Right time to perform desiccation on soybeans

The key factor for the producer to be successful with desiccation lies in getting the exact moment to carry it out. Early applications can significantly reduce soybean productivity, while those carried out late will not present good results in bringing forward the harvest, the main objective of the operation.

Soybean desiccation was widely used in Brazil in the 90s. Due to failures in weed management, mechanized harvesting became a very difficult practice. Thus, desiccation was essential to facilitate grain harvesting and reduce losses. After the insertion of RR soybeans into the field and expansion, weed control improved, reducing the need to use desiccation for this purpose.

In the current scenario, soybean desiccation provides important benefits for producers, such as: facilitating grain harvesting, scheduling harvesting and sowing (corn 2^a harvest) with consequent optimization of the use of machines (harvester and seeder), standardization of maturation, reduction of impurities in the grains, reduction of the cost of drying the grains, taking advantage of better commercial conditions for soybeans, bringing forward the sowing of winter corn, increasing the productive potential and reducing the risk of losses due to frost and drought of this crop. It is estimated that in the western region of the state of Paraná, more than 30% of areas are desiccated annually. In the region where Copacol operates, in the 2014/15 harvest, 45% of commercial soybean areas were desiccated. (Figure 1).

The most used and registered active ingredients for desiccating RR soybeans are: paraquat, diquat and ammonium glufosinate, present in several commercial brands (Table 1). The application of these products to dry soybeans represents an increase in production costs. In the last harvest, the average cost for a producer in Paraná to dry one hectare of soybeans was R$41,82. This cost was already higher, but with the registration of several brands of generic paraquat products there was an increase in the supply of brands and a decrease in the price for the producer (Figure 2).

Given this scenario, the team at the Copacol Agricultural Research Center (CPA) prepared a trial in the 2014/15 harvest to evaluate the best product and appropriate time for desiccating soybeans. The trial was set up in randomized blocks and factorial treatment design. Three different products were evaluated: paraquat, diquat, and ammonium glufosinate, at a dose of 2,0 L/ha of the commercial product and three moments of desiccation: 20 days, 15 days and 10 days before soybean harvest maturation, with four repetitions.

The cultivar used was BMX Alvo RR, which has a cycle of approximately 120 days for the region. Sowing took place on 12/10/2014, using 330 kg/ha of fertilizer formulated 04-24-16 (N-P₂O₅-K₂O) and spacing between lines of 45cm. Cultural treatments were carried out in accordance with official recommendations for soybean cultivation in the western region of Paraná. On 23/01/2015, the plots were demarcated with five lines and 10 meters in length. On the same date, treatments were applied in the first application period (20 days before harvest maturity). On 28/01/2015 and 02/02/2015, the second and third seasons were applied, 15 days and 10 days before harvest maturity, respectively (Figure 3). The applications were carried out with CO pressurized equipment.₂, using an XR 110 02 spray tip, and a flow rate of 300 L/ha. 

On 12/02/2015, all plots were harvested, determining productivity and mass of one thousand grains (MMG). The mass of the plots was corrected for 13% humidity. The data were submitted to ANOVA using a regression test for the application periods.

There were no differences between the products applied, both for productivity and MMG. However, there was a significant response depending on the time of application of the products. The greater the anticipation, the greater the productivity losses and reduction in MMG. As can be seen in Figures 4 and 5, desiccation 20 days before harvest maturity caused a reduction of 904 kg/ha (15 sc/ha). Losses were observed at all desiccation times, but they were smaller after 10 days (<5% losses).

The reduction in productivity is related to premature leaf fall. The total loss of leaf area stops the production and translocation of photoassimilates from the leaves to the grains. When translocation is interrupted, there is a reduction in MMG, which is one of the yield components that directly affects soybean productivity. In Figure 5, it is observed that with early desiccation of 20 days, 22,83 g grains were reduced¹⁰⁰⁰.

Table 2 presents the estimated percentage of daily loss (days before harvest maturity), calculated from the equation obtained in the test (Figure 4). Applications carried out before 7 days of harvest maturation have a higher rate of daily productivity loss. Therefore, the benefits and subtractions obtained by desiccation must be properly calculated.

Assessing the real need for desiccation is essential. Therefore, understanding the dynamics of the cultivation systems (crop succession) adopted on the property, the supply of machines, species and quantity of weeds present, uniformity of the soybean crop and forecast of the climatic conditions that will be faced, are some of the points that must be evaluated to make a decision on the need for desiccation.

 When adopted, the key success factor for desiccation is finding the right time to perform it. Early applications can significantly reduce soybean productivity, thus reducing rural producer profitability. On the other hand, delayed application will not present significant results in bringing forward the harvest, the main objective of the operation, increasing production costs without the expected return. 

Fernando Fávero, Tiago Madalosso, CPA/Copacol

Article published in issue 202 of Cultivar Grandes Culturas.