Field studies were conducted at Rosemount, MN, in 1992 and 1993 to quantify the demographic processes regulating the population dynamics of velvetleaf in soybean as part of a corn-soybean rotation. A consistent 6.8 ± 0.5% of the total velvetleaf seedbank emerged each year. Less than 21% of all velvetleaf seedlings survived each year in mixture with soybean, due in part to Verticillium spp wilt infection. The probability of seedling survival varied across time of emergence. Velvetleaf seed production in the absence of crop competition was 125 and 227 seeds plant−1 in 1992 and 1993, respectively. Velvetleaf plants that emerged early produced greater numbers of seed than later emerging plants. Velvetleaf survival and seed production were reduced up to 82% in the presence of crop competition. Soybean yield varied across soybean densities in both years, but was not reduced across velvetleaf densities.

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.

Sustainable rangeland management will require successional strategies to deal with the expanding weed problem. These strategies must be consistent with the view that plant communities are dynamic and technology is used to enhance the natural processes and mechanisms that direct vegetation change. The goal is to shift the dynamics toward a desired plant community. A unified conceptual model is necessary to direct the development and application of successional weed management systems. We propose using a resource management model as a conceptual basis for successional weed management. This model is based on the primary causes of succession: site availability, differential species availability, and differential species performance. This model provides the mechanistic framework necessary for developing successional weed management systems and it is meant to enhance communication among rangeland weed managers and scientists.

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.

A theoretical model was developed to assess the relative importance of weed competition and seed dispersal in determining long-term crop yield reductions imposed by weeds. Weed seed dispersal by cultivation, natural means, and harvest machines was incorporated into a weed and crop population model. Model simulations were conducted with green foxtail and spring wheat demographic data from the literature. Simulations indicated that seed dispersal from a weed species invading a field may have a more important influence on crop yield than the relative competitive ability of the weed with respect to the crop. However, if a weed species has a uniform or a high frequency random distribution, there is less difference in the relative importance of dispersal versus competitive ability in determining crop yield reductions. Harvest machines may be an important mechanism for dispersing weed seeds that are harvested with the crop. The practical implication is to make harvesters weed seed ‘predators’ rather than dispersal agents and thereby reduce herbicide and mechanical weed control inputs.

Repeated use of the preemergence herbicide triallate has selected for wild oat populations that are resistant (R) to field use rates. Field collections and an inbred R line were shown in greenhouse and petri dish dose response experiments to be 6- to 20-fold more tolerant to triallate than susceptible (S) lines. R populations and the inbred line were also resistant (8-fold) to the related thiocarbamate herbicide diallate, as well as to the chemically unrelated postemergence herbicide difenzoquat (60-fold). 14C-triallate uptake and translocation patterns were similar between R and S lines for the first 24 h after application. However, translocation of radioactivity was more rapid in S lines than R lines from 24 through 60 h after application. 14C-difenzoquat uptake was the same in R and S lines 12 h after application, but was 10 to 20% higher in R lines than S lines by 24 through 96 h after application. Similarly, translocation of radioactivity after 14C-difenzoquat application was 7 to 14% greater in R than S lines after 12 h, although translocated radioactivity amounts were not significantly different between R and S lines. The relatively minor differences in triallate and difenzoquat uptake and translocation patterns between R and S lines are most likely not of sufficient magnitude to explain the observed resistance levels.

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.

Crop yield loss–weed density relationships critically influence calculation of economic thresholds and the resulting management recommendations made by a bioeconomic model. To examine site-to-site and year-to-year variation in winter Triticum aestivum L. (winter wheat)–Aegilops cylindrica Host. (jointed goatgrass) interference relationships, the rectangular hyperbolic yield loss function was fit to data sets from multiyear field experiments conducted at Colorado, Idaho, Kansas, Montana, Nebraska, Utah, Washington, and Wyoming. The model was fit to three measures of A. cylindrica density: fall seedling, spring seedling, and reproductive tiller densities. Two parameters: i, the slope of the yield loss curve as A. cylindrica density approaches zero, and a, the maximum percentage yield loss as A. cylindrica density becomes very large, were estimated for each data set using nonlinear regression. Fit of the model to the data was better using spring seedling densities than fall seedling densities, but it was similar for spring seedling and reproductive tiller densities based on the residual mean square (RMS) values. Yield loss functions were less variable among years within a site than among sites for all measures of weed density. For the one site where year-to-year variation was observed (Archer, WY), parameter a varied significantly among years, but parameter i did not. Yield loss functions differed significantly among sites for 7 of 10 comparisons. Site-to-site statistical differences were generally due to variation in estimates of parameter i. Site-to-site and year-to-year variation in winter T. aestivum–A. cylindrica yield loss parameter estimates indicated that management recommendations made by a bioeconomic model cannot be based on a single yield loss function with the same parameter values for the winter T. aestivum-producing region. The predictive ability of a bioeconomic model is likely to be improved when yield loss functions incorporating time of emergence and crop density are built into the model's structure.

Field experiments were conducted to determine the effect of a short-term spring-seeded smother plant on corn development and weed control. Yellow mustard was managed to provide interference durations of 2,4,6, or 8 wk, and maximum height of 10 or 20 cm. Three yellow mustard planting patterns and eight seeding rates were evaluated during 1989 and 1990 at St. Paul and Rosemount, MN. Yellow mustard seeded at 2120 seeds m−2 with an interference duration of 4 wk and a maximum height of 10 cm decreased corn yield 17% and reduced weed dry weight 4 wk after yellow mustard emergence an average of 66%. Yellow mustard with a 2-wk interference duration did not reduce weed dry weight. Yellow mustard seeded at 2120 seeds m−2 with a 6- or 8-wk life cycle and 10-cm height reduced weed dry weight at the end of the interference period an average of 82% but delayed corn silk emergence an average of 5.3 d and reduced average grain yield 19%. Increasing yellow mustard height from 10 to 20 cm delayed corn silk emergence and reduced grain yield but did not decrease weed dry weight. Yellow mustard with an interference duration of 4 wk and a maximum height of 10 cm, seeded over the corn row at 530 seeds m−2, reduced weed dry weight 4 wk after mustard emergence an average of 51%, and resulted in an average corn grain yield reduction of 4%, compared with corn grown in monoculture averaged over weedy and weed-free treatments. These results suggest that it may be possible to develop spring-seeded smother plants that reduce weed biomass up to 80% but have only a small impact on corn yield.

This research presents a modified floatation method to extract weed seed from soil samples and an automated imaging technique to enumerate seed following extraction. Rapid floatation of weed seed from soil in a K2CO3 solution was facilitated by centrifugation. High recovery of seed from soil samples was accomplished in a relatively short time. Recovery of seed of giant foxtail and velvetleaf from a silt loam soil was near 100% and recovery of redroot pigweed was 86%. Germination of common lambsquarters, giant foxtail, and velvetleaf seed was reduced by exposure to K2CO3 under certain conditions. However, these reductions do not appear to reduce the utility of this extraction technique. The automated image capture technique for counting seed showed similar limitations as with manual counting. Seed count accuracy generally decreased with increased number of seed and smaller seed sizes. In general, the image capture technique compared favorably with manual seed counting and has the potential to further improve seedbank assessment methods.

Field experiments were conducted during 1992 and 1993 at Kalispell and Moccasin, MT, to determine the influence of application timing on the efficacy of reduced rate postemergence applications of imazamethabenz and diclofop in spring barley. Herbicides were applied at their respective 1 × and ½ × use rates at either 1, 2, or 3 weeks after crop emergence (WAE). While excellent wild oat control was sometimes achieved with reduced rates, there was no consistent relationship between wild oat growth stage and the level of control at either site regardless of the herbicide or rate applied. This response suggests that efficacy is governed not only by wild oat growth stage, but also by weed demographics and environmental considerations. Barley yield and adjusted gross return values were highest at Kalispell when imazamethabenz treatments were applied at 1 WAE, regardless of the level of wild oat control. Adjusted gross return values were similar for the 1 × and ½ × imazamethabenz treatments. Yields and adjusted gross returns with diclofop treatments were more related to the level of wild oat control at Kalispell, with the 1 × diclofop treatments providing the greatest yields and adjusted gross return values. The level of wild oat control at Moccasin had minimal effect on barley yield and adjusted gross returns, with both values being comparable to the nontreated check.

Kochia pollen dispersion was measured during 24 and 48 h periods from a kochia population in an 8- by 10-m area in the center of a 1.6 ha fallow field. Pollen counts from traps at 50- and 100-cm heights declined rapidly with increasing distance from the pollen source. Pollen deposition was highest along the prevailing wind direction: up to 23 pollen grains cm–2 were recovered 50 m from the pollen source along the southeast (SE) vector. Nonlinear regression analysis of pollen deposition along the SE vector was used to estimate that 99.9% of shed pollen would be deposited within 154.4 m of the source. Viability of pollen from greenhouse- and field-grown plants was measured using staining and germination assays. of four pollen stains tested, only 1,2,3-triphenyl tetrazolium chloride gave consistent results and did not stain heat-killed pollen. Depending on environmental conditions, kochia pollen remained viable from less than 1 d to 12 d. Length of kochia pollen viability was shortest under high temperatures (22 and 28 C) and low relative humidity (7 and 32%). Less than 0.5% germination was observed in 1.1% agar media with various additions; however, up to 17.8% germination was observed after incubation at 28 C in 100% relative humidity.

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.

An educational software program was developed to demonstrate the interactions among weed population demographic processes, plant community interactions, and grassland noxious weed management options. The software includes an example model parameterized to simulate yellow starthistle population dynamics and graphic output. The software allows students to change weed and competing grass population demographic parameters, herbicide, and biological control management options. The graphic output allows the user to observe the outcome of integration of weed biology and various weed management options.

Glyphosate [N-(phosphonomethyl)glycine] was applied in the spring and fall to leafy spurge (Euphorbia esula L. # EPHES) in the field at rates of 0.14 to 4.48 kg ai/ha. Fall applications of glyphosate at rates of 0.56 to 4.48 kg/ha stimulated axillary branching and caused an increase in the number of stems/m2 by the end of the following summer in a dense population. This was a result of shoot growth from buds on the crown region of the root system. The absorption and translocation of 14C-glyphosate applied to leaves of mature leafy spurge plants were evaluated at prebloom, full-bloom, and senescence phenological stages. Approximately 81% of the 14C-glyphosate applied to the leaves of senescing plants was absorbed. There was a decrease in the proportion of 14C translocated out of the treated leaf when applications were made after full bloom. Translocation of 14C to the treated stem, nontreated stems, root crown, and roots did not differ with phenological stage in 1983. Translocation and concentration of 14C in most plant parts in 1985 differed with phenological stage. Translocation to the crown buds as a percentage of the 14C absorbed was highest at the senescence stage. At senescence and before soil freezing, leafy spurge crown buds demonstrate transient but active elongation toward the soil surface. This active development may account for the enhanced translocation of 14C into the crown buds.

Measurements of above-ground plant volume were used to quantify corn interference with common cocklebur and velvetleaf. Separate experiments were carried out for each weed species in which neighborhoods with a radius of 50 cm were established around target plants of both species, selected from a range of corn plus cocklebur or velvetleaf densities. Height and canopy area of target plants and neighbor corn and weed populations were measured periodically during the growing season. Target plant (corn, cocklebur, or velvetleaf) size as well as corn and weed population size within each neighborhood were quantified as cylindrical volumes. Regression analysis was used to quantify the relationship between target plant seed production and cylindrical volumes of the target and neighbor species. Both target and neighbor plant volumes were correlated with target plant seed production for all species. The ratio of target plant volume to total neighborhood plant volume (volume ratio) was the independent variable that accounted for the most variation in target plant seed production. These volume-based variables may be used to develop competitive indices in physico-empirical based interference models.

As an extension of weed threshold models in which crop losses are based on weed density, an alternative model for grass weeds in cereal crops is proposed that incorporates the theoretical importance of selection for herbicide resistance, initial weed population frequency, and weed seed dispersal. Simulations suggest optimum weed population levels (thresholds) for maintaining genotypes that are susceptible to control practices and which minimize crop yield reductions. Weed population frequency, in combination with dispersal and competitive traits may determine optimum weed management strategies/Model simulations indicate that understanding how agricultural practices select for “weedy” traits (e.g. herbicide resistance, competitive ability, dispersal potential) may be important in determining weed density thresholds.

A simulation model was developed to predict the population dynamics and economics of velvetleaf control in a corn-soybean rotation. Data compiled from the literature were used to parameterize the model for two situations, one in which velvetleaf was infected by a Verticillium spp. wilt and one without infection. Verticillium was assumed to have no effect on corn or soybean yield. In the absence of control, simulated seedbank densities of a Verticillium-infected velvetleaf population were 5 to 50 times lower than for an uninfected velvetleaf population. The model was used to evaluate a threshold weed management strategy under the assumption that velvetleaf was the only weed and bentazon the only herbicide available for its control. In the absence of Verticillium, an economic optimum threshold of 2.5 seedlings 100 m−2 afforded the highest economic returns after 20 yr of simulation. Simulations in which velvetleaf was infected in 8 out of 20 randomly assigned years indicated a 6% increase in annualized net return and an 11 % reduction in the number of years that control was necessary. Sensitivity analysis indicated the parameter estimates having the greatest impact on economic optimum threshold were seedling emergence and survival, maximum seed production, and herbicide efficacy. Under an economic optimum threshold of 2.5 seedlings 100 m−2, management practices that manipulate the most sensitive demographic processes increased annualized net return by up to 13% and reduced long-term herbicide use by up to 26%. Results demonstrate that combining an economic optimum threshold with alternative weed management strategies may increase economic return and reduce herbicide use.

The potential of mourning doves (Zenaida macroura L.) as dispersers of leafy spurge (Euphorbia esula L. # EPHES) seeds was investigated by examing digestive tract contents and feces. Very few intact seeds passed through the digestive systems of wild or captive adult mourning doves. An average of less than one intact leafy spurge seed/g was recovered from fecal material found in dove nests. These results suggest that adult mourning doves are primarily consumers of leafy spurge seeds and rarely act as dispersal agents. It appears that dispersal is only possible when seeds passing through the digestive tracts of nestlings are deposited in the nest and the nest later falls.