The absorption, translocation, and metabolism of glufosinate were investigated in three differentially susceptible weeds, Xanthium strumarium (most susceptible), Ipomoea purpurea (intermediate susceptibility), and Commelina diffusa (least susceptible). Xanthium strumarium absorbed about three times more 14C-glufosinate than Ipomoea purpurea and about six times more 14C-glufosinate than Commelina diffusa. Translocation of the applied herbicide out of the treated leaf was low. No evidence of glufosinate metabolism, either in the treated leaves or roots, was found when the extracts were separated by HPLC.

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.

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.

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.

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.

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.

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.

Studies were conducted to develop a model from field and laboratory studies to predict the emergence phenology of Chenopodium album. A mechanistic model to predict the phenology of weed seedling emergence across locations, years, and tillage systems is presented. This was accomplished by the integration of hydrothermal time to describe germination and thermal time to describe shoot elongation. The interaction of soil moisture and temperature in the model was accounted for by the integration of hydrothermal time in algorithms predicting seed germination. Soil temperatures within the weed seed germination zone were predicted by temperature ranges at different depths in the soil. Emergence phenology of C. album seedlings was predicted with greater accuracy under no-till and moldboard plow systems than under a chisel plow system. We attributed this lower accuracy in the chisel plow system to increased heterogeneity in the soil matrix and vertical distribution of the seedbank caused by the chisel plow. The presence or absence of Zea mays did not affect model performance. The use of soil temperature to calculate thermal time was a better predictor of C. album seedling emergence than air temperature. The ability to predict weed seedling emergence phenology is an important component of an integrated weed management strategy.

Echinochloa oryzoides and E. phyllopogon have become the most serious weeds in California Oryza sativa since continuous flooding was used to suppress E. crus-galli. Continuous use of a limited number of available graminicides and an increasing number of control failures led to the investigation of herbicide resistance in E. oryzoides and E. phyllopogon. Greenhouse dose-response studies with postemergence (POST) applications of molinate, thiobencarb, fenoxaprop-ethyl, and bispyribac-sodium estimating GR50 (herbicide dose to inhibit growth by 50%) values suggested resistance to all herbicides in two E. phyllopogon accessions and to molinate and thiobencarb in one E. oryzoides accession when compared with susceptible E. phyllopogon and E. oryzoides controls, respectively. No resistance was detected in dose-response studies with propanil. Minimum and maximum ratios (R/S) of the GR50 values of resistant to susceptible E. phyllopogon plants (in two experiments involving two resistant accessions) were 7.8 and >13.3 for thiobencarb, 2.2 and 4.3 for molinate, 16.5 and 428.7 for fenoxaprop-ethyl, and 2.0 and 12.0 for bispyribac-sodium. Minimum and maximum E. oryzoides R/S ratios (average of two experiments) were 21.9 and 4.6 for thiobencarb and molinate, respectively. A resistant E. phyllopogon (one accession tested) and the susceptible control were killed by POST applications of glyphosate, glufosinate, and clomazone, and by a preemergence application of pendimethalin. Thus, the repeated use of the few available grass herbicides in the predominantly monocultured O. sativa of California has selected for herbicide resistance in E. oryzoides and E. phyllopogon. The introduction of herbicides with new mechanisms of action will be useful to manage herbicide-resistant E. oryzoides and E. phyllopogon. However, cross- and multiple resistance emphasize the need to integrate herbicide use with nonchemical means of weed management.

Few studies report animal grazing effects on weed populations. A study was conducted to assess weed populations in annual and perennial forage grasses grazed at various intensities by cattle over a 4-yr period. The perennial forages were Bromus inermis and Bromus riparius, and the annual forages were winter Triticosecale and a mixture of Hordeum vulgare and winter Triticosecale. With few exceptions, results from the two annual pastures could be adequately described as a group, as could the results from the two perennial pastures. The two most prevalent weed species were Capsella bursa-pastoris and Taraxacum officinale; other species encountered over the course of the study were analyzed as a group. Tillage (seedbed preparation) in the annual system supported a proliferation of annual weeds in the spring. In the perennial pasture system, a lack of tillage and spring MCPA allowed T. officinale to increase as the study progressed, especially at the highest grazing intensity. In the perennial pastures, each unit increase in grazing intensity led to 51 more C. bursa-pastoris m−2 and 4 more T. officinale m−2. At lower levels of grazing intensity, C. bursa-pastoris and other species were most abundant in the annual pastures. Weed population shifts in response to grazing pressure in the annual pasture systems were restricted because of annual tillage and MCPA. Therefore, pasture managers may subject annual pastures to heavy grazing pressure with less negative weed population consequences than perennial pastures where herbicides are not applied.

Sulfentrazone persistence in soil requires many crop rotational restrictions. The sorption and mobility of sulfentrazone play an important role in its soil persistence. Thus, a series of laboratory experiments were conducted to mimic the soil properties of cation and anion exchange with different intermediates. The molecular characterization and ionization shift of sulfentrazone from a neutral molecule to an anion were determined using a three-dimensional graphing technique and titration curve, respectively. Sorption and mobility of 2.6 × 10−5 M 14C-sulfentrazone were evaluated using a soil solution technique with ion exchange resins and polyacrylamide gel electrophoresis, respectively. Solution pH ranged from 4.0 to 7.4. As pH increased, sulfentrazone sorption to an anion resin increased and its sorption to a cation resin decreased. Percent sulfentrazone in solution was pH-dependent and ranged between 0 to 18% and 54 to 88% for the anion and cation resins, respectively. Mobility of sulfentrazone on a 20% polyacryalmide gel resulted in Rf values of +0.02 and +0.39 for pH of 4.0 and 7.4, respectively. A double peak for sulfentrazone was detected in the polyacrylamide gel when the pH (6.0 and 6.8) was near the reported pKa of 6.56. There was no clear interaction for the sorption of sulfentrazone at 1.0 mg kg−1 to Congaree loamy sand or Decatur silty clay loam saturated with either calcium or potassium. Sulfentrazone behavior with the polyacrylamide electrophoresis gels and ion resins indicate the potential for this herbicide to occur as a polar or Zwitter ion. Sulfentrazone was adsorbed by potassium, calcium, and sodium saturated resins and subsequently desorbed using variable pH solutions. The level of sulfentrazone adsorption will vary among soil types and the amount of desorption into solution may be soil cation-dependent.

Pesticide retention by eight inorganic soil amendments, the majority of which are used in turf, was evaluated using a laboratory-based technique with radiolabeled pesticides. Amendments evaluated were derived from various naturally-occurring deposits of zeolites, diatomaceous earths, and fired clays and are intended to provide long-lived, stable, and uniformly sized particles that can contribute favorable water- and nutrient-retention properties to the root zone. Sand, sedge peat, and a Marvyn loamy sand soil (Ap horizon) were included for comparative purposes. Pesticides evaluated included the herbicides imazaquin and oxadiazon and the fungicide/herbicide fenarimol. Pesticide retention was evaluated with a soil solution technique. Amendments evaluated had considerable variation in cation exchange capacity (CEC), effective CEC (ECEC), surface area (SA), and field capacity with lesser variation in particle size distribution and particle density. Scanning electron microscopy revealed that surface texture was variable but frequently rough and porous. Pesticide retention was also variable but generally more than that of sand and frequently equivalent to sedge peat. Only with fenarimol and amendments that had been Ca+2-saturated could retention be correlated with any of the individual physical or chemical parameters that are generally assumed to govern pesticide adsorption, which in this case were CEC and SA. Imazaquin retention by unaltered amendments was correlated only with the products of SA and CEC, and SA and ECEC. Retention of both oxadiazon and fenarimol by unaltered amendments could not be correlated with any individual physical and chemical parameters or products thereof. Pesticide retention by these amendments is probably the cumulative sum of both true adsorption and physical entrapment.

Rangeland and pastures comprise about 42% of the total land area of the United States. About three-quarters of all domestic livestock depend upon grazing lands for survival. Many ranges have had domestic stock grazing for more than 100 years and, as a result, the plant composition has changed greatly from the original ecosystems. Western rangelands previously dominated by perennial bunchgrasses have been converted, primarily through overgrazing, to annual grasslands that are susceptible to invasion by introduced dicots. Today there are more than 300 rangeland weeds in the United States. Some of the most problematic include Bromus tectorum, Euphorbia esula, Centaurea solstitialis, C. diffusa, C. maculosa, and a number of other Centaurea species. In total, weeds in rangeland cause an estimated loss of $2 billion annually in the United States, which is more than all other pests combined. They impact the livestock industry by lowering yield and quality of forage, interfering with grazing, poisoning animals, increasing costs of managing and producing livestock, and reducing land value. They also impact wildlife habitat and forage, deplete soil and water resources, and reduce plant and animal diversity. Numerous mechanical and cultural control options have been developed to manage noxious rangeland weeds, including mowing, prescribed burning, timely grazing, and perennial grass reseeding or interseeding. In addition, several herbicides are registered for use on rangelands and most biological control programs focus on noxious rangeland weed control. Successful management of noxious weeds on rangeland will require the development of a long-term strategic plan incorporating prevention programs, education materials and activities, and economical and sustainable multi-year integrated approaches that improve degraded rangeland communities, enhance the utility of the ecosystem, and prevent reinvasion or encroachment by other noxious weed species.