As cases of resistance to herbicides escalate worldwide, there is increasing demand from growers to test for weed resistance and learn how to manage it. Scientists have developed resistance-testing protocols for numerous herbicides and weed species. Growers need immediate answers and scientists are faced with the daunting task of testing an increasingly large number of samples across a variety of species and herbicides. Quick tests have been, and continue to be, developed to address this need, although classical tests are still the norm. Newer methods involve molecular techniques. Whereas the classical whole-plant assay tests for resistance regardless of the mechanism, many quick tests are limited by specificity to an herbicide, mode of action, or mechanism of resistance. Advancing knowledge in weed biology and genomics allows for refinements in sampling and testing protocols. Thus, approaches in resistance testing continue to diversify, which can confound the less experienced. We aim to help weed science practitioners resolve questions pertaining to the testing of herbicide resistance, starting with field surveys and sampling methods, herbicide screening methods, data analysis, and, finally, interpretation. More specifically, this article discusses approaches for sampling plants for resistance confirmation assays, provides brief overviews on the biological and statistical basis for designing and analyzing dose–response tests, and discusses alternative procedures for rapid resistance confirmation, including molecular-based assays. Resistance confirmation procedures often need to be slightly modified to suit a specific situation; thus, the general requirements as well as pros and cons of quick assays and DNA-based assays are contrasted. Ultimately, weed resistance testing research, as well as resistance management decisions arising from research, needs to be practical, feasible, and grounded in science-based methods.

Integrated weed management tactics are necessary to develop cropping systems that enhance soil quality using conservation tillage and reduced herbicide or organic weed management. In this study, we varied planting and termination date of two cereal rye cultivars (‘Aroostook’ and ‘Wheeler’) and a rye/hairy vetch mixture to evaluate cover-crop biomass production and subsequent weed suppression in no-till planted soybean. Cover crops were killed with a burn-down herbicide and roller-crimper and the weed-suppressive effects of the remaining mulch were studied. Cover-crop biomass increased approximately 2,000 kg ha−1 from latest to earliest fall planting dates (August 25–October 15) and for each 10-d incremental delay in spring termination date (May 1–June 1). Biomass accumulation for cereal rye was best estimated using a thermal-based model that separated the effects of fall and spring heat units. Cultivars differed in their total biomass accumulation; however, once established, their growth rates were similar, suggesting the difference was mainly due to the earlier emergence of Aroostook rye. The earlier emergence of Aroostook rye may have explained its greater weed suppression than Wheeler, whereas the rye/hairy vetch mixture was intermediate between the two rye cultivars. Delaying cover-crop termination reduced weed density, especially for early- and late-emerging summer annual weeds in 2006. Yellow nutsedge was not influenced by cover-crop type or the timing of cover-crop management. We found that the degree of synchrony between weed species emergence and accumulated cover-crop biomass played an important role in defining the extent of weed suppression.

In greenhouse studies, soil applications of 14C-methyl-labeled glyphosate [N-(phosphonomethyl)glycine] were not readily absorbed by corn (Zea mays L. ‘Michigan 400’) and soybean [Glycine max (L.) Merr. ‘Hark’]. However, glyphosate available to plants in sand culture was absorbed. Wheat (Triticum aestivum L. ‘Avon’) a sensitive bioassay plant, was used to detect the herbicide. Clay loam and muck soil rapidly inactivated 56 kg/ha of glyphosate. Autoclaving of the soil did not prevent the inactivation of glyphosate. In a sandy clay loam soil, application of 56 kg/ha of glyphosate decreased plant growth with increasing pH. Additions of 98 or 196 kg/ha of phosphate to the soil surface decreased glyphosate inactivation in the soil. It is postulated that initial inactivation of glyphosate in soil is by reversible adsorption to clay and organic matter through the phosphonic acid moiety.

Redroot pigweed is a major weed in corn throughout Ontario. Field experiments were conducted at two locations in 1991 and 1992 to determine the influence of selected densities and emergence times of redroot pigweed on corn growth and grain yield. Redroot pigweed densities of 0.5, 1, 2, 4 and 8 plants per m of row were established within 12.5 cm on either side of the corn row. In both years, redroot pigweed seeds were planted concurrently and with corn at the 3- to 5-leaf stage of corn growth. A density of 0.5 redroot pigweed per m of row from the first (earlier) emergence date of pigweed (in most cases, up to the 4-leaf stage of corn) or four redroot pigweed per m of row from the second (later) emergence date of pigweed (in most cases, between the 4- and 7-leaf stage of corn) reduced corn yield by 5%. Redroot pigweed emerging after the 7-leaf stage of corn growth did not reduce yield. Redroot pigweed seed production was dependent upon its density and time of emergence. The time of redroot pigweed emergence, relative to corn, may be more important than its density in assessing the need for postemergence control.

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.

Amaranthus species, commonly referred to as “pigweeds,” are among the most troublesome weeds in many crop production systems. Effective control of these species often begins with an understanding of their biological and reproductive characteristics. At two sites in Missouri, six pigweed species (redroot pigweed, common waterhemp, spiny amaranth, tumble pigweed, smooth pigweed, and Palmer amaranth) were established in 60-m rows spaced 1.5 m apart. At biweekly intervals, plant heights and dry weights were recorded for each species; seed numbers were estimated at the end of the growing season. Dry weight of Palmer amaranth was up to 65% greater than those of all other species 2 wk after planting (WAP). Palmer amaranth biomass accumulation remained greater than those of the other species throughout the season and at the end of the season was 1.2- and 2.7-fold greater than those of redroot and tumble pigweed, respectively. Palmer amaranth was approximately 10 cm tall 2 WAP (37% taller than the next tallest species, redroot pigweed) and approximately 24 cm tall 4 WAP (45% taller than redroot pigweed). In contrast, common waterhemp had not emerged 2 WAP, and plant dry weight 4 WAP was approximately 11 and 26% those of Palmer amaranth and redroot pigweed, respectively. Final plant height ranged from 58 (tumble pigweed) to 208 cm (Palmer amaranth). Redroot pigweed, smooth pigweed, common waterhemp, and Palmer amaranth plants each produced over 250,000 seeds plant−1. Spiny amaranth and tumble pigweed produced approximately 114,000 and 50,000 seeds plant−1, respectively. Common waterhemp produced 535 seeds g−1 of total plant dry weight; this seed production was 1.4-, 1.4-, 2.0-, 3.4-, and 3.4-fold greater than those of redroot pigweed, smooth pigweed, Palmer amaranth, tumble pigweed, and spiny amaranth, respectively. Because the timing for many postemergence herbicides depends on weed height, rapid growth shortly after emergence reduces the time frame for optimum control of species such as Palmer amaranth. Delayed emergence also could result in escaped common waterhemp. Escape of only a few plants could result in a rapid increase in seed populations in the soil seed bank and may select for late-emerging biotypes.

Under simulated no-till conditions in the greenhouse, rye (Secale cereale L. ‘Wheeler’) residues reduced emergence of lettuce (Lactuca sativa L. ‘Ithaca’) and proso millet (Panicum miliaceum L. # PANMI) by 58 and 35%, respectively, over that obtained with a wood shaving control mulch. Rye shoot tissue inhibited lettuce germination 52% more than root tissue. Petri dish bioassays of residue in soil confirmed phytotoxicity on cress (Lepidium sativum L. ‘Curly’), lettuce, barnyardgrass (Echinochloa crus-galli L. Beauv. # ECHCG), and proso millet. Radicle elongation was a more senstive measure of phytotoxicity than germination per se. Phytotoxicity increased as the distance from seeds to rye residues decreased. While rye shoot residues caused 40% greater inhibition of cress radicle elongation in sterile soil than nonsterile soil, barnyardgrass was equally inhibited in both soil regimes. Phytotoxicity decreased in nonsterile soil, indicating that the compounds were degraded by microorganisms. Phytotoxic compounds in shoots were water extractable. Cress and barnyardgrass responded similarly to both aqueous rye extracts and to residues added to soil.

Factors influencing dicamba drift, especially vapor drift, were examined in field and growth chamber studies. In field experiments, potted soybeans [Glycine max (L.) Merr.]. exposed to vapors arising from corn (Zea mays L.) foliarly treated with the sodium (Na), dimethylamine (DMA), diethanolamine (DEOA), or N-tallow-N,N1,N1-trimethyl-1,3-diaminopropane (TA) salts of dicamba (3,6-dichloro-o-anisic acid), developed dicamba injury symptoms. Dicamba volatilization from treated corn was detected with soybeans for 3 days after the application. Dicamba vapors caused symptoms on soybeans placed up to 60m downwind of the treated corn. When vapor and/or spray drift caused soybean terminal bud kill, yields were reduced. In growth chamber studies, dicamba volatility effects on soybeans could be reduced by lowering the temperature or increasing the relative humidity. Rainfall of 1mm or more on treated corn ended dicamba volatilization. The dicamba volatilization was greater from corn and soybean leaves than from velvetleaf (Abutilon theophrasti Medic.) leaves and blotter paper. The volatilization of dicamba formulations varied in growth chamber comparisons with the acid being most volatile and the inorganic salts being the least volatile. However, under field conditions, use of less volatile formulations did not eliminate dicamba symptoms on soybeans. The volatile component of the commercial DMA salt of dicamba was identified by gas chromatography-mass spectrometry as free dicamba acid.

Safeners are chemical agents that reduce the phytotoxicity of herbicides to crop plants by a physiological or molecular mechanism, without compromising weed control efficacy. Commercialized safeners are used for the protection of large-seeded grass crops, such as corn, grain sorghum, and wet-sown rice, against preplant-incorporated or preemergence-applied herbicides of the thiocarbamate and chloroacetanilide families. Safeners also have been developed to protect winter cereal crops such as wheat against postemergence applications of aryloxyphenoxypropionate and sulfonylurea herbicides. The use of safeners for the protection of corn and rice against sulfonylurea, imidazolinone, cyclohexanedione, isoxazole, and triketone herbicides also is well established. A safener-induced enhancement of herbicide detoxification in safened plants is widely accepted as the major mechanism involved in safener action. Safeners induce cofactors such as glutathione and herbicide-detoxifying enzymes such as glutathione S-transferases, cytochrome P450 monooxygenases, and glucosyl transferases. In addition, safeners enhance the vacuolar transport of glutathione or glucose conjugates of selected herbicides. The safener-mediated induction of herbicide-detoxifying enzymes appears to be part of a general stress response.

The extent of adsorption of five C14-labeled s-triazines, 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine), 2-methoxy-4,6-bis(isopropylamino)-s-triazine (prometone), and 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine (prometryne) from aqueous solutions by soil constituents was expressed as a distribution coefficient (Kd), which is the ratio of the amount of herbicide adsorbed to the amount in the equilibrium solution. The Kd value for a given s-triazine and exchanger remained relatively constant over a range of concentrations. The adsorption reaction was essentially at equilibrium within 1 hour. Increasing the temperature and pH resulted in decreased adsorption of simazine and atrazine. The order of increasing adsorption of these compounds by soils was propazine, atrazine, simazine, prometone and prometryne. Increased amounts of organic matter and/or clay in a soil generally were associated with increased adsorption of these s-triazines. No adsorption of simazine or atrazine was detected with kaolinite. Putnam clay, illite, and montmorillonite were increasingly adsorptive in that order. The organic materials were generally much more adsorptive than the clays. The adsorption reaction was reversible by increasing temperatures, water elution, and elution with various organic solvents. Fifty percent ethanol plus heat was very effective in eluting the chloro-s-triazines from soil.

Despite several decades of modern weed control practices, weeds continue to be a constant threat to agricultural productivity. Herbicide-resistant weeds and weed population shifts continue to generate new challenges for agriculture. Because of weed community complexity, integrated approaches to weed management may help reduce economic effects and improve weed control practices. Integrated weed management emphasizes the combination of management techniques and scientific knowledge in a manner that considers the causes of weed problems rather than reacts to existing weed populations. The goal of weed management is the integration of the best options and tools to make cropping systems unfavorable for weeds and to minimize the effect of weeds that survive. No single practice should be considered as more than a portion of an integrated weed management strategy. The best approach may be to integrate cropping system design and weed control strategies into a comprehensive system that is environmentally and economically viable. Management decisions must also be made on a site- and time-specific basis. Considering weeds in a broader ecological and management context may lead to the use of a wider range of cultural and management practices to regulate weed communities and prevent the buildup of adapted species. This will help producers manage herbicides and other inputs in a manner that preserves their effectiveness and move weed scientists toward the development of more diverse and integrated approaches to weed management.

Weeds are a significant problem in crop production and their management in modern agriculture is crucial to avoid yield losses and ensure food security. Intensive agricultural practices, changing climate, and natural disasters affect weed dynamics and that requires a change in weed management protocols. The existing manual control options are no longer viable because of labor shortages; chemical control options are limited by ecodegradation, health hazards, and development of herbicide resistance in weeds. We are therefore reviewing some potential nonconventional weed management strategies for modern agriculture that are viable, feasible, and efficient. Improvement in tillage regimes has long been identified as an impressive weed-control measure. Harvest weed seed control and seed predation have been shown as potential tools for reducing weed emergence and seed bank reserves. Development in the field of allelopathy for weed management has led to new techniques for weed control. The remarkable role of biotechnological advancements in developing herbicide-resistant crops, bioherbicides, and harnessing the allelopathic potential of crops is also worth mentioning in a modern weed management program. Thermal weed management has also been observed as a useful technique, especially under conservation agriculture systems. Last, precision weed management has been elaborated with sufficient details. The role of remote sensing, modeling, and robotics as an integral part of precision weed management has been highlighted in a realistic manner. All these strategies are viable for today's agriculture; however, site-specific selection and the use of right combinations will be the key to success. No single strategy is perfect, and therefore an integrated approach may provide better results. Future research is needed to explore the potential of these strategies and to optimize them on technological and cultural bases. The adoption of such methods may improve the efficiency of cropping systems under sustainable and conservation practices.

The critical period for weed control (CPWC) is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. Field studies were conducted in 1999 and 2000 in eastern Nebraska to evaluate the influence of nitrogen application on the CPWC in dryland corn in competition with a naturally occurring weed population. Nitrogen fertilizer was applied at rates equivalent to 0, 60, and 120 kg N ha−1. A quantitative series of treatments of both increasing duration of weed interference and length of weed-free period were imposed within each nitrogen main plot. The beginning and end of the CPWC based on an arbitrarily 5% acceptable yield loss level were determined by fitting the logistic and Gompertz equations to relative yield data representing increasing duration of weed interference and weed-free period, respectively. Despite an inconsistent response of corn grain yield to applied nitrogen, there was a noticeable influence on the CPWC. The addition of 120 kg N ha−1 delayed the beginning of the CPWC for all site–years when compared with the 0-kg N ha−1 rate and for three of the four site–years when compared with the 60-kg N ha−1 rate. The addition of 120 kg N ha−1 also hastened the end of the CPWC at three of the four site–years when compared with both reduced rates. The yield component most sensitive to both nitrogen and interference from weeds was seed number per ear. Practical implications of this study are that reductions in nitrogen use may create the need for more intensive weed management.

Because herbicide and cultivation efficacy is generally density independent, seedling density following these weed control practices will be proportional to the density of germinable seeds in the seedbank. Most farmers would therefore benefit from management practices that reduce seed inputs, increase seed losses, and reduce the probability that remaining seeds establish. Germination, predation, and decay are the primary sources of loss to the seedbank that may respond to management. Farmers have long prepared stale seedbeds in which shallow soil disturbance encourages germination losses. Postdispersal seed predation by vertebrate and invertebrate granivores may cause high rates of seed mortality in a wide range of cropping systems, but seed dispersal asynchronous with predator activity, and seed burial, may limit the overall effect on the seedbank. Although seeds would seem to be an ideal carbon source for soil microorganisms, limited evidence from a study of wild oat suggests that decay may be less responsive to management than germination, and likely predation. A final management objective, supporting a program that aims to reduce seedbank inputs and increase losses, is to reduce the size of the effective seedbank through manipulation of residues and disturbance to reduce the probability of establishment. Incorporation of green manures generally reduces weed establishment, whereas larger-seeded or transplanted crops may better tolerate the residue-mediated changes in the chemical, biological, and physical properties of the soil surface environment. Evidence from no-till systems further support the hypothesis that changes in soil surface conditions may regulate the abundance of “safe sites” for weed establishment, thereby modulating the size of the effective seedbank.

In a 6-yr study on four farms (36 fields) in Ontario, Canada, we tested the effects of tillage (moldboard, chisel plow, no tillage) and crop rotations (continuous corn, corn-soybean, corn-soybean-winter wheat) on emerged and seedbank weed species diversity and density. Aside from the imposed experimental treatments, all other management was generally consistent among farms. Tillage had the largest effect on weed diversity and density. No tillage promoted the highest weed species diversity, chisel plow was intermediate, and moldboard plow resulted in the lowest species diversity. These results are consistent with ecological succession theory. The increase in weed species diversity resulted from 20 species being associated with no tillage systems, 15 of which were winter annuals, biennials, or perennials. Emerged weed density was affected only by tillage. Over 6 yr, seedbank declined in no-tillage systems from 41,000 to 8,000 seeds m−3. Crop yields were not affected by tillage or crop rotation. In practical terms, reduced tillage in combination with a good crop rotation may reduce weed density and expenditures on weed management.

Laboratory and greenhouse studies were conducted to determine imazaquin and imazethapyr adsorption, mobility, and efficacy in Sharpsburg silty clay loam, Holdrege silt loam, and Tripp sandy loam soils after adjusting pH to 5, 6, and 7. Both herbicides were more strongly absorbed, less mobile, and less efficacious at a lower pH. Observations were attributed to ionic bonding resulting from protonation of basic functional groups on the herbicide molecules as pH decreased. Adsorption was greatest in the silty clay loam and least in the sandy loam soil. Conversely, the herbicides were more efficacious and mobile in the more coarse-textured soils. Imazethapyr was less mobile, more highly adsorbed, and more phytotoxic than imazaquin. Greater adsorption of imazethapyr was attributed to a stronger basic pKa and steric factors.

The arcsine and square root transformations were tested on 82 weed control data sets and 62 winter wheat winter survival data sets to determine effects on normality of the error terms, homogeneity of variance, and additivity of the model. Transformations appeared to correct deficiencies in these three parameters in the majority of data sets, but had adverse effects in certain other data sets. Performing the recommended transformation in conjunction with omitting treatments having identical replicate observations provided a high percentage of correction of non-normality, heterogeneity of variance, and nonadditivity. The arcsine transformation, not generally recommended for data sets having values from 0 to 20% or 80 to 100%, was as effective in correcting non-normality, heterogeneity of variance, and nonadditivity in these data sets as was the recommended square root transformation. A majority of data sets showed differences between transformed and nontransformed data in mean separations determined using LSD (0.05), although most of these differences were minor and had little effect on interpretation of results.

Current and projected increases in the concentrations of CO2 and other radiatively-active gases in the Earth's atmosphere lead to concern over possible impacts on agricultural pests. All pests would be affected by the global warming and consequent changes in precipitation, wind patterns, and frequencies of extreme weather events which may accompany the “greenhouse effect.” However, only weeds are likely to respond directly to the increasing CO2 concentration. Higher CO2 will stimulate photosynthesis and growth in C3 weeds and reduce stomatal aperture and increase water use efficiency in both C3 and C4 weeds. Respiration, and photosynthate composition, concentration, and translocation may be affected. Perennial weeds may become more difficult to control, if increased photosynthesis stimulates greater production of rhizomes and other storage organs. Changes in leaf surface characteristics and excess starch accumulation in the leaves of C3 weeds may interfere with herbicidal control. Global warming and other climatic changes will affect the growth, phenology, and geographical distribution of weeds. Aggressive species of tropical and subtropical origins, currently restricted to the southern U.S., may expand northward. Any direct or indirect consequences of the CO2 increase that differentially affect the growth or fitness of weeds and crops will alter weed-crop competitive interactions, sometimes to the detriment of the crop and sometimes to its benefit.

An in vivo shikimate accumulation assay with excised leaf tissue was developed to provide a fast and reliable method for identifying glyphosate-resistant plants. The assay is based on glyphosate-induced accumulation of shikimate. There was a linear accumulation of shikimate in excised leaf discs of soybean and canola treated with 250 μM glyphosate for 48 h. The IC50 for the accumulation of shikimate in soybean and corn leaf discs was 34 and 87 μM, respectively. Leaf discs excised from glyphosate-resistant corn or soybean did not accumulate shikimate when treated with 500 μM glyphosate. Leaf discs taken from a number of field-grown plants accumulated shikimate in a glyphosate dose–dependent manner. The accumulation of shikimate was dependent on light and the age of the leaf from which the disc was taken. The assay worked either in 96-well microtiter plates or in vials, and it clearly differentiated between glyphosate-resistant and -susceptible crops in which the resistance is due to an alteration of the target site for glyphosate. The assay was simple and robust and has the potential to be used as a high throughput assay to detect glyphosate resistance in weeds.

A 2-yr field study was conducted at Fayetteville, AR, to determine the effect of Palmer amaranth interference on soybean growth and yield. Palmer amaranth density had little effect on soybean height, but soybean canopy width ranged from 77 cm in the weed-free check to 35 cm in plots with 10 plants m–1 of row 12 wk after emergence. Soybean yield reduction was highly correlated to Palmer amaranth biomass at 8 wk after emergence and maturity, soybean biomass at 8 wk after emergence, and Palmer amaranth density. Soybean yield reduction was 17, 27, 32, 48, 64, and 68%, respectively, for Palmer amaranth densities of 033, 0.66, 1, 2, 333, and 10 plants m–1 of row. Soybean yield reduction and Palmer amaranth biomass were linear to approximately 2 Palmer amaranth m–1 of row, suggesting intraspecific interference between adjacent Palmer amaranth is initiated at Palmer amaranth densities between 2 and 3.33 plants m–1 of row.