Fossil fuel burning is the most widely-understood source of excess carbon dioxide in the air. Less well known are the contributions of deforestation and net losses of soil organic matter to the annual carbon dioxide burden. All living vegetation, including weeds, absorbs carbon dioxide from the air and converts it into plant tissue. When the plant dies or sheds its leaves, part of this fixed carbon is returned to the atmosphere as carbon dioxide during decay, while part remains behind in the soil as stabilized organic matter, from which additional carbon dioxide is released only slowly.
Farming systems that cause a net decrease in soil organic matter also entail a net release of carbon dioxide, whereas systems that achieve a net increase in soil organic matter sequester carbon into the soil.
What does this have to do with weed management? Every tillage operation stimulates some degree of organic matter oxidation and carbon dioxide release. The less bare soil in the crop rotation, and the less tillage used for weed control, the less carbon dioxide the farm adds to the air.
Weed-preventive measures that increase crop cover and build soil organic matter are beneficial to the global carbon balance. Research has shown that, with best weed and soil management practices, organic and sustainable farms can make a net contribution toward addressing the global climate crisis. Cover crops are especially effective in sequestering carbon. Fast-growing summer annuals like pearl millet, sorghum—sudangrass, forage soybean, and sunnhemp can generate two to four tons per acre of biomass over a two to three month growth cycle Schonbeck, ; Clark, This rivals or exceeds the carbon uptake of solid stands of the most aggressive weeds, and of course the cover crop is far easier to manage!
Thus, in the eastern United States, where natural plant communities consist of mixed hardwood forest, even the best managed vegetable field will have more weeds than a mature orchard with a grass—forbs understory. In the Midwest, a perennial grass—legume pasture or hayfield with several forage species mixed together will likely have fewer weeds than a corn—soybean rotation. Permaculture , an alternative food production system that aims to mimic local natural plant communities and ecological processes, strives to keep all available niches occupied by useful vegetation year round, thus leaving minimal space for troublesome weeds Mollison, Permaculture emphasizes the use of perennial food-producing crops, such as fruit- and nut-bearing trees and shrubs, with desired annual vegetables intercropped with perennials or grown in relatively small patches.
Agroforestry , the practice of integrating tree crops fruits, nuts, timber, wildlife habitat with pasture or hayfields, simulates the savanna ecosystems of subhumid climates. The permaculture approach may be impractical for farms whose main cash crops are annual vegetables that require a prepared seedbed to get established.
However, the grower can close off some weed niches through preventive measures based on an ecological understanding of weed—crop interactions. A diverse crop rotation tends to grow fewer weeds than continuous corn or other monoculture. Two permaculture concepts that are relevant to annual cropping systems are multifunctional components i. Cover crops that protect and restore soil, harbor desirable insects, and suppress weeds are an example of a multifunctional component.
Managing a problem weed with cover crops and livestock grazing as well as timely cultivation is an example of multiple tactics. Kudzu Pueraria thunbergiana , imported from Japan as a forage crop, is one dramatic example whose enormous vines can cover and kill large trees in the southeastern US. Some of our major agricultural weeds were intentionally brought to the United States from overseas to provide food or forage.
European colonists carried common lambsquarters Chenopodium album to the New World as a vital source of early season greens that prevented scurvy and other nutritional deficiency conditions. Livestock farmers imported bermudagrass Cynodon dactylon, 2nd worst weed , johnsongrass Sorghum halapense , 6th worst , and quackgrass Elytrigia repens , a major weed of vegetable crops in the northeastern US to this country for their utility as forages.
Still others arrived by accident as a seed contaminant in imported crop seed, feed grain, foods, bedding plants, or other materials. However, some newly introduced plants growing in the absence of the natural enemies with which they evolved may spread unchecked, choking out native vegetation as well as invading pastures or cultivated fields.
Examples include water hyacinth Eichhornia crassipes and purple loosestrife Lythrum salicaria in wetlands; Canada thistle Cirsium arvense , musk thistle Carduus nutans , spotted knapweed Centaurea maculosa , and St. Johnswort Hypericum perforatum in rangeland; and autumn olive Elaegnus umbellata and tree-of-heaven Ailanthus altissimus in eastern deciduous woodlands. Classical biological control—the introduction of herbivorous insects or microbial pathogens that are natural enemies of these weeds in their native lands—has been used with considerable success to combat some invasive exotic weeds.
Arrival of a new invasive exotic weed on an organic farm is one instance that may justify efforts to eradicate the new arrival. Early detection—combined with an understanding of the ecology of the weed—is vital for successful elimination of the invader. Most weeds are pioneer plant species that have evolved various traits that adapt them to thrive and reproduce successfully in recently-disturbed habitats.
Each kind of disturbance presents certain stresses temperature extremes, exposed subsoil, poor retention of moisture, or—in the case of agriculture—the likelihood of frequent habitat disruptions and certain opportunities removal of shade and competing vegetation, release of soluble plant nutrients. Weeds are plants that have evolved mechanisms to cope with stresses and exploit the opportunities of disturbance Mohler, a , including:.
Few plant species possess all of the above traits, and most weeds display combinations of several traits that equip them to thrive in certain kinds of disturbed habitats. Weed floras that develop in frequently-tilled fields differ from those that develop in pastures or rangeland, while a no-till row cropping system elicits yet another weed flora.
In addition, long-term use of certain herbicides such as atrazine not an option for organic farmers has elicited yet another evolutionary response from the weed community: herbicide resistance. Organic vegetable cropping systems often entail frequent rotary tillage and shallow cultivation, and certain annual weeds tend to multiply even when a diverse rotation is practiced. These seeds often germinate in response to light, even a brief flash of light, which serves as a signal that the soil has been tilled or otherwise disturbed, and that competing vegetation has been cleared away.
All of these responses help explain why tillage often leads to a flush of weed emergence. One interesting and useful fact of weed ecology is the importance of light color or light quality for many small weed seeds.
Whereas direct sunlight or even direct moonlight! A closed crop canopy can thus shut off weed seed germination as well as retard growth of those weeds that do emerge. Farmers who moldboard-plow their fields regularly may encounter a different weed flora from those who rely mainly on the rototiller Mohler, b. These include large-seeded annual weeds, such as velvetleaf Abutilon theophrasti and common cocklebur Xanthium strumarium , whose seeds can emerge from a couple inches depth and survive in a dormant condition for many years when buried more deeply.
Reduced till and no-till cropping systems may elicit an increase in weedy grasses that can emerge from seeds lying on the soil surface, and perennial weeds like nutsedge and bermudagrass that develop from overwintering rhizomes and tubers. Grass seedlings and stout shoots from vegetative propagules can more easily emerge through the surface residues of no-till fields than newly-germinating broadleaf seedlings.
Overgrazing of pastures imposes a different set of stresses that elicit a different suite of problem weeds—those adapted to occupying bare or nearly-bare soil that has undergone removal of aboveground vegetation and compaction by livestock, but has not been loosened and aerated by tillage. These weeds tend to become troublesome to farmers over wide geographic ranges, sometimes around the world. Others are adapted to specific stresses, such as the extreme soil acidity or toxic excesses of trace elements characteristic of mining wastes these pioneer plants may do more good than harm!
Knowing what conditions tend to favor a particular weed species, and how that weed functions in relation to the plant community and ecosystem, can help the organic grower identify and change management practices that may be giving that weed an advantage over crops. In addition, small seeded weeds require and respond to available nutrients immediately after emergence, while larger-seeded crops like corn utilize them somewhat later.
Therefore, overfertilizing, or fertilizing too early in the season, can give the weeds a jump on the crop Mohler, undated slide presentation. Part of understanding weed ecology is knowing the weaknesses of particular weed species, which can become opportunities for more effective control.
Because pioneer plant species are adapted to establish in empty niches, many of them do not tolerate competition early in their life cycle. Thus, a vigorous crop that rapidly closes its canopy can shut out many newly-emerging weeds.
Galinsoga Galinsoga spp. It is essential to consider the range of potential host plants for key diseases and their vectors, as well as insect pests. Weeds may be broadleaf or grasses, and the potential for management depends on the type of weed present and crop being grown. The key is to understand the weeds which will be a problem in your crop.
There are five possible stages in which weed control will be most effective. These are pre-planting, transplanting, early in-crop, late in-crop, and post-harvest, when the paddock is fallow.
Management is easier and more effective where longer rotations and suitable break crops to reduce weed seed bank are used.
Successful weed control commences with transplanting into a weed free seed bed. Best results are obtained using a combination of cultural and chemical controls. But this can be difficult in intensive production systems where crop rotation times are tight. This is the most critical time for obtaining good in-crop weed control, and is when most available herbicides for use are applied. Options for good control are limited if weeds develop in-crop. Broadleaf weeds in most vegetable crops can only be killed by cultivation or by inter-row spraying with a shielded spray unit.
Emerson plants that are competitive, persistent, pernicious, and interfere negatively with human activity Ross, et. Our human activities create weed problems since no plant is a "weed" in nature. Though we may try to manipulate nature for our own good, nature is persistent. Through the manipulation process, certain weeds are controlled, while, other more serious weeds may thrive because favorable growing conditions for them also have been meet.
Weeds are naturally strong competitors and those weeds that can best compete always tend to dominate. Both humans and nature are involved in plant breeding programs. The main difference between the two programs is that man breeds plants for yield, while nature breeds plants for survival.
Characteristics of weeds Certain characteristics are associated with and allow the survival of weeds. Weeds posses one or more of the following: a abundant seed production; b rapid population establishment; c seed dormancy; d long-term survival of buried seed; e adaptation for spread; f presence of vegetative reproductive structures; and g ability to occupy sites disturbed by human activities.
Other problems associated with weeds in agriculture include: a reduced crop quality by contaminating the commodity; b interference with harvest; c serve as hosts for crop diseases or provide shelter for insects to overwinter; d limit the choice of crop rotation sequences and cultural practices; and e production of chemical substances which are toxic to crop plants allelopathy , animals, or humans.
Costs of weeds Weeds are common on all million acres of U. Ross and Lembi, Benefits of weeds Despite the negative impacts of weeds, some plants usually thought of as weeds may actually provide some benefits. Some attributes include: soil stabilization; habitat and feed for wildlife, nectar for bees; aesthetic qualities; add organic matter; provide genetic reservoir; human consumption; and provide employment opportunities.
Weeds have a controversial nature. But to the agriculturist, they are plants that need to be controlled, in an economical and practical way, in order to produce food, feed, and fiber for humans and animals.
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