Selectivity after herbicide application in corn and soybeans

The emergence of numerous soybean and corn technologies that promote tolerance to some herbicide mechanisms of action has contributed to facilitating post-emergence management in these crops (Tables 1 and 2). However, with the appearance of resistant and difficult-to-control weeds, such as goosegrass (Eleusine indica) and pigweed (Amaranthus spp.), and the delay in the introduction of chemical control, many producers have opted to apply or use higher doses of herbicides, above the label recommendation, which modifies the selectivity of the soybean-corn combination.

To be selective, herbicides must affect the site of action in the weed or some metabolic process as a whole, while for the crop, the product can be metabolized in a way that does not cause phytotoxicity. Some products applied to certain agricultural crops may also be poorly absorbed or differentially translocated, causing a loss of toxic action even before the herbicide reaches the site of action.

Knowledge of the system

The importance of various technologies or genetic events that promote tolerance to more than one herbicide mechanism of action in agricultural environments is increasingly evident. However, it is important to highlight that the irrational use of herbicide molecules, resulting from numerous applications, leads to increased selection pressure on a given weed population. This indicates that the indiscriminate use of molecules can hinder integrated weed management, which tends to accelerate, through selection, the resistance process of certain biotypes.

There are several recommendations that can be made to extend the lifespan of chemical pest control, aiming to delay the emergence of herbicide-resistant weeds. For example, it is necessary to use herbicides with different mechanisms of action. Understanding how herbicidal molecules work helps define how to use the products, as well as diagnose problems in their performance and their relationship to the symptoms of injuries caused.

However, with the advent of several recently introduced technologies in Brazil, there is a possibility of increased reports of volunteer corn or soybean plants exhibiting tolerance, including to molecules primarily used in desiccation. This issue opens up new scenarios for soybean and corn producers, as it requires a more rational selection of cultivars and greater attention to chemical management.

Volunteer plants

The recommendation of certain pre-emergence herbicides, such as clomazone, for the control of volunteer species that have emerged in a production area, may result in a lack of phytotoxicity in these species, causing injury only to plants that emerged after the product application, especially at doses below those recommended in the product label.

In these applications, the damage may still depend on the climatic and agronomic conditions present during the product's effect. The application of other active ingredients recommended for pre-emergence use may lead to visible injuries in already emerged plants, as is the case with the use of pyroxasulfone and the characteristic leaf curling effect.

Regarding products recommended for post-emergence application on volunteer plants, significant damage may not be achieved, even with graminicide products (such as ACCase inhibitors) in controlling grasses, depending on the dose used, environmental factors (such as lower relative humidity), and the stage of application on the target plant. In more advanced stages of plant development subjected to graminicide application, a slight discoloration, tending towards purplish discoloration of the leaves of certain grasses, such as corn, may be observed; however, depending on the degree of injury, lethality will not be achieved.

Regarding the timing of application, certain herbicide mechanisms of action (such as Protox inhibitors), recommended both pre- and post-emergence, due to rapid absorption by leaves, as well as absorption by structures such as the coleoptile, hypocotyl, and epicotyl of seedlings, may have a phytotoxic effect on already emerged plants. For active ingredients with this mechanism of action, environmental factors, such as high air temperature, may increase efficiency. However, for applications made on plants in an advanced stage of development, plant death may not be confirmed.

Selectivity in management

In summary, it is possible to infer some recommendations aimed at establishing more effective strategies for herbicide application, considering the various scenarios of volunteer plant presence.

Among these, we can highlight: the importance of recommending desiccation with the use of active ingredients that are effective against volunteer plants, primarily in technologies with herbicide tolerance; management with the use of products with residual action, in the recommended pre-emergence application of the crop of interest; rotation of mechanisms of action throughout the entire crop cycle; and the use of methods that interrelate with chemical control, favoring it, such as the cultural method and the mechanical method.

* By Higor Meirelles Trovo, Julyana Vasconcelos Barbosa (UniRV); Renan Souza Silva (Unesp)

Article published in issue 320 of Cultivar Grandes Culturas Magazine