Use of pre-emergent herbicides requires attention to specificities

It is estimated that the economic losses of Brazilian agriculture, caused by the presence of herbicide-resistant weeds, could reach billions of reais. Currently, there are 53 unique cases of herbicide-resistant weed biotypes reported in the country. The best known and most severe: horseweed (Conyza spp..), bitter grass (Digitaria insularis), crow's foot grass (Eleusine indica) and caruru (Amaranthus spp.).

According to the MT Foundation researcher Lucas Barcellos, to act against this problem, one of the main solutions is for the producer to apply pre-emergence herbicides.

“These products are of great importance, as they act on the weed germination process, controlling the seed bank. The application of these herbicides provides more effective control of tolerant and resistant weeds,” he explains.

Another point to pay attention to is the right time of application. According to the researcher, the use of pre-emergent herbicides must be carried out after the weeds have dried out well and before the seeds emerge. This implies successful weed management and selectivity for the crop.

“When applied at the wrong time, there may be a loss of effectiveness in situations where the herbicide is leached into the soil profile, after heavy rain, or in cases of low humidity in the soil, the herbicide becomes adsorbed to the colloids", he explains.

In addition to paying attention to the right moment of combat, producers must pay attention to application management. Pre-emergent herbicides must be positioned depending on the weeds present in the area, chemical and physical attributes of the soil, soil moisture, selectivity for the crop and not have a residual effect on the successor crop (carryover).

Pay attention to the soil

Soil texture is very important when recommending pre-emergent herbicides for soybeans. Soils with a greater amount of clay and organic matter retain more herbicides in their colloids, leaving a lower concentration of the product in the soil solution. A very mobile herbicide, for example, can be applied to sandy soil and go directly into groundwater, which is harmful to the farmer and the environment. Therefore, it is important to pay attention to this detail, reinforces the doctor in phytotechnics.

In crops that have straw or green cover on the ground, the farmer needs to pay extra attention, after all, the pre-emergent herbicide is only effective if it reaches the soil. Therefore, any barrier between the soil and the product can impair its action. Furthermore, some products have very little ability to pass through the straw, being very dependent on rain to penetrate the straw.

Another fact little taken into consideration by soybean growers, according to the expert, is the level of infestation in the area at the time of application. “If at the time of application there are many weeds, with large soil coverage, they can absorb the pre-emergent herbicide before it reaches the soil, damaging its effect”, concludes the researcher.