The weedy Setaria species (giant, green, yellow, knotroot, and bristly foxtail) compose one of the worst weed groups interfering with world agriculture and in other disturbed and managed habitats. These species, together with their crop counterparts (foxtail millet, korali), form the foxtail species-group (spp.-gp). Five successive waves of Setaria spp. invasion from preagricultural times to the present have resulted in widespread infestation of the disturbed, arable, temperate regions of the earth. These invasions have resulted in considerable economic and environmental costs. The success of the Setaria spp.-gp is because of their intimate evolutionary relationship with humans, disturbance, agriculture, and land management. The ability to adapt rapidly to local conditions is the hallmark of this weedy group. Genotypic and phenotypic biodiversity provides this spp.-gp with traits that allow it to invade, colonize, adapt to, and endure in a wide range of habitats around the world. The phenotypic life-history traits important to the success of weedy Setaria spp. begin with the induction of dormancy in seed during embryogenesis. The formation of long-lived, heterogeneous seed pools in the soil is the inevitable consequence of the dormant seed rain. In soil seed pools, after-ripening, the occurrence and timing of seedling emergence, and the induction of secondary (summer) dormancy are regulated by seasonally and diurnally varying soil oxygen, water, and temperature signals. Precise and variable timing of seedling emergence ensures Setaria a dominant place in disturbed and managed communities during the growth and reproductive phases that follow. Once established in a community, phenotypic plasticity inherent in an individual weedy Setaria sp. allows it to maximize its growth, form, and reproduction to the specific local conditions it encounters, including competitive interactions with neighbors. Traits controlling the plastic development of plant architecture include the ability to form one or more tillering shoots whose stature and number are precisely sized to local conditions. A complex pattern of branching, from plant to spikelet, provides diverse microenvironments within which different levels of dormancy are induced in individual seeds on a panicle and among panicles on a common plant. Traits for adaptation to stress in weedy Setaria spp. include tolerance to many inhibitory chemicals (e.g., herbicides, salt), mechanical damage, and drought. Genetic traits such as self-pollination and small genome size contribute to a highly diverse collection of locally adapted genotypes and phenotypes ready to exploit any opportunities provided by a cropping system. Self-pollinating Setaria spp. exist in wild, weed, and crop variants, an ideal genetic condition ensuring both long-term stability and novelty. Weedy Setaria spp. populations have low to exceedingly low amounts of total genetic variation, unusually low intrapopulation genetic diversity, and unusually high genetic diversity between populations compared with an average plant species. These traits result spatially in local populations that are unusually homogeneous, typically consisting of a single multilocus genotype. Either a generally or a specifically adapted genotype of an individual species might predominate in that local population. Across the landscape, different single-genotype populations dominate particular local sites, providing novel genetics to the region by dispersal and gene flow when conditions change. Across North America, populations of green foxtail and knotroot foxtail are genetically differentiated along a north–south gradient. The history of invasion and colonization, the successful life histories of locally adapted weedy Setaria spp., and the evolutionary potential of this weed group emphasize the need for accurate prediction of its behavior. Weedy Setaria spp. management is the management of local selection pressure and consequent adaptation. Farmers, land managers, policy makers and regulators, homeowners, and consumers need accurate information about weedy Setaria spp. to predict and guide management decisions based on economics, risk, and environmental sustainability.

Laboratory and field experiments were conducted to assess the allelopathic potential of buckwheat. In the field, buckwheat demonstrated strong inhibitory activity by suppressing weeds. In laboratory studies, aqueous and organic solvent extracts of the aerial parts of common buckwheat inhibited the root and shoot growth of lettuce seedlings. The chloroform and ethyl acetate extracts showed maximum activity, and plants grown in the presence of the ethyl acetate extract showed severe root browning. The allelopathic constituents of the ethyl acetate phase were isolated and identified as gallic acid and (+)-catechin by nuclear magnetic resonance spectroscopy. Gallic acid and (+)-catechin were present in the upper part of buckwheat at concentrations of 0.02 and 0.01%, of fresh weight, respectively. Gallic acid was found to be selectively and strongly inhibitory to root and shoot growth of tested plants at 100 and 10 μg ml−1. (+)-Catechin, however, inhibited plant growth to a lesser extent. These results suggest that buckwheat may have allelopathic potential and that when used as a ground cover crop or green manure may produce inhibitors, which could suppress weeds.

The effects of three postemergence herbicides that inhibit carotenoid biosynthesis were tested on field dodder. Flurochloridone, sulcotrione, and mesotrione treatments led to bleaching symptoms in field dodder stems. The effect of flurochloridone was rapid; 2 d after treatment (DAT) the stem was bleached and contained only 2% β-carotene, with a massive accumulation of phytoene in comparison with the control. However, flurochloridone treatment did not inhibit stem elongation, and full recovery of pigment composition at newly elongated stems was recorded 6 DAT. The effects of sulcotrione and mesotrione were similar, but the recovery was slower than with flurochloridone. The developing stems were fully bleached 6 DAT, with no detectable β-carotene, and subsequently no recovery was observed. All three herbicides led to mass destruction of the plastids in the parenchyma cells of the cortex and pith tissue. This disruption of the plastids was associated with depletion in starch content. Sulcotrione and mesotrione treatments reduced field dodder biomass accumulation, whereas flurochloridone delayed biomass accumulation by 6 d only. The differences in phytotoxicity between flurochloridone, sulcotrione, and mesotrione might be due to their different translocation patterns. Flurochloridone is xylem-mobile and is therefore localized at the applied area because of the low transpiration rate of field dodder. Sulcotrione and mesotrione are phloem-mobile and tend to accumulate in sinks such as the primordial tissue. The massive destruction of the amyloplasts and the rapid decline in starch content in response to all the herbicide treatments may indicate the possible involvement of carotenoids in stabilizing the amyloplast outer membrane in field dodder.

Greenhouse and laboratory experiments were conducted to determine the effect of CGA 362622 on the herbicidal activity of clethodim on goosegrass. CGA 362622 did not affect absorption and translocation of 14C-clethodim by goosegrass. Averaged across the two treatments of clethodim alone and clethodim plus CGA 362622, absorption was 27 and 85% of the applied 14C-clethodim at 0.5 and 96 h, respectively. By 96 HAT, only 0.8% of applied 14C had translocated to the shoot below the treated leaf. Metabolism of clethodim was not affected by the presence of CGA 362622. Three metabolites of clethodim were detected in treated tissue at all harvest intervals. By 96 HAT, 56% of absorbed 14C converted to a relatively polar form when clethodim was applied alone or in the presence of CGA 362622. One day after treatment, the photosynthetic rate in plants treated with CGA 362622 had decreased below the rate in the nontreated check and remained lower until 6 d after treatment. These data suggest that the antagonism of clethodim by CGA 362622 may result from CGA 362622 altering the photosynthetic rate of goosegrass and therefore the sensitivity of acetyl-coenzyme A carboxylase to clethodim.

Glyphosate-resistant rigid ryegrass has been identified in California, but research has yet to elucidate the resistance mechanism. The objectives of this study were to examine the differences between sensitive and resistant rigid ryegrass in absorption and distribution of glyphosate, in vivo and in vitro absorption by chloroplasts, and shikimic acid accumulation after glyphosate treatment. Foliar absorption and distribution of 14C-glyphosate did not differ 1 to 3 d after treatment (DAT) between the susceptible (S) and resistant (R) biotypes. Absorption of 14C-glyphosate by isolated chloroplasts also did not differ between the S and R biotypes. After foliar application of 14C-glyphosate, chloroplasts were isolated from treated leaves from both biotypes. Accumulation of 14C-glyphosate in the chloroplasts did not differ between the two biotypes. Shikimic acid level increased significantly in the S biotype after treatment with glyphosate at 2.24 kg ai ha−1 to levels 10-fold greater than in the R biotype 11 DAT. Shikimic acid in the germination media at 2 to 5 mM did not affect seed germination of S and R biotypes but drastically decreased the length of coleoptiles of both at 5 DAT. Thus, biotype differences in sensitivity or metabolism of shikimic acid do not explain differences in sensitivity to glyphosate.

Laboratory and greenhouse studies were conducted to determine the effect of temperature, solution pH, water stress, and planting depth on broadleaf signalgrass germination. Broadleaf signalgrass seed required removal of the husk for germination. When treated with constant temperature, broadleaf signalgrass germinated over a range of 20 to 35 C, with optimum germination occurring at 30 and 35 C. Onset, rate, and total germination (87%) was greatest in an alternating 20/30 C temperature regime. Germination decreased as solution pH increased, with greatest germination occurring at pH values of 4 and 5. Germination decreased with increasing water potential, and no germination occurred below −0.8 mPa. Emergence was above 42% when seed were placed on the soil surface or buried 0.5 cm deep. Germination decreased with burial depth, but 10% of broadleaf signalgrass seed emerged from 6.0-cm depth. No seed emerged from 10-cm depth. These data suggest that broadleaf signalgrass may emerge later in the season, after rains, and could germinate rapidly and in high numbers. These attributes could contribute to poor control later in the season by soil-applied herbicides or allow broadleaf signalgrass to emerge after final postemergence treatments were made.

The seasonal emergence characteristics and seedbank ecology of annual bluegrass were evaluated in a vegetable field in the central coast of California. The emergence and germinability of annual bluegrass were monitored continuously for over 3 yr to detect seasonal variation in weed emergence. Weed emergence and seedbank densities were measured every 45 d for 41 mo. Weed emergence was monitored simultaneously in the field and in a growth chamber that was adjusted seasonally for day length and temperature. Samples were incubated in the growth chamber for 45 d and then elutriated to measure remaining viable ungerminated caryopses and relative germination potential. Seedbank densities ranged from 2,000 to 20,000 caryopses m−2 during the study period. Weed emergence and germinability were highest from October to November and lowest from March to July. Emergence from soils collected in the spring and fall and incubated under both spring and fall conditions in the growth chamber indicated that seed dormancy state determines germinability rather than environmental conditions during incubation. Soil samples collected in the spring had no emergence (<1%) when incubated under spring or fall conditions, whereas samples collected in the fall had high emergence (>95%) under both spring and fall conditions. A waveform regression fit the repeated emergence pattern with a cycle length of 364 d (365 d for the growth chamber). This model predicted that weed emergence would peak on November 5 and be lowest on June 20. A survey of organic vegetable fields found that caryopses were more common in soils with higher clay content than in lighter soils (R2 = 0.77, P = 0.02). Our data suggest that annual bluegrass is more likely to emerge when the soil is wet during winter. The timing and location of annual bluegrass emergence may increase the probability of reproductive success during periods when seasonal rains and wet conditions limit tillage, weeding activities, and competition with other weeds.

Interspecific hybridization of Palmer amaranth and common waterhemp produce hybrids with unique DNA fragments not found in either parent. The objective of this research was to investigate the mechanisms involved in the formation of the polymorphic fragments. Six novel fragments were cloned and sequenced. Five of the six were significantly similar to plant transposons, the sixth was similar to squamosa promoter–binding proteins from other plant species. Southern blot analysis using one of the novel fragments as probe revealed a consistent pattern of repetitive DNA that was species and biotype specific. These results indicate that transposon-like elements may play an important role in the formation of new fragments in Amaranthus hybrids derived from interspecific hybridization, suggesting that considerable instability of the hybrid genome may occur.

Crenate broomrape is a major constraint for legume production in Mediterranean and East Asian countries. Resistance to this parasitic weed is scarce in many legumes but is common in chickpea germ plasm. A detailed in vitro study has shown that resistance in chickpea is the result of the combination of at least two mechanisms. First, and most importantly, the two chickpea lines studied have been identified with low rates of stimulant production. Once germination is induced by exogenous applications of the synthetic germination stimulant GR24, thus overcoming the primary resistance mechanism in these lines, a second resistance mechanism is apparent. This is evidenced by a darkening of host cell tissue in contact with the broomrape radicle, leading to failure of establishment, which was frequently observed in the chickpea accessions. Anatomical studies have shown that this apparently “hypersensitive” response does not correspond with the death of host cells in contact with the parasite cells but corresponds to blocking and death of the penetration structures of the parasite.

Blackgrass is a common winter annual grass weed in autumn-sown crop rotations in Atlantic European countries. Control with a minimum amount of herbicides in integrated cropping systems would be facilitated by modeling the effect of cropping systems on its demography. To develop the submodel relating weed seed bank to emerged seedlings, laboratory experiments were conducted to analyze and quantify seed germination and shoot elongation. These processes were studied as a function of environmental conditions during seed production (nitrogen availability, water deficit, plant density, and crop) and of seed characteristics (seed weight, harvest date, storage length, and dry-stored/soil-buried). Nonlinear equations relating germination and shoot elongation to time calculated as cumulated degree-days were fitted to the observed germination and shoot elongation data. These were used to estimate parameters for germination proportion and rate, as well as final shoot length and elongation rate. Recently harvested seeds germinated best and fastest when they were collected in spring crops compared with winter crops. Germination proportion and rate increased with seed storage length, especially for seeds collected in winter crops. Midgermination time decreased with seed weight and water deficit during seed production; it increased with nitrogen amounts available to the mother plants. Maximum shoot length increased with seed weight and plant density during seed production. It decreased with nitrogen availability to mother plants and with storage length, irrespective of whether the seeds were dry-stored or buried in soil. Elongation rate was highest for early- and fast-germinating seeds. Time to midelongation increased with maximum shoot length. These germination and preemergence growth models can now be combined with other submodels to develop a blackgrass emergence model.

A mathematical model was developed to predict common lambsquarters seedling emergence in southwestern Quebec. The model was based on the thermal-time concept, using air temperatures in the double-sine calculation method. The model was built using data from five experiment-years for corn naturally infested with weed populations. Once developed, the model was calibrated using different crop seedbed preparation times. The base temperature was then adjusted for each time of seedbed preparation. A power regression function was used to relate adjusted base temperatures and the accumulated thermal units at seedbed preparation time. A modified Weibull function was then fitted to the field emergence data, expressed as the cumulative proportion of the total seedling emergence over the growing season as a function of cumulative thermal units. The simplicity and accuracy of this model would make it an excellent tool to predict common lambsquarters seedling emergence in field situations, facilitating the determination of the timing of scouting in integrated weed management systems.

Weed seedling emergence is partially dependent on biotic and abiotic conditions directly surrounding the seed. When environmental conditions are appropriate, seed germination and emergence occurs. We studied the impact of seeding depth (surface, 1 to 2, 3 to 4, and 6 to 7 cm) and fluctuating soil moisture regimes (field capacity [FC]–1/3FC–FC; FC–1/6FC–FC) on percent weed emergence in a greenhouse. At FC, wild mustard and field pennycress had the greatest percent emergence when seeds were placed on or near the soil surface, whereas percent emergence of barnyardgrass and round-leaved mallow was unaffected by seeding depth. All the perennials tested had the greatest percent emergence at FC when seeds were placed near or on the soil surface, except for common milkweed which only emerged below the soil surface. When soil moisture levels fluctuated, surface seeds of barnyardgrass, catchweed bedstraw, green foxtail, wheat, and wild oat had less emergence than seeds below the soil surface; field pennycress had increased emergence when the seeds were placed on the surface; and round-leaved mallow and wild mustard emergence was unaffected by seeding depth. The emergence of curly dock, dandelion, and perennial sowthistle was unaffected by seeding depth, whereas foxtail barley and quackgrass emergence was reduced when seeds were placed on the surface and soil moisture fluctuated.

The identities of two novel perennial nutsedge biotypes collected near Bakersfield, CA, were assessed using isozyme and random-amplified polymorphic deoxyribonucleic acid markers in conjunction with morphological analysis. The two biotypes, designated as CK (Cyperus rotundus cv. ‘Kempeni’) and CR (Cyperus esculentus cv. ‘Robusta’), morphologically resemble purple nutsedge and yellow nutsedge, respectively. Plants from both biotypes exhibited more prolific growth than the typical forms and possessed some traits that are not characteristic of the species they resemble. The morphological study was conducted on a total of 15 purple nutsedge, yellow nutsedge, CK, and CR populations collected in the first year and on 20 additional nutsedge populations were collected in the second year. The genetic analysis was performed on populations from the first year only. In general, there was agreement among the results obtained from the morphometric, isozymatic, and deoxyribonucleic acid studies. Populations of CR clustered with yellow nutsedge populations, indicating that CR is within the normal range of variation of this species and may represent a new introduction. Populations of CK, however, were distinct from both purple nutsedge and yellow nutsedge populations. Considering the low level of genetic variation reported in purple nutsedge and its strict vegetative mode of reproduction, CK might represent a sexually reproducing ecotype of purple nutsedge or a hybrid with yellow nutsedge.

Late watergrass is a serious weed of California rice that has evolved resistance to molinate, thiobencarb, fenoxaprop-ethyl, and bispyribac-sodium. To obtain an insight into the origin and spread of resistant (R) late watergrass in California rice fields, we evaluated similarities in morphological traits and amplified fragment length polymorphism (AFLP) fingerprints among 15 R strains compared with susceptible (S) strains. All strains were derived by inbreeding from accessions collected in rice fields of the Sacramento Valley, CA. In the field, R plants were shorter than S plants; they also had narrower and shorter flag leaves and thinner culms. Spikelets also appeared smaller and more slender in R plants. There was greater morphological similarity among the 15 R strains than among the eight S strains. The mean coefficients of variation for morphological traits were much smaller among R strains, which in a cluster analysis (Ward's method) were grouped morphologically apart at early clustering stages from the more variable S strains. AFLP electropherograms also showed greater similarity between R strains. R strains were grouped separately from the S strains in a cluster analysis based on calculated Nei and Li coefficients used in an unweighted pair group method using arithmetic means. However, small genetic differences also existed because the R strains were grouped into six clusters, suggesting that R strains were not samples from an identical strain. It was concluded that R strains originated from a preexisting and preadapted mutant late watergrass population in the Sacramento Valley. This study establishes that resistance moved by spikelet dispersal, not independent mutation events, most likely defined the geographical distribution of R late watergrass in California. Prevention and control of this dispersal combined with elimination of seed-producing survivors after herbicide treatment should be relevant components of the integrated management of herbicide-resistant late watergrass in California rice.

Yellow starthistle is an invasive plant species common in the semiarid climate of central Idaho and other western states. Early detection of yellow starthistle and estimation of its infestation potential in semiarid grasslands have important scientific and managerial implications. Weed detection and delineation of infestations are often carried out by using ground survey techniques. However, such methods can be inefficient and expensive in detecting sparse infestations. The distribution of yellow starthistle over a large region may be affected by various landscape variables such as elevation, slope, and aspect. These exogenous variables may be used to develop prediction models to estimate the potential for yellow starthistle invasion into new areas. A nonlinear prediction model has been developed using a polar coordinate transformation of landscape characteristics to predict the likelihood of yellow starthistle occurrence in north-central Idaho. The study region included the lower Snake River and parts of the Salmon and Clearwater basins encompassing various land-use (range, pasture, and forest) categories. The model provided accurate estimates of yellow starthistle incidence within each specified land-use category and performed well in subsequent statistical validations. This prediction model can assist land managers in focusing their efforts by identifying specific areas for survey.

The effects of temperature and light on the dormancy of velvetleaf, common waterhemp, and giant foxtail seeds were studied under controlled growth chamber conditions. Seeds were either kept chilled at 4 C for 12 wk under wet conditions or nonchilled at 4 C in dry storage. Then, seeds were germinated under increasing and decreasing temperatures and under continuous red light (R) and far-red light (FR). In addition, chilled and nonchilled seeds were germinated in the dark after being exposed to alternating R and FR flashes. Velvetleaf germination was increased by exposure to high temperatures (36 C) immediately after exposure to low temperatures (4 C), but light had no effect. Chilling increased common waterhemp seed germination and sensitivity to light and temperature. R promoted common waterhemp seed germination, whereas FR inhibited germination and maintained dormancy. In addition, the effect of light was reversible. Therefore, common waterhemp dormancy was phytochrome regulated. However, high temperatures (36 C) promoted the germination of chilled seeds, even when exposed to FR. The germination of chilled giant foxtail seeds was reduced by FR. Giant foxtail seed dormancy was partially phytochrome regulated, but dormancy regulation was more dependent on mean temperature.

The genus Orobanche includes chlorophyll-lacking root parasites that parasitize many dicotyledonous species and cause severe damage to vegetable and field crops worldwide. In the United States, small broomrape is a federally listed noxious weed that has been found in red clover seed production fields in Oregon. In this study, we classified the susceptibility of red clover, white clover, and crimson clover to small broomrape infection under controlled conditions. Red, white, and crimson clover seeds were sown in pots containing small broomrape–inoculated soil. Differences in susceptibility to infection were observed among clover species; red clover was severely infected, white clover was slightly infected, and crimson clover was not infected.

The diversity of resistance among wild oat collected before and after commercial introduction of imazamethabenz, difenzoquat, diclofop, fenoxaprop-P, sethoxydim, and tralkoxydim was evaluated. Wild oat sampled in 1964 and 2000 from the Red River Valley of Minnesota and North Dakota were screened for resistance. Nearly 43% of the 1964 collections were susceptible (S) to all six herbicides, whereas only 9% were S by 2000. The frequency of resistance in 2000 compared with 1964 increased for all six herbicides, and 27 phenotype response groups to the six herbicides occurred in 2000 vs. 14 phenotype response groups for the 1964 collection. The proportion of resistant (R) plants increased faster for the aryloxyphenoxypropionate (APP) herbicides, diclofop and fenoxaprop-P, than for the cyclohexanedione (CHD) herbicides, sethoxydim and tralkoxydim. High diversity of resistance responses was observed in wild oat to acetyl-coenzyme A carboxylase–inhibitor herbicides, suggesting that there may be multiple APP herbicide– or CHD herbicide–specific resistance mechanisms in addition to those that confer cross-resistance. The trend of resistance response generally indicates that increased exposure to herbicides in wild oat may confer resistance to newly introduced but unrelated herbicides.

Weeds are a major constraint to crop production in smallholder farms in tropical Africa. The weed seedbank and annual recruitment are the main sources of weed infestation in crops. This study was carried out in Ibadan, Nigeria, to evaluate the effect on the seedbank of two types of planted fallow (alley cropping with leucaena and live mulch with tropical kudzu) and a natural bush fallow under four land-use intensities. Type of fallow was the main plot. Land-use intensities, consisting of continuous cropping of maize intercropped with cassava, 1 cropping yr of maize–cassava followed by 1, 2, and 3 yr of fallow, were the subplots. Averaged over a 3-yr period, the seedbank was 55% lower in the tropical kudzu plots and 43% lower in the leucaena plots compared with natural bush. The difference in seedbanks between plots cultivated after leucaena and natural fallow was 23%. Seed density of annual broadleaf weeds was high and dominated the seedbank of both planted and natural fallow. Overall, seeds of grasses occurred at low densities in all plots cultivated after 2 to 3 yr of fallow, whereas seeds of sedges occurred more in continuously cultivated plots and plots cultivated after 1 yr of fallow in all the fallow types. Seed density of perennial broadleaf weeds increased as land-use intensity decreased. Live mulch with tropical kudzu, especially when combined with 2 to 3 yr of fallow, lowered the seedbank more than the leucaena and traditional bush fallow systems.

Field research was conducted for 2 yr to determine the effect of reduced rates of glufosinate on growth and yield of non–glufosinate-resistant cotton. Rates of 3.4, 6.7, 13, 26.5, 52.5, and 105 g ha−1, representing 0.008, 0.016, 0.031, 0.063, 0.125, and 0.25 of an effective use rate (420 g ha−1), were applied to cotton at the two-, five-, or nine-node growth stage. Based on analysis of visual injury, cotton response decreased as application timing was delayed in one of the three experiments. Injury response was increased slightly with application at the five- compared with the two-node growth stage and was not significant for the latest application timing (nine-node stage) in two of three experiments. In two of the three experiments, plant height reduction response was lowest at the five-node stage and greatest at the nine-node stage. Regardless of application timing, plant dry weight was negatively affected only with the highest rate of glufosinate. Glufosinate application, based on node above white flower number and percent open boll, did not result in a delay in maturity. Final plant population was reduced in all experiments at the two-node application and in one of the three experiments at the five-node stage. Glufosinate application did not adversely affect final plant population when applied to nine-node cotton. Negative effects on cotton growth were not manifested in seedcotton yield reduction after glufosinate application.