Wild oat is the second-most abundant, but most economically important, weed across the Canadian Prairies of western Canada. Despite the serious economic effects of resistance to acetyl-CoA carboxylase (ACC) or acetolactate synthase (ALS) inhibitors or both in this weed throughout the Northern Great Plains of North America, little research has examined the basis for herbicide resistance. We investigated target-site and nontarget-site mechanisms conferring ACC- and ALS-inhibitor resistance in 16 wild oat populations from across western Canada (four ACC-inhibitor resistant, four ALS-inhibitor resistant, and eight ACC- and ALS-inhibitor resistant). The ACC1 mutations were found in 8 of the 12 ACC inhibitor-resistant populations. The Ile1781Leu mutation was detected in three populations, the Trp2027Cys and Asp2078Gly mutations were in two populations each, and the Trp1999Cys, Ile2041Asn, Cys2088Arg, and Gly2096Ser substitutions were in one population each. Three populations had two ACC1 mutations. Only 2 of the 12 ALS inhibitor-resistant populations had an ALS target-site mutation—Ser653Thr and Ser653Asn substitutions. This is the first global report of ALS target-site mutations in Avena spp. and four previously undocumented ACC1 mutations in wild oat. Based on these molecular analyses, seedlings of five ACC + ALS inhibitor-resistant populations (one with an ACC1 mutation; four with no ACC or ALS mutations) were treated with malathion, a known cytochrome P450 monooxygenase inhibitor, followed by application of one of four ACC- or ALS-inhibiting herbicides. Malathion treatment often resulted in control or suppression of these populations, suggesting involvement of this enzyme system in contributing to resistance to both ACC and ALS inhibitors.

Resistance to protoporphyrinogen oxidase (protox)-inhibiting herbicides was identified in a population of common waterhemp that had been treated with acifluorfen for several years. The protox-resistant biotype of common waterhemp was approximately 34, 82, 8, and 4 times more resistant than a susceptible common waterhemp biotype to acifluorfen, lactofen, fomesafen, and sulfentrazone, respectively. The resistant biotype also showed a high level of resistance to acetolactate synthase–inhibiting herbicides thifensulfuron and imazethapyr but not to glyphosate or paraquat. An organophosphate insecticide was applied with acifluorfen or lactofen to determine if metabolism could be the mechanism of resistance. No differences were observed between resistant plants treated with an organophosphate plus a protox-inhibiting herbicide and plants treated with a protox-inhibiting herbicide alone.

Glyphosate [N-(phosphonomethyl)glycine] was less phytotoxic when applied in 190 L/ha of well water with ionic strengths of 0.005 or greater than in distilled water. At a carrier volume of 24 L/ha, this inhibition of glyphosate was reduced or eliminated. Glyphosate was less phytotoxic when applied in 10-mM solutions of CaCl2, FeSO4, Fe2(SO4)3, MgSO4, NaHCO3, Na2CO3, and ZnSO4 than in distilled water. Adding H2SO4, HCl, HClO4, acetic, and lactic acids at 5, 10, and 50 mM to the spray solution resulted in variable increases in glyphosate phytotoxicity. H2SO4 was the most effective acid tested in increasing glyphosate phytotoxicity when added to well water. Lowering the pH of the spray solution by using potassium biphthalate and phosphoric acid buffers did not increase glyphosate phytotoxicity.

Solarization is a method of heating moist soil by covering it with plastic sheets to trap solar radiation. In field experiments in Israel during the summer, maximum soil temperature under plastic cover at the 5-cm depth averaged 46 to 49C. No weeds emerged under the plastic cover during solarization and weed emergence was reduced after its removal. The heating effect from solarization decreased with soil depth. Concentration of O2 in soil under plastic was similar to that in uncovered controls, but the concentration of CO2 was markedly higher than in control soil, rising up to 2.4%. Higher temperatures and better residual weed control were produced by transparent than by black plastic, with best results from thin (0.03 mm), transparent polyethylene. Under Israeli summer conditions, 2 to 4 weeks of solarization produced effective control of annual weeds that was still appreciable after 1 yr. Narrow sheets of 20 to 50 cm produced effective weed control in bands. on soil irrigated once before placing the plastic sheets, there was no need to irrigate during solarization. The response of weed species to solarization differed. Many annual weeds, both summer species such as pigweed (Amaranthus spp.) and common purslane (Portulaca oleracea L.) and winter species as henbit (Lamium amplexicaule L.) were well controlled by solarization. Broomrape (Orobanche crenata Forsk.) was controlled in one experiment. on the other hand, horseweed [Conyza canadensis (L.) Cronq.] and bull mallow (Malva niceaensis All.) were relatively resistant, and established perennials escaped the treatment.

Hydrophobic exudate from roots of sorghum [Sorghum bicolor (L.) Moench. #3 SORVU ‘IS 8768’] contain four p-benzoquinones which in the dihydroquinone form are active as germination stimulants of witchweed [Striga asiatica (L.) Kuntz. # STRLU]. The three minor p-benzoquinones were partially characterized and found to be structurally similar to sorgoleone, the major p-benzoquinone of this exudate. Herbicidal activity of the hydrophobic exudate was due to concentration- and pH-dependent inhibition of root elongation in some but not all weeds tested. Witchweed has apparently adapted these “defense” compounds of sorghum as host-specific germination stimulants.

Competitive outcome between crops and weeds is affected by partitioning of new biomass to above- and belowground plant organs in response to nutrient supply. This study determined the fraction of biomass partitioned to roots vs. shoots in corn and velvetleaf in response to nitrogen (N) supply. Pots measuring 28 cm in diam and 60 cm deep were embedded in the ground and each contained one plant of either corn or velvetleaf. Each plant received one of three N treatments: 0, 1, or 3 g N applied as ammonium nitrate in 2001, and 0, 2, or 6 g N in 2002. Measurements of total above- and belowground biomass were made at 10 sampling dates during each growing season. The root:shoot ratio decreased over time for both corn and velvetleaf as a result of normal plant growth and as N supply increased. Root:shoot ratio was greater for corn than for velvetleaf at comparable stages of development and at all levels of N supply. Both corn and velvetleaf display true plasticity in biomass partitioning patterns in response to N supply. Velvetleaf root:shoot ratio increased by 46 to 82% when N was limiting in 2001 and 2002, respectively, whereas corn root:shoot ratio increased by only 29 to 45%. The greater increase in biomass partitioned to roots by velvetleaf might negatively impact its ability to compete with corn for light when N supply is limited.

Field and greenhouse studies of 2,4-D [(2,4-dichlorophenoxy)acetic acid] and glyphosate [N-(phosphonomethyl) glycine] absorption, translocation, and metabolism were initiated to explain field observations which indicated susceptibility of common milkweed (Asclepias syriaca L.) but not hemp dogbane (Apocynum cannabinum L.) to glyphosate and the reverse response to 2,4-D. Glyphosate was absorbed less than 2,4-D in both species with absorption of both herbicides being greater in common milkweed. Greater herbicide absorption by common milkweed was attributed to less epicuticular wax, less cuticle, lower contact angle of the herbicide spray, and the presence of stomata and trichomes on the adaxial leaf surface. No major translocation differences of the herbicides were noted between species. Translocation of glyphosate was more rapid than that of 2,4-D. More glyphosate than 2,4-D accumulated in areas of high meristematic and metabolic activity. Rapid 2,4-D metabolism occurred in common milkweed leaves above treated leaves. There was no detectable 2,4-D metabolism in hemp dogbane roots after 20 days, while 60% of the 2,4-D in common milkweed roots was metabolized. Limited absorption of glyphosate but not 2,4-D by hemp dogbane and metabolism of 2,4-D but not glyphosate by common milkweed were considered the primary factors involved in explaining observed susceptibility differences.

Contrasting cover-cropping systems were compared to determine whether fundamental differences in cover-cropping strategies affect weed seed predators and resulting seed predation. We conducted typical “feeding” trials in which 25 seeds of each of six weed species, including velvetleaf, wild mustard, yellow foxtail, common lambsquarters, redroot pigweed, and hairy galinsoga, were placed in the field. Exclosures showed that the majority of seed predation could be attributed to invertebrates: 43% out of a total of 56% seed predation for 11 d in 2002 and 40% out of a total of 58% seed predation for 4 d in 2003. The predominant invertebrate seed predator across all entry points of four cropping systems was a ground-dwelling carabid beetle, Harpalus rufipes, which was more abundant in vegetated treatments, particularly red clover, compared with treatments recently tilled and planted to a fall cover crop. In the absence of vertebrates, H. rufipes activity–density was positively correlated with mean seed predation in 2002 (Spearman ρ = 0.489; P < 0.001) but not in 2003 (Spearman ρ = 0.090; P = 0.504), possibly because of a delay between pitfall trapping and predation assay. The activity–density of invertebrate seed predators measured in these systems and the high level of predation imposed on weed seeds at the soil surface indicate that cover-cropping strategies should consider late-season weed management, which maintains seeds on the soil surface and provides desirable habitat for invertebrate predators.

Imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} sorption and mobility were studied in five Alabama soils ranging from sandy loam to clay. Techniques included thinlayer soil chromatography, batch equilibrium, and soil solution recovery. Imazaquin was mobile in all soils with Rf values of 0.8 to 0.9. Sorption based on batch equilibrium was minimal with Kd values ranging from 0.001 to 0.21. The soil solution recovery technique was used to evaluate imazaquin sorption in each soil as influenced by imazaquin concentration, wetting and drying, and pH. As herbicide concentration added to the soils was increased from 0.1 to 10 mg/kg, the amount of 14C-imazaquin in soil solution increased. Temporarily drying each soil to 25 or 50% of field capacity resulted in maximum sorption of imazaquin. Lowering the pH enhanced sorption in all soils such that the amount of imazaquin in solution ranged from 38 (low pH) to 100% (high pH). Soil sorption appeared to be governed by the pH-dependent charge surfaces from aluminum and iron oxyhydroxides (specifically hematite and gibbsite) and kaolinite.

Germination of weed seed and time of emergence are greatly affected by temperature. The effects of temperature on seed germination of tumble pigweed, prostrate pigweed, smooth pigweed, Palmer amaranth, Powell amaranth, spiny amaranth, redroot pigweed, common waterhemp, and tall waterhemp were examined under constant and alternating temperature regimens at 5, 10, 15, 20, 25, 30, and 35 C. Averaged over all temperatures, alternating temperature regimens increased total germination of all species, except Powell amaranth, which germinated similarly under both constant and alternating temperatures. In addition, Powell amaranth seed exhibited the highest total germination across all temperatures compared with the other amaranth species. Prostrate pigweed seed demonstrated the lowest total germination. Optimal temperatures for maximum germination were greater than 20 C for all species, except prostrate pigweed. The alternating temperature regimen centering at 30 C was used to compare the germination rates of the nine species. Palmer amaranth and smooth pigweed attained complete germination on the first day. The rate of germination for these species was much more rapid than the other Amaranthus spp., which took 3 to 8 d to reach 50% germination.

Gene flow among herbicide-resistant (HR) canola varieties can lead to the development of multiple HR canola plants, creating volunteer canola management challenges for producers. In western Canada, escaped populations of HR canola are ubiquitous outside of cultivated fields, yet the extent of gene flow resulting in herbicide resistance trait stacking in individuals within these populations remains unknown. The objectives of this study were to document the presence of single and multiple herbicide resistance traits and assess the extent of gene flow within escaped canola populations. Seed was collected from 16 escaped canola populations along the verges of fields and roadways in four agricultural regions in southern Manitoba from 2004 to 2006. Glyphosate resistance was found in 14 (88%) of these populations, glufosinate resistance in 13 (81%) populations, and imidazolinone resistance in five (31%) populations. Multiple herbicide resistance was observed at levels consistent with previously published canola outcrossing rates in 10 (62%) of the tested populations. In 2005 and 2006, maternal plants from two escaped populations were tested using trait indicator test strips for glyphosate and glufosinate resistance to confirm outcrossing events. In 2005, two of 13 tested maternal plants with single herbicide resistance traits produced progeny with both glyphosate and glufosinate resistance. In 2006, of 21 tested plants, 10 single HR maternal plants produced multiple HR progeny, and five nonresistant maternal plants produced resistant offspring. This is the first report indicating that intraspecific gene flow results in stacking of herbicide resistance traits in individuals within escaped canola populations, confirming that multiple HR canola volunteers are not confined to agricultural fields. Results of this study suggest that escaped populations of crop plants can contribute to the spread of genetically engineered novel traits, which has important implications for containment, especially for highly controversial pharmaceutical and industrial traits in crop plants.

Trials were carried out to determine the degree of light transmittance through soil and its role in photo-induced germination of buried weed seeds. Transmittance was found to be strongly dependent on soil type and decreased with decreasing particle size and with an increase in darker components. In each soil examined, light penetration fell below 0.01% at a depth of no more than 4 mm. It was also noted that with increasing soil depth, light permeability was proportional to wavelength, leading to progressive decline in the red-far red ratio. Low light penetration played a crucial role in germination of jimsonweed seeds, which had became light-sensitive after prolonged burial. Although this effect is a function of soil type, it may represent an important ecological factor influencing seed emergence in the first few mm of upper soil layers.

Herbicide-resistant weeds are a constraint to weed management in many cropping regions around the world. Of the numerous populations of weeds with resistance to herbicides, it appears that most have resistance due to an alteration to the target enzyme. Use of herbicides with alternative modes of action has relatively easily controlled these populations. In stark contrast are a much smaller number of populations with resistance due to increased rates of herbicide detoxification. These populations may be cross-resistant to herbicides with other modes of action. Such cross-resistance can severely compromise weed control because alternative herbicides may fail on their first use. It has proved extremely difficult to predict cross-resistance due to increased herbicide detoxification in weed populations and hence, difficult to provide adequate advice to growers on how to avoid or manage the problem. Most commonly, such cross-resistance has been selected by certain aryloxyphenoxypropanoate herbicides such as diclofop-methyl and phenylurea herbicides such as chlorotoluron and isoproturon; however, other herbicides can also act as selecting agents for this type of resistance. Illustrative examples from rigid ryegrass in Australia and blackgrass in Europe demonstrate the breadth of the problem and the magnitude of the effort required to understand increased herbicide detoxification as a resistance mechanism. Recent work has elucidated the genetic basis of cross-resistance in some populations, but this has so far not provided new predictive tools useful to growers. Despite more than a decade of research aimed at unraveling the complexities of cross-resistance due to increased herbicide detoxification, management of these cross-resistant populations remains a significant challenge.

Rising demand for organic soybeans and high price premiums for organic products have stimulated producer interest in organic soybean production. However, organic soybean producers and those making the transition to organic production cite weed management as their main limitation. Current weed management practices heavily rely on cultivation. Repeated cultivation is expensive and has negative consequences on soil health. Research is needed to improve organic reduced tillage production. Rye cover crop mulches were evaluated for weed suppression abilities and effects on soybean yield. Experiments were planted in 2008 and 2009 at three sites. Rye was planted in the fall of each year and killed at soybean planting with a roller/crimper or flail mower, creating a thick weed-suppressing mulch with potential allelopathic properties. The mulch was augmented with one of three additional weed control tactics: preemergence (PRE) corn gluten meal (CGM), postemergence (POST) clove oil, or postemergence high-residue cultivation. Roll-crimped and flail-mowed treatments had similar weed suppression abilities at most sites. There were no differences between CGM, clove oil, or cultivation at most sites. Sites with rye biomass above 9,000 kg ha−1 of dry matter provided weed control that precluded soybean yield loss from competition. In Goldsboro 2008, where rye biomass was 10,854 kg ha−1 of dry matter, the soybean yield in the rolled rye treatment was not significantly different from the weed-free treatment, yielding at 2,190 and 2,143 kg ha−1, respectively. Likewise, no difference in soybean yield was found in Plymouth 2008 with a rye biomass of 9,256 kg ha−1 and yields of 2,694 kg ha−1 and 2,809 kg ha−1 in the rolled rye and weed-free treatments, respectively. At low rye biomass levels (4,450 to 6,606 kg ha−1), the rolled rye treatment soybean yield was 628 to 822 kg ha−1 less than the weed-free treatment. High rye biomass levels are critical to the success of this production system. However, high rye biomass was, in some cases, also correlated with soybean lodging severe enough to cause concern with this system.

Herbicides have been the principal means of weed control in developed countries for approximately 50 yr because they are the most cost-effective method. Such general use of herbicides has resulted in weed resistance to herbicides, which continues to be a growing problem. Within the past decade, the evolution of resistance to the once-dominant herbicide glyphosate has resulted in major concerns about the future ability to control weeds in many crop systems. Moreover, many weed species have evolved resistance to multiple mechanisms of herbicide action. Given the dearth of new herbicides with novel mechanisms of action, it appears inevitable that weed management programs will need to be supplemented by the use of tactics other than herbicides. However, the inclusion of more diversity for weed management also introduces complexity, cost, and time constraints to current crop production systems. This paper describes broadly the considerations, opportunities, and constraints of diverse weed management tactics to address the burgeoning problems with herbicide resistance.

A plant's ability to detect and adjust morphologically to changes in light quality (red–far-red [R:FR] ratio) is one mechanism by which a crop plant responds to weeds. To test this hypothesis, two experiments were conducted where corn was grown in growth cabinets under different light environments. First, to determine the effect of R:FR ratio on corn growth and development, treatments of high R:FR (1.37) and low R:FR (0.67) ratio were compared. These were established by planting corn in pots and then placing trays of either turface (a baked clay medium with high R:FR) or commercial grass sod (low R:FR) on each side of a row of corn pots. Grass sod was used to simulate low-growing weeds. The low R:FR sod treatment resulted in corn plants which were taller, had larger leaves, and greater shoot–root ratio than plants growing in the high R:FR turface treatment. In the second experiment, the effect of R:FR ratio on corn leaf azimuth position was examined. This was accomplished by adding a third treatment where each corn row had sod placed on one side and turface on the other. The proportion of leaves in four azimuthal classes was recorded. In the presence of sod, the proportion of leaves perpendicular to the corn row decreased, and this altered the proportion of leaves in other classes. Therefore, corn seedlings detected changes in light quality caused by the presence of sod (which simulated low-growing weeds) and responded by adjusting carbon allocation and leaf orientation to optimize the interception of light quantity and quality. These results support our hypothesis that low-lying vegetation can alter the growth of corn seedlings before competition for resources occurs. This change in growth may help explain the importance of early-season weed control in corn.

Weeds are an important plant resource for insects, although feeding by insects on weeds can have both positive and negative effects on crop productivity. Weeds also indirectly affect crops via their influence on beneficial insects, and by harboring plant and insect diseases. Weeds may affect the ability of dispersing insects to locate crop plants. The host relationship between insects and plants is highly variable, ranging from very specialized to generalized feeding behaviors. Despite the myriad interactions of weeds and insects, many aspects of the relationship are predictable. Most insects, including crop pests, are specialists, and preadapted to feed only on some plants, often within a single plant family. Even polyphagous insects often have a distinct preference hierarchy, feeding more widely only when preferred hosts are unavailable. Use of plants by insects is a dynamic interaction, with characteristics of the insect (e.g., mandible structure) and the plant (e.g., allelochemicals) affecting feeding behavior. Thus, weeds that are closely related to crops are particularly important in harboring insects that attack those crops. Crop production practices should seek to sever the taxonomic association between the crop and the weeds found within the crop, and nearby, by eliminating weeds related to the crop. This will make it less likely that insects will move easily from weed to crop plants, that damaging population densities of insects will develop in the field, and that insect vectors that harbor plant diseases will be harbored in the field. Particularly important integrated pest management practices include crop rotation, reduced use of chemical herbicides, and management of weeds in noncultivated areas.

Adsorption of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] benzenesulfonamide} on four soils having different pH values (6.2 to 8.1) was low. Log/log Freundlich isotherms for all soils at 8 and 30 C were linear, the extent of adsorption being greater at 8 than at 30 C. There was an inverse relationship between pH and degradation rate with a half-life of 88.5 days at pH 6.2 and 144 days at pH 8.1 at 20 C. Rapid disappearance was noted during the first 15 days after application. With decreasing moisture and temperature, chlorsulfuron degraded more slowly. The half-life ranged from 229 days at 10 C and moisture content of 75% field capacity to 62.5 days at 40 C and 75% field capacity at pH 7.7.

The impact of two weed management systems on the weed seed reserves of the soil, on the yearly weed problem, and on corn (Zea mays L.) production was assessed where corn was grown under furrow irrigation for 6 consecutive years. In one system, 2.2 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was applied annually to the same plots as a preemergence treatment. In the other system, a mixture of 1.7 kg/ha of atrazine plus 2.2 kg/ha of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was applied preemergence, followed by a postemergence application of 0.6 kg/ha of the alkanolamine salts of 2,4-D [(2,4-dichlorophenoxy)acetic acid]. The response of weeds and corn is presented only where atrazine was applied annually because the results were similar between both weed management systems. Weed seeds from eight annual species were identified, with redroot pigweed (Amaranthus retroflexus L. ♯ AMARE) and common lambsquarters (Chenopodium album ♯ CHEAL) comprising 82 and 12%, respectively, of the initial 1.3 billion weed seeds/ha that were present in the upper 25 cm of the soil profile. After the sixth cropping year, the overall decline in the total number of redroot pigweed and common lambsquarters seeds was 99 and 94%, respectively. Very few weeds produced seeds during the first 5 yr, and no weed seeds were produced during the sixth year where atrazine was applied annually. When the use of atrazine was discontinued on one-half of each plot at the beginning of the fourth year, the weed seed reserve in soil began to increase due to an increase in the weed population. After 3 yr of not using atrazine, the weed seed reserve in soil had built up to over 648 million seeds/ha, and was then within 50% of the initial weed seed population. In the fifth and sixth years, grain yields were reduced 39 and 14%, respectively, where atrazine had been discontinued after 3 yr.

Narrower corn row widths, higher crop densities, and interrow cultivation may be used as part of integrated weed management (IWM). During a three-year study, we tested whether these factors affected corn growth, development and grain yield at final harvest, and weed biomass when weeds were late-emerging (after the three-leaf stage of corn). Increasing corn density from 7 to 10 plants m−2 or decreasing row width from 75 to 50 cm significantly increased corn leaf area index (LAI), and reduced photosynthetic photon flux density (PPFD) available for a mixture of weed species located below the corn canopy. Narrower rows and higher corn density significantly reduced biomass of late-emerging weeds. Corn yields increased significantly (10 to 15%) only when narrower rows were used. Intraspecific competition between corn plants in the higher density significantly reduced early corn growth and offset any gain in yield from reduced weed competition. In comparison to plots where late-emerging weeds grew uncontrolled, interrow cultivation did not decrease biomass of late-emerging weeds, hence did not increase corn yield. We recommend using narrower row widths to reduce weed competition and increase corn yield. Increased corn densities had no net yield benefit. Early-season weed management is crucial as the later-emerging weeds were less important in terms of their effect on yield.