In laboratory experiments, we studied barnyardgrass, common lambsquarters, and redroot pigweed seed dormancy release by alternating temperatures under different soil water regimes, to determine whether that response serves as a mechanism for assuring seedling establishment. Alternating temperatures were important in breaking seed dormancy in the three weeds, but the species' response to alternating temperatures differed with soil water content. For redroot pigweed and newly dispersed seeds of barnyardgrass, dormancy breaking by alternating temperatures occurred only when soil water content was high enough for germination. Moreover, adverse incubation conditions (constant temperature and low water content) induced secondary dormancy in these seeds. Seventy percent of seeds incubated in open boxes emerged before the soil reached wilting point, when seeds were incubated at temperatures high enough for a high germination rate. A dormancy breaking mechanism that triggers a high rate of germination may be important in assuring success of seedling establishment where rainfall patterns result in wetting and drying cycles of the soil, especially in soil types that lose water rapidly from the surface layers.

Flower production and pollen dispersal patterns of diclofop-methyl-susceptible and -resistant Italian ryegrass were examined to determine whether evolution of herbicide resistance could be controlled by cross-fertilization between biotypes. The susceptible and resistant ryegrass biotypes differed in timing and abundance of both ovule production and pollen release such that pollen from the susceptible plants had a much greater chance of fertilizing the resistant plant population than vice versa. Susceptible ryegrass, growing with or without wheat competition, produced more than 60% of its seeds before any pollen from resistant plants was released. In contrast, throughout the course of resistant plant seed generation, pollen from susceptible plants composed at least 30% of the total pollen load. These phenological differences, in conjunction with herbicide treatment results for third-generation ryegrass seedlings, suggest that gene flow can be used to reduce development of diclofop-methyl resistance in ryegrass populations within wheat cropping systems. In a wheat field infested with resistant ryegrass, cessation of herbicide application and sowing of a susceptible ryegrass biotype can be expected to reduce the evolution of diclofop-methyl resistance by at least 6% per year.

Laboratory and field experiments were conducted to determine whether manipulation of radiation environment during the period of winter wheat establishment can be used to improve wheat yields in Italian ryegrass-infested fields. Percentage of total irradiation and ratio of red (R) light (around 660 ηm) to far-red (FR) light (around 730 ηm) reaching the soil surface were important factors in regulating Italian ryegrass germination, growth, and competitive interactions with wheat. Reducing total irradiation to about 10% of full sunlight while maintaining the normal R/FR ratio of about 1.0 reduced wheat grain production in the presence of Italian ryegrass by about 40% compared with weed-free wheat in full sunlight. Further, reducing total irradiation to 3% of full sunlight plus reducing the R/FR ratio to about 0.2 reduced wheat grain production competing with Italian ryegrass by about 35% compared with production in the control conditions. Wheat production of dry matter, spikes, and seeds in the presence of Italian ryegrass increased more than threefold under both shading treatments compared with production in full sunlight.

The objective of this research was to modify the senior author's previously developed method for predicting yield losses in maize crops attributable to johnsongrass. The new method is based on the following assumptions: a) relative leaf frequency determines relative biomass; b) total biomass is constant despite the crop/weed ratio; and c) biomass, and therefore leaf frequency, are related to grain yield. Experiments in Argentina from 1986 to 1989 supported the above hypotheses, and the new method was more accurate than the old for predicting relative species biomass in johnsongrass/maize mixtures. Maize grain yield reductions associated with weed interference were also more accurately predicted.

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.

Overall efficacy of control practices for Johnsongrass was evaluated in soybean and sunflower cropping systems where timing of cultural and chemical control practices was adjusted to weed phenological stages by using visually estimated indices (stems 15 to 30 cm in height with 4 to 8 leaves) and with a thermal calendar model to forecast the crucial period for Johnsongrass propagation (when the population's rhizome mass is at a minimum). When visually estimated indices were used, weed infestation at the end of one growing season remained constant for the soybean cropping system and was reduced by about 70% in the sunflower crop. With the thermal calendar, weed infestation was reduced in both cropping systems by more than 90% in one growing season, and Johnsongrass was eradicated in two growing seasons. Although timing the control procedures to the crucial period severely hampered persistence of the weed in both cropping systems, the crop yield of sunflower was reduced by 35%. This yield reduction was caused by the delay in sowing date needed in order to adjust control procedures to the crucial period for propagation.

In Argentinean crop fields, weed control is mainly achieved by intense use of glyphosate as a nonselective and/or selective herbicide. Glyphosate use is very high as more than 95% of the 16 million ha soybean crop consists of glyphosate-resistant cultivars, always treated with this herbicide. From initial success, inconsistent glyphosate control of Johnsongrass, an invading C4 perennial grass of soybean crops, has become evident to producers from northern Argentina over the last 3 yr. Prior to this, glyphosate provided good control. This study evaluated the nature of these recurrent glyphosate failures in Johnsongrass. Experiments conducted with Johnsongrass plants obtained from seed and rhizome phytomers collected from fields with intense glyphosate use history showed that these populations showed differential survival and biomass productivity when glyphosate treated than Johnsongrass plants obtained from similar propagules collected from field sites with no history of glyphosate use. This empirical evidence establishes that the Johnsongrass survival in glyphosate-treated transgenic soybean fields from northern Argentina is due to evolved glyphosate resistance.

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.