One of the factors that most affect agricultural yield and productivity is the occurrence of weeds. Such plants assume great importance because they have direct effects on the main crop, such as interference (competition and allelopathy combined) and consequent loss of yield, as well as indirect effects, such as increased production cost, obstacles to the harvest, depreciation of product quality, and hosting pests and diseases. Estimated losses caused by weeds can, in cases no control is made, reach over 90%, and in grain production such losses average 13% to 15% of the total.

The first concept of weeds appeared in ancient times, when man started agricultural activities and selected plants that were considered useful (cultivated plants) from those considered useless (invasive plants). Nowadays, weeds encompass all the plants that persistently interfere with the growth of cultivated ones and that have a negative impact on human activities, and are thus deemed undesirable. This type of plant usually grows in adverse conditions, such as dry or wet environments, at low or high temperatures and varied types of soil. These plants have the capacity to abundantly produce viable seeds, with varied kinds of dispersion, and present resistance to pests and diseases.

Out of the 350,000 known plant species, only 3,000 are cultivated; and approximately 250 are universally considered weeds, of which about 40% belong to only two families: Poaceae (grasses) and Asteraceae (composites)

Weed characteristics

Weeds are able to adapt to different places, under the most diverse types of limitations to growth and development. In light of this trait, such plants obtain necessary natural resources (water, light and nutrients) more easily, thus becoming large competitors amidst the crops. Therefore, if the crop emerges first, this competition can become less aggressive, depending on the habits and the density of the weeds in the field. Some of these plants decrease their competition with the crop through the allelopathic effect they produce. Such effect occurs through the release of toxins that penetrate the soil and hinder the normal growth of other plants, including the crop.

Due to their competitive nature, weeds ensure their perpetuity through dormancy and disuniform seed germination. Such capabilities make the control of invasive species difficult due to the fact they do not all  germinate at the same time, even in ideal conditions of temperature, moisture and light.

The development of the invading plants is fast, capable of reaching maturity in little time. Seed production is high (it is produced in large amounts), however, this is not the only way invasive species reproduce; some species also have the ability to reproduce through bulbs, tubercles, rhizomes and rooting.

Weed interference

Weed infestation in croplands decreases their commercial value, and can even make agricultural exploration impracticable. The negative effects of their presence in farmlands include competition for limited resources. An example of the latter is the fact of that nematodes were found in the roots of invasive species, such as Alternanthera tenella, Indian goosegrass (Eleusine indica), hairy indigo (Indigofera hirsuta) and billygoat weed (Ageratum conyzoides), thus representing potential risk for maize and succeeding crops.

The level of interference weeds impose on crops is determined by the species that occur in the area, the spatial distribution of the infesting community, the period weed and crop coexist, and the environment. Competition for essential nutrients is of great importance since they are often limited.

For the appropriate realization of Integrated Weed Management (IWD), it is important and necessary that the species are correctly identified and so is their frequency in the area, since each species presents different potential to establish themselves and levels of aggressiveness, which can characteristically interfere with the crop.

The spectrum of infesting species that occur in Brazilian farms emcompasses both monocotyledonous plants, like plantain signalgrass (Urochloa plantaginea), signalgrass (Urochloa decumbens); southern sandbur (Cenchrus echinatus); crabgrass (Digitaria spp.), sourgrass (Digitaria insularis) and Indian goosegrass (Eleusine indica); and dicotyledons, which categorize the species: Alternanthera tenella; amaranth (Amaranthus spp.); balloonvine (Cardiospermum halicacabum); Spanish needles (Bidens pilosa); dayflower (Commelina spp.); horseweed (Conyza spp.); painted euphorbia or milkweed (Euphorbia heterophylla); morning glory (Ipomoea spp.); wild radish (Raphanus raphanistrum); Mexican clover (Richardia brasiliensis); sida (Sida spp.); and broadleaf buttonweed (Spermacoce latifolia), among many others.

Weed management goals

Integrated management intends to successfully reduce undesirable species during the critical period of competition, stage when co-living with weeds can cause irreversible damages to the crop, with subsequent damage to yield.

Management also provides for the optimization of the mechanized harvest, preventing the proliferation of weeds, guaranteing the production of maize in the following harvests.

Therefore, by using some method of weed control in the cultivation of maize, the grower should remember that the main goals are:

1. Preventing yield losses through competition

Production losses can vary from one year to another, due to weather conditions, soil variations (which vary from property to property), the weed population, and the management systems (crop rotation and no-till farming). Assessing a likely loss in production caused by the presence of weeds will help growers in the choice of the most efficient control method to be applied.

2. Optimizing the harvest

Weed control methods must be used to benefit the harvest and not only prevent their initial competition. The weeds that germinate, emerge and grow in the middle of the maize crop, after the critical period of competition, do not cause production losses. However, manual and/or the mechanized harvest can be hampered by the presence of such plants, as they can injure the hands of workers (as is the case of mimosa), or prevent machines from performing because the high infestation of morning glories and dayflowers, for instance, can jam the machine's cutting platform components.

3. Preventing increased infestation

The soil works as a seed bank for weeds, and, if nothing is done to prevent the production of seeds, the number of weedsd emerging every year will increase, causing reduced crop yield and increased dependence on the use of herbicides and, consequently, higher production costs, which could even cause the land to be abandoned. Therefore, one of the most important factors for management is maintaining the weed population at low infestation levels. For this purpose, some techniques can be adopted such as crop rotation and sowing cover crops and green manure. Cover crops, such as garden radish, oats, fodder vetch and millet, when cultivated between harvests, inhibit the emergence and development of weeds. The use of a brushcutter and/or the application of herbicides to desiccate weeds post-harvest can also be adopted so that the production of seeds and/or other propagules does not occur .

4. Protecting the environment

Integrated management is directly related to chemical control, which in the maize production system is performed in approximately 30% of the planted area. Herbicides are chemical substances that present different physical-chemical characteristics and thus a differentiated environmental behavior. Depending on the adsorption coefficient (K_d), on Henry's law constant, and especially on the half-life of the compound in the soil, air and water (T_1/2), the herbicide used can be a source of contamination of the environment. Volatile products, which turn into gases, can contaminate the air, while leacheable products, which move within the soil profile, can reach the water table, and the herbicides strongly embedded into the sediments can reach surface water deposits through erosion.

Weed resistance to herbicides

The use of the herbicides for chemical weed control has been a frequent tool for farmers, due to their practicity, economy and efficiency when compared with other methods. However, the indiscriminate use of herbicides caused the development of many cases of resistance to such compounds among several weed species. This process compromises obtaining high crop yields, resulting in increased production costs and making the use of certain herbicides impracticable.

Weed resistance is a biotype's ability to survive and reproduce after the application of a dose of herbicide that would normally control a normal population of this species. Tolerance, on the other hand, is given plants' capacity to withstand recommended doses of herbicide that control other species without suffering changes to their growth and/or development. Species like Commelina spp. (dayflower), Ipomoea spp. (morning glory), Spermacoce latifolia (broadleaf buttonweed) and Richardia brasilienses (Mexican clover) present differentiated levels of tolerance to 5-enolpyruvoylshikimate-3-phosphate (EPSP) inhibiting herbicides, which cannot thus be mistaken for resistance.

The occurrence of herbicide-resistant weeds will always be associated to genetic changes to the population in light of the selection caused by herbicide application. Genetic variability is present in infesting populations, and in case there is continued application of a product or of herbicides with the same mechanism of action, resistant plants will survive, increasing their frequency in the population in subsequent years.

Resistance can be simple, which only occurs for a herbicide; cross-resistance, which occurs for several herbicides with the same mechanism of action; and multiple resistance, which occurs for herbicides of different action mechanisms.

Weed resistance was first notified in 1957, with Commelina diffusa (dayflower) plants infesting sugarcane crops in Hawaii, and the herbicide 2,4D, which has the mechanism of action of mimicking auxins. After such selection, new cases of resistance have been reported, and globally there are 486 biotypes, in 253 species, present in 92 crops and 70 countries. Of the current 26 known herbicide mechanisms of action, 23 have already been mentioned in cases of weed resistance, in which 163 herbicides have already been reported.

In Brazil, the first stories of weed resistance to herbicides date from the 1980s, with the species Euphorbia heterophilla becoming resistant to the herbicides that inhibit the acetolactate synthase (ALS) enzyme. Currently, 27 species from different botanical families are already known to have resistance to herbicides that have been registered for agricultural use in Brazil.

Embrapa's studies on weeds

The areas pursued by weed research at Embrapa include:

1. Weed survey and mapping

2. Genetic characterization of weeds

3. Weed population dynamics (competition and allelopathy)

4. Study of weed seed bank

5. Weed control through cropping with emphasis on production systems with crop rotation and succession 

6. Chemical weed control

7. Integrated Management of Weeds

8. Herbicide application technology

9. Studies on the residual effect of herbicides in succession crops (carryover)

10. Herbicide mechanisms of action

11. Adjustments to the use of precision agriculture techniques in weed control

Cost of weed resistance to herbicides

The main costs of weed resistance to herbicide products are related to the need to use alternative products and also to crop yield losses, because of their competition, as they remain in the land after the application (Figure 1). The cost from alternative herbicides is variable according to the option adopted by the farmer, once there is often more than one product recommended for the management of resistant populations.

Figure 1. Percentage variation of the cost of the management of herbicide resistant weeds.