The emergence of weeds occurred together with the development of agriculture about 12,000 years ago. In the beginning, there was greater balance between the various species. With population growth and, consequently, greater human interference with crops, a gradual process of selection began. Species without agricultural interest began to be neglected and their multiplication and high adaptability allowed them to survive in various environments, making it necessary to adopt control measures.
Currently, there is no doubt that the presence of weeds causes damage to farmers. On average, about 20-30% of a crop's production cost is due to the cost of weed control. They are usually fast-growing and easy to spread, produce large numbers of seeds and grow under adverse conditions.
Its losses are directly related to losses in productivity and product quality. A typical example is what occurs with black prickle seeds (Bidens pilosa) in cotton cultivation. Many species are also extremely harmful in seed fields as they may impede their certification. In grazing areas, species such as coffee (Palicourea marcgravii) and room officer (Asclepias curassavica) intoxicate the animals, causing them to die.
Other species may also parasitize plants. A common example in Brazil occurs with the chickweed (Phoradendron rubrum) in citrus, causing losses in plant production and death. In other countries, parasitism occurs in maize plants caused by witchweed (Striga lutea), a hard-to-control weed due to its development below the ground surface.
Indirectly, weeds can also affect crops, being host to pests and diseases. In the case of guanxumas (Sida ssp.), Most of them are host to the golden bean virus transmitted by the whitefly. Massambará grass (Sorghum halepense), host of the sugarcane mosaic virus, is generally present in sugarcane fields. Other indirect damages occur due to the difficulty of harvesting in the presence of weeds such as viola string (Ipomoea sp), tick (Cenchrus echinatus), cat's claw (Acassia plumose) and mucuna (Mucuna pruriens) or even after management. , depreciating the value of the land, as is the case of the tyrant (Cyperus rotundus).
In addition to agricultural areas, weed management is also important in industrial areas, railways, public spaces or even dams for power generation. In these bodies of water, species such as cattail (Typha angustifolia) and water hyacinth (Eichornia crassipes) proliferate rapidly. During the process of water passing through the turbines, the dense mass of these species interrupts the normal flow, damages and costs the maintenance of the equipment. Other plants such as Salvinia (Salvinia molesta) and water lettuce (Pistia stratiotes) increase water losses as they reduce their use.
However, many benefits can also be gained from knowing and using weeds correctly. Due to their growth rate, many are used to control erosion in degraded areas such as grasses (Brachiaria brizantha, B. decumbens, B. ruziziensis and Panicum maximum). Others are used in phytoremediation processes, allowing the removal or degradation of residual compounds in the soil, ie, in addition to absorbing water and nutrients that make them grow, some species are capable of absorbing polluting elements, acting as biological filters, as an example for Stizolobium atrium, Lupinus albus and Canavalia ensiform, among others.
Many weed species can also be used in medicine for herbal treatment from leaf infusion, such as the popular use of stone breakers (Phyllanthus niruri) or nettle (Urtica dioica), or providing numerous compounds for their extraction. Some weeds also serve as food for natural enemies or even honey bees, such as the asapex (Vernonia polyanthes) and the button broom (Borreria verticillata).
Thus, with so many advantages and disadvantages, we should use good weed management practices, especially in the agricultural environment. Integrated management is one of the practices that begins with the choice of sowing site and variety, which should be appropriate to fertility and soil pH. Seeding density and correct spacing also facilitate its management during cultivation. Chemical management, although very important, must take into account plant dynamics, cultural practices and the sowing system used, as well as available technology.
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Post-cultivation management is another very important issue in reducing
