Advantages of using pre-emergent herbicides

Since the first reports of glyphosate-resistant weeds began to emerge in 1996, the number of cases of resistance to this herbicide has increased considerably worldwide, covering 45 different species, 25 of which are broadleaf species and 20 of which are narrowleaf species.

In Brazil, due to the introduction of the no-till system and glyphosate-resistant crops, weed management began to be carried out primarily with this herbicide. Therefore, eight resistant weed species were selected, with four narrow-leaf and four broad-leaf species (Heap, 2019).

Among the resistant weeds, bitter grass stands out. (Digitaria insularis), the crow's foot grass (Eleusine indica) and the buva (Conyza spp.) that have high adaptability in the country's agricultural regions, generating losses in crop productivity and high production costs. In a Technical Circular published by Embrapa, evaluating the economic impact of weed resistance to herbicides in Brazil, it was estimated that the average cost for management in the soybean production system is almost R$ 5 billion reais per year, without considering crop losses due to weed competition, which can increase losses to R$ 9 billion annually.

In relation to the control of grasses, such as resistant sourgrass, the costs for the rural producer with management can be up to 2,6 times higher compared to an area without the presence of this weed.

The high cost of managing glyphosate-resistant grasses is due to the limited number of herbicides available for post-emergence control and the need for sequential applications. Currently, the options are limited to five mechanisms of action for post-emergence applications: EPSP inhibitors (glyphosate), ACCase inhibitors (DIMs and FOPs), photosystem I inhibitors (paraquat, diquat), glutamine synthetase inhibitors (ammonium-glufosinate) and carotenoid synthesis inhibitors (clomazone, for example). In situations with glyphosate resistance, ACCase inhibitors are generally the most commonly used for control.

However, with multiple resistance of crowfoot and ryegrass to glyphosate and ACCase inhibitors, options have become even more restricted, increasing the need to employ pre-emergent herbicides to reduce the selection pressure for weeds resistant to herbicides applied post-emergence.

Another benefit of pre-emergent herbicides is to prevent initial infestation, since, to preserve the productive potential of soybeans, the crop must be free of weeds from the beginning of the cycle (Constantin et al., 2007). Herbicides generally provide a significant residual effect for at least 20 days after application, significantly reducing weed emergence during the vegetative phase of the crop.

There are 23 herbicides registered on the Brazilian market for controlling grasses with glyphosate resistance (ryegrass, sourgrass and goosegrass). However, only seven herbicides are selective for soybeans and can be used in pre-emergence applications.

Among the herbicides described above, the following stand out: early growth inhibitors (S-metolachlor), ALS inhibitors (imazethapyr and diclosulam), carotenoid synthesis inhibitor (clomazone) and microtubule formation inhibitor (trifluralin), which are highly efficient in controlling grasses. However, for the control of broadleaf plants, each herbicide has its own peculiarities. One example is S-metolachlor, which, in addition to being selective for the registered crops, provides control for grasses and some other species such as ragweed. (Comellina benghalensis) and hot herb (Spermacoce latifolia).

In an experiment aimed at evaluating the efficiency of pre-emergent herbicides applied before soybean sowing, it was evident that the use of any of the herbicides evaluated significantly reduced the density of sourgrass plants per square meter (Figure 1).

In treatments composed of Dual Gold (S-metolachlor) and Premerlin (pendimethalin), no infestation flows occurred up to ten days after soybean emergence. Regarding productivity, the management adopted with application prior to sowing, using pre-emergent herbicides, followed by post-emergence application in the crop preserved the productive potential, without losses in productivity. If sourgrass management is not carried out, productivity losses can exceed 80%, and it is essential to adopt strategies that minimize weed competition.

Post-emergence application is generally performed in soybean crops at stages V3 to V5, depending on the species, phenological stage and level of weed infestation. In the example shown in Figure 2, the soybean was at stage V3-V4 (21 DAE), the ideal time for weed control in situation (B), when the plants are young and have low infestation.

Furthermore, the species present in the area is not resistant to glyphosate, which facilitates management and reduces application costs. On the other hand, in situation (A), where no pre-emergent application was carried out, there is a significant weed infestation, with the main infestation being glyphosate-resistant sourgrass. In this situation, management is very difficult, since it is necessary to use sequential applications of graminicides. Furthermore, because the weed is in an advanced phenological stage, shading occurs in the crop, resulting in weed interference and, consequently, reduced productivity.

In this way, adopting weed management that uses pre-emergent herbicides facilitates the management of weeds that would emerge during crop development, presenting several benefits such as:

(a) reduction of initial weed competition;

(b) control of glyphosate-resistant weeds, even at the initial stage;

(c) low weed infestation at the time of post-emergence application;

(d) prevention of selection of herbicide-resistant biotypes;

(e) reducing the cost of weed management by not applying expensive herbicides;

(f) reduction of the seed bank in the soil.

By Felipe Goulart Machado, Lucas Matheus Padovese, John Vitor Scarlon Martoneto, Rubem Silvério de Oliveira Jr. (Center for Advanced Studies in Weed Science, UEM)