Weed scenario in corn cultivation in the Midwest region

Corn cultivation in the Center-West of Brazil predominantly occurs after the cultivation of early soybeans, in the off-season period. Among the factors that limit productivity, one of the main problems faced by farmers is the presence of weeds which, if not controlled, can cause losses of over 80%.

There are several weeds present in corn crops, such as: viola string (Ipomoeas spp.), fire extinguisher (Alternanthera tenella), and crabgrass (Digitaria horizontalis). In addition to these species, wild sorghum (Sorghum arundinaceum), the button broom (Spermacoce verticillata), and the caruru (Amaranthus hybridus). Both the broom and the pigweed become important, mainly due to their aggressiveness and difficulty in controlling them.

Due to the continuous use of the same mechanism of action, the number of herbicide-resistant weeds has increased in the world and in Brazil. Worldwide, 262 species are already known, infesting 93 crops in 70 countries. In Brazil there are 29.

The introduction of glyphosate-tolerant soybean, corn and cotton cultivars has contributed to the increase in the number of species resistant to this herbicide. In Brazil, corn tolerance to herbicide is 73% in the summer and 66% in the second harvest. In soybeans, above 90%. The first weed resistant to glyphosate in Brazil, in corn, was described in 2005, in Rio Grande do Sul and São Paulo. Since then, five more species, including those with multiple resistances, have been added.  

The Brazilian Center-West stands out as the main corn producing region in the country, mostly occurring during the off-season. This fact indicates that weed management in crops is carried out, on a larger scale, in places with more restricted water regimes and higher temperatures. These conditions differ from summer planting conditions, when precipitation and temperature are higher and the environment is more humid. Therefore, the presence of weeds in the dry winter tends to be smaller and present less diversity than in the summer. Second-crop corn cultivation in the Midwest begins at the end of summer and continues into winter.

Low precipitation associated with high temperatures can be an ally in reducing weed infestation. Chemical control is the method most used by farmers. However, application is often carried out under unfavorable conditions. It is common for applications to occur outside the recommended stage, reducing their effectiveness and favoring the selection of resistant biotypes. 

Another factor for reducing the effectiveness of herbicides is the timing of spraying. Under conditions of water stress, plants present an imbalance between water absorption through the roots and transpiration through the leaves. This fact can lead to the closure of the stomata, consequently causing the plant to stop growing. As a result, the penetration, absorption and translocation of herbicides are impaired. High temperatures and low relative humidity also contribute to the loss of effectiveness due to increased volatilization and evaporation of spray droplets. However, there are adjuvants and spray tips on the market that mitigate these effects.

Care must also be taken regarding wind and speed at the time of application, which can contribute to increased drift of herbicides outside the spraying area, causing damage to sensitive crops/plants or even intoxication of people.

For chemical control, rural producers must bear in mind that there is current legislation that needs to be respected. These products must only be purchased through an agronomic recipe issued by a qualified professional, who must indicate the necessary PPE, as well as technical recommendations for correct use. 366 commercial brands of herbicides are registered in corn, referring to 32 active ingredients.

Among the main weeds present in the Midwest are:

Trapoeraba (Commelina spp.): a species that is difficult to control, due to its reproductive capacity through aerial and underground seeds and rooting of portions of the stem. Several products are registered to control this species and the timing of application makes a difference.

 Bittergrass (Digitaria insularis): perennial species, which is spread by seeds and rhizomes. Its seeds are light and easily dispersed by the wind. Currently, glyphosate-resistant bittergrass is present in practically all soybean areas and, consequently, in corn crops sown in succession.

Buva (Conyza spp.): in Brazil three species of Conyza (1) bonariensis (2) canadensis and (3) sumatrensis. Horseweed is an annual plant that propagates by seeds. The three species already have biotypes resistant to the herbicide glyphosate.

Crow's foot grass (Eleusine indica): It is classified as one of the five most problematic weed species in the world. It is present in all regions of the country, but with few areas with high infestation, when compared to other species.

Button broom (Spermacoce verticillata): it is a perennial plant, that is, it can have a life cycle of more than two years when left uncontrolled. The tendency of this species is to increase its incidence in crops, if producers do not adopt more appropriate management in production systems.

Custodio grass (Pennisetum setosum): perennial plant that can reproduce by seeds or rhizomes. The difficulty of these species is associated in the same way as other perennial grasses. In advanced stages, this plant becomes clumpy, making it difficult to control.

Volunteer Plants: This is a common problem that farmers are facing, especially when they are sowing crops with the same transgenic plant in succession. Soybean and cotton plants have been found in corn crops, which will cause loss of crop yield, in addition to being hosts for pests and diseases. Control of these volunteers must be carried out early: in corn, weed management must be carried out between 20 and 40 days.

Off-season management: weed management during the off-season is often neglected by producers, which can contribute to an increase in the area's seed bank and crop production costs. Furthermore, the producer must pay attention to the growth stage of the plants, since younger plants are easier to control.

Use of soil cover (integration, brachiaria, millet, among others): The crop control method is a weed management strategy that has been used by an increasing number of growers. The effect of mulch on the soil has been known for a long time, but in recent times this practice has been compromised. Currently, the producer has the possibility of working with brachiaria associated or not with other species. The producer may also make use of other species, such as millet, jack beans, mucuna, black oats, white oats, forage turnips and sorghum. For weed management, the quantity and distribution of straw that the cover species will leave on the soil is important.