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* A strategy that minimizes the number of cultivations often requires longer to achieve complete eradication. 6 Sample size for means shown in the other columns. c Interval between cultivations during the first year. Usually the interval was increased in subsequent years as the weeds weakened. d Plants buried to controlled depths. * A strategy that minimizes the number of cultivations often requires longer to achieve complete eradication. 6 Sample size for means shown in the other columns. c...

Seasonal and aseasonal germination

Weed seeds often undergo several changes in dormancy state between seed shed and germination (Figure 2.1) (Baskin & Baskin, 1985). These changes represent an adaptive response to the problem of immobility a seed has little control over where it lands, but through dormancy response to environment, it can choose when to germinate. When first shed from the parent plant, seeds may lack dormancy and be ready to germinate if environmental conditions are favorable. This is commonly the case for...

Knowledge and technology for weed management an historical perspective

Humans have been managing weeds for over 10000 years. The distinction between crops and weeds was one of the earliest human concepts distinguishing the beginning of agriculture (Rindos, 1984, pp. 137-43). Crops such as wheat and squash were among the first plants to be cultivated rather than simply gathered. Other species such as rye and maize were selected for deliberate planting and weeding somewhat later (Minc & Vandermeer, 1990). Certain species, after undergoing initial domestication,...

Effects of tillage practices on established weeds

Different tillage implements move the soil in different ways and therefore have substantially different effects on weed populations (Table 4.1). Moldboard plows invert the soil (Nichols & Reed, 1934), and consequently tend to bury growing weeds with relatively little dismemberment. Chisel plows and field cultivators invert the soil to a lesser extent than moldboard plows, but still tend to bury weeds. The primary action of a chisel plow is to create a wake of soil rolling back from the tool....

Management

Only a few studies have compared current herbicide programs with modern cultivation programs that include full-field and in-row implements. VanGessel et al. (1995b) examined various combinations of rotary hoeing, alachlor, in-row cultivation, and a post-emergence herbicide chosen by a decision aid. One rotary hoeing plus in-row cultivation resulted in weed control and yields similar to alachlor plus post-emergence herbicide. Schweizer, Westra & Lybecker (1994) similarly concluded that in-row...

T

Figure 4.17 Spinners. (Redrawn from Schweizer, Westra & Lybecker, 1994.) from properly set tools will be no more than a few percent, and is often negligible. Cultivation late on a sunny day when crop stems are less turgid reduces mortality. Several studies have examined spinners in conjunction with other in-row and near-row tools (Schweizer, Westra & Lybecker, 1994 VanGessel et al., 1995b Mohler, Frisch & Mt. Pleasant, 1997), but so far, controlled investigations ofthese tools have...

Weed management objectives

From the standpoint of crop protection, weed management has three principal objectives (1) Weed density should be reduced to tolerable levels. Experimental studies with a range of species indicate that the relationship between crop yield loss and weed density can be described by a rectangular hyperbola (Cousens, 1985 Weaver, Smits & Tan, 1987 Norris, 1992 Blackshaw, 1993 Knezevic, Weise & Swanton, 1994 Chikoye,Weise & Swanton, 1995).The specific parameters of this relationship change...

References

Commentary the idea of precision agriculture. Consortium News, 9,5-11. Madison, WI Consortium for Sustainable Agriculture Research and Extension. Amanor, K. (1993). Farmer experimentation and changing fallow ecology in the Krobo District of Ghana. In Cultivating Knowledge, ed. W. de Boef, K. Amanor & K. Wellard, pp. 35-43. London Intermediate Technology Publications. Ashby, J., Braun, A., Brekelbaum, T., Gracia, T., Guerrero, M., Quiros, C., & Roa, J. (1999). Investing...

Tillage pros and cons

Tillage prior to planting a crop can be used to meet a variety of objectives, including weed control, seedbed preparation, and residue management (Buckingham & Pauli, 1993, p. 2). From a weed management perspective, tillage re-initiates ecological succession, allowing dominance by early succes-sional annual crops rather than the perennial species that naturally come to dominate undisturbed vegetation. Tillage has been criticized as a cause of erosion and destroyer of soil tilth. Indeed, when...

Survival of weed seeds in the soil

The seeds of most annual and stationary perennial weeds persist in the seed bank for at least a few years, and many remain viable for decades if conditions are favorable. Excavations from dated archaeological strata indicate that some agricultural weed species, including Chenopodium album, Stellaria media, and Lamium purpureum, probably remain viable for several hundred years (Odum, 1965), although movement of younger seeds into the strata by soil animals cannot be excluded with certainty....

Life span and seed production

Life Span Annual Weed

The potential postgermination life span of weeds in agricultural systems varies from a few months to decades. In most arable cropping systems, actual life span is rarely more than a few years due to periodic tillage. Some annuals are truly monocarpic resources in vegetative tissues are remobilized to fill seeds and the plant senesces after seed set (e.g., Chenopodium album, Setariafaberi). However, many annuals shed seeds continuously through much of the growing season and for a substantial...

Conclusions

The discussion in the preceding sections indicates that agricultural weeds generally share certain properties, including small seed size, high relative growth rate, low early absolute growth rate, intolerance to stress, and high reproductive capacity. They differ from crops in most of these respects, and these differences form the basis for a variety of weed management tactics. Despite similarities among weeds, weed species differ with respect to longevity, ability to spread vegetatively,...

Tillage and the emergence of weed seedlings

Germination of weed seeds is more likely near the soil surface because seeds are more likely to experience light, fluctuating temperatures, and other factors that commonly promote germination (see Chapter 2). Moreover, if a seed does germinate, emergence from the soil is more likely if the seed is near Figure 4.3 Vertical distribution of seeds and soil markers following one or two operations with various tillage implements. Figure 4.3 Vertical distribution of seeds and soil markers following...

Weed patchiness and uncertainty the challenge to improving weed management

Weeds in a crop field are distributed irregularly, with patches of high density as well as patches with few weeds (Cardina, Johnson & Sparrow, 1997). Spatially these patches may be relatively stable from year to year, a product of localized seed rain, a relatively immobile seed bank, the clonal spread of vege-tatively propagated weeds, and the patchiness of the soil environment (Colbach, Forcella & Johnson, 2000). Although many weed species may be present in a field, only a limited number...

Survival after emergence

Rates of natural mortality due to disease, herbivory, and drought are usually low for established weeds in annual crops. In the absence of post-emergence weed control, survival rates for annual weeds from the cotyledon stage to maturity usually lie between 25 and 75 (Table 2.8) (Chancellor & Peters, 1972 Naylor, 1972 Sagar & Mortimer, 1976 Weiss, 1981 Mack & Pyke, 1983 Lapointe et al., 1984 Mohler & Callaway, 1992). Sometimes, however, rates of survival to maturity exceed 90 (Young,...

Why tillage promotes the germination of weed seeds

Tillage promotes germination of most agricultural weeds, provided the soil disturbance comes at a time of year when the seeds are not innately dormant. Agricultural weeds have adapted to respond to cues associated with soil disturbance because their small seedlings make them poor competitors early in life. Vigorous, well-established plants are unlikely to be present immediately after soil disturbance, and hence weedy species have been selected for germination under conditions that indicate soil...

Weeds from an ecological perspective

Weeds share certain ecological characteristics that distinguish them from other plants. Specifically, weeds are plants that are especially successful at colonizing disturbed, but potentially productive, sites and at maintaining their abundance under conditions of repeated disturbance. That is, weeds are the plants that thrive where soil and climate are favorable to plant growth, but disturbance frequently reduces competition among plants to low levels. Unlike previous conceptions of weediness...

Agroecosystem redesign

The agroecosystem redesign approach is characteristic of ecological weed management and involves a shift from linear, one-to-one relationships between target weeds and a particular weed management tactic, to webs of relationships between weeds, multiple weed management tactics, and other farming practices Liebman amp Gallandt, 1997 . Emphasis is placed on preventing weed problems and reducing requirements for purchased inputs through better use of ecological factors that stress and kill weeds....

Acute and chronic effects of herbicides on human health

Although much remains to be learned about the acute and chronic health impacts of herbicide use, public health reports and epidemiological studies indicate that certain herbicides can be responsible for direct, unintentional poisoning and may be associated with increased incidence of cancer and other disorders. Farmers, farm families, and agricultural workers are exposed to herbicides at higher concentrations than the general public and consequently may be subjected to greater health risks....

Seed persistence and weed management

Persistence of seeds in the soil has consequences for many aspects of weed management. By allowing a given generation of seeds to test the suitability of several growing seasons, a seed bank buffers annual species against a year in which little reproduction is possible Cohen, 1966 . This protects the weed against local extinction, but from the grower's point of view, it makes weeds with seed banks highly resistant to eradication. However, complete eradication is rarely necessary, and knowledge...

Human dispersal of weeds risks and potential reductions

Weed contamination of crop seed has been a major source of introductions at all scales from continents to individual fields. Weed seeds are still regularly transported between countries in seed shipments Tasrif et al., 1991 Huelma, Moody amp Mew, 1996 . Probably the only effective method for preventing this is inspection of large samples from every international shipment Tasrif et al., 1991 . This could be facilitated by computer visualization procedures that identify contaminated samples....

Stale seedbed and false seedbed

Since tillage promotes germination of many weed species see Chapter 2 , tillage followed by destruction of weed seedlings with minimal further soil disturbance often leads to lower weed density in the crop. This is referred to as the stale seedbed method of planting. The technique is especially useful for providing reduced competition early in the development of small-seeded or slowly establishing crops like onion and carrot. Much recent work has explored application of the technique to...

Fullfield and inrow cultivation tools weeding harrows rotary hoes rubberfinger weeders spinners torsion weeders and

The ecology behind all cultivators that attack weeds in the crop row is essentially the same disturb the shallow surface layer of soil above the rooting depth of the crop, thereby killing very small weeds without uprooting crop plants. All in-row and full-field cultivating tools are primarily successful against small-seeded weeds that must germinate near the soil surface to emerge, and in large-seeded crops that can be planted relatively deeply. Conversely, these tools are ineffective against...

Proposal

The development of ecological weed management depends on the collective ability of farmers, extensionists, and scientists to convert local weed information into an improved understanding of weed ecology. Special attention must be taken to develop principles of weed ecology that are applicable in improved farmer planning and decision-making. The following proposal offers a starting point for a learning to learn process on farmers' management of weed patchiness and uncertainty. The process should...

Basic principles of mechanical weeding

This and succeeding sections discuss methods and implements for physically removing weeds from crops. Most of these implements act by cutting or uprooting the weeds with tools that disturb the soil. These implements are commonly referred to as cultivators. In addition to this large class of implements, thermal and electric weeders damage weed tissues by a discharge of heat, cold, or electricity. The most common of these are the various types of flame weeders. Other implements include weed...

Population dynamics of annual weeds in ridge tillage

Weed Population Dynamics

The effect of ridge tillage on weeds is distinctive from other types of tillage in that seeds do not move randomly relative to the crop row. In ridge tillage sometimes referred to as till-plant a crop is planted on ridges formed the previous growing season Figure 4.8 . During planting, the surface of the ridge is scraped into the inter-row valleys. Seeds that were shed onto the ridge the previous season are thus moved to the valleys where seedlings can be destroyed by inter-row cultivation....

Nearrow tools vegetable knives disk hillers spyders basket weeders and brush weeders

Several tools have been invented to cultivate 5 to 12 cm from the crop. To avoid root damage, typical working depth for near-row tools is shallow 2 to 5 cm. Furthermore, when working close to the crop, soil must flow either parallel with, or away from, the row to avoid burying small crop plants. Basket weeders and brush weeders can work closer to the row than disk hillers and spyders because their rotation is parallel to the row and thus the leading edge is no closer to the row than is the...

Ground cover in coffee farmers observe weeds and propose management alternatives

In Central America coffee is often produced on sloped land, either under shade with low inputs or in open sun with pesticides and high levels of fertilizers Rice amp Ward, 1996 . Yields vary from 200 to 2700 kg of green beans ha_1, depending on soils, climates, farm size, and farmer resources. The diversity of growing conditions and the layout of coffee fields create spatial and temporal variability in weeds, insect pests, and diseases. Improving yields and lowering production costs requires...

Cultivator guidance systems

Cultivation is an exacting task that is often tiring for the tractor operator. It causes crop loss if not done carefully. These problems are multiplied when using tools that have to travel at a precise position relative to the crop row. Consequently, some rear-mounted implements are designed to be steered by a rider Geier amp Vogtmann, 1987 Melander, 1997 , but this is a laborintensive solution. Fortunately, automation of implement and tractor guidance is advancing rapidly. The simplest...

Herbicide drift

Herbicides can contaminate off-target sites by moving in air as well as in water. Generally, herbicide drift from tractor-mounted sprayers is about 5 to 10 of the material applied, with most off-site deposition occurring within 20m of field edges Freemark amp Boutin, 1995 . However, depending on meteorological conditions, application equipment, and physical characteristics of herbicide products, spray drift concentrations of 0.02 to 2 of application rates may occur at distances as great as 400...

Exposure of perennating organs to desiccation and freezing

Desiccation of roots and rhizomes by summer fallowing was a traditional method of controlling wandering perennials in Europe. The soil was plowed and allowed to dry into clods. These were stirred occasionally with a plow or heavy cultivator to completely desiccate roots and rhizomes Bates, 1948 Travers, 1950 . Foster 1989 indicated that the same procedure can be used to manage Rumex obtusifolius and R. crispus. In Botswana, Phillips 1993 found that an extra moldboard plowing substantially...

Interrow cultivators shovels and sweeps rolling cultivators and rotary tillers

Rotary Weeder

Tools designed to work between crop rows can dig moderately deeply typically 5 to 10 cm without harming the crop. Consequently, complete destruction of even large weeds is often possible during inter-row cultivation. However, unless precautions are taken, vigorous soil movement can bury young crop plants, and deep digging can damage the roots of larger crops. To avoid crop damage, these tools usually work only 50 to 70 of the soil surface, leaving weeds in the crop row unharmed. However,...

Thermal and electric weeders

Thermal Weeder

Flame weeders briefly expose weeds to a propane or butane flame at 800-1000 C Ascard, 1995a , which disrupts cell membranes and leads to rapid dehydration Ellwanger, Bingham amp Chapell, 1973 Ellwanger et al., 1973 . A bank of burners can flame a wide area to kill weeds before crop planting or crop emergence, or to defoliate plants prior to harvest Tawczynski, 1990 . Shielding such machines to contain the heat increases their efficiency. Irrigation a few days before planting ensures that the...

Adaptations for seed dispersal

Weed species have a variety of adaptations for dispersal. Many members of the Compositae and other groups e.g., Asclepias and Epilobium spp. have hairs attached to the seeds that provide buoyancy to aid dispersal by wind. In other species, the plant breaks off at ground level e.g., Amaranthus albus, Sisymbrium altissimum , or the inflorescence detaches as a unit e.g., Panicum capillare and rolls with the wind over the ground surface. Seeds of some species have hairs, spines, or hooks that adapt...