Differential activities of BOA, DIBOA, and crude water extract of Secale cereale ‘Elbon’ were studied in culture dish bioassays using several vegetable and weed species. On average, DIBOA was about seven times more inhibitory to root growth and four times more inhibitory to shoot growth than BOA. Allelochemicals from S. cereale inhibited shoot more than root elongation of cucurbits Cucumis melo, Cucumis sativus, and Cucurbita pepo. Small-seeded crops Lycopersicon esculentum and Lactuca sativa were sensitive to S. cereale. Large-seeded crops, including the cucurbits and Zea mays var. rogusa, were tolerant. Among the small-seeded weeds Amaranthus palmeri, Digitaria sanguinalis, Echinochloa crus-galli, and Eleusine indica, E. crus-galli was least susceptible. Inhibition of germination by BOA or DIBOA occurred only in small- to medium-seeded species, including A. palmeri, D. sanguinalis, E. indica, L. sativa, L. esculentum, and Sida spinosa. Large-seeded species C. melo, C. sativus, C. melopepo, Z. mays var. rogusa, Ipomoea hederacea var. integriuscula, Ipomoea lacunosa, and Senna obtusifolia were tolerant to allelochemicals from S. cereale. This bioassay indicated a promising potential for controlling small-seeded weeds in large-seeded crops.

Seed production of weedy species of Setaria in crops of Zea mays and Glycine max was studied for 2 yr in western Minnesota and eastern South Dakota. Viable seed production was curvilinearly related to panicle length. A 100-mm-long panicle of S. pumila, S. faberi, and S. viridis produced 129, 323, and 851 viable seeds, respectively. Values were consistent across years, crops, and herbicide treatments. Frequency distributions of panicle lengths of all panicles within a population closely followed nonlinear Weibull functions and were stable across years and crops but not species or herbicide treatment. Positive skewness of these distributions decreased, and median panicle size (mm) increased, in the following order: S. viridis (41), S. pumila (52), and S. faberi (78). Postemergence herbicides applied at full label rates increased skewness and reduced median panicle size (to 11 mm) and seed production of S. viridis. Skewness lessens the reliability of using average panicle size as a measure of seed production for the entire population. However, integration of panicle size–frequency and panicle size–fecundity relationships provided estimates of the number of seeds per panicle that were more representative of the population than the statistical average panicle. These estimates were 52, 242, and 246 seeds per panicle for S. pumila, S. viridis, and S. faberi, respectively. Multiplication of these values by panicle densities generated seed production estimates that were similar to actual counts of seeds. Setaria seed production tended to be higher in Z. mays than in G. max only because of higher plant and panicle densities. Early-maturing panicles tended to be larger than those maturing later, but seed viability generally was stable across maturity times.

Identification of associations between site properties and weed species abundance led to the generation of hypotheses as to why weed populations occur where they do, or do not, in agricultural fields. The objective of this research was to use a multivariate statistical technique, canonical correlation analysis, to identify the associations. Two continuous Zea mays production fields under center-pivot irrigation in the central Platte River Valley of Nebraska were grid-sampled between 1994 and 1997 for nine site properties and six to seven weed species. Weed species were identified and counted just prior to postemergence weed control in two adjacent quadrats (1 by 0.38 m) at each grid sampling point. These quadrats represented untreated weed populations emerging between crop rows and treated populations that survived preemergence herbicide banded within the crop row. Canonical correlation analysis identified one to five significant correlations between linear combinations of site properties and weed species abundance depending on field site, years, and between- vs. on-crop row weed populations. The first pair of linear combinations consistently described an association that separated weed species across a gradient of topography and soil type. The second pair of linear combinations described associations between weed species and soil fertility. In all cases, it was hypothesized that management practices strongly interacted with site properties to create the observed associations with weed populations. Other hypothesized mechanisms for weed patchiness include patchiness in available soil moisture that would influence weed seed germination, emergence, and seedling growth. Additional variation in plant-available preemergence herbicide concentration across the field site would vary weed control efficacy. Another mechanism would be variation in soil fertility that affects the growth, reproduction, and competitive ability of both the crop and the weed.

A field in which Brassica napus volunteers were not controlled by several applications of glyphosate was investigated in 1998. This field had been planted with glufosinate-resistant and imidazolinone-resistant B. napus in 1997 and was adjacent to a field that had grown glyphosate-resistant B. napus. Mature volunteer B. napus were collected on a 50- by 100-m grid in the field. Progeny from 34 volunteers were sprayed with glyphosate at 440 g ae ha−1, and the survivors were sprayed with either glufosinate or imazethapyr at 400 or 50 g ai ha−1, respectively. Where seed numbers permitted (14 volunteers), seedlings were also sprayed sequentially with glyphosate, glufosinate, and imazethapyr, at 440 g ae ha−1, 400 g ai ha−1, and 50 g ai ha−1, respectively. In total, 15 volunteers had progeny that were between 66 and 82% resistant to glyphosate, consistent with the predicted 3:1 resistant : susceptible ratio. Volunteer B. napus plants with glyphosate-resistant seedlings were most common close to the putative pollen source; however, a plant with glyphosate-resistant progeny was collected 500 m from the adjacent field edge. Seedlings from all nine volunteers collected from the glufosinate-resistant area showed multiple resistance to glyphosate and glufosinate, whereas seedlings from 10 of 20 volunteers collected from the imidazolinone-resistant area showed resistance to imazethapyr and glyphosate. DNA extraction and restriction fragment length polymorphism (RFLP) analysis of seedlings confirmed that mature B. napus volunteers were hybrids resulting from pollen transfer rather than inadvertent seed movement between fields. Two seedlings from the 924 screened were resistant to all three herbicides. Progeny from these self-pollinated individuals were resistant to glyphosate and glufosinate at the predicted 3:1 resistant : susceptible ratio and resistant to imazethapyr at the predicted 15:1 resistant : susceptible ratio. Sequential crossing of three herbicide-resistant varieties is the most likely explanation for the observed multiple herbicide resistance. Integrated management techniques, including suitable crop and herbicide rotations, herbicide mixtures, and nonchemical controls should be used to reduce the incidence and negative effect of B. napus volunteers with multiple herbicide resistance.

A Lolium multiflorum Lam. biotype resistant to diclofop-methyl was found in a Triticum aestivum field in France (Normandy) that had been treated for several years with diclofop-methyl. Based on plant survival evaluated 21 d after herbicide application, the biotype exhibited a high level of resistance to diclofop-methyl and moderate resistance to CGA-184927-propargil and PP-604. The resistant biotype exhibited a small increase in tolerance to haloxyfop-methyl, quizalofop-ethyl, sethoxydim, and BAS-517-H, but was controlled by recommended field rates for these herbicides. The mechanism of resistance was investigated for diclofop-methyl. There was little or no difference in diclofop-methyl absorption by leaves of resistant and susceptible biotypes measured 48 h after treatment. For both biotypes, less than 1% of absorbed radiolabel was translocated during 48 h following foliar application of 14C-diclofop-methyl. Metabolism of diclofop-methyl was not significantly altered in the resistant biotype. In both biotypes, diclofop-methyl was rapidly metabolized to diclofop acid followed by a slow rate of formation of a polar conjugate. Two multifunctional acetyl coenzyme A carboxylase isoforms (ACCase I and ACCase II) were isolated from leaf tissue of resistant and susceptible biotypes. Both isoforms exhibited a subunit molecular mass of 203 kDa. For both resistant and susceptible biotypes, ACCase I constituted approximately 80% of total ACCase activity. Graminicide concentrations required to inhibit ACCase activity by 50% (I50 values) were determined for both ACCase isoforms from resistant and susceptible biotypes. The ACCase II isoform was highly resistant to graminicides in both biotypes. In contrast, the I50 value for diclofop inhibition of ACCase I was 19-fold greater for the enzyme isolated from the resistant biotype compared with the susceptible biotype. It is concluded that diclofop resistance in the L. multiflorum biotype from Normandy is caused by the presence of a resistant form of the ACCase I isoform.

This study was conducted in lowland fields at the International Rice Research Institute in the 1994 wet season and the 1995 dry season to determine Oryza sativa plant traits that confer competitive ability against weeds when pregerminated seeds are sown on puddled soil. Initial biomass (IB), crop growth rate (CGR), leaf area index (LAI), and biomass at tillering of O. sativa plants were associated with their competitiveness against weeds, whereas relative growth rate, net assimilation rate, and tillering capacity of O. sativa were not. Biomass at tillering affected weed biomass directly, and IB, CGR, and LAI indirectly affected weed biomass through O. sativa biomass. Biomass at tillering was the best predictor of modern cultivar competitiveness against weeds.

Traditionally, herbicide efficacy has been evaluated by visual ratings, but these data provide little insight to the biological response of weeds to herbicides. Field studies were conducted in 1995 and 1996 to determine the rate response of Setaria faberi seedling survival, seed production, and biomass to postemergence herbicides in Zea mays and Glycine max. Nicosulfuron and sethoxydim were applied to Z. mays and G. max, respectively, at 1×, 12×, 14×, 18×, 116×, 132×, and 0× the label rate. Mature plant density of S. faberi was linearly related to seedling density, indicating that seedling survival was not density dependent. Based on a nonlinear dose–response analysis, maximum S. faberi survival was 55% in Z. mays across years and 60 and 45% in G. max in 1995 and 1996, respectively. Minimum survival was 0% except for Z. mays in 1996 when it was 13%. The minimum survival was greater in Z. mays in 1996 due to greater survival of late cohorts than in 1995. Setaria faberi seedling survival was greater in 12× than 1× herbicide treatments in Z. mays and G. max each year. Setaria faberi seed production was related to mature plant density with a negative exponential function. Seed production per plant was similar between 1× and 12× rates in Z. mays and among 1×, 12×, and 14× rates in G. max each year. However, seed production per square meter was greater in 12× than 1× treatments due to greater seedling survival. Regardless, seed production per square meter was 95% less in the 12× herbicide treatment compared to seed production by untreated plants in Z. mays and G. max.

Abutilon theophrasti is one of the worst agricultural weeds in North America, yet it has not reached that status in California in the 80 yr since it was first reported. The research reported here examined the distribution and modeled climatic requirements of A. theophrasti to determine whether it is likely to spread more widely in the state. Herbaria records and weed literature were surveyed to determine the historical occurrences of A. theophrasti in the state; current distribution was assessed through surveys sent to University of California personnel in each county. Combined results showed 42 counties out of 58 with A. theophrasti present historically or currently. A plot of the cumulative number of counties containing A. theophrasti by decade fit a logistic equation. The maximum rate of spread of this species occurred in 1962 and it is likely that its final distribution by county in California is leveling off and not likely to increase further. The climate-matching/mapping software CLIMEX® was used with observed and estimated parameters of environmental requirements of A. theophrasti to model its current distribution from India through China to Japan. The same model parameters were then used to map its potential distribution in California. Areas where A. theophrasti has been reported were predicted by CLIMEX to be poorly suited for its growth and development without added soil moisture in the form of irrigation. It appears that the Mediterranean climate is a deterrent to the integration of A. theophrasti into California. The climate-matching approach provided a biologically reasonable assessment of potential distribution of A. theophrasti in California. The approach also allowed assessment of the effects of common agricultural practices on potential distribution given the environmental requirements and limitations of A. theophrasti.

Field experiments were conducted to determine if seeds would be produced on Triticum aestivum by Aegilops cylindrica hybrids in the field and, if it were, to determine the viability of the seeds produced. One, five, or 10 hybrids were planted into varying proportions of A. cylindrica and T. aestivum in a replacement series design. Percent seed set ranged from 0 to 5.5% in 1996 and from 0 to 9.2% in 1997. Seeds were set in all treatments. The average seed set was 2.3% in 1996 and 3.8% in 1997. No differences in seed set were found among treatments. The seeds produced were separated according to seed condition, either full or shriveled, and tested for germination. The germination of the seeds produced on the hybrids was not significantly different between years. The average germination for full seeds was 94% in both years and 79 and 84% for shriveled seeds in 1995 and 1996, respectively. Greenhouse studies were conducted to evaluate the rate of self-fertility of the BC1 generation and to identify morphological characteristics that could be used to identify the probable pollen donor parent and to predict the occurrence of seed set. In 1997 4.1% and in 1998 2.1% of BC1 plants set seeds. The average seed set was 0.3% in 1997 and 0.06% in 1998. It was not possible, using any morphological characteristic measured, to determine the identity of the parent serving as the pollen donor in the previous generation or to predict the occurrence of seed set in the BC1 generation. This is the first reported study to show that hybrids between T. aestivum and A. cylindrica have the ability, although limited, to backcross under field conditions and set seeds. Furthermore, the seeds produced are viable and will germinate and produce plants. With the millions of hectares of T. aestivum infested with A. cylindrica, even the limited ability to backcross is of concern for the movement of a herbicide-resistance gene.

The metabolism of the herbicide 14C-glufosinate (GA) was studied in excised shoots and leaves of 20 weed and nonweed species; GA was fed through the xylem. After 24 or 48 h of incubation, the plant material was examined for phytotoxic symptoms, analyzed by autoradiography, and extracted. The extract was cleaned up and analyzed by high-performance liquid chromatography. GA and its metabolites were identified by cochromatography with authentic 14C-labelled reference compounds. 14C-GA was rapidly absorbed by the excised plant parts. Most of the radioactivity (91.3 to 99.7%) in the shoots and leaves was extractable. The main metabolite observed with all species was 3-(hydroxymethylphosphinyl)propionic acid (MPP); lower amounts of 2-hydroxy-4-(hydroxymethylphosphinyl)butanoic acid (MHB) were also found in 14 species. Metabolic rates of GA varied considerably ranging between 13.1 and 2,836.5 ng GA h−1 mg−1 protein. The species with the highest rates of metabolism of GA were Galium verum (2,836.5 ng GA h−1 mg−1 protein), Lythrum hyssopifolia (846.0 ng GA h−1 mg−1 protein), and Digitalis purpurea (494.8 ng GA h−1 mg−1 protein). The mean value across all species was 275.9 ng GA h−1 mg−1 protein.

Full-count random sampling has been the traditional method of obtaining weed densities. Currently it is the recommended scouting procedure when using HERB, a herbicide selection decision aid. However, alternative methods of scouting that are quicker and more economical need to be investigated. One possibility that has been considered is binomial sampling. Binomial sampling is the procedure by which density is estimated from the number of random quadrats in which the count of individuals is equal to or less than a specified cutoff value. This sampling method has been widely used for insect scouting. There has also been interest in using binomial sampling for weed scouting. However, an economic analysis of this sampling method for weeds has not been performed. In this paper, the results of an economic analysis using simulations with binomial sampling and the HERB model are presented. Full-count sampling was included in the simulations to provide a benchmark for comparison. The comparison was made in terms of economic losses incurred when the estimated weed density obtained from sampling was inaccurate and a herbicide treatment was selected that did not maximize profits. These types of losses are referred to as opportunity losses. The opportunity losses obtained from the simulations indicate that in some situations binomial sampling may be a viable economic alternative to full-count sampling for fields with weed populations that follow a negative binomial distribution, assuming no prior knowledge of weed densities or negative binomial k values.

Rapid detection methods were developed for discriminating between resistant (R) and susceptible (S) biotypes of Echinochloa colona to either propanil or fenoxaprop-P at all growth stages. In the pregerminated seed assay for fenoxaprop-P, seeds were placed on 1.0% agar containing a range of concentrations of herbicides and kept under humid conditions. For propanil, pregerminated seeds were placed on moist filter paper in the lid of a petri dish and, when one leaf had developed, the lid was inverted for 1 min into propanil solutions at a range of concentrations. For the fenoxaprop-P and propanil test, seedling length and fresh weight were measured after 1 wk. For juvenile plants with four-leaf to one-tiller, shoots and roots were trimmed and placed in 20-ml glass tubes containing 0.2% (wt/v) agar and a range of concentrations of herbicides. Shoot extension and weight were recorded after 7 d. Larger plants with several small tillers were also assayed by this method. Tillers were removed from larger plants and were evaluated as described for juvenile plants. At later growth stages from ear emergence to flowering, excised stem node segments (8 cm) were soaked in water to stimulate rooting. Rooted nodes were placed in 30-ml glass bottles containing 0.2% agar with a range of concentrations of herbicides, and the test was conducted as described for juvenile plants but with the final assessment of new shoot extension and weight being recorded after 10 d. Discrimination between R and S biotypes was possible on the basis of GR50 values for shoot length and fresh weight in all testing methods. With few exceptions, GR50 values for the length of new shoot were very similar to those for the new shoot fresh weight. We concluded that all testing methods in our study provided reliable and quick discrimination between biotypes for both propanil and fenoxaprop-P susceptibility, covering various growth stages from seed to flowering stage. Trimming plants before herbicide treatment gives a rapid method of discrimination by measuring not only newly grown shoot fresh weight but also shoot length.

The floristic diversity and the vertical distribution of the weed seedbank were studied in ridge tillage (RT) and conventional tillage (CT) systems in clay and clay loam soils. Viable seedbank populations were monitored during 3 yr using germination in a greenhouse. Ridge-tilled fields had a larger soil seedbank (2,992 seeds m−2) than moldboard-plowed fields (1,481 seeds m−2) in the top 15 cm. This result can be explained by the larger perennial seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Annual dicot seeds were more abundant in clay soils than in clay loams at the two soil depths. Annual grass seeds were more abundant under CT than under RT in clay soils at the two sampled depths. In clay loams, the density of annual grass seeds in RT fields was six times greater than in CT fields in the top 5 cm of soil and two times greater at the 5- to 15-cm depth. The vertical distribution of total seeds in soil did not differ between tillage systems. The top 5 cm of the 15-cm soil core contained 35 and 46% of all weed seed in CT and RT systems, respectively. However, the CT system had the highest concentration of annual dicot seeds 5 to 15 cm deep, whereas in the RT system, the same depth contained the highest concentration of perennial seeds. These results confirm that tillage systems and soil types can regulate seedbanks. Weed management programs must take this information into account.

Field studies conducted over 2 yr in Louisiana determined critical periods of Rottboellia cochinchinensis interference in Zea mays. In a duration of interference study, R. cochinchinensis was allowed to compete with Z. mays for 0 (weed-free, season-long), 2, 4, 6, 8, 10, 12, or 14 (weedy, season-long) wk, after which plots were maintained weed-free for the rest of the growing season. Rottboellia cochinchinensis biomass at time of initial removal increased linearly as weeks of interference increased. For 2 wk of interference, R. cochinchinensis biomass was greater in 1993 than 1994, but for 4 wk or more of interference, biomass was greater the second year indicating environmental conditions were more conducive to R. cochinchinensis growth in 1994. Season-long R. cochinchinensis interference reduced Z. mays height by 18% compared with the weed-free check. For both years, R. cochinchinensis reduced yields 125 kg ha−1 for each week of interference. In weed-free maintenance studies, 0, 2, 4, 6, 8, 10, 12, or 14 (weed-free, season-long) wk of weed-free conditions were provided, after which R. cochinchinensis was allowed to repopulate. Zea mays yield was equivalent for the weed-free control and plots maintained free of R. cochinchinensis for 2 wk or more. In the interference studies, season-long R. cochinchinensis interference reduced Z. mays yield at least 33% compared with the season-long weed-free check.

The effects of environmental factors on germination and emergence of Campsis radicans seeds were examined in laboratory and greenhouse experiments. Campsis radicans pods produced numerous, papery, and small seeds (696 seeds/pod; 4 mg/seed). Seeds exhibited dormancy that was relieved (74% germination) after 2 wk of prechilling. Fluctuating temperatures and a 12-h photoperiod were required for maximum germination. Optimum conditions for C. radicans seed germination (74%) were 35/25 C (day/night, 12/12 h) with a 12-h photoperiod. Temperatures below 25/15 C or above 40/30 C were unfavorable for germination. Germination in constant temperatures or in continuous darkness was less than 15%. More than 59% of C. radicans seeds germinated at pH 5 to 9, but at pH 4 or 10 seed germination was totally inhibited. Germination was totally inhibited at osmotic stress higher than −0.2 MPa. Germination was 60% at 40 mM NaCl and 20% at 160 mM NaCl. Emergence was maximum (68%) for seeds that were placed on the soil surface, but no seedlings emerged from a soil depth at 4 cm. About 10% of seeds were still viable even after 20 wk of prechilling. Each pod contained about 700 seeds and each plant produced 20 to 40 pods. These results suggest that the spread potential of C. radicans by seeds would be at least 1,400 to 2,800 seeds plant−1. However, only seeds near the soil surface would be able to germinate.

Differences in the depth of weed seedling recruitment due to agronomic management practices, such as reduced tillage, have implications for weed competitive ability and management strategies. Depth of seedling recruitment of Avena fatua, Triticum aestivum, Setaria viridis, Polygonum convolvulus, and Echinochloa crus-galli was measured in situ in 1997 and 1998 prior to seeding (preseeding) and before in-crop spraying (prespray) in a total of 44 zero-tillage and 44 conventional-tillage fields located across approximately 3 million ha of southern Manitoba, Canada. For the monocot species, depth of recruitment was measured from the soil surface to the intact seed coats, which marked the point of germination. For P. convolvulus, a dicot, greenhouse studies were conducted prior to sampling in the field to identify a reliable morphological marker indicating the point of germination. For all species, mean recruitment depth was found to be significantly shallower in zero- vs. conventional-tillage fields and significantly shallower in the preseeding vs. the prespray period. There were relatively few differences in mean recruitment depth among weed species. Within a sampling period and tillage system, for example, the greatest difference in mean recruitment depth between species was less than 1.2 cm, and the maximum mean recruitment depth across species, sampling times, and tillage practice was very shallow (less than 4.2 cm). Locating weed seedling recruitment depth is the first step in characterizing weed seedling recruitment microsites. Results indicate this information should be specific to tillage and sampling time.

Unique long-term historical emergence records were used to assess the association between weed seedling emergence and various elements of meteorological data. These elements included both temperature-based and rainfall-related variables in the 7-d periods before and during which emergence occurred. Five weed species (Stellaria media, Chenopodium album, Capsella bursa-pastoris, Matricaria perforata, and Veronica hederifolia) with contrasting emergence patterns were studied in disturbed soil. Logistic regression analysis was used to identify meteorological variables of interest and allowed their relative importance to be assessed and ranked. Logistic regression was further used to associate probabilities of emergence with observed levels of important individual meteorological elements. This approach enabled prediction of the probability of emergence following given meteorological conditions and hence an assessment of the risk of omitting weed control measures. Predictions were made based on single meteorological variables and compared with observed data. Results indicated that temperature was the dominant factor in predicting emergence. Soil moisture, while also important, was a secondary factor only becoming important once the species-specific temperature requirement had been satisfied. The potential for further development of the model is discussed.

The objective of this study was to identify some epidemiological conditions that affect the pathogenicity and weed control efficacy of the fungal pathogen Dactylaria higginsii on Cyperus rotundus. In controlled environments, the fungus required a minimum dew period of 12 h and a temperature of 25 C during the dew period to produce severe disease on four- and six-leaf-stage plants inoculated with 106 conidia ml−1. Under these conditions, 75% disease (percent leaf area damaged) and excellent weed control (nearly 100% plant mortality) were achieved. In experiments to test the interaction among dew period temperature, dew period duration, and plant growth stages, younger plants (four- to six-leaf) were more susceptible to D. higginsii than older (eight-leaf-stage) plants. At the dew period duration of 24 h and dew period temperature of 30 C, the number of days to obtain 50% disease severity on four- to six-leaf-stage plants was significantly less (10 d) compared with older plants (16 d). To achieve effective control, D. higginsii should be applied early in the growing season when C. rotundus plants are young and the temperature and dew period requirements are not limiting. The need for a long dew period (>12 h) for infection and disease development may be a limiting factor in this pathosystem. This limitation may be overcome by using inoculum amended with moisture-retaining gels.

Phytoremediation depends on the ability of plants to tolerate and assimilate contaminants. We have been interested in the use of common ornamental plants to ameliorate the effects of pesticide waste on golf courses and ornamental plant nurseries. This research characterized the interaction between two ornamentals, Acorus gramenius and Pontederia cordata, and the herbicide simazine. Simazine tolerance levels for the ornamentals were determined by exposing plants to 0, 0.01, 0.03, 0.1, 0.3, 1.0, or 3.0 mg L−1 in aqueous nutrient media for 7 d. Response endpoints included fresh weight gains for both species after 7 d exposure and 7 d postexposure. Quantum efficiency of P. cordata was also measured using dark-adapted (Fv/Fm) and light-adapted (fluorescence yields) plants. Simazine uptake and distribution within each plant was determined by exposing plants to [14C]-simazine in nutrient media (0.24 mg L−1) for 1, 3, 5, or 7 d. Plant tissues were combusted and analyzed by liquid scintillation counting. Pontederia cordata fresh weight gains were reduced 76 and 70% by 1.0 and 3.0 mg L−1, respectively, after 7 d exposure. Acorus gramenius fresh weight gains were reduced 103, 124, and 144% at 0.3, 1.0, and 3.0 mg L−1 following 7 d exposure, respectively. Photosynthetic efficiency measured on dark-adapted P. cordata (Fv/Fm) was reduced 21, 47, and 71% by 0.3, 1.0, and 3.0 mg L−1, respectively, whereas photosynthetic efficiency measured on light-adapted plants (fluorescence yields) were reduced 9, 25, 59, 87, and 96%, respectively, by 0.03, 0.1, 0.3, 1, and 3 mg L−1. Simazine activity in solution was reduced 45 and 34% over 7 d with A. gramenius and P. cordata, respectively. By day 7, activity was distributed throughout the plants, but predominantly in the leaves. Uptake of simazine was correlated with water uptake throughout the 7 d. These results suggest that A. gramenius and P. cordata may be good candidates for incorporation in a phytoremediation scheme for simazine.

Field studies were conducted to determine the effect of early-season and early- plus late-season acetolactate synthase–resistant Helianthus annuus interference on Glycine max and H. annuus growth and yield at two sites in Missouri. Helianthus annuus densities of 3 plants m−2 were established shortly after G. max emergence in all plots except the weed-free check. To study early-season interference, H. annuus were removed with postemergence glyphosate (0.84 kg ae ha−1) 2, 4, 6, and 8 wk after planting (WAP) and kept weed-free for the rest of the growing season. Glycine max yields were not different with 2, 4, 6, or 8 wk of early-season interference at either location. To study early- plus late-season interference, H. annuus densities were established at 3 plants m−2. They were then removed 2, 4, 6, or 8 WAP with glyphosate and subsequently reestablished at the same density within 2 wk after removal by newly emerging and transplanted H. annuus. These H. annuus were allowed to remain in the field for the remainder of the growing season. This provided a weed-free period of approximately 2 wk during the growing season beginning 2, 4, 6, or 8 WAP. Season-long interference and no-interference treatments were also included. Glycine max yields were reduced 47 to 72% with season-long interference. Helianthus annuus vegetative dry matter was approximately 56% lower at Columbia than at Miami. Glycine max yields tended to increase as the weed-free period was delayed into the growing season. Early-season weed-free periods (2 to 4 and 4 to 6 WAP) allowed H. annuus to become re-established before G. max formed a canopy and resulted in larger amounts of H. annuus biomass and seed production as well as G. max yield losses of 15 to 80%. Re-establishment of H. annuus in 6 to 8 WAP and 8 to 10 WAP weed-free treatments generally resulted in the plants surviving for only a few weeks after establishment and not producing seed or reducing G. max yield.