Weed control in the off-season

The off-season is not only used to help reduce the weed seed bank, but also to manage some difficult-to-control species such as bittergrass and horseweed. These two weeds should not be combated by one product alone, but by a set of actions and herbicides applied sequentially. 

There is a consensus that to obtain good weed control in soybean crops, it is necessary to be attentive throughout the year. And the off-season is the best time to eliminate some species such as horseweed (Conyza spp.) and bittergrass (Digitaria insularis).

Soybeans in Brazil are cultivated over large territorial extensions, in different regions, each with very different characteristics in terms of climate and water regime, especially in the off-season. The South of Brazil, with its different micro-regions, is characterized by having lower temperatures during this period, while the Central-West stands out for having a more limited water regime and higher temperatures. Regardless of the characteristics of the regions and their climate and soil conditions, the off-season is a favorable time for weed plants to multiply and increase the seed bank in the soil. Some microregions may have little rainfall to the point of not allowing the cultivation or development of a crop of economic interest, but it may be enough for the germination and development of some weed species. For many species, such as wild peanut (Euphorbia heterophylla), black pickerel (Bidens spp.) and others, this is a great opportunity to replenish the seed bank, which will allow them to express their full interference potential during the summer period.

Controlling weeds is of fundamental importance for obtaining high yields, both in soybeans and in second-crop crops, as these plants can interfere not only with productivity, but also with the technical harvest coefficient, the percentage of impurities and moisture in grains.

The off season

Weed control in the second harvest is as important as during soybean cultivation in the main harvest. In the fallow period, between the cultivation of the second crop and the sowing of soybeans for the main crop, or even in autumn, there are good conditions for the multiplication of weed species. The weed seeds produced at this time, in addition to those already existing in the soil bank, infest the soybeans cultivated later with greater pressure than in areas well managed during this period. Although highly competitive, soybeans suffer the effects of this coexistence, as the weed seed bank is normally large enough to nullify its competitive capacity.

To demonstrate the effect of the potential for increasing the weed seed bank in the soil, a simulation was carried out during the off-season in a production area awaiting wheat sowing. The replenishment of the black picão and wild peanut seed bank was analyzed based on the number of plants (m-2) and the average number of seeds (m-2) produced by the plants present. 71 plants m-2 of black picão and 33 plants m-2 of wild peanut were counted. After this count, a calculation was made, simulating that 80% of the seeds produced were able to germinate and that two applications of herbicides were made with 90% efficiency. The increase in the seed bank would be 1954 and 144 seeds m-2 of black picão and wild peanut respectively. These species can have at least 2 to 3 germination flows, which allows for even greater replacement throughout the year. The results give an idea of ​​how easy it is to increase the seed bank, and indicate the importance of being concerned about this factor when establishing weed control programs. 

Management in the off-season also represents the opportunity to work the area at the ideal time to eliminate invasive plants such as horseweed and bittergrass, through the opportunity to use herbicides and doses without the risk of soybean phytotoxicity. This is the time when non-selective products such as paraquat, paraquat + diuron, glyphosate, 2,4-D, saflufenacil, ammonium glufosinate can be used, in addition to being possible to combine these herbicides with others, with or without residual action on the soil . The number of applications and doses to be used vary depending on the community present in the area and the stage of development of the weed plants.

Sequential applications in the off-season have provided excellent results, and are essential, especially when the species are more developed. In areas with a long interval between harvesting a crop (such as safrinha corn) and soybean sowing, the first application can be made approximately 15 days to 20 days after corn harvesting and the second before soybean sowing. .

Bittergrass

The biggest challenge that farmers face with this species occurs when they need to manage adult and clumped plants. Under these conditions, control with a single application may leave something to be desired and allow plants to resprout. Research results have shown that planning sequential applications is essential for these cases. The ideal time to start controlling bittergrass is in the off-season, using glyphosate (normally intended to eliminate other species) and a post-emergence graminicide (essential when dealing with glyphosate-resistant biotypes). Pre-emergent herbicides can also be used to prevent seed emergence. The program can be complemented with contact herbicides to accelerate control or to facilitate seeding in the area. A sequential application may be necessary before soybean sowing, if elimination is not complete and regrowth occurs. After the emergence of the crop and if there is the presence of bittergrass plants, a new application of graminicide must be made, but now in smaller doses, as new plants with up to 3 to 4 tillers are more sensitive. The application of herbicides to plants mowed mechanically or by the harvester's cutting bar should only be carried out when the plants show good vegetative development, with regrowth, approximately 30 cm in height, and in suitable climatic conditions. The mowing height should preferably be around 10 cm.

Control in areas also infested with horseweed may involve the use of latifolicides (such as 2,4-D) and graminicides. Depending on the graminicide, working conditions, climate, age of the plants, size of the clumps and the dose used, this combination can result in incompatibility problems and reduce the efficiency of graminicides (ACCase inhibitors).

Horseweed

The difficulty of chemical control of horseweed is associated with the size of the plants, especially when they are above 10 cm, and when the population is resistant to glyphosate. In areas with the presence of resistant biotypes, glyphosate must be associated with other mechanisms of action, such as 2,4-D and/or chlorimuron, in addition to residual products such as diclosulam, imazaquin, metribuzin or flumioxazin. Herbicides with residual action can help reduce weed emergence in the period prior to soybean sowing. This application can be complemented to control leaks with the use of non-selective contact desiccants, such as saflufenacil, paraquat +diuron, glufosinate ammonium. It is important to emphasize that horseweed must be controlled before soybean sowing, as combating it in the post-emergence of the crop has limitations, due to the low efficiency of the herbicides that can be used for this purpose. It is necessary to be alert, as there are already areas infested with horseweed that present multiple resistance to ALS inhibitors + EPSPs. 

ryegrass

The presence of ryegrass biotypes resistant to glyphosate, ALS and ACCase inhibitors represents a major economic and technical impact on Brazilian agriculture. Glyphosate has a low cost for the producer and high control efficiency, which is the reason for its intensive use. The molecules iodosulfuron and nicosulfuron, ALS inhibitors, are important herbicides used in wheat and corn, respectively, and, due to the resistance of ryegrass biotypes to ALS-inhibiting herbicides, these herbicides have lost their efficiency in controlling this species. Likewise, ACCase inhibitors (clethodim, sethoxydim, among others) were the main alternatives for ryegrass control, and now no longer. In situations of simple resistance to glyphosate, alternative products that inhibit ACCase (clethodim, sethoxidim, haloxifop, clodinafop, among others) and ALS inhibitors (iodosulfuron, nicosulfuron, among others) are efficient and, if applied appropriately, prevent them from occurring. crop yield losses due to competition. In situations of multiple resistance (glyphosate + ACCase or glyphosate + ALS), alternative herbicides are non-selective products such as paraquat and paraquat + diuron for use in pre-sowing desiccation of crops, with lower efficiency and possibility of losses due to competition due to of control failures.  

Volunteer corn

In the safrinha corn harvesting operation, it is normal for grain and cob losses to occur, which can lead to volunteer plants that compete with soybeans grown in succession. Therefore, the smaller the harvest loss, the smaller the problem will be. The germination of volunteer corn can be uneven, depending on whether or not the grain is detached from the cob, or whether or not it is buried in the ground. This may result in the need for chemical control to be carried out in more than one application. In the recent past, chemical control of voluntary corn was carried out with the herbicide glyphosate, applied in the off-season management operation or post-emergence of the soybean crop, on RR cultivars. This application is no longer viable when the volunteer corn plants come from hybrids that are also resistant to the herbicide glyphosate, whose cultivation area has gradually increased. Therefore, the recommendation for chemical control of volunteer corn is based on the application of post-emergent graminicides from the ACCase inhibitors group.

Final considerations

The off-season period is not only used to help reduce the weed seed bank, but also to manage some difficult-to-control species such as bittergrass and horseweed. These two weeds are not controlled by a single product, but by a set of actions and products applied sequentially. The spread of weeds resistant to herbicides has resulted in problems for farmers, making control more laborious and more expensive, and in some cases an increase of 200% to 400% compared to areas without these biotypes. However, there are practices that can help with control, such as the use of crops that produce straw in the off-season, especially wheat and oats in the South and brachiaria in the Central-West. Mulch has made it possible to reduce the use of herbicides in direct seeding and has reduced the emergence of weeds by 60% to 90% when compared to areas not cultivated in the off-season or cultivated with single off-season corn. 

Dionysus Luiz Pisa Gazziero, Fernando Storniolo Adegas, Alexandre Ferreira, Leandro Vargas, Decio Karan, Antonio Luiz Cerdeira, Elemar Voll, Embrapa

Article published in issue 207 of Cultivar Grandes Culturas.