As soybean harvest gets into full swing, harvest aids are often applied to desiccate weeds and expedite harvest. While weeds can create problems for harvest equipment, many producers see the benefit of a harvest aid in creating a more rapid and efficient harvest in the absence of weeds. Soybean plants can reach physiological maturity but still have an excess of green stems and leaves. By removing this “green matter”, the combine can operate at higher speeds and more effectively thresh the soybean and move it into the hopper instead of out the rear. Another, and possibly more important, advantage is earliness of harvest. A harvest aid may allow a producer to get into the field several days sooner than allowing the plant to senesce naturally. With the threat of hurricanes always looming this time of year, two or three days might mean the difference between harvesting the crop or not.
While there are several viable options for soybean defoliation/desiccation, label restrictions for certain products can often delay harvest due to the pre-harvest interval (PHI). For example, the PHI of Gramoxone SL in soybean is 15 days after application. Sharpen has received a supplemental label as a soybean harvest aid/desiccant and allows up to 2 oz/A to be applied three days prior to harvest. Aim is another option which also has a three day PHI. Although both Aim and Sharpen permit harvesting within three days of application, it may take longer to reach optimal defoliation/desiccation of soybean depending on environmental conditions and soybean maturity at the time of application.
A recent study was conducted at the Delta Research and Extension Center to evaluate soybean defoliation/desiccation with commercially-available products. Treatments included Sharpen at 1.5 oz/A plus 1% v/v methylated seed oil (MSO) or Sharpen at 1.5 and 2 oz/A plus 1% v/v crop oil concentrate (COC). These treatments were compared to industry standard of 16 oz/A Gramoxone SL plus 0.25% nonionic surfactant (NIS). Aim at 1.5 oz/A plus COC was also included. Treatments were applied when approximately 65% of soybean pods were yellow to brown in color. Visual ratings of soybean defoliation/desiccation were taken at 3, 5, 7, and 10 days after application. Average air temperature was 81 F with high humidity and no precipitation.
Gramoxone SL provided the greatest desiccation of soybean 3 and 5 days after application at 63 and 88% defoliation, respectively (see Harvest Aid Graph). Sharpen applied at 1.5 oz/A plus MSO provided 94% defoliation and was comparable to Gramoxone SL at 7 days after application. When Sharpen was applied with COC only the highest rate of 2 oz/A was comparable to Sharpen plus MSO at 7 days after application. It is important to note that the use of Sharpen with COC as an adjuvant is not currently labeled as a harvest aid. Aim, while excellent as a weed desiccant in other studies, was not as effective at defoliating/desiccating soybean plants and provided only 78% defoliation 10 days after application.
This data suggests that Sharpen at 1.5 oz/A plus MSO provides similar defoliation/desiccation to Gramoxone SL 7 days after application and could be an alternative to Gramoxone SL as a soybean harvest aid. While the Sharpen label permits harvest within three days of application, this study and others suggest that as many as 7 days may be required for optimal results.