In order to clarify the translocation and metabolism of glyphosate in lead tree, a mixture of glyphosate (0.5 mmol) and 14C-glyphosate was injected into mini tree plants (1.5-cm trunk diameter and 120 cm tall), simulating a grown lead tree in a greenhouse experiment. Within 6 d after treatment (DAT), all leaves on the lead tree plants dropped. Analysis of nonlabeled glyphosate in both xylem and phloem every 15 d showed that glyphosate residues accumulated mainly in phloem, similar to the findings in established lead tree. In this experiment, glyphosate concentration in the phloem of three parts of the trunk, i.e., upper (15 cm above the upper injection hole), middle (between the two injection holes), and lower (15 cm below the lower injection hole) parts, increased rapidly within 45 DAT, and decreased thereafter. Obviously, significant glyphosate metabolism occurred during the latter period. Further 14C radioactivity measurement also exhibited similar time-course changes of glyphosate dissipation, except that a low level of radioactivity in the phloem of the upper part and a high level of radioactivity in the lower part were detected from 45 to 90 DAT, suggesting that more glyphosate metabolites in the phloem of the upper part might have translocated with photosynthetic assimilates and reallocated to the lower part of the mini tree. Thin-layer chromatography (TLC) analysis of metabolites derived from 14C-glyphosate also revealed that about 70% of the radioactivity recovered in the phloem of the lower part comprised the unknown metabolites M1 (retention factor [Rf] : 0.83) and M2 (Rf : 0.94), with nearly 28% of the radioactivity from sarcosine (Rf : 0.76), and less than 2% of the radioactivity from aminomethylphosphoric acid (AMPA) (Rf : 0.49) and glyphosate (Rf : 0.36). It is concluded that active metabolism of glyphosate in this 90-d simulation experiment occurred mainly from 45 to 90 DAT, and the primary metabolism was modulated by C-P lyase.

Junglerice is one of the most serious grass weeds of rice in the tropics. Experiments were conducted in the laboratory and screenhouse to determine the influence of environmental factors on seed germination and seedling emergence of junglerice in the Philippines. In the laboratory, germination was stimulated by light, suggesting that seeds of this species are positively photoblastic. The tested temperatures (35/25, 30/20, and 25/15 C alternating day/night temperatures), however, did not influence germination. Germination in the laboratory was not affected by a soil pH range of 4 to 9, but was decreased by salinity (> 50 mM NaCl) and moisture stress (< −0.2 MPa osmotic potential). In the screenhouse, germination of junglerice was greatest (97%) for seeds at the soil surface, but emergence declined exponentially with increasing seed burial depth, and no seedlings emerged from seeds buried at 6 cm. In pots, seedling emergence declined markedly with the addition of rice residue to the soil surface at rates equivalent to 4 to 6 tonnes (t) ha−1. As germination of junglerice was strongly stimulated by light, and seedling emergence was optimal at shallow burial depths, this species is likely to be problematic in reduced tillage systems.

The ecological significance of species diversity within agroecosystems has become a point of interest in recent years. Although the time and spatial scale at which diversity is measured may influence the interpretation of its functional importance, little research has been conducted on methodological approaches to assess the number and relative abundance of agricultural plant species. In this study, we (1) evaluated the applicability of the species-area curve to examine plant species richness and α and β diversity in conventional no-tillage and organically managed spring wheat systems, and (2) assessed temporal changes in plant species richness across systems. Measurements were obtained at three times during two growing seasons in experimental plots and at three times during one growing season on commercial farms in Montana. In accordance with previous studies, management system affected species richness and diversity. In eight of nine studied cases, we detected a positive relationship between species richness and sampled area. In these eight cases, intercepts (α diversity) were higher for the organic systems than for the conventional no-tillage systems. Slopes (β diversity) were higher for the organic system in six of nine cases studied. Species richness declined as the season progressed for both systems, with the organic systems consistently having more species than the conventional no-tillage systems. Despite differences in the species composition and between the experimental plots and commercial farm field size, the patterns of species richness and α and β diversity were relatively constant, suggesting that the processes responsible for the assembly of plant communities in agroecosystems of the Northern Great Plains are similar at a range of spatial scales.

The temporal dynamics of spatial heterogeneity was studied for the weed communities in a seashore paspalum turf with the use of a power-law model. Surveys were conducted in January, March, May, July, September, and November in 2007. In every survey, we set 100 quadrats (50 by 50 cm) referred to as L quadrats on a 50-m line transect at the same position in the turf. Each L quadrat was then divided into four S quadrats (25 by 25 cm) and all plant species occurring in each of these S quadrats were identified and recorded. These data were summarized into frequency distributions and the percentage of S quadrats containing a given species, and the variance of each species was estimated. The power law was used to evaluate the spatial heterogeneity (δ) and frequency of occurrence (p) for each species in the weed communities in six survey months. The results showed that weeds emerged more frequently in the summer–spring season than in winter–autumn, and the spatial heterogeneity was much higher in summer–spring than winter–autumn, especially in summer. The Shannon–Wiener diversity indexes (H') from large to small were July (5.9202) > May (5.6775) > September (5.6631) > March (5.5727) > January (5.1742) > November (4.9668). Likewise, the spatial heterogeneity index (δc) of the whole community was also different in different months. The biggest δc (0.2790) was in July, and the smallest (0.1811) in November. Meanwhile, manilagrass had a high p (= 1.0), indicating that it occurred in all S quadrats in every weed community of every month. However, the turfgrass, seashore paspalum, only emerged in March, May, July, and November, and possessed a low p, indicating the seashore paspalum turf has been naturally replaced by manilagrass.

To assess the influence of long-term fertilization on weed communities of early and late rice crops, the weed species composition was investigated in experimental plots initiated in 1981 at the Key Field Experimental Monitoring Station of the Reddish Paddy Soil Eco-Environment in Wangcheng, China. The treatments were (1) a control (CK), no fertilizer; (2) N–P, no K; (3) N–K, no P; (4) P–K, no N; (5) N–P–K; (6) N–P–K + Ca, N, P, and K plus lime; (7) N–P + S, N and P plus additional rice straw return; (8) N–P–K + S, N, P, and K plus additional rice straw; (9) N–K + M, N and K plus swine manure. The results indicated that weed flora composition and density were influenced by the different fertilization treatments. Multivariate analyses indicated that changes in the weed community composition were primarily due to soil-available N, followed by light intensity on the field surface, and soil-available P. More weed species and total weed density were observed in the control and P–K plots than in plots in which N, P, and K were applied together. Omission of N application had a greater effect on the weed community than the omission of P or K applications. Nutrients derived from synthetic fertilizers and organic manure or the additional application of lime had no obvious effect on the weed community of late rice crops.

The compositions of the germinable weed seedbank and aboveground weed communities in a long-term tillage and rotation study were characterized 4, 5, and 6 yr (2002 to 2004) after the adoption of glyphosate-tolerant corn and soybean. Averaged across rotation, mean germinable weed seed density and diversity were greatest in the no-tillage treatment as compared to the minimum- and conventional-tillage treatments. Averaged over tillage, density and diversity were greater in the corn–oat–hay (ryegrass + alfalfa) system as compared to the continuous corn and corn–soybean rotations. Similar trends in density and diversity were observed for the aboveground weed communities. Differences in community composition among treatments were quantified with the use of a multiresponse permutation procedure. Results indicated that the weed seedbank community in a corn–oat–hay rotational system differed from the communities associated with the continuous corn and corn–soybean rotational systems. Weed seedbank communities developing under a no-tillage operation differed from those in minimum- and conventional-tillage scenarios. Compositional differences among the aboveground weed communities were less pronounced in response to tillage and rotation. Indicator species analyses indicated that the number of significant indicator weed species was generally higher for no tillage than minimum or conventional tillage for both the seedbank and the aboveground weed communities. The number of significant indicator species for the seedbank and weed communities was generally greater in the three-crop rotation as compared to the continuous corn and corn–soybean rotations. The trends observed in density, diversity, and community composition after the adoption of glyphosate-tolerant corn and soybeans, and a glyphosate-dominated weed management program, were also observed when soil-applied herbicides were included in the study. We suggest that the switch to a POST-glyphosate protocol did not significantly alter weed communities in the short term in this study.

Field experiments were conducted to determine whether exposure to reduced red : far-red light ratios (R : FR) typical of crop–weed environments was associated with adaptive changes in morphology, productivity, and fecundity of common lambsquarters. Plants were grown in reduced or ambient R : FR environments (both in full sunlight) until initiation of flowering, after which plants were grown in full sunlight or partial shade. At initiation of flowering, plants that had been exposed to reduced R : FR exhibited greater specific leaf area, stem elongation, main stem leaf area, specific stem length, and main stem mass compared with plants exposed to ambient R : FR. However, biomass allocation to stems, leaves, and roots did not differ between vegetative-stage R : FR treatments. At the end of flowering, morphology and productivity of plants exposed to partial shade did not differ between vegetative-stage R : FR treatments. In contrast, plants exposed to full sunlight during flowering after exposure to reduced R : FR during the vegetative stage had less total plant mass, less total leaf area, greater stem elongation, greater specific stem length, and a greater ratio of main stem to total stem mass compared with plants exposed to ambient R : FR during the vegetative stage. At physiological maturity, plants exposed to reduced R : FR during the vegetative stage and to partial shade during the reproductive stage had less total seed mass and fewer seeds compared with plants exposed to ambient R : FR during the vegetative stage and to partial shade during the reproductive stage. Fecundity of plants exposed to full sunlight during the reproductive stage did not differ between vegetative-stage R : FR treatments. These results indicate that exposure of common lambsquarters to reduced R : FR during the vegetative stage was maladaptive at later stages of growth in competitive environments, and suggest that interactions of light quality and quantity are important determinants of common lambsquarters fecundity.

Glyphosate-resistant (GR) crops have been rapidly adopted in the United States and the evolution of GR weeds throughout the world has also been on the rise. With experience, weed scientists and crop advisers develop “intuition” on the basis of field history and current in-field conditions for predicting whether escaped weed biotypes may be herbicide resistant. However, there are no previous reports on the association of in-field crop management factors with the prediction of herbicide resistance. By using in-field survey data, we tested the accuracy of predicting glyphosate resistance in late-season horseweed escapes. We hypothesized that glyphosate resistance in late-season horseweed populations found in soybean fields could be predicted using in-field knowledge of crop residues and the appearance and distribution of weeds in the field. Field survey data were collected to determine the distribution and frequency of GR horseweed populations in Indiana soybean fields during September and October of 2003, 2004, and 2005. After the in-field survey, soil properties for sampled field locations were also collected from the U.S. Department of Agriculture Natural Resources Conservation Service Web Soil Survey. GR horseweed predictions used in-field presence of crop residues and the appearance, abundance, and distribution of weeds in the field. The significance of independent data factors were determined by chi-square statistics. The interactions and relative significance of multiple factors were modeled using classification and regression tree analysis. Our results indicated that the most important factor for predicting GR populations was the identification of an altered plant phenotype after injury from POST glyphosate. This was followed by crop rotation, field distribution, and the presence of other escaped weed species in the field in a model with a classification rate of 0.68.

Demand for organic food products has consistently increased for more than 20 yr. The largest obstacle to organic soybean production in the southeastern United States is weed management. Current organic soybean production relies on mechanical weed control, including multiple postplant rotary hoe uses. Although postplant rotary hoe use is effective at the weed germination stage, its efficacy is severely compromised by delays due to weather. Preplant rotary hoeing is also a practice that has been utilized for weed control but the effectiveness of this practice to reduce the need for multiple postplant rotary hoeing for organic soybean production in the southeastern United States has not been investigated. Preplant rotary hoe treatments included a weekly rotary hoeing 4 wk before planting, 2 wk before planting, and none. Postplant rotary hoe treatments consisted of zero, one, two, three, and four postplant rotary hoe uses. Weed control was increased with preplant rotary hoeing at Plymouth in 2006 and 2007 but this effect disappeared with the first postplant rotary hoeing. Multiple postplant rotary hoe uses decreased soybean plant populations, decreased soybean canopy height, lowered soybean pod position, and decreased soybean yield. Plant mapping revealed that the percentage of total nodes and pods below 30 cm was increased by increased frequency of postplant rotary hoe use.

Univariate analyses fail to account for covariance among phenomorphological traits implicated in crop competitive ability. A more complete analysis of cultivar–weed interactions would reduce a number of important traits to a few underlying principal factors responsible for sweet corn competitiveness. Twenty-three commercial sweet corn hybrids from nine seed companies were grown in the presence and absence of wild-proso millet to (1) quantify the extent to which phenomorphological traits vary in sweet corn, (2) identify underlying principal factors that describe variation in crop canopy development, and (3) determine functional relationships between crop canopy factors and competitive ability. A principal component factor analysis revealed that 7 of the 18 weed-free crop traits measured at silking loaded highly (0.65 to 0.90) into the first factor, including plant height, shoot biomass, per plant leaf area, leaf area index, and intercepted light, as well as thermal time from emergence to silking and emergence to maturity. All seven traits were highly correlated (0.38 to 0.93) and were interpreted as a “late canopy and maturity” factor. Another five traits formed two additional principal factors that were interpreted as an early “seedling quality” factor (e.g., kernel mass, seedling vigor, and height at two-leaf stage) and a mid-season “canopy closure” factor (e.g., leaf area index and intercepted photosynthetically active radiation at six-leaf stage). Relationships between principal factors and competitive abilities were quantified using least-squares linear regression. Cultivars with greater loadings in the late canopy and maturity and canopy closure factors were more competitive with wild-proso millet. In contrast, crop competitive ability declined with cultivars that loaded highly into the seedling quality factor. The analyses showed that sweet corn's ability to endure weed interference and suppress weed fitness relates uniquely to three underlying principal factors that capture crop canopy development around emergence and near canopy closure and during the reproductive phase.

Restoration of historic fire regimes is complicated by concerns about exotic plant invasions, yet little is known of how the two may interact. Seeds of Japanese brome, spotted knapweed, Russian knapweed, and leafy spurge were subjected to fire at six fuel loads (100 to 700 g m−2) and a nonburned control. Fires were simulated with field-cured grass and time–temperature profiles were developed from thermocouples at the soil surface. Emergence was determined by species and fuel load in growth chambers. Fuel load explained 98% of variation in mean heat dosage and emergence decreased with increasing fuel load across species. Emergence was reduced 79 to 88% relative to nonburned treatment with 100 g m−2 of fuel and at least 97% with 200 g m−2 of fuel. Emergence probabilities were less than 0.01 for all species but spotted knapweed with a 300 g m−2 fuel load. Results indicate high potential for fire to disrupt the life cycle of invasive species through direct seed mortality. The relationship between fuel load and seedling emergence provides good predictability of fire effects on surface-deposited seeds. A single fire is unlikely to eradicate many invasive species because they often produce abundant seeds and some will undoubtedly escape fire. However, abrupt reductions in seedling emergence with relatively light fuel loads indicate that fire may be an effective tool for increasing mortality of invasive plant seed across a broad range of habitats.

Strategic fertilizer management is an important component of integrated weed management systems. A field study was conducted to determine the effect of various application methods of phosphorus (P) fertilizer on weed growth and wheat yield. Weed species were chosen to represent species that varied in their growth responsiveness to P: redroot pigweed (medium), wild mustard (medium), wild oat (medium), green foxtail (high), redstem filaree (high), and round-leaved mallow (high). P fertilizer application methods were seed placed at a 5-cm depth, midrow banded at a 10-cm depth, surface broadcast immediately before seeding, and surface broadcast immediately after seeding of wheat. An unfertilized control was included. P treatments were applied to the same plot in four consecutive years to determine annual and cumulative effects over years. Shoot P concentration and biomass of weeds were often lower with seed-placed or subsurface-banded P fertilizer compared with either surface-broadcast application method. This result occurred more frequently with the highly P-responsive weeds and was more evident in the latter study years. P application method had little effect on weed-free wheat yield but often had a large effect on weed-infested wheat yield. Seed-placed or midrow-banded P compared with surface-broadcast P fertilizer often resulted in higher yields when wheat was in the presence of competitive weeds. Seedbank determinations at the conclusion of the study indicated that the seed density of five of six weed species was reduced with seed-placed or subsurface-banded P compared with surface-broadcast P. Information gained in this study will aid development of more effective weed management systems in wheat.

Sweet corn is planted over a long season to temporally extend the perishable supply of ears for fresh and processing markets. Most growers' fields have weeds persisting to harvest (hereafter called residual weeds), and evidence suggests the crop's ability to endure competitive stress from residual weeds (i.e., crop tolerance) is not constant over the planting season. Field studies were conducted to characterize changes in the residual weed community over the long planting season and determine the extent to which planting date influences crop tolerance to weed interference in growth and yield traits. Total weed density at harvest was similar across five planting dates from mid-April to early-July; however, some changes in composition of species common to the midwestern United States were observed. Production of viable weed seed within the relatively short growth period of individual sweet corn plantings showed weed seedbank additions are influenced by species and planting date. Crop tolerances in growth and yield were variable in the mid-April and both May plantings, and the crop was least affected by weed interference in the mid-June and early-July planting dates. As the planting season progressed from late-May to early-July, sweet corn accounted for a great proportion of the total crop–weed biomass. Based on results from Illinois, a risk management perspective to weeds should recognize the significance of planting date on sweet corn competitive ability. This work suggests risk of yield loss from weed control failure is lower in late-season sweet corn plantings (June and July) than earlier plantings (April and May).

Wild oat causes more crop yield losses and accounts for more herbicide expenditures than any other weed species on the Canadian Prairies. A study was conducted from 2001 to 2005 at four Canadian Prairie locations to determine the influence of repeated cultural and herbicidal management practices on wild oat population density, biomass, and seed production, and on barley biomass and seed yield. Short or tall cultivars of barley were combined with normal or double barley seeding rates in continuous barley or a barley–canola–barley–field-pea rotation under three herbicide rate regimes. The same herbicide rate regime was applied to the same plots in all crops each year. In barley, cultivar type and seeding rate were also repeated on the same plots year after year. Optimal cultural practices (tall cultivars, double seeding rates, and crop rotation) reduced wild oat emergence, biomass, and seed production, and increased barley biomass and seed yield, especially at low herbicide rates. Wild oat seed production at the quarter herbicide rate was reduced by 91, 95, and 97% in 2001, 2003, and 2005, respectively, when tall barley cultivars at double seeding rates were rotated with canola and field pea (high management) compared to short barley cultivars at normal seeding rates continuously planted to barley (low management). Combinations of favorable cultural practices interacted synergistically to reduce wild oat emergence, biomass and seed production, and to increase barley yield. For example, at the quarter herbicide rate, wild oat biomass was reduced 2- to 3-, 6- to 7-, or 19-fold when optimal single, double, or triple treatments were combined, respectively. Barley yield reductions in the low-management scenario were somewhat compensated for by full herbicide rates. However, high management at low herbicide rates often produced more barley than low management in higher herbicide rate regimes.

POST weed harrowing and other cultivation methods to control weeds in early crop growth stages may result in crop damage due to low selectivity between crop and weeds. Crop tolerance to cultivation plays an important role but it has not been clearly defined and analyzed. We introduce a procedure for analyzing crop tolerance on the basis of digital image analysis. Crop tolerance is defined as the ability of the crop to avoid yield loss from cultivation in the absence of weeds, and it has two components: resistance and recovery. Resistance is the ability of the crop to resist soil covering and recovery is the ability to recover from it. Soil covering is the percentage of the crop that has been buried because of cultivation. We analyzed data from six field experiments, four experiments with species of small grains, barley, oat, wheat, and triticale, and two experiments with barley cultivars with different abilities to suppress weeds. The order of species' tolerance to weed harrowing was triticale > wheat > barley > oat and the differences were mainly caused by different abilities to recover from soil covering. At 25% soil covering, grain yield loss in triticale was 0.5%, in wheat 2.5%, in barley 3.7%, and in oat 6.5%. Tolerance, resistance, and recovery, however, were influenced by year, especially for oat and barley. There was no evidence of differences between barley cultivars in terms of tolerance indicating that differences among species are more important than differences among cultivars. Selectivity analysis made it possible to calculate the crop yield loss due to crop damage associated with a certain percentage of weed control. In triticale, 80% weed control was associated with 22% crop soil cover on average, which reduced grain yield 0.4% on average in the absence of weeds. Corresponding values for wheat, barley, and oat were 23, 21, and 20% crop soil cover and 2.3, 3.6, and 5.1% grain yield loss.

Field studies were conducted to examine both density and duration of glyphosate-resistant (GR) horseweed interference in cotton. Two studies, one examining the effect of horseweed density and a second the duration of horseweed interference, were conducted on a site with a natural population of horseweed that were treated with glyphosate at 0.84 kg ae ha−1 prior to planting and at the 2nd and 4th cotton node growth stages. GR horseweed density effect on cotton height, maturity, and lint yield was determined at horseweed densities of 0, 5, 10, 15, 20, and 25 plants m−2. Duration of horseweed interference was evaluated when 20 horseweed m−2 were allowed to interfere with cotton from emergence to 2nd node, 6th node, 10th node, 12th node, and 1st bloom stage of cotton. The maximum cotton lint yield loss (46%) occurred when horseweed was allowed to compete with cotton from emergence to maturity at the two highest densities (20 and 25 horseweed m−2). When the data were fit to the Cousens model the estimated a (maximum yield loss) and i (yield loss per unit density as density approaches zero) were 53 ± 7.3 and 2.8 ± 0.6 SE, respectively. In both years of the study, horseweed interference from emergence to the 2nd cotton node did not reduce cotton lint yields. In 2006, cotton lint yield loss was 28% compared to 39% in 2005 when horseweed interfered with cotton from emergence until the 6th cotton node. Cotton lint yield loss was 37 and 44% when horseweed competed to the 8th cotton node in 2005 and 2006, respectively. Maximum horseweed seed production was 134,000 to 148,000 seeds m−2.

Field and laboratory studies were conducted to examine herbicide dissipation when applied to low density polyethylene (LDPE) mulch for dry scenarios vs. irrigation. Analytical chemical analysis was used for quantification. In field studies, halosulfuron, paraquat, carfentrazone, glyphosate, and flumioxazin were surface applied to black 32-μm-thick (1.25 mil) LDPE mulch. LDPE mulch harvest began 1 h after treatment (HAT) then continued every 24 h for five consecutive rain-free days after treatment (DAT) to determine the level of herbicide dissipation from the LDPE mulch surface. In a related study, treated LDPE mulch was harvested 1 HAT, then sprinkler irrigation was applied, followed by a sampling five HAT, then the same irrigation and sampling procedure was repeated every 24 h for five consecutive DAT. The order for half-life, as defined as time for 50% dissipation (DT50), varied by herbicide and method of dissipation for dry and irrigated studies. Data indicated that glyphosate and paraquat dissipation was rapid following irrigation. Glyphosate and paraquat DT50 were both 1 h in the irrigated study, but 84 and 32 h for the dry scenario, respectively. This indicated that glyphosate and paraquat could be removed from LDPE mulch with rainfall or irrigation, primarily due to their high water solubility. Halosulfuron and flumioxazin DT50 were 3 and 6 h in the irrigated study, and 18 and 57 h for the dry study, respectively. Carfentrazone DT50 was similar at 28 and 30 h for the irrigated and dry studies, respectively. This indicated that carfentrazone was adsorbed to the LDPE mulch, and irrigation water did not remove it from the LDPE mulch. Results from 14C-herbicide laboratory studies were similar to those from field studies for halosulfuron, glyphosate, paraquat, and flumioxazin.