Herbicides associated with desiccation

At desiccation of areas for cultivation association of residual herbicides with management becomes an alternative interesting. Among the products that have a residual effect and that can be associated with glyphosate in the management of weeds that are difficult to control during desiccation it is possible to list diclosulam, flumioxazin, chlorimuron and imazethapyr.

In Brazil, the direct planting system (SPD), which is based on the non-disturbing of the soil, its permanent cover and crop rotation, gained considerable space among soybean producers in the last 50 years. The area cultivated with oilseed in this system is estimated at 32 million hectares.

For the establishment of direct planting, a good straw formation on the soil. This material, when distributed uniformly over the soil surface, can act physically, chemically and biological impact on the weed seed bank, with consequent decrease in the germination rate and reinfestation of the area. However, not all weed species are suppressed by the effects of mulch. Like this, in the desiccation of areas for crop cultivation, the association of herbicides residues to management, such as glyphosate, becomes an alternative interesting, as it allows the initial establishment of cultures in the clean, that is, without the presence of weeds and their consequent competition with the cultivation. Among the herbicides that have a residual effect and that can be associated to glyphosate in the management of weeds that are difficult to control in the desiccation of These areas include diclosulam, flumioxazin, chlorimuron and imazethapyr.  

In field research, the soybean cultivar Monsoy 9144 RR was sown in the plant-apply system, on 4,4 t/ha of vegetation cover provided by Urochloa ruziziensis. When desiccating the vegetation cover, the following were adopted: herbicide treatments: glyphosate+diclosulam without complementary application post-emergence with glyphosate (G+D); glyphosate+diclosulam pre-sowing with a complementary post-emergence application of glyphosate (G+D+G) and the application of glyphosate in  pre-sowing with more a post-emergence application of glyphosate in complementation (G+G) (Table 1). All supplementation applications were carried out 35 days after sowing (DAS), V4 stage of soybeans. By harvest time, in the assessment of reinfestation of weeds in the area, the superiority of control was notable where there was supplementation with glyphosate in the post-emergence of weeds, carried out at 35 DAS for soybeans (Figure 1). In this research it was evident that the effect residual of the herbicide diclosulam was not sufficient to exclude the complementary application. Despite the residual effect of diclosulam to be 67 days when applied in a direct planting area, the reduction of this effect associated with the absence of glyphosate complementation allowed the successful germination of weed seeding, in addition to regrowth of brachiaria, with consequent reinfestation of the area. In the presence of straw may have interfered with the efficiency of diclosulam, since the layer straw can prevent it from reaching the ground through retention, exposing herbicide molecules to conditions favorable to different forms of degradation, until it is taken to the ground by the occurrence of rain.

In assessing the presence of weeds which was carried out 31 days after desiccation, it was observed that the black walleye (bidens pilosa) was the species dominant in the area. The high population of the black picão indicates the big adaptation of this weed to SPD, which was not suppressed by straw present on the soil surface, in addition to highlighting the low effectiveness of diclosulam in residual control for this species. Composing the community weed, there were still other weeds that were found in lesser quantity, such as trapoeraba (Commelina benghalensis), yerba-de-santa-luzia (Chamaesyce hirta), brachiaria (Brachiaria ruziziensis), crabgrass (I would digitize insularis) and horseweed (Conyza canadensis) (Figure 2). The glyphosate+diclosulam mixture in pre-sowing without supplementation also reduced the infestation of yerba-de-santa-luzia and horseweed in the installments.

Treatments with the application of glyphosate + diclosulam with and without complementation (G+D and G+D+G) decreased the average population density of plants weeds at 31 days after desiccation. Treatment with the application of glyphosate no residual herbicide at 35 DAS resulted in the highest initial reinfestation, with the presence of 63,6 plants m-2 (Figure 1 and 2). Many weed species have seeds with varying degrees of dormancy, in order to guarantee successive emergence. As a result, the absence of herbicide with a residual effect on desiccation allowed the germination of the weed seed bank a few days after desiccation.

To the portions of the treatment without glyphosate supplementation resulted in 50% difficulty in harvesting, with most of this infestation coming from the vegetation cover (brachiaria) and black picão (bidens pilosa). The presence of vegetation cover provided by weed community at the time of harvest can make harvesting difficult mechanized, in addition to influencing the quality of the harvested grain.

The plots treated with G+D, without complementation, resulted in lower productivity (Figure 3), in due to the higher incidence of weeds, and consequent competition with culture. The residual effect of diclosulam allowed the establishment initial clean up of soybean cultivation, although it was not enough to suppress the competition of weeds with the crop until the moment of harvest.

Only the association of herbicides do not ensure weed control throughout the crop cycle. soy. Therefore, complementary post-emergence application cannot be dispensed with. More studies with associations for the residual control without complementary application must be carried out considering the use of more herbicides with residual effects. Furthermore, the dynamics of these herbicides in the straw must be investigated, as it can influence the efficiency of herbicides in controlling weeds depending on the physicochemical characteristic of each molecule and plant species weeds that make up the weed community.

Article published in issue 222 of Cultivar Grandes Culturas, November, 2017.