Whole-plant, negative cross-resistance was studied in Conyza canadensis and Echinochloa crus-galli, important global weeds. Negative cross-resistance can be a most useful preemptive, cost-effective tool for delaying the evolution of resistance, as well as for resistance management, after resistant populations evolve. Seeds of triazine-resistant and -susceptible biotypes were collected in or near orchards that had been continuously treated with atrazine for more than 10 yr. Plants grown from the seeds were treated, in a greenhouse, with herbicides from the following chemical families: triazine, benzothiadiazole, phenyl-pyridazine, arylophenoxy-propionate, cyclohexanedione, phenoxycarboxylic acid, pyridine carboxylic acid, phosphinic acid, glycine phosphate, chloroacetamide, sulfonylurea, and bipyridylium. Eleven of the 18 herbicides tested exerted significant negative cross-resistance against atrazine-resistant weeds, ranging from 0.03 to 0.67 of the concentration required to affect the triazine-sensitive type. No synergism was found between bentazon and fluroxypyr in mixture on Conyza, even though both separately exerted negative cross-resistance. Using a mixture with half the amount of each component lowers the environmental effect of each component while controlling a broader spectrum of other weeds.

Poa annua control with norflurazon was different between two field trials in 1999. At the field site with suspected diuron-resistant P. annua, control was less than at the site with known diuron-susceptible P. annua. Diuron-resistant and -susceptible biotypes of P. annua were subjected to a range of norflurazon rates in the greenhouse. The diuron-resistant P. annua biotype (R/S : of 2.5) was collected in 1995 from a Lolium perenne L. seed production field near Tangent, OR. The R/S : for norflurazon in these experiments was 4.8; thus, confirming that diuron-resistant P. annua is resistant to norflurazon.

Carfentrazone-ethyl absorption, translocation, and metabolism was determined in Glycine max, Zea mays, and Abutilon theophrasti. Glycine max absorbed greater than 90% of applied carfentrazone-ethyl within 2 h after treatment (HAT) when nonionic surfactant (NIS) or crop oil concentrate (COC) were included in the treatment solution. The addition of 28% urea ammonium nitrate (UAN) did not improve carfentrazone-ethyl absorption in G. max, but in Z. mays and A. theophrasti, UAN combined with NIS or COC increased the rate of carfentrazone-ethyl absorption. Carfentrazone absorption in A. theophrasti 2 HAT was 70% when UAN was combined with NIS or COC compared to 40% with NIS or COC alone; however, 24 HAT, absorption with NIS and COC were similar to treatments with UAN. Carfentrazone-ethyl did not translocate from the treated leaf to other plant parts in Z. mays and only small amounts of radiolabeled product were detected in the rest of the shoots of A. theophrasti (5%) and G. max (12%). Herbicide metabolism in Z. mays and G. max was greater than in A. theophrasti. All three species converted carfentrazone-ethyl to its phytotoxic metabolite carfentrazone-chloropropionic acid; therefore, the parent molecule was considered to be the sum of the ethyl ester and its hydrolysis product. Estimated half-lives of carfentrazone in Z. mays, G. max and A. theophrasti were 1, 7, and 40 h, respectively. The rate of carfentrazone metabolism corresponded to plant sensitivity (sensitivity to carfentrazone: Z. mays<G. max<<A. theophrasti); however, rapid absorption and translocation of carfentrazone may reduce the tolerance of G. max.

The emergence of Sorghum bicolor seedlings and the dormancy status and survival of S. bicolor seeds buried in soil in the midwestern United States were studied from 1992 to 1995. Possible causes of seed mortality were also investigated. Emergence of seedlings from seeds buried in November averaged less than 2%, compared with 28% from seeds buried in March. Seeds showed little ability to survive winter, with more than 80% dying between November and March and virtually none surviving a second winter. Many seeds exhibited signs of cold damage. Further evidence of the effect of cold was the decreased mortality of seeds protected somewhat from the cold. The decline in the viability of seeds was modeled well by negative exponential curves with an exponent of between 0.011 and 0.016.

The extensive use of propanil for weed control in Oryza sativa over the past 30 yr has led to the development of propanil-resistant Echinochloa crus-galli. This problem was first identified in one Arkansas county in 1990. Since that time, 171 resistant populations have been identified in 20 counties. Random amplified polymorphic DNA (RAPD) analysis was used to assess genetic variation among populations of propanil-resistant and -susceptible E. crus-galli in Arkansas and to elucidate the origin of resistance and its dispersal. Sixteen E. crus-galli populations from seven Arkansas counties and one population from Mississippi were analyzed using 13 primers, which amplified a total of 159 repeatable fragments. Genetic distance was calculated among the populations, and the populations were clustered. The analysis revealed two distinct clusters with an average between-cluster genetic distance of 0.436 and average within-cluster distances of 0.012 and 0.029. These clusters may represent different Echinochloa species. Each cluster contained at least one resistant population and populations from different regions of the state. In addition, several resistant populations from different regions of the state were genetically nearly identical. Thus, it is likely that resistant biotypes have spread in Arkansas by seed dispersal and by independent mutation events. This suggests that there are multiple origins of the resistance, and simple control of weed seed dispersal will not stop the spread of resistance.

The isolation of a Glycine max cytochrome P450 monooxygenase (P450) cDNA designated CYP71A10 that conferred linuron resistance to laboratory-grown, transgenic Nicotiana tabacum seedlings was previously reported. A nonsegregating transgenic N. tabacum line has been established that possesses two independent copies of the G. max CYP71A10 transgene. Five-week-old progeny plants of this selected line were grown in a controlled environmental chamber and treated with linuron using either pretransplant incorporated (PTI) or postemergence (POST) applications. CYP71A10-transformed N. tabacum was more tolerant to linuron than the wild type for both application methods. The transgenic N. tabacum line tolerated an approximately 16-fold and 12-fold higher rate of linuron than wild-type N. tabacum when the herbicide was applied PTI or POST, respectively. These results provide evidence that plant-derived P450 genes can be employed effectively to confer herbicide resistance to transgenic plants.

Avena fatua seeds remaining on the plant at harvest and taken into the combine harvester may be dispersed over large areas. The objective of this study was to characterize the development of A. fatua in comparison to spring Triticum aestivum. As part of this objective, the rate of seed shed in A. fatua relative to development of T. aestivum was determined. Avena fatua and T. aestivum had similar phyllochron intervals within locations but differed between locations. Plant development as measured by the Zadoks plant development scale was consistent within plant species between locations. Seed shed in A. fatua was also consistent between locations. Most of the seed shed occurred within 2 wk, and the cumulative seed shed followed a sigmoidal pattern. The seed shed occurred as T. aestivum was ripening, and the percentage of seed shed appears to be related to the water content of the T. aestivum spike. Because of this relationship, the proportion of seed remaining on A. fatua at harvest could be managed by changing the timing of crop harvest.

Additive and replacement series studies were conducted to investigate the effect of Glycine max cultivar canopy characteristics, interference, and weed-free period on the vegetative and reproductive growth of Solanum ptycanthum. Solanum ptycanthum established 0, 2, and 4 wk after planting (WAP) did not reduce seed yield of either Group II (Pioneer 9273) or Group IV (Asgrow 4715) varieties. The Group IV variety intercepted a greater percentage of light throughout the season. However, both varieties reduced growth of S. ptycanthum established 2 and 4 WAP, suggesting no disadvantage of Group II in suppressing S ptycanthum growth. The greenhouse and field replacement series experiments indicated that S. ptycanthum plants experienced greater intraspecific than interspecific competition; that is, S. ptycanthum grew more in mixed species competition than in monoculture. Under field conditions, G. max grew equally well in competition with S. ptycanthum or in monoculture. However, in the greenhouse G. max accumulated more biomass and fruit weight in monoculture vs. competition with S. ptycanthum. Relative yield total values (RYT) in the field supported greater productivity per unit area for mixed species competition when compared to G. max or S. ptycanthum grown in monoculture. Data from both studies suggested that S. ptycanthum did not reduce G. max yield when grown in the field. Regardless of maturity group, a fully developed G. max canopy reduced S. ptycanthum growth.

The effect of two commonly used desiccant tank mixtures on Glycine max seed production and viability when applied at G. max growth stages R5 to R8 (R5 = beginning pod fill, R6 = full green pod, R7 = beginning maturity, R8 = full maturity) was evaluated. Two mid-Group IV G. max cultivars, one conventional (‘Northrup King 4260’) and one glyphosate-tolerant (‘Asgrow 4701RR’), were evaluated. Tank mixtures containing either glyphosate + sodium chlorate or paraquat + sodium chlorate + surfactant reduced yield, seed weight, and subsequent germination, emergence, and seedling growth when applied at R5 or R6 to either G. max cultivar. Glyphosate + sodium chlorate applied at R7 reduced germination of the glyphosate-tolerant cultivar. When applied at R6 or R7, glyphosate + sodium chlorate reduced the next generation's seedling length compared to paraquat + sodium chlorate. Glyphosate + sodium chlorate increased the number of abnormal seedlings produced in the conventional cultivar compared to paraquat + sodium chlorate. These results indicate that preharvest desiccant applications should not be made prior to G. max maturity (R7).

Greenhouse and laboratory research was conducted to determine the antagonistic effects of various tank mixtures on BAS 625 efficacy. Bensulfuron at 60 g ai ha−1 and BAS 635 at 40 g ai ha−1 did not antagonize control of Echinochloa crus-galli or Brachiaria platyphylla by BAS 625 at 30 g ai ha−1 in greenhouse experiments. Tank mixtures of BAS 625 with 1,000 g ai ha−1 bentazon reduced BAS 625 control of E. crus-galli from 100 to 40%. Antagonism of BAS 625 activity by bentazon or chlorimuron at 10 g ai ha−1 was similar with B. platyphylla, reducing control from 100 to 28 and 32%, respectively. Addition of 5% (v/v) ethanol eliminated all antagonism with any of the herbicides used with either weed species. Uptake and translocation of 14C-BAS 625 1 and 12 h after treatment was not enhanced, either alone or in tank mixtures, with the addition of ethanol. Uptake of 14C-BAS 625 1 and 12 h after treatment was lower in both species when tank-mixed with bentazon. There was no effect of any of the antagonizing herbicides or ethanol on the metabolic degradation of the BAS 625 that was taken up by the plant. The herbicide concentration for 50% inhibition of activity (I50) for BAS 625 on Triticum aestivum acetyl coenzyme A carboxylase (ACCase) was 125 µM. Bentazon, BAS 635, and NC-311 at 1 mM each did not alter the inhibition on ACCase by BAS 625. BAS 635, NC-311, and bentazon at 1 mM inhibited the activity of ACCase 12, 16, and 29%, respectively. Our results indicate that antagonism of the weed control activity of BAS 625 by bentazon may be partly caused by reduced uptake. Other mechanisms may be involved to explain the antagonism of BAS 625 by bentazon and the sulfonylurea herbicides used in this study.

Absorption, translocation, and metabolism studies using 14C-quinclorac were conducted with quinclorac-sensitive Digitaria sanguinalis and quinclorac-tolerant Eleusine indica at the one- to two-tiller growth stage cultured under hydroponic conditions. After an 80-h exposure time, both species had absorbed nearly equal amounts of 14C-quinclorac (27 and 22% for D. sanguinalis and E. indica, respectively). Over the exposure period, the absorption curve for D. sanguinalis was curvilinear, with the maximum absorption occurring approximately 48 h after exposure. The response curve for E. indica was linear across the exposure period. Results from the translocation studies showed that 95% of the absorbed 14C-quinclorac remained in the treated leaf for D. sanguinalis after 80 h. However, only 58% of the absorbed 14C remained in the treated leaf of E. indica. Most of the 14C translocated out of the leaves moved to the tiller, the crown, and new leaf tissue. There was no appreciable exudation of 14C-quinclorac by either species during the absorption period. Results of the metabolism studies showed that neither the susceptible species (D. sanguinalis) nor the tolerant species (E. indica) metabolized the parent quinclorac herbicide. Spray retention studies showed that E. indica (tolerant) retained more applied quinclorac than D. sanguinalis (sensitive). Overall results suggested that a large difference in tolerance of the two species to quinclorac involves mechanisms other than absorption, metabolism, or spray retention.

Growth chamber and laboratory experiments were conducted to evaluate foliar absorption and translocation of the isopropylamine (Ipa) or trimethylsulfonium (Tms) salts of glyphosate in Abutilon theophrasti and Setaria faberi. Glyphosate was applied at 250 g ai ha−1. Absorption of 14C-glyphosate in both formulations increased similarly in both species over 72 h. Seventy-two hours after treatment (HAT), A. theophrasti absorbed 26% of applied 14C of both formulations, whereas S. faberi absorbed 43% of both formulations. The addition of 1% ammonium sulfate (AMS) to both formulations increased the amount of herbicide absorbed to 35% in A. theophrasti and 60% in S. faberi 72 HAT. Translocation of absorbed 14C-glyphosate in both formulations (with or without AMS) was greater for S. faberi than A. theophrasti 72 HAT. For A. theophrasti, 17% of 14C absorbed in Ipa and Tms formulations were translocated out of the treated leaf, whereas additions of AMS increased the amount translocated to 25 and 23% for Ipa and Tms, respectively. The largest portion of translocated 14C moved to tissues above the treated leaf. For S. faberi, 37% of absorbed 14C in both Ipa and Tms formulations were translocated out of the treated leaf 72 HAT, whereas 42% translocated when 1% AMS was added to both formulations. In S. faberi, the largest portion of 14C-glyphosate translocated to tissues below the treated leaf. In summary, no differences were found between Ipa and Tms formulations in both weed species.

A model is presented of the spread of a weed from a point source, with dispersal being both unaided and aided by a combine harvester. Four “type” weeds were modeled, chosen to represent species of differing population and dispersal ecologies and based broadly on Avena spp. (wild oats), Raphanus raphanistrum (wild radish), Bromus spp. (bromegrass), and Fumaria spp. (fumitory). Wind-adapted species were excluded. The greatest rate of spread was predicted to be for R. raphanistrum and the least for Fumaria, regardless of whether dispersal by combine harvester was included. Rate of spread was more sensitive to dispersal parameters than to demographic parameters and increased up to 16-fold as soon as any seeds were dispersed by the harvester. Given that uptake by the harvester is a function of phenology of seed production and dispersal, better data on such processes is required for weeds. Because rate of spread is more dependent on dispersal than on demographic factors, greater attention needs to be given to describing dispersal frequency distributions and especially to analysis of their shapes.

Translocation and metabolism of radiolabeled RPA 201772 was studied in Setaria faberi. Tissue-killing heat girdles were used to determine the extent of RPA 201772 transport in the apoplast and symplast. In leaf uptake studies, girdling was performed above and below the treated area on the leaf. In root uptake studies, girdling was performed on the stem just above the crown. Girdling restricted translocation of 14C following both the root and leaf applications. However, translocation occurred past the girdles suggesting that both the symplast and apoplast are involved in translocation of 14C from RPA 201772. Translocation of 14C out of the treated leaf was reduced 85% with a girdle below the 14C spotting area. In root metabolism studies, 27, 40, and 33% of recovered 14C were identified as parent RPA 201772, diketonitrile, and other metabolites, respectively, 24 h after treatment (HAT). Conversion from parent RPA 201772 to diketonitrile was more extensive in leaf tissue than in roots with 10, 68, and 22% of recovered 14C identified as parent RPA 201772, diketonitrile, and other metabolites, respectively, in the treated area of the leaf 24 HAT. Metabolite analysis demonstrated RPA 201772 is mobile in both the apoplast and symplast.

Group IV Glycine max cultivars, chlorimuron postemergence, and paraquat + sodium chlorate applied as preharvest desiccants were evaluated for their effect on weed seed production and viability. Early-maturing G. max cultivars reduced seed weight, seed production, and seedling growth in Ipomoea lacunosa in most instances and usually reduced seed weight, germination, emergence, and growth in Sesbania exaltata by allowing harvest prior to physiological maturity of these weeds. Tall, late-maturing G. max decreased weed seed production and seed weight of both species, presumably through increased competitiveness of G. max. The seeds produced by weeds that emerged after chlorimuron application were usually lower in weight, germination, and emergence, and growth of seedlings was reduced because most weeds present at the time of application were controlled. Thus, only late-emerging or stunted plants were present to produce seed. Preharvest desiccation of late-maturing S. exaltata parent plants with paraquat + sodium chlorate reduced height and fresh weight of the resulting S. exaltata seedlings 35 to 50% by reducing the amount of time for the S. exaltata seeds to mature.

At the end of the cropping season in November 1994, Striga hermonthica seed populations were collected in northern Bénin (in the Atacora and Borgou departments). Host crops included Zea mays L. (corn), Pennisetum americanum (pearl millet), and Sorghum bicolor (L.) Moench (sorghum). The seed populations were kept dry in the laboratory, and germination was tested regularly after 2 wk conditioning in the laboratory. The seeds passed through a state of primary dormancy, which was not the same for seed populations collected from Z. mays and S. bicolor fields and those collected from P. americanum fields. The length of the primary dormancy was approximately 6 mo. After passing through primary dormancy (after-ripening), the seeds later went through annual, recurrent states of secondary dormancy. Primary dormancy coincided with the dry season directly after maturity (i.e., between December 1994 and April 1995), and secondary dormancy coincided more or less with the subsequent 1995/1996 and 1996/1997 dry seasons. It is concluded that the secondary dormancy pattern was endogenous. Germination percentages during the period that coincided with the first rainy season after collection were generally higher than during the period that coincided with the second rainy season.

The effect of three spray variables (droplet number, active ingredient concentration, and droplet size) on uptake and translocation of formulated 14C-triclopyr ester was studied in greenhouse-grown Populus tremuloides seedlings. The dose per plant in all treatments was held constant. In all experiments, absorption (as a percentage of dose applied) was much greater than translocation (as percentage of dose absorbed). Absorption and translocation decreased as concentration (ai) was increased and droplet number decreased. Absorption and translocation also decreased as droplet number decreased and droplet size increased. When concentration (ai) was increased and droplet size decreased, absorption again decreased but to a much lesser extent than in the other two experiments; there was no significant effect on translocation. A time-course experiment indicated that uptake rate began to decrease within 1.5 h of application. The rate of decrease was greater at the higher concentration (ai), suggesting that the decrease was associated with contact injury. A model to integrate the application parameters and translocation gave a high correlation between dose per unit droplet stain circumference and translocation.

Field experiments were conducted in 1996 and 1997 to determine the effect of the rate and time of glyphosate application on weed emergence, survival, biomass, and Glycine max yield in reduced-tillage (RT) and no-tillage (NT) glyphosate-resistant G. max planted in rows spaced 18 (narrow-row) and 76 cm (wide-row). Glyphosate was applied at 0.42, 0.63, and 0.84 kg ae ha−1 at V2, V4, R1, and R4 growth stages. On separate plots, 0.84 kg ha−1 glyphosate was applied at each growth stage with hand weeding. A weed-free check was maintained with preemergence imazethapyr plus metolachlor supplemented with hand weeding, and a nontreated check was included. Weed population density before glyphosate application ranged from 239 to 606 plants m−2 in RT and 33 to 500 plants m−2 in NT systems. Setaria faberi and Chenopodium album were the predominant species. Weed control efficacy and crop yield were influenced more by application time than by glyphosate rate. Glyphosate applied at V2, V4, and R1 gave season-long control of weeds in 18-cm rows. In 76-cm rows, glyphosate applied at V2, V4, and R1 gave almost complete control of weeds, but broadleaf weeds emerged after application at V2. The critical time of weed removal, the time beyond which weed competition reduced G. max yield by 3% or more compared to the weed-free check, was at R1 and V4 in 18-cm RT G. max in 1996 and 1997, respectively, and at V2 in 76-cm RT G. max in both years. The predicted critical time of weed removal in 18- and 76-cm NT G. max was R1 and V4, respectively, in 1996 and R1 in 1997. This research showed that there was variation in the onset of the critical time of weed removal between tillage systems, as well as within tillage systems across years. The results indicate a single glyphosate application can prevent yield loss in narrow-row, glyphosate-resistant G. max under favorable conditions, but application timing becomes more critical in wide rows because the critical period of weed removal occurs earlier. Late-emerging weeds may warrant a second glyphosate application in wide-row G. max.

Absorption and translocation of 14C-glyphosate was studied in greenhouse-grown Erythroxylum coca and E. novogranatense. Autoradiography indicated that translocation patterns were similar for both species and that E. novogranatense absorbed and translocated more glyphosate than E. coca. In both young and mature plants, absorption of leaf-applied 14C-glyphosate increased with increased exposure time, and 288 h after application, absorption was higher in E. novogranatense (79 and 52% of applied, respectively) compared with E. coca (60 and 14% of applied, respectively). Similarly, translocation of 14C-glyphosate increased with time in both species. In mature plants, after 288 h more 14C-glyphosate translocated in E. novogranatense (6.9% of applied) than E. coca (2.5%), but the opposite occurred in young plants. Most of the radioactivity translocated from the treated leaf accumulated in the main stems and roots of both species with little accumulation in tissues above the treated leaf. However, most of the applied radioactivity remained in the treated leaf regardless of growth stage and species. The absorption of 14C-glyphosate in young and mature plants of E. coca was 1.3 and 3.6 times lower, respectively, than in E. novogranatense 288 h after treatment. Differences in absorption and translocation of glyphosate in E. coca and E. novogranatense may partially explain the reported differential response to glyphosate.

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.