What is pre-plant soybean desiccation?
Pre-planting desiccation or anticipated desiccation is nothing more than the management to eliminate the weeds present in the area, in order to enable the sowing of the crop "in the clean", facilitating sowing operations and avoiding the initial competition of the crop with the weeds. It is noteworthy here that it is not advisable to carry out this practice on a scheduled basis, that is, pre-set a number of days before sowing to carry out desiccation without prior analysis of the weeds present in the area, as well as available management tools.
Is managing weeds more complex this season?
The continuous evolution of herbicide-resistant weeds, increasingly reduced planting windows, failures in application technology, among other factors, make desiccation management increasingly complex, which demands high technical knowledge. In addition to these factors, the ban on the use of paraquat herbicide, restrictions on hormonal herbicide applications in the state, added to the scarcity of pesticides (including herbicides) in the global market, has made weed management an even more challenging activity in this 2021 crop. /2022.
The complexity of pre-sowing soybean desiccation
The complexity of desiccation to solve the problems mentioned above results in the need to mix one, two or even three herbicides with different mechanisms of action, in order to achieve satisfactory control of the various weeds present in the area, which was previously achieved with a single application of glyphosate (the main herbicide used worldwide) in most areas. Within this scenario, our main challenge is to control weeds resistant to glyphosate (plants that, by selection processes, are no longer controlled with the recommended dose of this herbicide) and those weeds considered "tolerant" (plants that due to innate characteristics of the species have never been satisfactorily controlled with this herbicide).
The first step for successful desiccation is to survey the weeds present in the area, determining the dominant class (grasses or broad leaves), identifying the species and its growth stage, and seeking information regarding the history of the area in failures in control or resistance to any herbicide. From this information it is possible to choose the herbicides to be used, which can be sprayed directly to the soil (the herbicides move from the roots to the shoot) or herbicides applied via the leaves, and this second group is divided into herbicides of “contact”, that is, those whose action is restricted to leaves (does not move internally in the plant) and “systemic” herbicides, which move (translocate) to other internal parts of the plant.
The importance of integrated management
The effectiveness of desiccation will also depend on the coverage of the area prior to planting, thus, areas with the presence of winter crops have the potential to reduce the weed seed bank in the soil, in addition to contributing to a reduction in the size of weeds, it also facilitates the effectiveness of the herbicide. On the other hand, in fallow areas there will be taller or perennial plants, which are difficult to control and usually require sequential application of herbicides. In addition, taller plants contribute to the known “umbrella” effect on smaller species, decreasing their control effectiveness. Sequential application (more than one application of herbicides before sowing) is usually indicated in situations with high infestation of weeds, large plants and/or plants considered to be “difficult to control”.
The main weeds in RS
Among the species currently considered problematic in Rio Grande do Sul, which have always shown a certain tolerance to glyphosate, we can mention:
• Viola string;
• Donkey grass.
In addition, horseweed and ryegrass are old problems for the farmer, showing resistance to herbicides and, recently, other weed species have been occupying space in the agricultural scenario, such as bittersweet, wormwood, chicken grass and pigweed (recently reported with suspicion of resistance). These species can be considered as “hard to control plants” and usually require more than one herbicide application.
What herbicide do I use to desiccate weeds?
In the cases mentioned above, in addition to glyphosate, a mixture with another herbicide with a different mechanism of action and which has a systemic action on the plant has been indicated, such as 2.4 D, dicamba or fluroxypyr in the case of broad leaves, or graminicides (herbicides of ACCase inhibitor class) with a focus on grasses. About 7-10 days after this first application, sequential application is recommended, normally