While the majority of soybeans are harvested without the benefit of a harvest aid, there are a few situations where they make sense. Using the right product in the right field at the right timing is key to achieving enhanced dry down without impacting seed quality.
Select the Right Field: Most fields mature without benefit of a harvest aid. Harvest aids work primarily on leaves and exposed weed surfaces, and are typically best used to kill soybean leaves and hasten dry down in MG 2 or 3 fields where early harvest is needed, or to reduce moisture from weeds. They will not change the maturity of the crop and do very little to dry down green stems in my experience. Also, a separate trip across the field adds to cost and unless similar sized spray equipment is used (running in the old wheel tracks), will knock down additional soybeans reducing yields by approximately 1 to 2% depending on spray boom width.
Use the Right Product: In Tennessee the labeled options are Gramoxone SL at 8 to 16 oz/A, sodium chlorate at 3.2 to 4.8 quarts/A depending on formulation, Aim at 1 to 2 oz/A, or glyphosate at 22 to 48 oz/A depending on formulation. If the intent is to dry up soybean leaves and weeds, true desiccant products such as paraquat or sodium chlorate, or a combination of the two products, are more effective than glyphosate, while Aim may enhance dry down of vines. Glyphosate might be a good choice for control of some late season grass weeds but will not desiccate a Roundup Ready soybean, and would be slower acting on LL beans. Some products require a high volume of water (20 GPA) for thorough spray coverage so check product label. Observe pre harvest intervals for all products. Aim has the shortest pre harvest interval at 3 days and Gramoxone the longest at 15 days.
Apply at the Right Time: Treat soybeans anywhere from 3 to 15 days prior to intended harvest depending on product used. Apply when the soybean crop is mature enough so that seed are not affected by premature termination. The majority of seed pods should be a mature brown color or at least turning yellow, but different products require different levels of maturity. For example, with indeterminate soybeans, plants should be ‘mature and ready for harvest’ as per the sodium chlorate label or ‘when 65% of pods are mature brown color or when seed moisture is 30% or less’ as per the paraquat label. Remember, a harvest aid will not change the maturity of the crop, it only enhances dry down of a mostly mature field of soybeans. If a desiccant is sprayed when pods are too young (green pods, seed are not full size, and are not separating easily from the pod wall), seed will not mature normally and often show up as ‘butter beans’ and damaged seed at harvest.
Refer to the 2019 Weed Control Manual for Tennessee for additional information on harvest aids.