Results from Mississippi Research Using Paraquat as a Harvest Aid for Soybeans
A White Paper on this website titled “Using Harvest Aids for Soybeans” provides information and links to information about using paraquat as a harvest aid/desiccant to improve soybean harvest. Results from recent research about this practice are reported in an article titled “Responses of seed yield, quality, and composition to the harvest-aid paraquat in soybean grown in Mississippi” by Bellaloui et al. that appears in the journal Agrosystems, Geosciences, & Environment (2022;5:e20262). This research was supported by MSPB funds granted to Project No. 32-2019/2020. The following narrative is a summary of the above research conduct and results.
• Field experiments were conducted in 2019 and 2020 at Stoneville, Miss. (lat. 33°25'N).
• Maturity Group IV soybean varieties (P46A57BX and P48A60X) were planted on May 1, 2019 and May 13, 2020.
• Paraquat (Gramoxone SL2.0) was applied on the same date to both varieties at growth stages R6, R6.5, and R7. A control treatment (CONT) did not receive paraquat.
• Seed samples were collected at R8 (maturity/mature seeds) of each treatment for determination of germination, viability, and number of hard seed, plus protein, oil, fatty acid, and sugar contents.
• The R8 sample dates in 2019 were Sept. 6 (R6), Sept. 10 (R6.5), Sept. 17 (R7), and Sept. 30 (CONT). In 2020, R8 sample dates were Sept. 14 (R6), Sept. 17 (R6.5), Sept. 21 (R7), and Oct. 6 (CONT). Thus, R8 or maturity was hastened by 22-24, 19-20, and 13-15 days by paraquat applications at R6, R6.5, and R7, respectively, compared to the no-paraquat treatment.
• Length of the reproductive period, 100-seed weight, and seed yield were both increased by delaying paraquat application from R6 through the R8 control. Conversely, early paraquat applications made in this study always resulted in lower seed weight and reduced seed yield.
• When paraquat was applied at stage R7, seed yield was reduced by an average of 1.8 bu/acre across both varieties and years.
• Time of paraquat application significantly affected mature seed damage, but all values were below the 2% threshold that is generally the level below which no dockage discount is assessed.
• These results indicate that paraquat should not be applied to soybeans earlier than stage R7 to ensure the least yield reduction from its application. The slight yield reduction resulting from the R7 paraquat application should be considered by producers to ensure that the resulting increased harvest efficiency and hastened maturity from its application are more important than the potentially lower yield that may result from that application.
Take Home Message. If paraquat is applied to soybeans as a harvest aid before full seed maturity (R8) and complete translocation of metabolites to seeds, yield losses should be expected, especially if applied before R7. Thus, producers who decide that a paraquat application is necessary to expedite maturity and/or increase harvest efficiency should wait as long as possible in relation to seed maturity to make the application. Since R8 occurred 13-15 days after the R7 paraquat application in this study, and the pre-harvest interval for paraquat is 15 days, it is likely that stage R7 is the latest time for paraquat to be applied to soybeans as a pre-harvest desiccant. If it is anticipated that harvest will be delayed by several days after R8, then the application of paraquat can be made later than R7. This is a decision that individual producers will have to make based on their harvest capacity and anticipated weather conditions during harvest season. Remember, stage R8 does not equate to harvest maturity–i.e., harvest maturity will most likely occur 5-10 days later than stage R8 following suitable drying weather that has matured all pods.
Click here for resources related to soybean reproductive stages.
Click here to access a White Paper on this website that provides details about using harvest aids for soybeans.
Composed by Larry G. Heatherly, June 2022, larryh91746@gmail.com