Field studies were conducted for three consecutive years to determine if PRE and/or POST herbicides were needed in addition to preplant foliar-applied glyphosate and POST cultivation for maximum seed yield of irrigated and nonirrigated soybean planted in stale and undisturbed seedbeds on clay soil. Soybean seed yields following the use of PRE and POST herbicides alone or in combination were similar in all years, and exceeded seed yield following the use of glyphosate plus POST cultivation only. Plantings made in no-till and fall-till seedbeds produced similar seed yields when both PRE and POST herbicides were used. These results indicate that glyphosate plus cultivation was not adequate for soybean in stale seedbed plantings, and that either PRE or POST herbicides, but not both, were required for maximum seed yield.

In field trials conducted in 1989 and 1990, fifteen postemergence herbicides were tested for phytotoxicity to kenaf. Clethodim (110 g ai ha–1), fluazifop (220 g ai ha–1), quizalofop (70 g ai ha–1), and sethoxydim (210 g ai ha–1) were not phytotoxic to kenaf in the cotyledonary stage. MSMA (2.2 kg ha–1) was not phytotoxic to kenaf at the cotyledonary or 35-cm stage. All other herbicides applied postemergence to kenaf caused significant injury.

The effect of barley seeding rate and row spacing, and triallate, diclofop, and difenzoquat herbicide rate on barley grain yield and quality, and wild oat control were evaluated in field experiments near Bonners Ferry, Idaho, in 1989 and 1990. The purpose of the study was to develop integrated control strategies for wild oat in spring barley. Barley row spacing (9 and 18 cm) did not affect barley grain yield. Barley grain yield was greatest when barley was seeded at 134 or 201 kg ha–1 compared to 67 kg ha–1. Wild oat control increased as wild oat herbicide rate increased and barley grain yield was greatest when wild oat herbicides were applied. However, barley grain yield was similar when wild oat biomass was reduced by either 65 or 85% by applications of half and full herbicide rates, respectively. Net return was greatest when the half rate of herbicide was applied to 100 wild oat plants per m2 and was greatest when half or full herbicide rates were applied to 290 wild oat plants per m2. Net return increased when the seeding rate was increased to 134 or 201 kg ha–1 when no herbicide was applied and when 290 wild oat plants per m2 were present.

Studies were conducted to determine the response of ‘Jewel’ sweetpotato in propagation beds to DCPA and napropamide. Napropamide at 2.2 and 4.4 kg ha–1 caused slight injury, and reduced plant number and weight when compared with the untreated control. Napropamide at 1.1 kg ha–1 combined with chloramben at 3.3 kg ha–1 was not injurious; plant production was equal to the untreated control, diphenamid at 4.4 kg ha–1, and chloramben at 3.3 kg ha–1. DCPA at 8.3 and 10.0 kg ha–1 caused unacceptable injury and reduced plant number and weight.

Factors affecting the germination and seedling growth of leafflower were evaluated. Germination required exposure to light. Seeds covered with a 1-cm deep layer of pine bark mulch failed to emerge. Maximum seedling development occurred in full exposure to sunlight; shading that reduced light intensity more than 26% was inhibitory. Maximum seed germination (79%) occurred between 25 and 35 C; temperatures outside this range were inhibitory. Germination was strongly influenced by osmotic potential. Osmotic potentials of 0, 304, and 507 kPa resulted in 85, 24 and 14% germination, respectively. PRE applications of either granular formulated oxadiazon at 4.5 kg ha–1, or premixed granular formulated oxyfluorfen + oryzalin at 4.6 + 2.3 kg ha–1, provided a minimum of 85% control. The following POST treatments provided a minimum of 90% control of mature plants: paraquat at 0.21 kg ha–1, glyphosate at 0.56 kg ha–1, oxyfluorfen at 1.12 kg ha–1, and acifluorfen at 2.24 kg ha–1.

Experiments were conducted to evaluate imazaquin and selected herbicides for POST control of wild garlic. At the same dosages, imazaquin applied in late November/early December (late fall) controlled wild garlic better than when applied in early March (late winter) when assessed in May. Imazaquin provided control equivalent to 1120 g ae ha–1 2,4-D plus imazaquin. Imazapyr, imazamethabenz, or imazethapyr at 280, 1120, or 560 g ai ha–1, respectively, provided control equivalent to 560 g ai ha–1 imazaquin. Thifensulfuron at 70 g ai ha–1 provided better control applied in late winter (85%) than late fall (45%). All herbicides consistently provided better control than 2,4-D plus mecoprop plus dicamba, except thifensulfuron applied in late fall, at the final evaluation. Common bermudagrass growth in early spring was delayed 2 mo by ≥ 560 g ha–1 imazaquin, 70 g ha–1 thifensulfuron, imazethapyr, and imazapyr or 2,4-D plus mecoprop plus dicamba applied in late winter. Imazamethabenz did not delay common bermudagrass growth. Late fall applications of 1120 g ha–1 imazaquin did not delay growth while ≥ 560 g ha–1 imazethapyr delayed common bermudagrass growth through April.

Dicamba and 2,4-D, which are used prior to seeding in conservation tillage systems, were applied in late fall and early spring to determine their residual effect on subsequent spring-seeded crops. The herbicides 2,4-D ester and 2,4-D amine with dicamba or glyphosate, applied 0 or 15 d prior to spring seeding, damaged canola, peas, lentils, and alfalfa. The legumes were damaged by spring-applied dicamba. There was also a slight reduction in the total dry matter yield of wheat and barley by 2,4-D applied in spring prior to seeding. All crops tested, except lentils, were tolerant of recommended fall applications of 2,4-D for the control of winter annual flixweed and field pennycress.

Crop enterprise budgets for eight crops prepared from a sample of 131 farms in the Chaouia region of Settat province, Morocco show that weed management is a component of crop production. Although all farmers used herbicides on small grains, weed control was accomplished primarily with hired and family labor. In general, labor is more plentiful than capital and the per hectare cost of using hired labor is low. The percentage of total cash expenditures assigned to weed management averaged more than 5% in each farm size group. However, there was substantial variability among crops. Average hours spent to manage weeds was nearly constant for the three groups. Barley received the least labor and corn or a food legume the most. Farmers of small farms used more family labor and those of large farms used more hired labor.

The discipline of weed science is entering a critical period in its development. Decisions made in the next few years will determine if weed science will remain weed control technology oriented or develop into a broad-based scientific discipline. Over the last 30 yr the evolution of weed science has been based on the development of weed control methods. Currently, most crop production systems rely very heavily on one weed control technology, herbicides. Agriculture is now in a debate with society over how food and fiber should be produced. Society is asking weed science, because of food safety and water quality concerns, to develop new methods to control weeds. To do this, weed science must put more emphasis on principles-based research that can provide the basic knowledge required to develop new weed control technology. Weed scientists must join forces with private citizens and organizations concerned with the effect of current agricultural practices on the environment, to obtain, through the political process, new resources for weed science research. These resources are needed to develop the basic principles that will support the development of new weed control technologies with reduced environmental impact. Weed scientists must decide if they are going to lead the direction of weed science research or be led reluctantly by others.

Weed science has been and will continue to be an evolving discipline. The evolution and future of weed science will depend greatly on our visions of the future and our willingness to effect and implement change. Thus, the principal question we must ask ourselves is, what do we want weed science to be in the future? This paper, which is only one part of the symposium on the Future of Weed Science, will address the issues involved with teaching. This requires identification of future students and employers, their educational needs based on professional goals, and packaging and presenting the information. Most future agriculture undergraduates and weed science graduate students will need to be recruited, will have minimal background in agriculture, and will include more minorities and women than in the past. We will offer a more interdisciplinary and internationalized curriculum to future students. The curriculum will include traditional courses with greater emphasis on communication skills, business and economics, computer science, agricultural ethics, and interpersonal skills. Courses will be taught to future students by a regional teaching concept that uses electronic media. Teaching and learning about weed science in the future will be exciting, challenging, and unquestionably different from today.

Weed Science Extension is in a period of change. Though there is decreased funding for extension in the United States and Europe, the number of programs including extension activities is increasing in developing countries as shown by World Bank funding. There is a perception problem with the name weed science; we need a strong, recognizable science name. Decreases in one-on-one contact method for teaching, fewer extension weed specialists, and the increased need for interdisciplinary projects to support Integrated Pest Management and Sustainable Agriculture will increase stresses on our positions. These factors also increase the challenge for weed specialists and extension advisors (agents) to become leaders in current programs, initiate new multidisciplinary programs, and choose those programs that will benefit our discipline. Our clientele and the method of communicating with them will change, presenting yet another challenge.

The agricultural-chemical industry has been offering solutions in plant protection for several decades. The primary role of Industry in weed control has mainly been to provide chemicals with increasingly attractive profiles regarding safe handling, ecological acceptance, and economical attractiveness. These objectives have been reached by significant developments, e.g. in rates, persistence, and specificity for non-target organisms. It is therefore safe to assume that in the year 2015 and beyond chemicals will continue to maintain a major role in weed control. However, as cropping systems and criteria for desirable control levels change, industry will have to change from a re-active to a pro-active participant in the development of integrated systems. Chemical solutions will be complemented by biological and agronomical methods and will be further supported by biotechnological successes in the crop area. In addition, it is anticipated that sophisticated computer models now in development will help exploit the potential of products as well as of integrated systems. Thus, fully integrated companies active in chemical, biological, and molecular-biological research and having branches in the agro-chemical as well as in the seed business will be especially suited to be driving forces in the changing world of modern weed control. The practice will ask for services much more than for single products. Industry will not only have to offer those services but at the same time assist in the education of growers to enable efficient use of the increasingly intricate methods of future weed control.

Threshold weed management methods have recently been elaborated to consider effects of threshold management on weed population dynamics. Such economic optimum thresholds are calculated using population-dynamics models which require detailed information about weed demography, including seed production (as affected by events between germination and seed dispersal), seed dispersal, and seed survival and movement in soil. Factors affecting any of these aspects of demography appear likely to modulate the growth rate of a sub-threshold population and therefore to influence the economic optimum threshold value. To test this conjecture and evaluate weed threshold management, including associated risk, improved understanding is particularly needed of weed seed dispersal, seedbank processes, and unpredictable demographic variation.

The concept of thresholds has many applications in weed science, depending on the response being measured. The most common adjectives used to describe thresholds are damage, economic, period, and action. Damage threshold is the term used to define the weed population at which a negative crop yield response is detected. An economic threshold is the weed population at which the cost of control is equal to the crop value increase from control of the weeds present. Economic threshold may be used to describe short-term effects of weed interference occurring in a single growing season, or multiple-season effects including some cost associated with seed produced by uncontrolled plants. The term period threshold implies that there are times during the crop cycle in which weeds are more or less damaging than at others. Action threshold is the point at which some control action is initiated, and usually includes economic considerations along with other less tangible factors such as aesthetics, risk aversion, or sociological pressures. Regardless of the type, thresholds imply that weed effects are population dependent, and as such, allow some type of prediction to be made relative to the consequences of control decisions. One successful approach to the implementation of thresholds has been through the development of computerized decision-aid software. These programs allow users to compare economic and environmental consequences of potential control actions before committing to one particular decision.

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.

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.

According to Grime's model, velvetleaf can be classified as a competitive-ruderal species. Its biological and ecological characteristics make it particularly competitive and persistent in cultivated fields. Herbicide control is difficult and, once established, it is almost impossible to avoid seed production. The economic threshold, on a single year basis, varies between 0.3 and 1.7 plants m–2. When the competitive relationships and their effects are evaluated, it is very difficult to apply a weed management system based on a single-season threshold. When one considers the agronomic implications of the ecological characteristics of velvetleaf over a longer period, it can be concluded that the use of a single-season economic threshold is probably not the best management strategy.

Barnyardgrass at 10 or more plants per 1 m of row caused over 80% root yield loss in spring-sown sugarbeets in California. The yield loss decreased to between 5 and 20% when barnyardgrass densities were one plant ever 2 to 3 m of crop row. The relationship between crop yield and barnyardgrass density was described by a rectangular hyperbolic function. The economic threshold density was in the range of one barnyardgrass plant every 5 to 20 m or row. Barnyardgrass plants growing at densities of less than one plant per 1 m in sugarbeets experienced only interspecific competition. Such individual barnyardgrass plants were estimated to produce from 30 000 to over 200 000 seeds, with an average of 100 000 seeds per plant. This seed production equated to a seed rain of about 4000 to nearly 20 000 seeds per m from a density of one weed every 10 m of crop row. These data suggest that using single-year economic thresholds for barnyardgrass is not a sound management practice because even low densities can produce large quantities of seed.