Efficient natural dispersal of herbicide-resistance alleles via seed and pollen can markedly accelerate the incidence of herbicide-resistant weed populations across an agroecoregion. Studies were conducted in western Canada in 2014 and 2015 to investigate pollen- and seed-mediated gene flow in kochia. Pollen-mediated gene flow (PMGF) from glyphosate-resistant (GR) to non-GR kochia was quantified in a field trial (hub and spoke design) at Saskatoon, Saskatchewan. Seed-mediated gene flow of acetolactate synthase (ALS) inhibitor-resistant kochia as a function of tumbleweed speed and distance was estimated in cereal stubble fields at Lethbridge, Alberta and Scott, Saskatchewan. Regression analysis indicated that outcrossing from GR to adjacent non-GR kochia ranged from 5.3 to 7.5%, declining exponentially to 0.1 to 0.4% at 96 m distance. However, PMGF was significantly influenced by prevailing wind direction during pollination (maximum of 11 to 17% outcrossing down-wind). Seed dropped by tumbleweeds varied with distance and plant speed, approaching 90% or more (ca. 100,000 seeds or more) at distances of up to 1,000 m and plant speeds of up to 300 cm s–1. This study highlights the efficient proximal (pollen) and distal (seed) gene movement of this important GR weed.

Wild oat is a problematic weed species that requires new management techniques in the face of herbicide resistance; harvest weed-seed control (HWSC) may be an option. Wild oat demographic information was collected in long-term, rotational field studies in Lacombe, AB, Canada, in 2006 and 2007, and a periodic matrix model was parameterized using management extremes (no IPM, no herbicide to high IPM, and full herbicide). Population growth rates were calculated for each treatment and year. Prospective (elasticity) and retrospective (LTRE) analyses were conducted alongside a rearrangement of the model equation in which population growth rates were designated and the required proportion of newly shed seed survival that gives that growth rate was solved for. All populations had λ > 1 or increasing populations. Elasticity analyses indicated that λ was most-highly elastic to the overwinter seedbank (Esw = 1), followed by seedling survival, fecundity, and survival of newly shed seed (0.63 to 0.86 across treatments). The latter may be the most-accessible vital rate for management of herbicide resistant populations. LTRE exposed the stochasticity of wild oat population growth rates between years and their ability to take advantage of lapses in control. Decreasing the proportion of newly shed seeds (snew) that survives was the most-effective and available control strategy until reduced to 0.1 to 0.3 when the summer seedbank becomes more critical. When averaged across treatments, > 80% of newly shed seed must be eliminated to stop the population from growing, resulting in a stable population, but not a decline. Because of preharvest shattering, HWSC will likely not be effective enough alone to cause wild oat populations to decline. New management techniques for wild oat control that can be used in combination with HWSC and integrated weed management strategies are needed.

Recovery of common agricultural weeds after burial by soil was studied in four greenhouse and three field experiments. Species studied included velvetleaf, Powell amaranth, common lambsquarters, barnyardgrass, and giant foxtail. Seedlings were bent over before burial to simulate the effect of the impact of soil thrown by a cultivator. Altogether, more than 35,000 seedlings were marked and observed for recovery. No seedlings recovered from 4 cm of burial. Recovery from complete burial under 2 cm of soil ranged from 0 to 24% depending on the experiment, species, and watering treatment, but recovery greater than 5% was rare. Large-seeded species tended to recover from complete burial under 2 cm of soil better than small-seeded species. The study did not reveal a difference in recovery of grasses relative to broadleaf weeds. Overall, seedlings tended to recover best when water was applied daily after burial, worst when water was applied once on the day of burial, and to an intermediate extent when no water was applied. However, difference in recovery between the no-water and watering-once treatments were usually small. Also, many experiment by species combinations showed no significant differences among watering treatments. When even a small portion of the seedling was left exposed, recovery generally exceeded 50%. Organic weed management systems commonly use burial of weed seedlings with tine weeders and soil thrown by sweeps and hilling disks to control weeds in crop rows. Recovery from burial could pose a substantial weed management problem in some circumstances, particularly for large-seeded weed species. Maximizing burial depth is important for limiting recovery. Recovery from burial can be minimized by withholding irrigation for several days after hilling-up operations.

The evolution of herbicide-resistant weeds is a major concern in the corn- and soybean-producing Pampas region of Argentina, where growers predominantly plant glyphosate-resistant crop varieties and depend heavily on glyphosate for weed control. Currently, 16 weed species in Argentina are resistant to one or more of three different herbicide mechanisms of action, and resistant weed populations continue to increase, posing a serious threat to agricultural production. Implementation of integrated weed management to address herbicide resistance faces significant barriers in Argentina, especially current land ownership and rental patterns in the Pampas. More than 60% of Pampas cropland is rented to tenants for periods that rarely exceed 1 yr, resulting in crop rotation being largely abandoned, and crop export taxes and quotas have further discouraged wheat and corn production in favor of continuous soybean production. In this paper we discuss ways to facilitate new approaches to weed management in Argentina, including legal and economic reforms and the formation of a national committee of stakeholders from public and private agricultural sectors.

The net selection effect of herbicides on herbicide-resistance traits in weeds is conditioned by the fitness benefits and costs associated with resistance alleles. Fitness costs play an important evolutionary role preventing the fixation of adaptive alleles and contributing to the maintenance of genetic polymorphisms within populations. Glyphosate is widely used in world agriculture, which has led to the evolution of widespread glyphosate resistance in many weed species. The fitness of glyphosate-resistant and -susceptible perennial ryegrass plants selected from within a single population were studied in two field experiments conducted during 2011 and 2012 under different soil water availability. Glyphosate-resistant plants showed a reduction in height of 12 and 16%, leaf blade area of 16 and 33%, shoot biomass of 45 and 55%, seed number of 33 and 53%, and total seed mass of 16 and 5% compared to glyphosate-susceptible plants in 2011 and 2012, respectively. The reduction in seed number per plant resulted in a 40% fitness cost associated with the glyphosate-resistance trait in perennial ryegrass. Fitness costs of glyphosate-resistant plants were expressed under both conditions of water availability. These results could be useful for designing management strategies and exploiting the reduced glyphosate-resistant perennial ryegrass fitness in the absence of glyphosate selection.

We used laboratory and field feeding trials to investigate adult carabid beetle preferences for three brassicaceous weed species (rapeseed, wild mustard, and field pennycress) that are pests in canola. All carabid species preferred seeds of rapeseed most and those of field pennycress least and showed intermediate preference for wild mustard seeds. Beetles highly preferred imbibed seeds of all three weed species. Activity–density of carabids and mean weed seed removal were highly correlated in field plots of canola, with activity–density accounting for 67% of the observed variation in seed removal. Our study indicates that seed consumption among carabids is influenced by several factors, including weed species, physiological state of seeds, and carabid activity–density. Carabid seed predation is significant in canola agroecosystems; therefore, understanding these influences has implications for ecological weed management.

Weeds are often spatially aggregated in maize fields, and the level of aggregation varies across and within fields. Several annual weed species are present in maize fields before postemergence herbicide application, and herbicides applied will control several species at a time. The goal of this study was to assess the spatial distribution of multispecies weed infestation in maize fields. Ground-based imagery was used to map weed infestations in rain-fed maize fields. Image segmentation was used to extract weed cover information from geocoded images, and an expert-based threshold of 0.102% weed cover was used to generate maps of weed presence/absence. From 19 site-years, 13 (68%) demonstrated a random spatial distribution, whereas six site-years demonstrated an aggregated spatial pattern of either monocotyledons, dicotyledons, or both groups. The results of this study indicated that monocotyledonous and dicotyledonous weed groups were not spatially segregated, but discriminating these weed groups slightly increased the chances of detecting an aggregated pattern. It was concluded that weeds were not always spatially aggregated in maize fields. These findings emphasize the need for techniques allowing the assessment of weed aggregation prior to conducting site-specific weed management.

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.

Weeds are selected to produce overwhelming propagule pressure, and while vertebrate and invertebrate seed predators destroy a large percentage of seeds, their ecosystem services may not be sufficient to overcome germination site limitations. Cover crops are suggested to facilitate seed predation, but it is difficult to disentangle reductions in weed recruitment attributable to granivores from those due to plant competition. Using common lambsquarters as a focal weed species, we used experimental seed subsidies and differential seed predator exclusion to evaluate the utility of vertebrate and invertebrate seed predators in fallow, killed cover crop, and living mulch systems. Over two growing seasons, we found that seed predators were responsible for a 38% reduction in seedling emergence and 81% reduction in weed biomass in fallow plots following simulated seed rain, suggesting that granivory indeed overcomes safe-site limitation and suppresses weeds. However, the common lambsquarters densities in ambient seedbanks across fallow and cover crop treatments were high, and seed predators did not impact their abundance. Across the study, we found either neutral or negative effects of vertebrate seed predators on seed predation, suggesting that invertebrate seed predators contribute most to common lamnsquarters regulation in our system. These results imply that weed seed biocontrol by invertebrates can reduce propagule pressure initially following senescence, but other tools must be leveraged for long-term seedbank management.

The influence of temperature, light, solution pH, water stress, salt stress, and burial depth on seed germination and seedling emergence of catchweed bedstraw and the sensitivity of that weed to commonly available herbicides in China were studied in laboratory and greenhouse. Germination occurred at day/night temperatures from 5/0 C to 30/25 C, with optimum germination at 15/10 C. Catchweed bedstraw germinated equally well under a 12-h photoperiod and continuous darkness; however, a 24-h photoperiod inhibited seed germination. Catchweed bedstraw seed is moderately sensitive to osmotic potential and salt stress, with 15 and 3% germination rates at an osmotic potential of −0.5 Mpa and salinity level of 120 mM, respectively. Maximum seed germination was observed in near neutral pH; germination was greater than 80% over a broad pH range from 5 to 8. Seedling emergence of the seeds buried at a depth of 1 cm was higher (74%) than those placed on the soil surface (20%), but declined with burial depth increasing. Few (10%) seedlings emerged when seeds were placed at a depth of 5 cm. Bensulfuron-methyl, and ethametsulfuron-methyl applied PRE and tribenuron-methyl, fluroxypyr, and florasulam applied POST can be used to provide greater than 80% control of catchweed bedstraw. The results of this study have contributed to more complete understanding of the germination and emergence of catchweed bedstraw.

Effective outreach is critical to achieving success in managing herbicide-resistant weeds. Interdisciplinary collaboration is needed to adapt information delivery and to engage communities to address the herbicide-resistance problem. Weed scientists must partner with the production community to adapt herbicide-resistance practices for local needs, to work collaboratively with state and regional stakeholders to create effective resistance-management practices, and to provide an overarching national message as to the causes of, and solutions to, resistance.

Integrated weed management (IWM) relies upon multiple chemical, physical, or biological weed management techniques to achieve an acceptable level of weed control. Agents that selectively suppress weeds but not crops and that can be manipulated in agriculture will be promising components for inclusion in IWM. We used a meta-analytic approach to investigate the potential of arbuscular mycorrhizal fungi (AMF) to contribute to IWM. We quantified the effect of crop and weed host status (strong and weak AMF hosts are divided in this study by a 10% root length colonization threshold), AMF diversity (single vs. mixed), and soil N and P fertility management on plant mycorrhizal growth responses (MGRs). Our results indicated that weak host weeds had consistently lower MGRs than strong host crops in both controlled and field conditions. Moreover, these differences in MGRs between weak host weeds and strong host crops were more pronounced under mixed AMF inoculum and low N and P nutrient availability. In contrast, MGR of strong host weeds was not different from strong host crops in general. However, we observed a wide range of MGRs among strong host weeds, some of which had much lower MGRs than strong host crops. In addition, in the presence of N and P fertilizers, strong host crops had a stronger positive response to AMF than strong host weeds. Thus, our meta-analysis indicates that AMF have potential to contribute to weed control by direct and indirect pathways: directly suppress weak host weeds, and indirectly suppress some strong host weeds mediating by competitive effects exerted by strong host crops. We suggest that management practices affecting AMF diversity and crop and weed mycorrhizal responses could be chosen to improve the contribution of AMF to IWM. Better understanding is needed of crop–weed–AMF interactions and management practices that enhance this form of weed management.

Recommended rates of glyphosate for noncultivated areas destroy the aboveground shoots of the perennial plant leafy spurge. However, such applications cause little or no damage to underground adventitious buds (UABs), and thus the plant readily regenerates vegetatively. High concentrations of glyphosate, applied under controlled environmental conditions, have been shown to cause sublethal effects in UABs of leafy spurge that produce stunted and bushy phenotypes in subsequent generations of shoots. We treated leafy spurge plants in the field with glyphosate (0, 1.1, 3.4, or 6.7 kg ai ha−1) to determine its effects on vegetative growth from UABs and on molecular processes. The number of shoots derived from UABs of glyphosate-treated plants was significantly increased compared to controls in subsequent years after application, and new shoots displayed various phenotypical changes, such as stunted and bushy phenotypes. Quantifying the abundance of a selected set of transcripts in UABs of nontreated vs. treated plants (0 vs. 6.7 kg ha−1) indicated that glyphosate impacted molecular processes involved in biosynthesis or signaling of tryptophan or auxin (ARF4, CYP79B2, PIN3, TAA1, TRP6, YUC4), gibberellic acid (GA1/CPS1, GA2/KS), ethylene (ACO1, ACS10), cytokinins (AHP1, AK2, CKX1), and the cell cycle (CDC2A, CDC2B, CYCD3;1). Glyphosate-induced effects on vegetative growth and transcript abundance were persistent for at least 2 yr after treatment. Determining the molecular mechanisms associated with vegetative reproduction in leafy spurge following foliar glyphosate-treatment could identify limiting factors or new targets for manipulation of plant growth and development in perennial weeds.

Green galenia is a South African woody prostrate perennial that was first recorded in Australia in the early 1900s and has since become a serious threat to indigenous temperate grasslands and surrounding agricultural areas. Laboratory and field based experiments were conducted to examine the effect of environmental factors on the germination and viability of green galenia seed. It was shown that green galenia was able to germinate over a broad range of temperatures, but short bursts (5 min) of high temperatures (80 C to 120 C replicating possible exposures to a fire) reduced seed germination. Seed germination was positively favored by light, declined rapidly in darkness, and decreased by > 80% at a depth of only 0.5 cm in soil. Water stress greatly reduced seed germination (45% germination at osmotic potentials below −0.2 MPa). Germination was completely inhibited at water potentials of −0.4 to −1.0 MPa. This species is moderately tolerant to salinity, with over 50% of seeds germinating at low levels of salinity (60 mM NaCl), and moderate germination (49%) occurring at 120 mM NaCl, it can germinate well in both alkaline (pH 10–83%) and acidic (pH 4–80%) conditions. The results of this study have contributed to our understanding of the germination and emergence of green galenia, and this will assist in developing tools and strategies for the long term management of this noxious weed in Victoria and other parts of Australia.

A field study was undertaken to investigate the influence of different management strategies on rigid ryegrass plant density and seedbank dynamics over 4 yr. Even though weed seedbank declined by 86% after oaten hay in year 1, the residual seedbank enabled rigid ryegrass to reinfest field peas the next year, and the population rebounded sharply when weed control relied solely on PPI trifluralin. However, use of POST clethodim followed by crop-topping for seed-set prevention of rigid ryegrass in field pea was highly effective and caused a further decline in the weed seedbank. Integration of effective management tactics over 3 yr significantly reduced rigid ryegrass weed and spike density (90 and 81%) in the final year of the 4-yr cropping sequence. Use of oaten hay in year 1, followed by effective weed control in field pea and wheat crops, depleted the high initial seedbank (4,820 seeds m−2) to moderate levels (< 200 seeds m−2) within 3 yr. Effective weed-management treatments depleted the rigid ryegrass seedbank, reduced in-crop weed infestation, and returned higher grain yields and profitability. The results of this study clearly show that large rigid ryegrass populations can be managed effectively without reducing crop productivity and profitability provided multiyear weed-management programs are implemented effectively.

Management of herbicide resistance can be most effectively accomplished if every person and organization involved in agricultural production takes an ownership position and participates in solving the growing problem of weed resistance to herbicides. Growers and other pest management practitioners are keys to effective herbicide resistance management since they make the final decisions on practices used. However, many other people and organizations have an important role to play as well. Agricultural input supply networks, including chemical companies, are a widely used information source for growers' decisions through company marketing efforts. Government agencies may influence decisions through regulations or incentive programs. University scientists through their research, education, and outreach programs may impact management decisions, and organizations such as professional societies, farm and commodity groups, public interest organizations, and the agricultural press play roles as well. It is critically important that all of these groups impacting herbicide resistance management decisions are sending the same message and that message is based on sound science. The time to act is now.

The widespread adoption of glyphosate-resistant corn and soybean in cropping rotations often results in volunteer plants from the previous season becoming problem weeds that require alternative herbicides for control. Corn yield losses due to season-long volunteer soybean competition at several densities in two growing seasons were used to define a hyperbolic yield loss function. The maximum corn yield loss observed at high volunteer soybean densities was about 56%, whereas, the incremental yield loss (I) at low densities was 3.2%. Corn yield loss at low volunteer soybean densities was similar to losses reported for low densities of velvetleaf and redroot pigweed, with 10% yield loss estimated to occur at 3 to 4 volunteer soybean plants m−2. Several herbicides, including dicamba with or without diflufenzopyr applied at the V2 growth stage of volunteer soybean, provided > 90% control, demonstrating several economical options to control volunteer glyphosate-resistant soybean in glyphosate-resistant corn. Reevaluation of control recommendations may be needed with commercialization of other genetically modified herbicide-resistant soybean varieties.

Palmer amaranth has greatly disrupted agricultural practices in the United States with its rapid growth and rapid evolution of herbicide resistance. This weed species is now suspected in Argentina. To document whether the suspected plant populations are indeed Palmer amaranth, molecular comparisons to known standards were conducted. Additionally, these same plant populations were screened for possible herbicide resistance to several acetolactate synthase (ALS)-inhibiting herbicides. Sequencing data confirmed that suspected populations (A2, A3, A4) were indeed Palmer amaranth. Another population (A1) was tested to determine whether hybridization had occurred between Palmer amaranth and mucronate amaranth the native amaranth species of the region. Tests confirmed that no hybridization had occurred and that A1 was simply a unique phenotype of mucronate amaranth. Each population was screened for resistance to imazapic, nicosulfuron, and diclosulam. All Palmer amaranth populations from Argentina were shown to be resistant to at least one ALS-inhibiting herbicide. The populations were then subjected to further testing to identify the mutation responsible for the observed ALS resistance. All mucronate amaranth populations exhibited a mutation previously documented to confer ALS resistance (S653N). No known resistance-conferring mutations were found in Palmer amaranth.

Herbicide-resistant (R biotype) and -sensitive (S biotype) individuals were identified from the same population, and seed was increased for each biotype for three generations. We conducted laboratory experiments to determine the effects of temperature, light, salt stress, osmotic stress, pH, and burial depth on the germination and emergence of resistant and sensitive biotypes of Japanese foxtail. The results revealed that there was no difference in the final germination rate between the two biotypes under different temperature conditions, but time to obtain 50% germination or emergence (tE50) and mean germination time of the R biotype were higher than that of the S biotype at 10 C and 15/10 C 12-h day/night regime. In dark conditions, the final germination rate of the S biotype was higher and lower than that of the R biotype at 10 and 25 C, respectively. The overall germination rate of the R biotype was lower than that in the S biotype, and extended germination time was required in extreme conditions, such as 250 mM NaCl and −0.4 MPa osmotic potential. The change in environmental pH had no effect on the germination of the two biotypes. Emergence of the R biotype was lower than the S biotype when seed was buried at least 8 cm deep in an organic matter substrate. This study demonstrated the pleiotropic effects of a resistance allele on seed germination and emergence under different environmental conditions. Deep tillage could be used to reduce the growth and spread of resistant Japanese foxtail individuals.

Glyphosate-resistant (GR) horseweed has become an especially problematic weed in different crop production systems across the United States and the world. In this field study, we used a nondestructive measurement system to analyze the pollen production, deposition, and dispersion of a Tennessee glyphosate resistant (TNR) horseweed biotype in Knoxville, TN during the 2013 pollination season. We observed that the pollination season of TNR horseweed lasted about 2 mo (54 d). About 78.93% of horseweed pollen was released between 9:00 A.M. and 7:00 P.M. during each sampling day and the release peak was at about 1:30 P.M. The seasonal release of pollen grains was estimated to be 5.11 million grains plant−1. The release rate data indicated that the integrated horizontal flux density and deposition flux density contributed to 78.17% and 21.83% of the release rate, respectively. We also found that pollen concentration decreased with distance from the source field; the average pollen concentration decreased to 50.69% at a distance of 16 m from the source plot. This is the first result of a systematic, direct examination of the release rate (emission and deposition), release pattern (daily and seasonal), and dispersion pattern of GR horseweed pollen.