Amaranth challenges soybean management and becomes a topic of debate

Technical and commercial aspects related to the difficulty of managing the purple amaranth weed (Amaranthus hybridusThe panel discussion on "Difficult-to-Control Weeds - Management Challenges," focusing on weeds in soybean cultivation, will take place on April 13th, from 10:30 AM to 12:00 PM, in the auditorium of the SmartAgro Pavilion at Expo Londrina, held in Londrina (PR).  

Organized by Embrapa Soja, with the participation of representatives from the Cocamar, Coamo, and Integrada cooperatives, the panel aims to discuss the main problems faced by producers in the last harvest regarding weed management in the crop.

Management challenges  

Embrapa Soja researcher Rafael Romero Mendes will lead the discussion, contextualizing the topic. According to him, the infestation of amaranth has increased significantly in Rio Grande do Sul over the past four harvests; however, in the last harvest, the difficulty in managing the weed also put producers in Santa Catarina, Paraná, and São Paulo on alert. There are several species of amaranth, but the main ones share similar aggressive characteristics. In general, amaranth tends to have rapid and aggressive growth, small seeds, ease of dispersal, and a high germination capacity.

According to Mendes, the increase in infested areas is due to the weed's resistance to some herbicides and, mainly, the dissemination of seeds via machinery. "Therefore, we recommend preventive management measures such as cleaning the machinery, which is a simple action that can reduce seed dispersal. Another valid measure is to keep the soil always covered with mulch to reduce pigweed germination," Mendes indicates.

According to the researcher, the use of cultivars with new biotechnologies is also an alternative management strategy. And, when it comes to the use of chemicals, he says there are herbicide options that can be used in pre-emergence of soybeans to prevent the germination of weeds. “The indication for use will depend on whether the area itself or neighboring areas have pigweed plants. In the case of glyphosate resistance, the use of pre-emergent herbicides is indispensable,” he explains.

Use of pre-emergents 

The increased use of pre-emergent herbicides in controlling weeds, such as pigweed (Amaranthus hybridusPre-emergent herbicides have been used in recent harvests, but the practice requires specific care and can pose risks to the soybean crop if not properly managed. Mendes says that pre-emergent herbicides are recommended for their efficiency in preventing weed germination soon after planting.

However, each product has specific characteristics and presents distinct effects depending on the type of soil, climatic conditions, and the soybean cultivar used. "One of the main points of attention is the risk of phytotoxicity—that is, damage that the herbicide can cause to the crop itself," he warns. "Although herbicides are developed not to affect soybeans, product residues in the soil can cause symptoms such as plant injuries, irregular emergence, and stand failures," he assesses.

Impurities in soybean batches 

In addition to agronomic challenges, another issue that stands out is the cases of Brazilian soybean shipments rejected by international buyers, such as China, due to the presence of weed seeds, including pigweed. "Even within levels considered acceptable, some shipments were refused, highlighting the commercial rigor and decision-making power of importers," says Mendes.

This scenario has reignited the debate about the importance of efficient weed management throughout the entire crop cycle. "There are no isolated solutions, but control must be well executed from the beginning, preventing invasive plants from completing their cycle and producing seeds that could contaminate the harvest," he advises.