Weed population models can serve as a framework to organize weed biology information and to develop weed control strategies. Models help to identify information gaps, to set research priorities, to develop hypotheses pertinent to weed population regulation, and to suggest control strategies. A population simulation model of leafy spurge (Euphorbia esula L. # EPHES) was used to demonstrate the applicability of population models to weed science. Sensitivity analysis of an existing leafy spurge model indicated that transition from basal buds to vegetative shoots, survival of vegetative shoots, and survival of basal buds over winter were important transition parameters influencing population growth of this weed species. Possible mechanisms (intraspecific competition and environmental factors) that influence the transition from basal buds to vegetative shoots were shown. Intraspecific density effects on basal bud transition and production were included to show model refinement and second-generation model development. Four control strategies were simulated and were compared to field studies to show the predictive and management potential of the modeling approach. Simulations of population response to foliage feeding herbivores was highly correlated (r = 0.98) with field data for sheep grazing on leafy spurge. Simulation of picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) applied to leafy spurge also was correlated (r = 0.97) with field results.

Field studies were established in 1974 and 1975 at various locations in San Diego County, CA, to evaluate several herbicides for the control of chamise (Adenostoma fasciculatum H.&A.) and redshank chamise (Adenostoma sparsifolium Torr.). Herbicide effects on vegetation were reevaluated in 1982. Plots originally treated with 2.2 or 4.5 kg ae/ha of glyphosate [N-(phosphonomethyl)glycine] had virtually no chamise regrowth 8 yr after treatment. The herbicides 2,4-D [(2,4-dichlorophenoxy)acetic acid], combinations of 2,4-D with 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] or dichlorprop [2-(2,4-dichlorophenoxy) propionic acid], fosamine [ethyl hydrogen (aminocarbonyl)phosphonate], triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea}, and picloram (4-amino-3,5,6-trichloropicolinic acid) had no long-term effects on chamise. Soil collected in 1982 from glyphosate-treated plots contained similar numbers of chamise seeds but fewer seeds of other species compared to untreated plots. Canopy cover of redshank chamise was similar in treated and untreated plots after 8 yr. These results document the effectiveness of glyphosate in killing the underground root-crown of chamise, which prevented sprouting, but not redshank chamise.

Herbicide resistance jeopardizes the usefulness of valuable chemical tools and, therefore, weed management in many crop systems. Models must be developed to evaluate management tactics that prevent, delay, or reduce resistance. The complexity of biological processes involved in herbicide resistance also requires models to focus research and to integrate experiments. A population model was developed that improves upon previous attempts to predict herbicide resistance dynamics. The model incorporates plant population demographics with the Hardy-Weinberg concept for gene segregation. The model simulates the evolution, spread, and subsequent dynamics of resistance in the presence and absence of a herbicide. Analysis of model simulations identified two sets of biological processes as key factors in the evolution and dynamics of herbicide-resistant weed populations. These are processes that influence ecological fitness and gene flow. Several options are suggested as examples for the management of resistant weed populations.

An experiment was established in 1961 to determine the influence of bearmat (Chamaebatia foliolosa Benth.) competition on ponderosa pine (Pinus ponderosa Laws.) survival and growth. Ponderosa pine seedlings were planted in bearmat which was: (A) untreated, (B) sprayed with a mixture of 2,4-D [(2,4-dichlorophenoxy)acetic acid] and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], and (C) eliminated by a combination of herbicide, clipping sprouts, and trenching to prevent root and rhizome invasion. Ponderosa pine survival after 19 yr averaged 9%, 66%, and 90%, respectively, for the three treatments. Tree height after 19 yr averaged 1.6, 1.9, and 5.7 m for treatments A, B, and C, respectively. Soil moisture use was initially less on the herbicide-treated than on the untreated plots, but bearmat quickly sprouted after application to compete with the pine seedlings for moisture. After 19 yr the bearmat was more dense and appeared to be more vigorous on the sprayed plots than on those receiving no treatment. We estimate that 75% reduction in net wood production could result after 50 yr on this site from bearmat competition.

Three approaches to data analysis were compared to describe competitive interactions between wheat and Italian ryegrass. Replacement series were performed using the two species at total densities of 100, 200, and 400 plants/ m2, and separate monoculture experiments for each species at densities from 33 to 800 plants/m2. Approaches to data analysis included: 1) conventional analysis of replacement series experiments, 2) development of synthetic no-interaction responses from monoculture experiments for comparison with results from mixtures, and 3) responses of the reciprocal yield of individual plants to variation in densities of the two species. Wheat was the superior competitor to ryegrass; however, the three approaches varied in ability to quantify this competitive relationship. The conventional replacement series analysis was least sensitive in describing the influences of either density or proportion on the plant association. The synthetic no-interaction approach provided the most detailed analysis of the influence of proportion on the species interaction. The reciprocal yield approach provided the simplest and most sensitive analysis of the joint influences of density and proportion. The latter approach also provided the most quantitative analysis of the influence of density on the species interaction. Plant density and species proportion are important variables for interpreting the process of plant competition.

Primary root cells from five dinitroaniline-resistant (R) and three susceptible (S) goosegrass biotypes from North Carolina and South Carolina were observed by transmission electron microscopy to determine whether resistance was associated with changes in cell wall formation. Cell wall malformations were found in some cells from two of the R-biotypes and in one of the S-biotypes. Malformations consisted of partially deposited cell walls and the inclusion of cell wall material in the cytoplasm. Some of the affected cells also had abnormal, lobed nuclei and malformed mitochondria. There seems to be little or no correlation between dinitroaniline resistance and cell wall malformations.

Comparative growth, development, and resource allocation of triazine - susceptible and triazine - resistant biotypes of common groundsel (Senecio vulgaris L.), grown under two light regimes, were studied over a 7 - week period. Dry-matter production, height, number of leaves, and leaf area of the susceptible biotype were greater than those of the resistant biotype at all harvests under both light regimes. Root/shoot ratios were lower in the resistant than in the susceptible biotype under high light. Lower values for these parameters in resistant plants are due to lowered photosynthetic capacity, which limits growth and lowers relative root production. Net assimilation rate (NAR) was lower in resistant than susceptible plants when grown under high light, but mean leaf area ratio over a harvest interval (L) of resistant plants was higher than that of susceptible plants under high light. Relative growth rate (RGR) was higher for susceptible plants before day 42 and higher for resistant plants after day 42 under high light. A lower plastochron index (PI) of resistant plants up to 5 weeks old, relative to susceptible plants of the same age, suggests that, when grown under high light, resistant plants are developmentally at least 2 days behind susceptible plants. Shading lowered dry-weight production, height, number of leaves, leaf area, NAR, RGR, PI and root/shoot ratios of both biotypes. Values for these growth parameters for susceptible plants were similar to or larger than for resistant plants when grown under low light. In both biotypes, resource allocation patterns and L shifted towards increased shoot and leaf production in response to shading.

The interference between dry bean (Phaseolus vulgaris L. var. Red Kidney), black nightshade (Solanum nigrum ♯3 SOLNI), and barnyardgrass (Echinochloa crus-galli ♯ ECHCR) was examined in replacement series experiments. A modified replacement series experiment also was performed to examine the effects of no interference, and intraspecific and interspecific interference on the height, leaf area, and dry weight of bean, black nightshade, and barnyardgrass. In both types of competition experiments, bean germinated earlier than either weed species and caused significant reductions in weed height, leaf area, and dry weight. The impact of bean upon itself was always greater than the effect of either weed species. In laboratory studies, barnyardgrass and black nightshade seeds were subjected to eight concentrations of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine). Black nightshade was 100 times more tolerant to trifluralin than barnyardgrass. Field studies, conducted at several locations in the Central Valley of California in which trifluralin-treated plots were compared with nontreated plots, demonstrated that trifluralin favors trifluralin-tolerant weeds (black nightshade), but trifluralin-sensitive weeds (barnyardgrass) have not been eliminated despite 20 yr of trifluralin use.

Management of herbicide resistance will require interdisciplinary research to understand the mechanisms and dynamics of resistance. We believe that models are valuable tools. Models propose hypotheses about biological processes that influence the evolution of resistance and management tools that may reduce resistance in weed populations. Hypotheses help to direct research. For example, our model proposes that fitness and gene flow dramatically influence the evolution of and recovery from resistance in weed populations. Therefore, our research agenda should include ecological investigations of the life-history traits involved in fitness and gene flow. The model suggests thresholds of susceptible weed abundance to reduce selection pressure and to delay resistance. These thresholds will be related closely to economic weed thresholds. While the resistance issue emphasizes the need for basic biological and ecological research, resistance is only a part of the broader research agenda, which calls for holistic and interdisciplinary approaches to understanding and managing crop systems.

Correlations between herbicide damage and several physiological factors were examined in the field for ponderosa pine (Pinus ponderosa Dougl. ex P&C Lawson) and greenleaf manzanita (Arctostaphylos patula Greene). Pine injury caused by 2,4-D [(2,4-dichlorophenoxy) acetic acid], glyphosate [N-(phosphonomethyl) glycine], or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid]} was compared to leader growth rate, needle growth rate, predawn xylem potential, daytime xylem potential, and photo synthetic rate occurring on the dates of herbicide application. Shrub injury for each of the three herbicides was compared to predawn xylem potential, daytime xylem potential, and photo synthetic rates. Both species exhibited less injury from herbicide applications made at the end of September than from any applications made from April through October. Comparison of factors highly correlated to herbicide damage indicates that highest herbicide selectivity occurs when pine has ceased growing, the xylem potential of the pine is relatively low (high water stress), and the xylem potential of the manzanita is relatively high.

Herbicide injury to five coniferous species was determined for the butoxyethanol ester of 2,4-D [(2,4-dichlorophenoxy)acetic acid], the isopropylamine salt of glyphosate [N-(phosphonomethyl)glycine], and the triethylamine salt of triclopyr {[(3,5,6-trichloro-2-pyridinyl) oxy] acetic acid} throughout the growing season of 1981. The relationship of herbicide tolerance to growth rate, water stress, and photosynthesis was determined for Jeffrey pine (Pinus jeffreyii Grev. & Balif), sugar pine (P. lambertiana Dougl.), red fir (Abies magnifica A. Murr.), white fir [A. concolor (Gord. & Glend) Lindl.], and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco]. Although most of the species showed a high correlation of injury to leader or needle growth rate and xylem pressure potential, the seasonal trend in the degree of injury and the relationship of herbicide tolerance to various physiological factors were unique for each species. Criteria for determining herbicide application periods depended on species and herbicide.

The selectivity of five foliage-applied herbicides, 2,4-D [(2,4-dichlorophenoxy)acetic acid], dichlorprop [2-(2,4-dichlorophenoxy)propionic acid], glyphosate [N-(phosphonomethyl)glycine], fosamine [ethyl hydrogen (aminocarbonyl)phosphonate], and triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, applied at three phenological stages of development to five Sierran shrub species, deerbrush [Ceanothus integerrimus var. californicus (Kell.) G. T. Benson.], greenleaf manzanita (Arctostaphylos patula Greene), bearmat (Chamaebatia foliolosa Benth.), snowbrush ceanothus (Ceanothus velutinus Dougl.), and whiteleaf manzanita (Arctostaphylos viscida Parry), was compared. Treatments in the summer and fall were less effective than spring applications. Periods of herbicide susceptibility generally corresponded to times when moisture stress was low and photosynthesis was high.

Studies of weed and crop competition can be used to predict yield losses from weed presence and to determine optimum levels or periods of weed control. However, competition is a complex phenomenon that is governed by various biological, environmental, and proximity factors. The factors of proximity include plant density, species proportion, and spatial arrangement among individuals. Several experimental methods have been developed that attach different levels of importance to proximity factors. These methods are described, and the advantages and disadvantages of each are discussed. Density, proportion, and arrangement of plants influence the outcome of competition experiments and should be incorporated into studies of crop-weed interference, since differing estimates for the effects of weeds on crop productivity can be obtained, depending upon the experimental method used.

There are three primary activities that characterize the discipline of Weed Science. These activities are weed technology, weed biology, and the ethics of weed control. Each of these activities needs to be considered as herbicide-tolerant crops (HTCs) are introduced. HTCs are the most recent refinement in the existing technology to control weeds. The potential benefits from the improved weed control must be weighed against possible increased costs of production and potential for genes that control herbicide tolerance to escape into non-tolerant plant populations. These questions about herbicide resistance are primarily technological and biological. They demonstrate the paucity of information in Weed Science on weed genetics, gene flow, fitness, and other aspects of weed-crop population dynamics. Other questions about HTCs are ethical. They require that we ask who benefits from the technology and what are the economic, ecological, and social consequences of it.

The agricultural community is currently involved in a debate with other members of society concerning many of the tools and tactics used to grow food. The discipline of weed science represents a microcosm of this larger societal debate, and thus is indicative of many other applied agricultural disciplines. Weed science comprises six fundamental disciplines, which have been arranged to depict its three major areas of research: weed technology, weed biology, and the ethics of weed control. Insight into those components of the debate that involve weed science is gained by examining each area. It seems unlikely that solutions to the “neckriddle” can be found through traditional research because of the integrative nature of agriculture, which involves environmental, economic, and social components. It is hoped that movement toward more holistic approaches to research will lead to resolution of the debate between society and the agricultural community.

Sulfur cinquefoil is an exotic, perennial forb that invades a wide range of ecosystems in western North America. It forms dense populations and often threatens native plant species and communities. In this study, we address the following questions: (1) what herbicides, rates, and application times are most effective at reducing sulfur cinquefoil abundance while having the least impact on native plants; and (2) does postherbicide seeding with native grass species increase native plant abundance? In 2002, we experimentally examined the effects of five herbicides (dicamba + 2,4-D; metsulfuron-methyl; triclopyr; glyphosate; and picloram) at two rates of application (low and high), three application times (early summer, fall, and a combined early summer–fall treatment), and two postherbicide seed addition treatments (seeded or not seeded) on sulfur cinquefoil abundance, plant community composition, and species richness. Experimental plots were monitored through 2005. Picloram was the most effective herbicide at reducing sulfur cinquefoil density, the proportion of remaining adult plants, and seed production. The effects of picloram continued to be evident after 3 yr, with 80% reduction of sulfur cinquefoil in 2005. In addition, seeding of native grass seeds alone (no herbicide application) reduced the proportion of sulfur cinquefoil plants that were reproductively active. Despite reductions in sulfur cinquefoil abundance, all treatments remained dominated by exotic species because treated areas transitioned from exotic forb- to exotic grass-dominated communities. However, a one-time herbicide application controlled sulfur cinquefoil for at least 3 yr, and therefore might provide a foundation to begin ecological restoration. Herbicide applications alone likely are to be insufficient for long-term sulfur cinquefoil control without further modification of sites through native grass or forb seeding. Integrating herbicides with native plant seeding to promote the development of plant communities that are resistant to sulfur cinquefoil invasion is a promising management approach to ecological restoration.

Herbicides are the primary method used to control exotic, invasive plants. This study evaluated restoration efforts applied to grasslands dominated by an invasive plant, sulfur cinquefoil, 6 yr after treatments. Of the five herbicides we evaluated, picloram continued to provide the best control of sulfur cinquefoil over 6 yr. We found the timing of picloram applications to be important to the native forb community. Plots with picloram applied in the fall had greater native forb cover. However, without the addition of native perennial grass seeds, the sites became dominated by exotic grasses. Seeding resulted in a 20% decrease in exotic grass cover. Successful establishment of native perennial grasses was not apparent until 6 yr after seeding. Our study found integrating herbicide application and the addition of native grass seed to be an effective grassland restoration strategy, at least in the case where livestock are excluded.

Over a decade ago, the Weed Science Society of America sponsored a symposium on the then emerging technology of herbicide-tolerant crops (HTCs). The symposium and subsequent proceedings addressed potential benefits and concerns about that new technology to control weeds. Technological, biological, and ethical questions were addressed that were likely to emerge from the widespread adoption of HTCs. It was suggested at that time that if such questions were answered, HTC development would proceed on a more solid foundation, whereas continued uncertainty and criticism would probably result if the questions were not answered. We now review developments in HTC technology. Questions and concerns posed one decade ago are still pertinent, but current knowledge is still insufficient to address them. Adoption of HTC has risen dramatically since their commercial introduction, but there is still no evidence of associated production cost reductions or enhanced yields

Plant invasions that arise from human introductions of new species to a region, or from range expansion of some native plant species, can profoundly affect biodiversity and alter the structure and function of ecosystems. Although preventive strategies may be an effective way to limit plant invasions, they are difficult to achieve because adequate descriptions of biological and environmental characteristics are often lacking, and truly predictive models of invasive biology have been elusive. On the basis of a history of repeated plant introductions and the absence of a general predictive theory, it may be best to assume that plant invasions will continue. This paper explores ways to empirically study and to predict plant invasions through a study of the invasion process. Several approaches are explored, such as species demography, DNA analysis, and geographic information system reconstructions, to characterize source and satellite populations and factors that influence the spread of these populations.