Modern weed control tactics have played a major role in the productivity of cropping systems. Herbicides have been an effective component of weed control for major crops, greatly reducing yield losses and facilitating reduced tillage systems. Although these benefits are important, weed problems, soil erosion, and environmental concerns persist. Herbicides will continue to play a key role in most production systems, but weed species will continue to evolve and weed communities shift in response to selection pressures. Weed science must develop and incorporate additional practices to create integrated management systems that diversify selection pressures and reduce environmental degradation. Integrated pest management (IPM) may provide a useful framework for the development of integrated weed management systems. The basic principles of IPM are well established and have been successfully applied to many agricultural pests. However, the application of IPM to weed management has lagged behind other pest management disciplines. Many of the concepts and approaches of IPM are relevant to weed management, but these were not developed specifically for weed management and are not sufficient to address it adequately. Principles of IPM unique to weed management need to be delineated, developed, and put into practice. Although IPM for other pests provides an excellent framework, weed science must develop its own theory, management tactics, and monitoring procedures based on the unique characteristics of weed communities.

Glycine max cultivars exhibit differential tolerance to soil-applied sulfentrazone. The intent of this study was to determine the physiological basis for this differential tolerance by evaluating sulfentrazone absorption and metabolism during the earliest stages of G. max development (i.e., germinating seeds, and germinal seedlings). Imbibed seeds (24 h) of the sulfentrazone-tolerant cultivar ‘Stonewall’ absorbed 37% less sulfentrazone than the sulfentrazone-sensitive cultivar ‘Asgrow 6785’. Similarly, germinal seedlings (i.e., 60 h from start of imbibition) of the sulfentrazone-tolerant cultivars Stonewall and ‘Pioneer 9593’ absorbed 22% less sulfentrazone than the sulfentrazone-sensitive cultivars Asgrow 6785 and ‘Carver’ when exposed to sulfentrazone-containing solution for either 24 or 48 h. The amount of root-absorbed 14C-sulfentrazone that was translocated into cotyledon or hypocotyl tissues did not exceed 11% of the amount absorbed and was similar for all four cultivars. Sulfentrazone metabolism by both imbibed seeds and by germinal seedlings was independent of cultivar. Increasing the sulfentrazone concentration in the seed imbibition solution and increasing the temperature resulted in greater seedling height reduction at 10 d in Asgrow 6758 than in Stonewall. Results indicate that differential absorption during the earliest stages of development is the basis for the differential response among G. max cultivars. Comparatively limited sulfentrazone absorption by Stonewall, as reflected in acceptable seedling injury, remained relatively consistent across the range of concentrations and temperatures evaluated.

Failure to control Xanthium strumarium with acetolactate synthase (ALS) inhibitor herbicides has been reported in Iowa and surrounding states. Single-seed descent techniques were used to isolate three X. strumarium biotypes: CAM-10 from near Cambridge, Iowa; Colo-25 from near Colo, Iowa; and Ohio-1 from Fulton County, Ohio. Ohio-1 and Colo-25 were selected because of apparent resistance to imazethapyr, whereas CAM-10 was selected for observed sensitivity to imazethapyr. The biotypes were assayed in vitro with three different ALS inhibitor herbicides, and ALS activity was measured. The 50% inhibition values (I50) of ALS for imazethapyr were determined to be ninefold or higher for the Ohio-1 and Colo-25 biotypes compared to the CAM-10 biotype. The I50 for imazaquin was determined to be about ninefold higher for the Colo-25 biotype and sixfold higher for the Ohio-1 biotype when compared to the CAM-10 biotype. All biotypes were equally sensitive to chlorimuron ethyl. The resistance was due to a single dominant nuclear gene.

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.

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.

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.

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.

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.

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.

If decision-aid software models of weed emergence and growth are ever to help producers better time weed management, these models must be able to predict perennial weed shoot emergence from vegetative propagules. In this research, Cirsium arvense shoot emergence from adventitious root buds in spring was modeled using degree-day heat sums. Fractional C. arvense shoot emergence was best modeled as a logistic dose–response function of degree-day heat sum as follows: Y = 1.108/(1+[X/488.344]−5.161) where Y = fractional C. arvense shoot emergence (0 to 1) and X = heat sum in degree-days above 0 C after day 91 of the year (April 1) with an upper limit of 800 degree (C) days (r2 = 0.83). This empirical model was validated by graphing observed vs. model-predicted C. arvense shoot emergence using two independently gathered data sets, one of C. arvense emergence in autumn chisel-plowed Triticum aestivum (r2 = 0.82) and the other in no-till fallow (r2 = 0.63). The model slightly overestimated emergence at low fractional emergence (< ∼7% at 0.1 fractional emergence) and underestimated emergence at high fractional emergence (10 to 20% at 0.8 to 1.0 fractional emergence). Below an emergence fraction of 0.8, the model adequately estimated observed emergence to within about 10% of the predicted regression line. Using the model, about 1% and 80% of C. arvense shoots should emerge from adventitious root buds after a heat sum accumulates of about 197 and 587 C d, respectively, starting from day 91 of the year. Consequently, farmers should begin monitoring C. arvense patches for emergence and height growth after about 197 C d accumulate and expect to control C. arvense before about 587 C d accumulate, which is when about 80% of shoots have emerged.

Parasitic plants, including the root holoparasites Orobanche spp., cause devastating damage to crops worldwide. Arabidopsis thaliana is widely used as an amenable model plant system to study host–pathogen interactions. Understanding the molecular basis involved in host–parasite interactions will provide practical tools for the detection of genes responsible for incompatibility and resistance responses. In preliminary petri dish experiments, A. thaliana induced seed germination of O. aegyptiaca, O. minor, and O. ramosa at the rate of 87, 72, and 67% of maximum seed germination, respectively. Arabidopsis thaliana induction of O. crenata and O. cumana seeds was negligible (less than 2% of maximum germination). In additional polyethylene bag studies, A. thaliana was parasitized by O. aegyptiaca, O. ramosa, and O. minor resulting in 12, 5, and 2 parasites per host plant, respectively. The results facilitate the use of A. thaliana in host-parasitic plant interaction research.

A 2-yr field study was conducted to compare the growth of Amaranthus palmeri, A. rudis, A. retroflexus, and A. albus planted in June and July. Rates of height increase (centimeters per growing degree day) were 0.21 and 0.18 for A. palmeri, 0.16 and 0.11 for A. rudis, 0.12 and 0.09 for A. retroflexus, and 0.08 and 0.09 for A. albus in 1994 and 1995, respectively, when planted in June. A. palmeri had among the highest values for plant volume, dry weight, and leaf area, while A. albus had the lowest. Specific leaf area values (cm2 g−1) ranged from 149 to 261 for A. palmeri, 160 to 205 for A. rudis, 150 to 208 for A. retroflexus, and 127 to 190 for A. albus. Maximum relative growth rates (g g−1 day−1) for any measured period were 0.32 for A. palmeri, 0.31 for A. rudis, 0.30 for A. retroflexus, and 0.26 for A. albus. Recent increases in species range and observed changes in weed community structure may be partially explained by the growth characteristics of A. palmeri and A. rudis. Herbicide rate and timing recommendations for mixed populations of these weeds should be based on A. palmeri because of its high growth rates.

To evaluate relationships among populations, phenotypic variation of morphological characters in one Gutierrezia microcephala and eight Gutierrezia sarothrae populations from New Mexico was quantified and compared with variation expressed when these same populations were grown in a common garden. During flowering, plants were randomly collected from each population across New Mexico during two growing seasons. A common garden of stem cuttings from these same populations was established in Las Cruces. Vegetative and reproductive characters were measured for each population at original and common sites. Vegetative characters did not differ between G. sarothrae and G. microcephala collected from the same location; however, reproductive characters were dissimilar between these two species. Vegetative and reproductive characters differed among G. sarothrae populations at original and common sites between years, although certain populations clustered. Based on morphological characters, more populations clustered when collected from original sites compared to when grown at the common site. Genetics and environment both played a role in the expression of G. sarothrae phenotype when measuring morphological characters; however, G. sarothrae genotypes maintained their general population phenotype expressed at original sites when grown in the common site. Apparently, vegetative and reproductive characters are fairly stable, and the Gutierrezia genotype has more influence on resulting phenotype than the environment in which it grows.

The size, location, and variation in time of weed patches within an arable field were analyzed with the ultimate goal of simplifying weed mapping. Annual and perennial weeds were sampled yearly from 1993 to 1997 at 410 permanent grid points in a 1.3-ha no-till field sown to row crops each year. Geostatistical techniques were used to examine the data as follows: (1) spatial structure within years; (2) relationships of spatial structure to literature-derived population parameters, such as seed production and seed longevity; and (3) stability of weed patches across years. Within years, densities were more variable across crop rows and patches were elongated along rows. Aggregation of seedlings into patches was strongest for annuals and, more generally, for species whose seeds were dispersed by combine harvesting. Patches were most persistent for perennials and, more generally, for species whose seeds dispersed prior to expected dates of combine harvesting. For the most abundant weed in the field, the annual, Setaria viridis, locations of patches in the current year could be used to predict patch locations in the following year, but not thereafter.

High Avena fatua control costs have caused some Hordeum vulgare growers to use reduced rates of herbicides without fully understanding the consequences. Field studies near Moscow and Genesee, ID, were conducted to determine the effect of A. fatua density and PP-604 rate on A. fatua seed production in H. vulgare and on H. vulgare yield. PP-604 treatments were 25, 50, 100, 150, and 200 (minimum labeled rate) g ha−1, and five A. fatua densities ranged from 0 to 386 plants m−2. Visual A. fatua control was greater than 85% with 100 g ha−1 PP-604 at all locations. Data from 1998 were used to construct nonlinear exponential decay and parabolic models to describe the effect of reduced herbicide rates on viable A. fatua seed production and relative H. vulgare grain yield, respectively. At A. fatua densities of 42 to 138 plants m−2, 46 to 71% of the minimum labeled rate of PP-604 reduced seed production 95%. However, an estimated 140 to 235 seeds m−2 were produced at this level of control, which may not ensure a decline in the A. fatua population over the long-term. Hordeum vulgare grain yield was maximum when 70 to 85% of the minimum labeled rate was applied to A. fatua densities of 42 to 138 plants m−2. A higher rate of PP-604 likely will be required to ensure a decline in A. fatua populations over the long-term than needed to obtain maximum H. vulgare grain yield in a single growing season.

Mulches on the soil surface are known to suppress weed emergence, but the quantitative relationships between emergence and mulch properties have not been clearly defined. A theoretical framework for describing the relationships among mulch mass, area index, height, cover, light extinction, and weed emergence is introduced. This theory is applied to data from experiments on emergence of four annual weed species through mulches of selected materials applied at six rates. Mulch materials, in order from lowest to highest surface-area-to-mass ratio, were bark chips, Zea mays stalks, Secale cereale, Trifolium incarnatum, Vicia villosa, Quercus leaves, and landscape fabric strips. The order of weed species' sensitivity to mulches was Amaranthus retroflexus > Chenopodium album > Setaria faberi > Abutilon theophrasti, regardless of mulch material. The success of emergence through mulches was related to the capacity of seedlings to grow around obstructing mulch elements under limiting light conditions. Mulch area index was a pivotal property for quantitatively defining mulch properties and understanding weed emergence through mulches. A two-parameter model of emergence as a function of mulch area index and fraction of mulch volume that was solid reasonably predicted emergence across the range of mulches investigated.

The objective of this research was to evaluate the accuracy of remote sensing for detecting weed infestation levels during early-season Glycine max production. Weed population estimates were collected from two G. max fields approximately 8 wk after planting during summer 1998. Seedling weed populations were sampled using a regular grid coordinate system on a 10- by 10-m grid. Two days later, multispectral digital images of the fields were recorded. Generally, infestations of Senna obtusifolia, Ipomoea lacunosa, and Solanum carolinense could be detected with remote sensing with at least 75% accuracy. Threshold populations of 10 or more S. obtusifolia or I. lacunosa plants m−2 were generally classified with at least 85% accuracy. Discriminant analysis functions formed for detecting weed populations in one field were at least 73% accurate in identifying S. obtusifolia and I. lacunosa infestations in independently collected data from another field. Due to highly variable soil conditions and their effects on the reflectance properties of the surrounding soil and vegetation, accurate classification of weed-free areas was generally much lower. Current remote sensing technology has potential for in-season weed detection; however, further advancements of the technology are needed to insure its use in future prescription weed management systems.

Solanum ptycanthum plants putatively resistant to acetolactate synthase (ALS) inhibitors were identified in a Wisconsin Glycine max field in 1999. Three- to four-leaf-stage S. ptycanthum plants in the greenhouse were 150, 120, and 5.9-fold resistant to imazethapyr, imazamox, and primisulfuron, respectively, compared with susceptible plants. In vivo ALS was 170- and less than 20-fold more resistant to imazethapyr and primisulfuron, respectively. These results suggested that the S. ptycanthum accession was highly resistant to imazethapyr and imazamox, and that resistance was associated with insensitive ALS. This is the first confirmed occurrence worldwide of S. ptycanthum resistance to ALS inhibitors.