Impacts of volunteer corn on soybeans and strategies for management

The production system that has been most widely used in Brazil is the soybean and corn succession, given the good profitability that both crops present. Although it has positive points, it is worth noting that the continued use of the soybean and corn succession can create production bottlenecks for the environments in which it is adopted. Among the obstacles generated by the exclusive use of this production system, we can highlight the occurrence of volunteer plants (tigeras) in the subsequent crop. When volunteer plants coexist with the crop, they begin to behave like true weeds, interfering with its development and productivity.

Volunteer plants can emerge from seeds with anomalies that were sown in previous harvests, or from grains resulting from harvest losses. Due to the aggressive characteristics of corn, it is clear that this species is more competitive than soybeans, given that it is a species with C4 photosynthetic metabolism compared to a C3 species. In this sense, the presence of volunteer corn plants in soybean crops has been one of the greatest challenges related to weed management in the crop (Figure 1).

The occurrence of volunteer corn in coexistence with soybean crops is not a recent occurrence, since the intensification of corn cultivation in off-season conditions has led to the presence of these volunteer plants. Despite this, prior to the introduction of glyphosate-tolerant (RR) hybrids, the management of volunteer corn plants in RR soybean crops was simple, given that glyphosate is highly effective in controlling conventional corn. With the introduction of RR hybrids, the management of volunteer plants of this species in soybean crops has become more complex.

In addition to the negative effects that the occurrence of volunteer corn brings when infesting soybeans, there are other points that should be highlighted, which can cause damage not only to the productivity of the crop, but also to the sustainability of production systems.

Currently, the vast majority of corn hybrids have transgenics that confer tolerance to attacks by chewing pests (Bt). This transgenic is also available for soybeans, a fact that has led to a decrease in the use of insecticides in this crop.

Situations in which volunteer corn plants are present in soybeans, combined with the lower frequency of insecticide applications in this crop, create a favorable scenario for the breakdown of the tolerance of corn hybrids to pest attacks, since the plant is exposed to insect infestation throughout the period in which it coexists with soybeans. Still within the scope of the problem of pests, it is worth noting that the corn leafhopper, which is a vector of the corn stunt complex, is one of the main phytosanitary problems for corn cultivation in Brazil and has the ability to survive on these volunteer plants.

Another factor that makes it difficult to manage volunteer corn is related to the emergence cycles that the species has had throughout the soybean crop. Unlike weeds that require water availability and temperature fluctuations to begin the germination process, for volunteer corn, the main stimulus for emergence is the presence of moisture in the soil. Therefore, whenever there is rain and there are corn grains in the soil, an emergency flow of volunteer plants may occur, which will interfere with the soybean crop.

The negative effect of volunteer corn on productivity is undeniable, however its intensity varies depending on factors such as the time of year in which the crop coexists, the density of corn plants, the cultivar and the population density of soybeans, among others. Reductions of nearly 90% in soybean productivity have been reported depending on the population density of volunteer corn coexisting with the crop (Figure 2).

Therefore, it is clear that it is necessary to manage volunteer corn plants, since they have a high potential to interfere with soybean crops. To ensure that management is successful, the measures used should not be restricted to situations where volunteer corn already coexists with soybeans, but rather to the adoption of control methods that prevent the increase in the seed bank with grains from losses in the second corn harvest, as well as inhibit the emergence of volunteer plants in soybean crops.

Integrated management of volunteer corn

Integrated management aims to jointly use different weed control methods, reducing losses caused to soybeans due to coexistence with the weed community.

The most commonly used control methods for soybeans include preventive, cultural, mechanical and chemical methods. In the context of preventive control of volunteer corn, the use of measures that avoid increasing the presence of these grains in the soil can contribute to reducing the occurrence of volunteer plants. One practice that can contribute to this reduction is related to the adequate harvesting of the crop when growing in the off-season. Poorly adjusted harvesters, operator unpreparedness and crops with grains outside the recommended moisture level for harvesting can increase losses. In this sense, harvesting operations carried out properly will contribute to reducing productivity losses in corn and reducing volunteer plants.

The choice of hybrid can also help reduce problems with volunteer corn, since RR hybrids will produce glyphosate-tolerant offspring, which will make management more difficult and consequently increase production costs due to the need to purchase other herbicides. Therefore, producers should be sure to choose materials that are glyphosate-tolerant, knowing the losses that the occurrence of volunteer plants in soybeans can cause.

Despite this, even producers who do not cultivate RR hybrids are not free from volunteer corn plants that are tolerant to glyphosate, since it is a species that has cross-pollination (allogamous), it is common for the flow of pollen from one crop to fertilize neighboring areas.

Regarding the mechanical method, the use of a mower attached to the tractor to eliminate plants that emerged before soybean sowing, in addition to a knife roller to split the ears, are viable alternatives. Although efficient, it is important to consider the high consumption of diesel oil and the operational performance of these methods before making their recommendation. In the context of cultural control, practices such as balanced fertilization, sowing density and spatial arrangement of plants that provide better soybean development, as well as the adoption of crop rotation, may contribute to minimizing the negative effects of volunteer corn.

Finally, the chemical method consists of using herbicides in different modalities, which are related to pre-sowing desiccation and pre- and post-emergence applications of soybeans. For the management of volunteer corn in pre-sowing desiccation, one of the alternatives is the use of glyphosate.

However, if corn plants are tolerant to glyphosate, it is necessary to use herbicides that are effective in controlling this species, and ACCase inhibitors (clethodim, haloxyfop) or contact products (diquat or glufosinate) may be indicated. Regarding herbicides applied in pre-emergence, studies available in the literature have shown that although herbicides do not provide effective control, they serve as a tool to suppress the growth rate of volunteer corn, a fact that will aid in post-emergence management of soybeans. Among the herbicides that have the potential to be used in this modality are diclosulam, imazethapyr, and chlorimuron (Buchling et al., 2022).

The most widely used application method for controlling volunteer corn in soybeans is the application of post-emergence herbicides. This practice is adopted due to the successive emergence of volunteer corn, which leads producers to wait for this to occur, aiming to reduce costs with sequential herbicide applications. Despite this, it has been demonstrated in the literature that late management of volunteer corn not only reduces the effectiveness of certain herbicides, but also allows these plants to interfere with soybean productivity (Ovejero et al., 2016).

Table 1 presents herbicides for post-emergence control of volunteer corn, in addition to the indication for which transgenic(s) each active ingredient is effective. It is worth noting that the effectiveness will be determined mainly according to the development cycle of the volunteer corn at the time of application, since plants in more advanced stages tend to be more tolerant to the action of herbicides.

For volunteer plants from conventional corn hybrids, glyphosate and ACCase inhibitors (DIMs and FOPs) are the best options for control. Glufosinate performs best when applied at early stages, with reduced control levels in late applications. The behavior of volunteer plants from RR hybrids in relation to herbicide sensitivity is similar, except for their tolerance to glyphosate. This also occurs for materials with transgenics that confer resistance to glufosinate (Liberty Link - LL), which cannot be used for the management of volunteer plants.

Regarding the chemical control of volunteer corn from hybrids with Enlist technology, the most effective alternatives consist of the use of ACCase inhibitors from the DIM chemical group (clethodim, sethoxydim), since this transgenic species confers tolerance to glyphosate, glufosinate and graminicides from the FOP chemical group. One fact to be mentioned about herbicides belonging to the DIM chemical group is that they tend to show reduced effectiveness when applied to plants in advanced stages, which makes it difficult to control volunteer corn when they are used. It is also worth noting that not all ACCase inhibitors are effective in controlling volunteer corn, and it is essential to consult a competent technician before selecting the herbicide that will be used in the crop for this purpose. 

By Guilherme Braga Pereira Braz (University of Rio Verde); Sergio de Oliveira Procopio (Embrapa Environment); Dieimisson Paulo Almeida (Cooperative With Me)

Article published in issue 288 of Cultivar Grandes Culturas Magazine