Occurrence of horseweed and bittergrass resistant to the herbicide glyphosate in the region where Grupo ABC operates

The theme “resistance” has been part of producers’ daily lives for some time, when soybeans were conventional and milkweed plants (Euphorbia heterophylla) and black pickerel plants (Bidens pilosa) were no longer controlled by ALS-inhibiting herbicides. .

Soon after, RR soybean tolerant to the application of the herbicide glyphosate in the post-emergence of the crop was widely adopted and this herbicide was used on a large scale, helping to control resistant milkweed and black pickerel plants. However, with the intensive use of this herbicide, other resistant species were once again selected and today problems with ryegrass are found in the southern region of Brazil (Annual ryegrass), horseweed (Conyza spp.) and bittergrass (Digitaria insularis) resistant to glyphosate.

New technologies will soon be available, but when not used properly, they may have a short useful life and result in increases in production costs. Therefore, weed management, when carried out in a sustainable way, guarantees greater survival of the products currently available and in the long term can result in lower costs in grain production.

An example are the strategies adopted by the technical assistance of cooperatives operating in the ABC Group Region, which comprises the region of Campos Gerais, south of São Paulo, Londrina and Goiás. In 2006 a small percentage of the assisted areas (28%) began using residual herbicides in soybean pre-emergence, even without the presence of glyphosate-resistant species; in 2008 there was a significant increase with 84% of the area adopting soybeans and to this day more than 70% of the area continues to adopt this strategy.

The use of residual herbicides, associated with crop rotation, soil cover in winter and weed control in the off-season resulted in a delay in cases of resistance to the herbicide glyphosate within the ABC Group's area of ​​operation. The first cases of glyphosate-resistant horseweed were reported in the country in 2005, while in areas that adopted these strategies they were only observed from 2012, that is, 7 years later. The same was observed for bittergrass, where the first cases occurred 5 years later.

Both in the management of horseweed and bittergrass, the off-season, especially in fallow areas, is a favorable period for their multiplication and in the case of dabuva the problem gets worse, since the longer the interval between the harvest of the summer crop and Winter sowing gives horseweed more time to develop and in the control of this species, the application stage is crucial to the success of the control.

In safrinha corn areas, autumn management is carried out immediately after harvest, when the horseweed plants are still small, ensuring good control with the available herbicides. It is important to remember that horseweed emerges in the field from April to September and soon after its emergence, chemical control is easier. In the 2017 winter harvest, the climatic conditions with the absence of precipitation in July led to the emergence of horseweed later only after the first rains in August, as a consequence we will be able to observe horseweed plants within the summer crop in the 17/18 harvest, which, If not managed at the appropriate time, they can result in greater production losses.

In addition to chemical control, the formation of straw in the system during the winter period is a tool used in horseweed management. In areas where the soil remains covered during the winter, straw forms a physical barrier that reduces the emergence of horseweed plants.

Unlike horseweed, bittergrass is a perennial plant and its emergency flow occurs throughout the year, but in February and March the largest flows may occur. The initial growth of this plant is slow, around 40 days, after this period the growth is very rapid and the plant perennializes and forms rhizomes. Therefore, management strategies must be adopted at the beginning of their development, ensuring their effectiveness.

Therefore, if we want sustainable weed management with lower costs and preserving the technologies currently available, we must take preventive measures throughout the agricultural year and not just within winter or summer crops.