biomass compared with no disturbance. Spotted knapweed generally had lower biomass where intermediate wheatgrass density and biomass was highest. One approach to rehabilitation is to design disturbances that favor desired species and then use high seeding rates that overwhelm the pool of available propagules and occupy a high percentage of safe sites.

A greenhouse experiment was conducted to determine the efficacy of glyphosate and acetyl-CoA carboxylase (ACCase)–inhibiting herbicides sethoxydim, clethodim, and quizalofop on prairie cupgrass and windmillgrass. Herbicides were applied at seedling, tillering, and heading growth stages. In addition, a study to determine glyphosate absorption and translocation in both species was conducted. Herbicide treatments were glyphosate at 541, 841, and 1121 g ha−1 and sethoxydim, clethodim, and quizalofop at 350, 210, and 70 g ha−1, respectively. In general, control of prairie cupgrass and windmillgrass increased as the rate of glyphosate increased. In addition, windmillgrass was less susceptible to glyphosate at heading and seedling stages than was prairie cupgrass. Efficacy of all ACCase-inhibiting herbicides applied at any growth stage was equal to or greater than efficacy of the highest rate of glyphosate applied at the same stage. Furthermore, all herbicide treatments were more phytotoxic to prairie cupgrass and windmillgrass at seedling stage than at tillering or heading. Differential response of prairie cupgrass and windmillgrass to glyphosate was attributed to differences in glyphosate translocation. Separate field experiments were conducted to evaluate preemergence (PRE) and postemergence (POST) herbicide treatments for prairie cupgrass and windmillgrass in no-till corn at Hays, KS, in 2001 and 2002. All PRE treatments provided at least 90% control of windmillgrass in both years 6 wk after treatment (WAT). Prairie cupgrass was controlled effectively by acetochlor plus atrazine, alachlor plus atrazine, and S-metolachlor plus atrazine in 2001. In 2002, the only PRE treatments that gave 90% control or greater of prairie cupgrass 6 WAT were alachlor plus atrazine and pendimethalin plus atrazine. Glyphosate sequential treatment was the only POST treatment that provided 100% control of prairie cupgrass and windmillgrass in both years.

Barnyardgrass (BYG) has been the most frequently reported troublesome weed in rice because it is an aggressive invader, is difficult to control, and reduces yields significantly. A replacement series study was conducted to determine how a naturally suppressive cultivar (T65∗2/TN 1; ‘PI 312777’), a nonsuppressive cultivar (‘Lemont’), and an F3 cross between the two (‘PI 312777 × Lemont’) would interfere with BYG in the southern United States. The rice cultivars did not differentially affect BYG height. The PI 312777 produced more tillers and greater shoot dry weight but was only moderately competitive (relative yield [RY]) or aggressive (relative crowding coefficient) against BYG. Competitiveness at 2:2 rice and BYG mixture proportion and replacement series illustrations on RYs for the number of tillers and shoot dry weights for the three rice cultivars indicated that PI 312777 suppressed BYG growth relatively better than the other two cultivars. Plant-for-plant, PI 312777 was more competitive than Lemont. PI 312777 × Lemont suppressed BYG relatively less than did PI 312777 and therefore would require significant genetic improvements before it is suitable for commercial use in a reduced herbicide production system.

Azafenidin (AZ), pendimethalin, simazine, and sulfometuron (SF) were applied alone and in combination to black cherry, black walnut, eastern white pine, flowering dogwood, northern bayberry, northern red oak, Siberian crabapple, white ash, white oak, and yellow poplar seedlings grown for 2 yr in the field. There were significant differences in diameter, height, and stem volume among the treatments for every species after the third growing season. Tillage significantly increased tree growth over controls for most species. No single herbicide treatment ranked best for all tree species but comparison of the mean ranks of the treatments for all species indicated that AZ resulted in the best growth. Post–bud-break applications of SF and AZ were no better than untreated controls for most species. Herbicide treatments did not affect tree survival over years but exhibited potential for better growth in establishment years.

The effect of ethametsulfuron on populations of canola was evaluated in field and laboratory trials. Three canola populations (TR4, CB9604, and BC86-18) and the open-pollinated canola cv. ‘AC Parkland’ were treated with several doses of ethametsulfuron at the two- to four-leaf stage. In the field experiment, plant densities were determined before spraying and again at plant maturity. Density reductions during the growing season were observed for all populations (6 to 17%), but reductions due to ethametsulfuron above 0.3 g ha−1 depended on year, being noted for TR4 in 1997 (40 to 65%) only. In laboratory trials, only TR4 demonstrated sensitivity to ethametsulfuron, up to 49% visual damage averaged over all plants. The response was binary, with about 40% of plants at any given herbicide rate surviving unharmed and the remaining 60% of plants exhibiting > 90% visual damage. It is concluded that TR4 is sensitive to ethametsulfuron and further work, including a comprehensive genetic study, is required to determine the inheritance of sensitivity.

Wild oat is a serious weed in cultivated oat because there are no herbicides to selectively control it. Considering the effect of time of emergence on weed–crop interference is critical for the development of accurate crop yield loss models and weed density thresholds. Therefore, field experiments were conducted at two locations in Saskatchewan, Canada, in 2002 and 2003 to determine the effect of wild oat density and time of emergence on cultivated oat yield and quality. Wild oat was planted at 50 growing degree day (GDD) intervals ranging from 100 GDD before to 100 GDD after crop planting. Wild oat density ranged from 0 to 320 plants m−2. High densities of early emerging wild oat greatly reduced cultivated oat yield and increased wild oat contamination, with observed oat yield losses as great as 70% and wild oat contamination levels of 15%. Wild oat that emerged before cultivated oat caused considerably more yield and quality loss and had higher reproductive output than wild oat that emerged after cultivated oat. The yield loss caused by individual wild oat plants at low densities (parameter I) ranged from 0.40 to 0.49%. The effect of relative time of wild oat emergence (parameter C) always varied significantly between site-years. However, little variation in absolute values within site-years was observed for cultivated oat yield loss, wild oat seed production, and wild oat contamination, suggesting that relative time of wild oat emergence influences all similarly. The results of this study emphasize both the need to control early emerging wild oat, as well as the importance of time of emergence in the prediction of crop yield loss. Furthermore, our approach of conducting an emergence study based on thermal time is novel and demonstrates a robust, mechanistic method of estimating crop yield losses due to relative time of emergence.

Although the effectiveness of vegetative filter strips (VFS) for reducing herbicide runoff is well documented, a comprehensive review of the literature does not exist. The objectives of this article are to denote the methods developed for evaluating herbicide retention in VFS; ascertain the efficacy of VFS regarding abating herbicide runoff; identify parameters that affect herbicide retention in VFS; review the environmental fate of herbicides retained by VFS; and identify future research needs. The retention of herbicide runoff by VFS has been evaluated in natural rainfall, simulated rainfall, and simulated run-on experiments. Parameters affecting herbicide retention in VFS include width of VFS, area ratio, species established in the VFS, time after establishment of the VFS, antecedent moisture content, nominal herbicide inflow concentration, and herbicide properties. Generally, subsequent transport of herbicides retained by VFS is reduced relative to adjacent cultivated soil because of enhanced sorption and degradation in the former.

Increasing concerns about pesticide use and a steadily increasing conversion to organic farming have been major factors driving research in physical and cultural weed control methods in Europe. This article reviews some of the major results achieved with nonchemical methods and strategies, especially adapted for row crops (e.g., corn, sugar beet, onion, leek, and carrot) and small-grain cereals (e.g., barley and wheat). In row crops, intrarow weeds constitute a major challenge, and research has mainly aimed at replacing laborious hand-weeding with mechanization. A number of investigations have focused on optimizing the use of thermal and mechanical weeding methods against intrarow weeds, such as flaming, harrowing, brush weeding, hoeing, torsion weeding, and finger weeding. And new methods are now under investigation such as robotic weeding for row crops with abundant spacing between individual plants and band-steaming for row crops developing dense crop stands. The strategic use of mechanical weed control methods in small-grain cereals has been another area of considerable interest. Weed harrowing and interrow hoeing provide promising results when they are part of a strategy that also involves cultural methods such as fertilizer placement, seed vigor, seed rate, and competitive varieties. Although research in preventive, cultural, and physical methods have improved weed control in row crops and small-grain cereals, effective long-term weed management in low external input and organic systems can only be achieved by tackling the problem in a wider context, i.e., at the cropping system level. Basic principles of this approach, examples of cover crop and intercropping use for weed suppression, and an application in a 2-yr rotation are presented and discussed.

To improve understanding of relationships between crop diversity, weed management practices, and weed population dynamics, we used data from a field experiment and matrix models to examine how contrasting crop rotations affect velvetleaf. We compared a 2-yr rotation system (corn–soybean) managed with conventional rates of herbicides with a 4-yr rotation (corn–soybean–triticale + alfalfa–alfalfa) that received 82% less herbicide. In November 2002, a pulse of velvetleaf seeds (500 seeds m−2) was added to 7- by 7-m areas within replicate plots of each crop phase–rotation system combination. Velvetleaf seed, seedling, and reproductive adult population densities, seed production, and seed losses to predators were measured during the next year. Velvetleaf seed production was greater in the 4-yr rotation than in the 2-yr rotation (460 vs. 16 seeds m−2). Averaged over 12 sampling periods from late May to mid-November 2003, loss of velvetleaf seeds to predators also was greater in the 4-yr rotation than in the 2-yr rotation (32 vs. 17% per 2 d). Modeling analyses indicated that velvetleaf density in the 4-yr rotation should decline if cumulative losses of seeds produced in the soybean phase exceeded 40%. Achieving such a level of predation appears possible, given the observed rates of velvetleaf seed predation. In addition, no tillage occurs in the 4-yr rotation for 26 mo after soybean harvest, thus favoring seed exposure on the soil surface to predators. Models that included estimates of seed predation indicated that to prevent increases in velvetleaf density, weed control efficacy in soybean must be ≥ 93% in the 2-yr rotation, but could drop to 86% in the 4-yr rotation. These results support the hypothesis that diverse rotations that exploit multiple stress and mortality factors, including weed seed predation, can contribute to effective weed suppression with less reliance on herbicides.

Scientific understanding of multitactic weed management systems (MTS) is complicated by (1) the large number of potential combinations among tactics, (2) potentially noisy and complex system behavior because of individually more moderate mortality events, and (3) possible transient system behavior of unknown duration. Therefore, decomposing the relative performance of MTS components is much more difficult than it is for single-tactic strategies (STS). Attempting to accommodate the increased complexity of system behavior while maintaining the generality of results requires analytical methods capable of accomplishing these tasks. We provide two examples of statistical procedures that may help gain understanding of MTS systems using previously published weed demographic time-series data. First, we demonstrate the use of mixed-effects models capable of representing and removing factors contributing uncontrolled variation to system behavior. Model selection criteria are used to highlight the importance of the increased flexibility the mixed-model framework provides. Second, by explicitly modeling the probabilistic process presumed to be generating the data, we demonstrate how different components of the MTS can be compared and how the methodology can facilitate integration of such information into a decision-making application.

Since commercialization of Collego™ and Devine™ in the early 1980s, there has been a small but consistent research effort in the area of bioherbicides. The bioherbicide approach has promised effective weed management in cropping systems where the classical approach (using exotic natural enemies) is largely unsuitable. The overriding principle of the bioherbicide approach has been that a host-specific, coevolved natural enemy can be used as a bioherbicide when applied in simple formulations at inundative levels; however, two decades of research has effectively disproven this principle. Although research has revealed weaknesses in the bioherbicide approach, it has also revealed potential in a number of areas. A number of niche situations will remain in which host-specific plant pathogens can be developed as bioherbicides, such as for parasitic weeds and narcotic plants, but more research should be conducted with virulent, broad host range organisms, and more effort should be devoted to developing techniques for the cultural and genetic enhancement of bioherbicidal organisms.