Field studies were conducted in 1998 and 1999 to evaluate crop response, weed control, Glycine max yield, and economic returns with sulfentrazone alone and tank-mixed with glyphosate, cloransulam, or chlorimuron at two preplant application timings in no-till, narrow-row, glyphosate-resistant G. max. No significant crop injury was observed. Setaria faberi and Polygonum pensylvanicum control 5 wk after planting (WAP) was generally greater with sulfentrazone applied early preplant (EPP) than with sulfentrazone applied at planting (AP). When applied AP, glyphosate plus sulfentrazone provided greater S. faberi control than sulfentrazone alone. Control of Amaranthus rudis, Ambrosia artemisiifolia, and Ipomoea hederacea was greater in 1998 than in 1999 because of more timely early-season precipitation. Sulfentrazone-based programs provided 80 to 100% control of A. rudis in 1998, but control in 1999 ranged from 72 to 95% at Columbia and 46 to 83% at Novelty. Cloransulam alone, at either application timing, was the only treatment that provided greater than 80% control of A. artemisiifolia at each site in each year. All sulfentrazone-based treatments provided greater than 80% control of I. hederacea in 1998, but control was less in 1999 and ranged from 54 to 91%. Xanthium strumarium control ranged from 5 to 94% with sulfentrazone alone; however, the addition of cloransulam or chlorimuron provided 75 to 99% control regardless of application timing. A blanket application of glyphosate was made 6 WAP over all preplant herbicide treatments, and weed control 5 wk after this treatment was greater than 79% with all sulfentrazone-based treatments. Sulfentrazone plus cloransulam or chlorimuron plus glyphosate EPP or AP followed by (fb) glyphosate postemergence (POST) generally provided the greatest weed control. Overall weed control was generally greater with the use of residual herbicides vs. glyphosate alone, although yield and net returns were not always greater. A greenhouse study was conducted to determine if altering the preplant application timing reduced sulfentrazone injury to G. max. Treatment variables included herbicide rate, temperature during a preplant incubation period, and application timing. Glycine max, Zea mays, and Sorghum bicolor were used as indicator species. Sulfentrazone caused less injury to G. max, Z. mays, and S. bicolor in soils incubated at 30 C when applied 20 d before planting compared to 0 d before planting. Equivalent amounts of crop injury were noted with sulfentrazone applied 20 or 0 d before planting in soils incubated at 5 C with all indicator species.

There is little information published on patch expansion of perennial weeds and none for Apocynum cannabinum. Studies were conducted to measure the between-season and in-season expansion patterns of natural A. cannabinum patches over three growing seasons. Regression analysis indicated strong relations between patch area in consecutive years 1996 to 1997 (r 2 = 0.81) and 1997 to 1998 (r 2 = 0.76). Patches less than 20 m2 in 1996 increased in area by more than 100% in 1997 during a fallow season. However, patches decreased in size 6 to 51% between 1997 and 1998 when Glycine max was grown. Evidence suggested that a late-season mowing of the A. cannabinum patches in 1997 contributed more to the decline in patch area than competition from G. max during the 1998 season. The relations between patch area and growing degree units (r 2 = 0.97) indicated that greater than 89% of the terminal patch expansion occurred prior to the accumulation of 435 growing degree units (GDU) (June 19, 1997; May 31, 1998; June 9, 30-yr average), with minimal patch expansion between 435 and 1,000 GDU. Patches were at 50% of their final area on May 27, 1997, and May 14, 1998, a time when only 22% of the A. cannabinum population had emerged (r 2 = 0.99). Knowledge of patch size and expansion could help growers time weed scouting, to account for the later emergence patterns of this species, as well as assist in timing appropriate weed management efforts. This information could also be used in conjunction with aerial photographs to project potential patch size for site-specific management of this weed.

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.

Interactions between weeds and arthropods occur frequently. This review covers the topic of weed/arthropod interactions, and provides the reader with access to literature in the subject area that is scattered in weed science, entomological, crop production, and ecological journals. We first analyze the current status of weed and arthropod management in the context of multidisciplinary integrated pest management (IPM). The remainder of the review is organized according to the mechanisms driving interactions. The first section deals with interactions driven by trophic relationships, and is subdivided into direct and indirect trophic interactions. Direct trophic interactions occur when pest or beneficial arthropods feed directly on weeds. Indirect trophic interactions occur when arthropod feeding damage to crops impacts weeds through alteration of ecosystem resource availability, or through weeds serving as hosts for alternate prey for beneficial arthropods, or via tritrophic interactions. The second mechanism driving interactions is considered in relation to alteration of the physical habitat by the presence of weeds, such as alteration of temperature within the plant canopy. The third major mechanism driving interactions is based on control tactics for the two types of pests. These are considered from the aspect of direct physical effects, such as tillage, and from the aspect of interactions resulting from the use of pesticides. The latter is divided into direct effects of herbicides and insecticides on non-target pests and beneficials, and on interactions that result from alteration of host plant physiology by pesticides. A conclusion section attempts to place the impact of interactions into an IPM framework, and to indicate where multidisciplinary research involving weed and arthropod management should be focused in the future.

Rhizobacteria isolated from the rhizospheres of dominant weed species in six representative cropping systems and one native prairie ecosystem in mid-Missouri were screened for phytotoxicity on Lactuca sativa seedlings and their host plants in the laboratory. The proportions of deleterious rhizobacteria (DRB) were compared among different cropping systems to determine possible effects of crop management practices on the occurrence of DRB. Phytotoxicity screening on L. sativa seedlings revealed that an integrated crop management system with a Zea mays–Glycine max–Triticum aestivum cover crop rotation under no-tillage had the highest proportion of DRB at 25.3%, followed by an organic farming system with continuous Fragaria virginiana (strawberry) and organic amendments under minimum tillage at 22.9%. A continuous cool-season grass–legume meadow with no agrochemical inputs had the lowest proportion of DRB at 13%. Crop management practices that maintained high soil organic matter had higher proportions of DRB compared to cropping systems with lower organic matter. Phytotoxicity screening on host plants greatly reduced the proportion of rhizobacteria characterized as DRB, likely because of the high sensitivity of L. sativa seedlings to phytotoxins. Although screening on L. sativa is an effective method to detect phytotoxic rhizobacteria, our research indicates that it is essential to test selected cultures on their host weed species for accurate assessment of their occurrence in the field. Using this approach, we found that crop management practices influence the occurrence of DRB naturally associated with weed seedlings. Results suggest that crop production systems can be developed to favor soil microorganisms such as DRB that affect weed growth and thereby become important considerations in overall weed management.

Field studies were conducted in 1996 and 1997 to determine the response of Cyperus rotundus and Cyperus esculentus, Kyllinga brevifolia, and Kyllinga gracillima to mowing regimens common to recreational turfgrass. Treatments were selected to simulate Cynodon dactylon golf course management and included mowing at 1.3 and 3.8 cm with mowing frequencies of three times per week and once a week, respectively. A nonmowed check was included for comparison. Reductions in C. rotundus shoot number were observed beginning 6 wk after initial treatment (WAIT) in 1996 and 9 WAIT in 1997 for the 1.3-cm mowing regime. The 3.8-cm mowing regime did not reduce C. rotundus shoot number until the final evaluation of each year. Reductions in C. rotundus rhizome length, tuber number, and tuber size were observed for both mowing regimes in both years. Cyperus esculentus shoot number was reduced by the 1.3-cm treatment at each evaluation date in 1996 and 1997. Cyperus esculentus shoot number reductions in the 3.8-cm regime were first observed 4 and 6 WAIT in the 2 yr and continued until termination. The 1.3-cm regime reduced C. esculentus spread beginning 6 WAIT in 1996 and 3 WAIT in 1997. Cyperus esculentus spread was also reduced by the 3.8-cm treatment, but reduction began at later evaluations (8 and 9 WAIT). Tuber production by C. esculentus was completely inhibited by the two mowing regimes in both years. The only treatment effect observed in K. brevifolia and K. gracillima in 1996 was a reduction in internode length of K. gracillima by the 1.3-cm mowing regime. In 1997, the 1.3-cm regime reduced K. brevifolia shoot number at 15 and 18 WAIT and plant spread beginning 6 WAIT and continuing until termination. The 3.8-cm treatment did not affect K. brevifolia shoot number and reductions in spread were only observed at the final evaluation. Kyllinga gracillima shoot number and plant spread were reduced by the 1.3-cm mowing regime at each 1997 evaluation. Reductions in K. gracillima shoot number occurred at the final evaluation, and reductions in spread began 12 WAIT when subjected to the 3.8-cm treatment. Both mowing regimes reduced K. brevifolia and K. gracillima internode length. Kyllinga brevifolia total rhizome length and total node number were reduced by the 1.3-cm regime only. Kyllinga gracillima rhizome length, internode length, and node number were reduced by both regimes in 1997.

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.

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.

To identify the most commonly regulated weedy plants in the United States and southern Canada, we compiled a database of noxious weed lists obtained from the 48 continental states and six bordering provinces. The 10 most frequently listed weeds are Cirsium arvense, Carduus nutans, Lythrum spp. (includes purple loosestrife), Convolvulus arvensis, Euphorbia esula, Acroptilon repens, Sorghum spp. (includes johnsongrass and shattercane), Cardaria spp. (includes hoary cress, also called whitetop), Centaurea maculosa, and Sonchus arvensis. When genera are ranked, the top genus is Centaurea, which includes C. maculosa, C. diffusa, and C. solstitalis. Biological control programs have targeted many of the top dicotyledonous weeds of national concern, but none of the weedy grasses and sedges. We recommend that exploratory studies be initiated to determine the feasibility of developing biological control agents for the latter species. The complete database of noxious weed lists is available on the Internet at http://invader.dbs.umt.edu. This information may be useful to resource managers and regulatory officials in assessing which weeds are problematic in adjacent geographic areas and by researchers to help select which weeds to target with new management strategies.

Parasitization by Orobanche is a complex process, one that is mediated by host-derived chemical signals that control parasite seed germination and haustorium initiation and one that ultimately results in the union of two plant species. Experiments were conducted to characterize Orobanche parasitization of the model plant Arabidopsis thaliana and to begin to explore the role of host flavonoid metabolism in the interaction. Arabidopsis thaliana stimulated seed germination and allowed tubercle development of O. aegyptiaca and O. ramosa but did not significantly stimulate seeds of O. crenata, O. minor, or O. cernua. However, if Orobanche seeds were artificially stimulated, O. crenata and O. minor successfully established tubercles on A. thaliana. When compared to the recognized crop hosts, Daucus carota and Nicotiana tabacum, A. thaliana stimulated less O. aegyptiaca germination but allowed for formation of equivalent numbers of tubercles. These findings indicate that A. thaliana is not a large-scale producer of germination stimulant but is highly susceptible to the parasite once Orobanche seeds have germinated. Experiments comparing wild-type A. thaliana plants to mutant lines deficient in flavonoid biosynthesis revealed no differences in the ability to stimulate germination or to allow tubercle formation, indicating that host flavonoid production is not essential for Orobanche parasitization. The results of this work support the use of A. thaliana as a valuable host in understanding Orobanche parasitization.

Field and greenhouse studies were conducted to evaluate the responses of several imidazolinone (IMI)-resistant Amaranthus hybridus populations to various acetolactate synthase (EC 4.1.3.18)-inhibiting herbicides. In 1996 field studies, IMI resistance was confirmed in one A. hybridus population (R4) that was not cross-resistant to the sulfonylurea herbicides CGA-277476, chlorimuron, or thifensulfuron. Amaranthus hybridus control with triazolopyrimidine herbicides was ≤ 35%, but control with cloransulam-methyl or flumetsulam plus cloransulam-methyl was higher than with IMI herbicides. Follow-up greenhouse studies were conducted in 1997 and 1998 to investigate the response of one IMI-susceptible (S) A. hybridus population collected near Painter, VA, and four IMI-resistant A. hybridus populations (R1, R2, R3, R4) collected from fields in Somerset County, MD, to postemergence imazethapyr, chlorimuron, thifensulfuron, pyrithiobac, and cloransulam-methyl applications. Resistance to imazethapyr was confirmed in all R populations, and no practical level of cross-resistance to chlorimuron, thifensulfuron, pyrithiobac, or cloransulam-methyl was detected. Based on resistance ratios, R populations were slightly more tolerant to chlorimuron and slightly more sensitive to pyrithiobac, thifensulfuron, and cloransulam-methyl than the S population.

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.

Sporisorium cruentum teliospores and sporidia (cultured from teliospores with about 30-fold spore increase) caused about equal, although variable, levels of smut infection of individual Sorghum halepense plants when applied to the foliage. Teliospores were applied in a water–surfactant suspension (106 spores ml−1) at 935 L ha−1 and sporidia in a water-in-oil invert emulsion (about 107 spores ml−1) at 468 L ha−1. In crop interference studies, a single foliar application of teliospores to space-planted S. halepense plants in a first-year crop of Saccharum sp. hybrids caused about 55% infection (at least one smutted panicle per plant) but did not reduce the level of S. halepense infestation or substantially improve Saccharum yield compared to noninoculated plants. Smutted panicles usually appeared 30 to 60 d after the foliar treatment, but many culms on a plant produced only healthy panicles. Two applications of teliospores, either 7 d apart in one experiment or 1 yr apart in another, significantly increased (P = 0.05) smut infection over one application but generally did not improve Saccharum yield substantially over plots infested with noninoculated S. halepense. In contrast to foliar spray inoculation, S. halepense plants injected with teliospores in the seedling stage prior to transplanting to the field were 98% infected with smut and were much less competitive with the crop than noninoculated transplants. In host range studies using several S. bicolor genotypes injected with teliospores in the greenhouse, the ratios of resistant to susceptible were 17 to 2 for grain types and 16 to 5, with 2 intermediate, for forage types, showing that genotypes have a relatively high level of resistance to the disease. The result of these experiments indicate that loose kernel smut has potential as a biocontrol agent for S. halepense, but only if infection by foliar treatment can be improved to levels at least comparable to injection.

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.

To determine simazine movement and dissipation in a drip-irrigated Vitis vinifera vineyard under two irrigation schedules [ grower standard (GS) and current evaporation/transpiration (CET)], field experiments were conducted in a Hanford fine sandy loam, a soil type prone to leaching. In experiment 1, simazine was surface-applied in a 1.7-m swath down the vine row, and chloride was applied as a tracer. Total recovery of simazine was < 1.0% under the irrigation emitters 51 and 57 d after simazine application in 1997 and 1999, respectively. Simazine was not detected in the soil profile from 0 to 150 cm deep, 1.0 m from the emitters. A chloride tracer moved to a soil depth of 90 cm but not deeper. In experiment 2, simazine moved 75 cm under the emitters in 7 d but did not move deeper into the soil. Under the emitter, 28% of applied simazine was found 0 to 45 cm deep and 3% was > 45 cm deep. In experiment 3, which was conducted in the absence of irrigation, total recovery of simazine was 30% when sheltered from rain and 8% when exposed to rain. Rapid dissipation and proper irrigation management were key factors preventing deep percolation of simazine in these studies.

Weed population estimates were collected from four Glycine max fields during the summers of 1997 and 1998. Seedling weed populations were sampled using a regular coordinate system on a grid either 50 by 50, 30 by 30, or 10 by 10 m. MSU-HERB and Mississippi Herbicide Application Decision Support System (HADSS) (yield loss prediction and herbicide recommendation models for G. max) were used to determine the estimated net gain resulting from simulated herbicide applications at each sample location in each field. When necessary, the appropriate data points from the 10- by 10-m grid were removed to form population data sets on grids 20 by 20, 40 by 40, and 80 by 80 m. The objectives of this research were to compare estimated economic returns of site-specific herbicide management and broadcast herbicide management in nontransgenic and glyphosate-tolerant G. max and to evaluate the effects of various weed sampling intensities on estimated economic returns from site-specific herbicide applications. Site-specific herbicide management was the compilation of simulated herbicide treatments giving the highest estimated net gains at each location within each field. Broadcast herbicide management was the simulated broadcast application giving the highest estimated net gain for each field. Sampling costs and the unattainable site-specific application costs were not included in the estimated net gain calculations. In nontransgenic G. max production, the estimated net gain for treating the four fields with site-specific technology was $104.76 ha−1 higher than when using the optimum broadcast herbicide. In glyphosate-tolerant G. max production, the average estimated net gain for site-specific treatment of the fields was $96.24 ha−1 higher than for treatment with the best broadcast herbicide application. In nontransgenic G. max, the estimated net gain resulting from site-specific applications on a 10-m grid was $77.17 ha−1 higher than from site-specific applications on a 20-m grid; however, in glyphosate-tolerant G. max, this difference was only $19.84 ha−1. Increased estimated net gain resulted primarily from the use of herbicides that maximized return for each field area and from the decrease of unnecessary herbicide applications because of below-threshold weed infestations.

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.

Laboratory and field experiments were conducted to determine the effects of light, temperature, oxygen, and seed burial depth on seed germination and seed production of Eriochloa villosa without the interference of a crop. Eriochloa villosa seed germination did not respond to light, although it is possible that experimental conditions were not appropriate for this response to develop. Seeds germinated above 10 C and below 45 C, with germination above 85% occurring for temperatures between 20 and 40 C. Seed germination rate and total germination increased as the oxygen concentration increased to the ambient level. Eriochloa villosa seed emerged from 15 cm deep, with maximum emergence from 1 to 4 cm. In the field, emergence was dependent on tillage that affected the vertical distribution of seeds in the soil. Seeds emerged in the reduced-tillage field from between 0.5 and 9 cm deep in soil, with maximum emergence from 2 cm. A delay in planting greatly reduced the growth and seed production of E. villosa. Seed production under noncompetitive conditions was reduced by at least 75% when planting was delayed 6 to 8 wk. However, plants that emerged after July 7 still produced approximately 3,000 seeds when grown under conditions without intraspecific and interspecific competition.

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.

Greenhouse and field studies were conducted to evaluate Oryza sativa response to the simulated carryover of fluometuron, imazethapyr, metolachlor, and norflurazon in dry- and water-seeded culture. In the greenhouse, fluometuron and norflurazon did not affect O. sativa emergence, but reduced O. sativa shoot and root dry weight at all the residue levels. Greater reduction in O. sativa emergence and shoot dry weight was observed with imazethapyr and metolachlor in dry- compared with water-seeded O. sativa. In the field, imazethapyr at 0.04 kg ai ha−1 injured dry-seeded O. sativa both years, but O. sativa yield was reduced only in the second year. Greater O. sativa injury and yield reduction were observed in dry- compared with water-seeded O. sativa with metolachlor at most residue levels. Norflurazon at 0.42 and 0.21 kg ai ha−1 caused greater O. sativa yield reduction in water-seeded O. sativa than in dry-seeded O. sativa in the first and second year, respectively. The results indicate that water-seeded O. sativa has less risk of being damaged by imazethapyr and metolachlor but is more susceptible to residues of norflurazon compared with dry-seeded O. sativa. Thus, choosing the right seeding method following a specific rotational crop herbicide could help to minimize the risk of O. sativa injury from herbicide carryover.