Desiccation is a practice that involves the application of a contact herbicide. After application, it promotes rapid uniformity of the plant for harvesting. This practice aims to accelerate the end of the soybean reproductive cycle by bringing forward the harvest, usually by one week, allowing for the subsequent sowing to be brought forward.
Some regions of Brazil allow for the cultivation of two crops in the same agricultural year, with early soybeans and off-season corn and/or cotton, and have the possibility of using the desiccation practice to sow the crop in succession while still within a good sowing window. Therefore, producers who cultivate within the recommended zoning for the region, such as in Mato Grosso, for example, bring forward soybean sowing as much as possible when soil moisture conditions are favorable.
The time interval between herbicide application and harvest depends on factors such as the moisture content of the grains at the time of application, the type of product used, and the weather conditions after application. Generally, hot and dry weather after desiccation favors an early harvest.
Two important factors that require attention from producers are the phenological stage of the crop and the weather conditions.
Ideal phenological stage for desiccation:
Pre-harvest desiccation of soybeans requires an ideal time to be carried out, with the R7.3 stage being the recommended time, when the plant reaches physiological maturity. Anticipating this practice too early can result in significant losses in productivity and/or seed quality. Therefore, it is important and necessary to also understand the phenology of the crop.
When making a decision about when to start applying herbicides for desiccation, the main point to be evaluated is the pods, choosing the ideal moment and being able to recognize the phenological stage of the soybean.
R7.1 – Beginning at 50% of yellowing of vegetables and pods.
R7.2 – Between 50 and 75% of yellowing of pods and vegetables.
R7.3 – More than 75% of yellowing of vegetables and leaves.
R8.1 – Beginning at 50% of defoliation.
Figure 1. Phenological stages of the soybean crop suitable for the practice of desiccation
Fonte: Thulio Mariotto
Other aspects to be observed are: leaves changing color from intense green to light green to yellow and grains detached from each other when opening the pod. Knowing this, it is worth emphasizing the importance of applying contact herbicides from the R7.3 phenological stage, when more than 75% of the grains are physiologically mature.
Studies conducted in the municipality of Jaboticaba/RS and at the Seed Production and Technology Laboratory of UFSM, Frederico Westphalen Campus/RS, in the 2010/11 harvest, show that the application of desiccant herbicide in R6 causes a 35% loss in productivity when compared to the control (without application). In R7.1 the reduction was 13% and in R7.3 there was no significant difference, demonstrating superiority over the other treatments and evidencing that it is the ideal phenological stage for desiccation. According to Pelúzzio (2008), the reduction in productivity is due to the fact that the plant may still be translocating photoassimilates to the seed at the time of desiccation. If this occurs, the translocation process is paralyzed, generating a decline in productivity.
Impacts of poor desiccation on grain quality:
What producers need to be aware of: if desiccation is performed before the R7.3 stage, significant losses in productivity may occur due to loss of PMS and physiological quality of the seed, considering that before this stage, grain filling is still occurring and the pods are not ready for harvest with good production yield. Therefore, the recommended stage for desiccation is R7.3.
Another important point that producers should be aware of is the climate conditions after crop desiccation. Crops that were desiccated before harvesting do not tolerate excessive rain, which can increase the number of burnt and sprouted grains. In other words, when deciding to desiccate, producers need to be aware of the possible occurrence of rain in the next fifteen days and analyze whether it is really possible to harvest within a window that does not harm final productivity. Therefore, the desiccation front must be very much in line with the harvest.
In view of all the above, the greatest care in pre-harvest desiccation of soybeans starts with choosing the time of application. Therefore, recognizing the phenological stage that presents the lowest loss rates and considering the weather conditions are key points to avoid damage caused by poor desiccation, which poses the risk of drastically reducing crop yield.
Text written by Eduarda Meneghello Gheller and Ueslen Barth, students of the Agronomy course at UFSM campus Frederico Westphalen, members of the Tutorial Education Program - PET Agricultural Sciences, under the supervision of the tutor, professor Dr. Claudir José Basso.
References
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FRANZONI, Maiara. Soybean desiccation: the right time and the best herbicides for a good harvest. Agroadvance, 2024. Available at: https://agroadvance.com.br/blog-dessecacao-da-soja/. Accessed on: February 26, 2025
LAMEGO, F. P. et al. Pre-harvest desiccation and effects on productivity and physiological quality of soybean seeds. Scielo Brasil, v. 31, n. 4, p.929-938, Dec. 2013. Available at: https://doi.org/10.1590/S0100-83582013000400019. Accessed on: February 27, 2025
Professors Alfredo & Leandro Albrecht. Soybean desiccation for harvesting! When and why to do it?. Youtube, January 26, 2020. Available at: https://www.youtube.com/watch?v=iuLGv_LqY_E&t=3s. Accessed on: February 27, 2025
