Genetically engineered (GE) herbicide-resistant crops have been widely adopted by farmers in the United States and other countries around the world, and these crops have caused significant changes in herbicide use patterns. GE crops have been blamed for increased problems with herbicide-resistant weeds (colloquially called by the misnomer “superweeds”); however, there has been no rigorous analysis of herbicide use or herbicide-resistant weed evolution to quantify the impact of GE crops on herbicide resistance. Here, I analyze data from the International Survey of Herbicide Resistant Weeds and the USDA and demonstrate that adoption of GE corn varieties did not reduce herbicide diversity, and therefore likely did not increase selection pressure for herbicide-resistant weeds in that crop. Adoption of GE herbicide-resistant varieties substantially reduced herbicide diversity in cotton and soybean. Increased glyphosate use in cotton and soybean largely displaced herbicides that are more likely to select for herbicide-resistant weeds, which at least partially mitigated the impact of reduced herbicide diversity. The overall rate of newly confirmed herbicide-resistant weed species to all herbicide sites of action (SOAs) has slowed in the United States since 2005. Although the number of glyphosate-resistant weeds has increased since 1998, the evolution of new glyphosate-resistant weed species as a function of area sprayed has remained relatively low compared with several other commonly used herbicide SOAs.

The size, location, and variation in time of weed patches within an arable field were analyzed with the ultimate goal of simplifying weed mapping. Annual and perennial weeds were sampled yearly from 1993 to 1997 at 410 permanent grid points in a 1.3-ha no-till field sown to row crops each year. Geostatistical techniques were used to examine the data as follows: (1) spatial structure within years; (2) relationships of spatial structure to literature-derived population parameters, such as seed production and seed longevity; and (3) stability of weed patches across years. Within years, densities were more variable across crop rows and patches were elongated along rows. Aggregation of seedlings into patches was strongest for annuals and, more generally, for species whose seeds were dispersed by combine harvesting. Patches were most persistent for perennials and, more generally, for species whose seeds dispersed prior to expected dates of combine harvesting. For the most abundant weed in the field, the annual, Setaria viridis, locations of patches in the current year could be used to predict patch locations in the following year, but not thereafter.

Field experiments were conducted to study the effects of varying velvetleaf (Abutilon theophrasti Medic.) densities and planting dates on the growth and yield of soybeans [Glycine max (L.) Merr. ‘Amsoy 71’]. Velvetleaf densities ranging from 2.5 to 40 plants/m2 caused reductions in the dry weight of soybean leaves, stems, roots, and pods and seeds, and in the leaf area index, pod number, and seed yield when velvetleaf and soybean plants emerged at the same time. The magnitude of reduction in soybean growth and the time at which the reduction was first observed was dependent on weed density. Soybean growth reductions caused by high velvetleaf densities were less when conditions of high soil moisture content minimized the effects of competition for water. Velvetleaf emerging 21 and 23 days after soybean emergence did not reduce crop growth or yield.

Natural herbicides approved in organic agriculture are primarily nonselective burn-down essential oils applied POST. Multiple applications are often required due to their low efficacy. To address this problem, the in vivo herbicidal activity of manuka oil, the essential oil distilled from manuka tree (Leptospermum scoparium J.R. and G. Forst), was tested on selected broadleaf and grass weeds. While manuka oil exhibited good POST activity when applied in combination with a commercial lemongrass oil–based herbicide, it ultimately demonstrated interesting PRE activity, providing control of large crabgrass seedlings at a rate of 3 L ha−1. Manuka oil and its main active ingredient, leptospermone, were stable in soil for up to 7 d and had half-lives of 18 and 15 d, respectively. The systemic activity of manuka oil addresses many of the current limitations associated with natural herbicides. Additionally, its soil persistence opens up a multitude of new possibilities for the use of manuka oil as a tool for weed management and may be a potential bridge between traditional and organic agriculture.

Since the beginning of agriculture, crops have been exposed to recurrent invasion by weeds that can impose severe reductions in crop quality and yield. There have been continuing efforts to reduce the impacts of weeds on production. More than 40 yr ago, overreliance on herbicide technology to reduce weed infestations resulted in the selection of adaptive traits that enabled weed survival and reproduction under herbicide treatments (Délye et al. 2007; Powles and Yu 2010; Vila-Aiub et al. 2008). As a result, herbicide resistance in > 200 weed species has evolved worldwide (Heap 2013; Powles 2008).

Since its discovery in 2005, glyphosate-resistant Palmer amaranth has become a major problem for many farmers in the southern United States. One mechanism of resistance found in a Georgia population of glyphosate-resistant Palmer amaranth is amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene throughout the genome, with some resistant plants containing and expressing more than 100 EPSPS genes. Such high numbers of EPSPS genes and protein production could result in a fitness cost to resistant plants due to (1) metabolic cost of overproduction of this enzyme and (2) disruption of other genes after insertion of the EPSPS gene. A greenhouse experiment was set up to investigate differences in growth and reproduction between glyphosate-susceptible and -resistant Palmer amaranth plants. Measurements included growth rate, plant height/volume ratio, final biomass, photosynthetic rate, inflorescence length, pollen viability, and seed set. This study found no significant fitness costs for plants with the resistance trait. This study also provided a clear example of how controlling for genetic background is important in fitness cost studies and how potentially misleading results can be obtained if only a few fitness traits are measured. These results indicate that glyphosate-resistant Palmer amaranth plants with high EPSPS gene copy numbers are likely to persist in field populations, even in the absence of glyphosate, potentially leading to long-term loss of glyphosate as a control option for Palmer amaranth.

Seeds of six weed species were buried in the field at depths of 2.5, 7.6, and 15.2 cm. Seed germination after various light and temperature treatments was determined before burial and after recoveries in darkness at 3-month intervals over 12 and 20-month periods. In one experiment the five species studied were ungerminable before burial. After 6 months burial, most species responded to light but became ungerminable again after this time. These results suggest cycles of dormancy where seeds pass from total dormancy at maturation to a phytochrome-controlled dormancy after burial and then return to total dormancy. During the phytochrome-controlled dormancy period, some species were highly light sensitive. In another experiment the response of yellow rocket (Barbarea vulgaris R. Br.) seeds to thirteen light and temperature treatments was determined before and after burial. Without light treatment, germination of the recovered seeds at various temperatures was low. Germination was usually high after brief exposures to red light, particularly in seeds recovered from 15.2 cm. Very large increases in germination were induced by light exposures of 3 × 10−9 Einsteins cm−2 incident in the 600 to 680-nm region (1 sec). The evidence suggests changes in total phytochrome (Ptot) are responsible for a seasonal change in light sensitivity and a previously undescribed type of dormancy.

Chinese sprangletop is a grass weed that has the ability to grow in both flooded and upland conditions, which makes it a widespread and abundant weed in rice and many other crops. Experiments were conducted to study the germination of this weed in the Philippines. Germination was strongly stimulated by light and warm fluctuating temperatures, suggesting high emergence of Chinese sprangletop on bare ground such as a crop seedbed. A high level of germination (95%) immediately after seed harvest suggests that the soil seed bank of this species could be depleted by stale-seedbed strategies. Seed burial depth in soil strongly influenced emergence of this weed, and no seedlings emerged from seeds buried at depths of ≥ 0.5 cm. Maximum emergence (80%) from seeds placed on the soil surface suggests that no-till practices would enhance the emergence of seedlings. Seedling emergence from surface-sown seeds was similar (80 to 82%) between saturated and aerobic-moist soil, demonstrating that this weed does not require a saturated soil and that it can emerge from a moist soil. Flooding, even though not continuous or deep, had a suppressive effect on the emergence and dry matter of Chinese sprangletop. Germination responses to light and seed burial depth were different between our study in the Philippines and the study conducted with seeds of an Italian population, suggesting that Chinese sprangletop might be polymorphic. The implications of this information for weed management strategies are discussed.

Although the problems with waterhyacinth (Eichhornia crassipes [Mart.] Solms; # EICCR) have diminished in the United States over the last decade, the weed continues to be troublesome and requires constant management on a maintenance basis (29, 30). Chemical herbicides are currently the principal means of control when an immediate solution to a waterhyacinth problem is needed. Preventive maintenance programs also rely on chemicals to keep the weed populations at acceptable levels and to prevent weed migration into unwanted areas. However, when control is not urgently required or is economically unfeasible due to the location of the weed, a combination of biological control agents is used. The latter may have been the important factor that contributed to the lessening of the weed problem in this country over the past decade (43).

Corn (Zea mays L.) was grown continuously without tillage (no-tillage) and with conventional tillage for 7 years to evaluate several herbicides for use in both crop culture systems. The only consistently satisfactory herbicide combinations for the no-tillage corn were 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 1,1′-dimethyl-4,4′bipyridinium ion (paraquat) or 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), simazine, and paraquat. Annual weed population shifted rapidly with different herbicide systems; fall panicum (Panicum dichotomiflorum Michx.) was the major annual weed where triazines were used as the residual herbicide. After several years of corn grown with no-tillage, hempdogbane (Apocynum cannabinum L.) became a significant problem in some plots. Corn yields with no-tillage were equal to yields in tilled areas provided weed control was satisfactory.

In a survey of herbicide responses among Illinois waterhemp half-sib populations, several were observed with differential responses to imazethapyr and thifensulfuron, two acetolactate synthase (ALS)–inhibiting herbicides. Plants from two waterhemp populations were verified resistant to imazethapyr, but susceptible to chlorimuron, using a nondestructive leaf-disc assay. Sequencing of the ALS gene revealed that imazethapyr-resistant waterhemp plants from both populations had inferred amino acid substitutions at position 653 of ALS. Depending on the population, the serine at position 653 of ALS was substituted with either asparagine (S653N) or threonine (S653T). Waterhemp lines were derived from each population to create uniformly imidazolinone-resistant (IR) waterhemp biotypes, designated IR-62 and IR-101. ALS-inhibitor responses of each IR biotype were compared with a previously identified ALS inhibitor–resistant biotype with a tryptophan to leucine substitution at position 574 (W574L) and an herbicide-susceptible control. Whole-plant dose–response experiments with waterhemp biotypes containing W574L, S653N, or S653T mutations indicated that each biotype was resistant to imazethapyr, but only the biotype with a W574L mutation was resistant to thifensulfuron. In vitro ALS-activity assays revealed unique patterns of cross-resistance among protein extracts derived from each biotype in response to imazethapyr, thifensulfuron, cloransulam, and pyrithiobac. In conclusion, three different forms of target-site–based resistance to ALS inhibitors have been identified in waterhemp.

Doses required to reduce fresh weights of seedling oats 50 percent (ED50) in 32 soils were determined for isopropyl N-(3-chlorophenyl)carbamate [CIPC], 3-(3,4-dichlorophenyl)-1,1-dimethylurea [diuron], and 2-chloro-4,6-bis(ethylamino)-s-triazine [simazine]. Physical and chemical properties of soils and the extent of adsorption of the three herbicides on the soils were also determined and evaluated by correlation and multiple regression analysis for usefulness in predicting dose requirements.

Extent of adsorption was most closely correlated with ED50 values for diuron and simazine, but organic matter content was most closely correlated with ED50 values for CIPC. None of the soil properties nor the extent of adsorption provided consistent predictive value for all of the soils and herbicides studied.

Weed seeds were germinated in incubators at constant temperatures ranging from 4 to 32 C increments. Percent germination per day was regressed against temperature with the x-intercept representing the low threshold temperature for development. Low threshold temperatures for development (low TTD's) calculated for wild proso millet (Panicum miliaceum L. # PANMI), barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG], common lambsquarters (Chenopodium album L. # CHEAL), and redroot pigweed (Amaranthus retroflexus L. # AMARE) were 6.9, 9.7, 6.0, and 10.0 C, respectively.

Metribuzin [4-amino-6-tert-butyl-3-(methylthio-as-triazin-5(4H)one] degradation by soil microorganisms decreased as the soil pH decreased. Metribuzin and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] mobility increased with increasing concentration and soil pH. 14C-metribuzin had greater mobility on soil thin-layer plates than 14C-atrazine because of differences in water solubility and basicity. Adsorption increased with decreasing soil pH. The pKa for metribuzin was 0.99 ± 0.08. Adsorption increased gradually as the soil pH decreased toward the pKa of metribuzin. Protonation of the amine group with subsequent adsorption to soil colloids was the postulated reason for the reduction in phytotoxicity, microbial degradation, and mobility of metribuzin at low soil pH levels.

Two separate open grassland areas within Sugarloaf Ridge State Park, Sonoma County, CA, were burned for three consecutive years (1993–1995 [Site A] and 1995–1997 [Site B]) for control of yellow starthistle. Burns were conducted in late June to early July following seed dispersal and senescence of desirable grasses and forbs but prior to viable seed production in yellow starthistle. After the first year burn, there was no significant reduction in yellow starthistle cover the following spring and summer. Despite the lack of control, the first year burn reduced the yellow starthistle soil seedbank by 74% and the number of seedlings the following spring by 83%. However, total plant diversity and species richness increased dramatically in the burned areas. This was due primarily to an increase in the number of native broadleaf species. A second burn the next summer (1995–1997 site) reduced seedbank, seedling density, and summer vegetative cover the following year by 94, 92, and 85%, respectively, while maintaining significantly higher native plant cover and richness. A third consecutive summer burn decreased yellow starthistle seedbank and seedling density by 96, 98, and 85%, respectively, in the 1995–1997 burn site. Three consecutive years of burning in the 1993–1995 site reduced yellow starthistle seedbank and seedling density by over 99% and summer vegetative cover by 91%. These results indicate that prescribed burning can be an effective tool for the management of yellow starthistle and can have a long-term benefit on native broadleaf diversity and richness.

Red rice, which grows taller and produces more tillers than domestic rice and shatters most of its seeds early, is a major weed in many rice-growing areas of the world. Field experiments were conducted at Stuttgart, AR in 1997 and 1998 to evaluate the growth response of the Kaybonnet (KBNT) rice cultivar to various population densities of three red rice ecotypes. The ecotypes tested were Louisiana3 (LA3), Stuttgart strawhull (Stgstraw), and Katy red rice (KatyRR). Compared with KBNT alone, LA3, the tallest of the three red rice ecotypes, reduced tiller density of KBNT 51%, aboveground biomass at 91 d after emergence (DAE) 35%, and yield 80%. Stgstraw, a medium-height red rice, reduced KBNT tiller density 49%, aboveground biomass 26%, and yield 61%. KatyRR, the shortest red rice, reduced KBNT tiller density 30%, aboveground biomass 16%, and yield 21%. Tiller density of rice was reduced by 20 to 48% when red rice density increased from 25 to 51 plants m−2. Rice biomass at 91 DAE was reduced by 9 and 44% when red rice densities were 16 and 51 plants m−2. Rice yield was reduced by 60 and 70% at red rice densities of 25 and 51 plants m−2, respectively. These results demonstrate that low populations of red rice can greatly reduce rice growth and yield and that short-statured red rice types may affect rice growth less than taller ecotypes.

A suspected glyphosate-resistant Italian ryegrass biotype was collected from a filbert orchard near Portland, OR, where glyphosate was applied multiple times per year for about 15 yr. Greenhouse studies were conducted to determine if this biotype was glyphosate resistant. The plants were sprayed with glyphosate (0.01 to 3.37 kg ae ha−1) 14 d after planting and shoot biomass was determined 3 wk after herbicide treatment. Based on the dose–response experiments conducted in the greenhouse, the suspected Italian ryegrass biotype was approximately fivefold more resistant to glyphosate than the susceptible biotype. Plants from both susceptible and resistant biotypes were treated with glyphosate (0.42 and 0.84 kg ha−1) and shikimic acid was extracted 12, 24, 48, and 96 h after treatment. The susceptible biotype accumulated between three and five times more shikimic acid than did the resistant biotype. Leaf segments from both susceptible and resistant biotypes were incubated with different glyphosate concentrations (0.5 to 3000 μM) for 14 h under continuous light. Shikimic acid was extracted from each leaf segment and quantified. At a concentration up to 100 μM, leaf segments from the susceptible biotype accumulated more shikimic acid than leaf segments from the resistant biotype. The epsps gene was amplified and sequenced in both susceptible and resistant biotypes; however, no amino acid change was found in the resistant biotype. The level of resistance in this biotype is similar to that reported for a glyphosate-resistant Italian ryegrass biotype from Chile.

Rigid ryegrass population VLR69 has become resistant to nine classes of herbicides after 21 yr of exposure to five herbicides in five different chemical classes. The population was exposed to diuron in 17 seasons and is resistant to diuron (4 fold) and chlorotoluron (8 fold) when compared with a reference susceptible population (VLR1). VLR69 had six seasons of exposure to chlorsulfuron and exhibits a high level of resistance to chlorsulfuron (> 20 fold) and triasulfuron (> 25 fold) and a lesser change in sensitivity to sulfometuron (7 fold); however, 4% of the population has a high level of resistance to sulfometuron. Resistance to atrazine (5 fold), simazine (6 fold), and ametryn (10 fold) was observed after five seasons of exposure to atrazine. There is a high level of resistance to all aryloxyphenoxypropionate herbicides after only two exposures to diclofop eight generations prior to testing the population. The population was cross-resistant to tralkoxydim (> 9.5 fold) and sethoxydim (1.8 fold). There was a small change in sensitivity to paraquat (1.4 fold) after three generations of exposure. The population displayed cross-resistance to: imazaquin (7 fold), imazapyr (2.5 fold), metribuzin (8.7 fold), and metolachlor (2 fold) but was susceptible to oxyfluorfen and dinitroaniline herbicides. There was also a small shift in sensitivity to tridiphane (1.6 fold).

Yield reductions from the heaviest stand of smooth pigweed (Amaranthus hybridus L.) averaged 39% for corn and 55% for soybeans. For corn the increase in pigweed dry matter about equalled the decrease in the dry matter from the crop; the total dry matter yield was relatively constant. Pigweed grew taller than soybeans, and the dry weight of crop plus pigweeds was 1⅓ times that of weed-free soybeans. As stand of pigweed was increased, there was a decrease in yield of grain, cobs and stalks or straw, diameter of cornstalks, height of crop, and soybean pods per plant. Yield reductions obtained emphasize the importance of both chemical and cultural measures for reducing competition between row crops and weeds.

The genetic basis of weedy and invasive traits and their evolution remain poorly understood, but genomic approaches offer tremendous promise for elucidating these important features of weed biology. However, the genomic tools and resources available for weed research are currently meager compared with those available for many crops. Because genomic methodologies are becoming increasingly accessible and less expensive, the time is ripe for weed scientists to incorporate these methods into their research programs. One example is next-generation sequencing technology, which has the advantage of enhancing the sequencing output from the transcriptome of a weedy plant at a reduced cost. Successful implementation of these approaches will require collaborative efforts that focus resources on common goals and bring together expertise in weed science, molecular biology, plant physiology, and bioinformatics. We outline how these large-scale genomic programs can aid both our understanding of the biology of weedy and invasive plants and our success at managing these species in agriculture. The judicious selection of species for developing weed genomics programs is needed, and we offer up choices, but no Arabidopsis-like model species exists in the world of weeds. We outline the roadmap for creating a powerful synergy of weed science and genomics, given well-placed effort and resources.