Soybean production throughout Oklahoma is very diverse. This is because of the wide array of production systems, ranging from early, full-season production through double-crop soybean production, with soybean being planted following wheat harvest. These diverse systems came with unique challenges that growers had to overcome in order to improve the success of these systems. One challenge that is present universally across all production systems is harvest. Harvest efficiency can be reduced by a number of factors, these can include late-season weeds present in the field, green leaves and petioles even with mature pods, and immature stems and stalks. In these cases, a harvest aid or desiccant may not only help improve the efficiency of harvest in the current season but also improve the fields for future years.
Determining the timing for harvest aids in soybean production systems is one of the most challenges aspects. Applying harvest aids too early could result in significant yield loss due to termination of the plant while the crop is still filling the seed and producing seed weight. Too late of application could result in delayed harvest, due to labeled restrictions (Table 1), resulting in higher amounts of seed loss due to shattering prior to or during harvest.
Determining an exact growth stage of a soybean field is the challenging aspect of harvest aid management. This is because the highly variable nature of the crop, especially with indeterminate soybean varieties. In many soybean fields, parts of the field or, even, parts of an individual plant could be in the early stages of maturity while other parts are still progressing through reproductive growth. Growth stage determination should be done on a whole field basis.
Harvest aids should be applied when the soybean plant has begun to mature. Maturity begins when the crop reaches R7 growth stage, or when one normal pod (non-damaged) on the main stem has reached its mature color. During this stage, most of the green color of the plant will begin to disappear and the crop will have a paler or yellow appearance. At the beginning of this stage seed moisture will typically vary around 60% moisture. This is still too early to apply harvest aids. Growers should allow the plant to reach 30-40% seed moisture prior to desiccating the crop. This will allow for rapid dry down of the vegetative material and allow for smoother dry down of the seed. It should be noted that the application of desiccants will not directly influence seed moisture and this practice should only be used to manage the green material in the field. Increased wind flow and higher light penetration associated with the use of desiccants can help to begin to dry down the seeds but the chemicals applied will have no impact.
Several products are available that can be successfully used as a harvest aid in soybean production. Care should be taken on two different standpoints: 1) label restrictions of the individual materials, and 2) what value these products actually provide. Table 1 highlights products that are available and can be used as a harvest aid as well as products, harvest restrictions, and any potential additive required/suggested.
The industry standard for soybean desiccation is paraquat. Paraquat does a very good job desiccating the soybean crop and weeds within a field given proper application and high enough carrier water. The primary flaw with paraquat is the long pre-harvest interval that is required, 15 days from application to harvest. Especially in Oklahoma conditions, soybean applied with paraquat should dry very quickly. This requires that producers should be ready to harvest following the harvest interval requirement to ensure minimal shatter loss.
One of the newest soybean desiccants is saflufenacil, going by the tradename Sharpen®. Original harvest interval required was 7 days; however, supplemental labels have allowed for the crop to be harvested 3 days following application. While quicker harvest is allowed, it will typically require 7 days to allow for the crop to desiccate if the harvest aid was applied at around 30-40% moisture. One of the largest benefits of this harvest aid is the ability to apply it later within the maturation window and allow for quicker harvest. If used correctly, growers can use this harvest aid to help manage harvest around adverse environmental conditions or labor restrictions. Work done by Dr. Jim Griffin at LSU AgCenter showed that, 7 days following application, the desiccation potential was similar between saflufenacil and paraquat.
Under the tradename of Aim® can also be used as soybean desiccant. Carfentrazone is actually a soybean herbicide that can be used in-season to control many weeds, specifically viney weeds. Unfortunately, it being an herbicide that can be applied in-season, it has limited ability to desiccate the soybean crop. It should be noted that if viney weeds (specifically Morningglories) are a problem late-season, Carfentrazone can be very effective at desiccating weeds that could be an issue.
As opposed to the other chemicals that have been highlighted above, sodium chlorate is an actual desiccant. As a true desiccant, not a harvest aid, sodium chlorate works to draw the moisture out of not only the crop but also any weeds that are present within the fields. This means, especially when paired with typical Oklahoma drying conditions, that the soybean crop will dry rapidly. The preharvest interval for sodium chlorate is 7-10 days depending on the formulation (Table 1). Similar to paraquat, following the preharvest interval, growers should be ready to harvest the crop immediately.
Management of late-season weeds:
One of the largest benefits of harvest aids in Oklahoma soybean production systems is late-season weed management. Several conventional herbicides are allowed to be applied later into reproductive growth (Lactofen, etc.); however, most common herbicides used in Oklahoma soybean production (glyphosate and dicamba) can only be utilized until early flower. This allows a long time between final herbicide applications until harvest. Harvest aids can be utilized as a means to help not only efficient harvest in the current season but also better weed management in future years. All chemicals that can be utilized as a harvest aid in soybean production can be successful at restricting late-season weed growth. Harvest aids, however, cannot be the last weed management strategy in the field during the season. As most weed plants are at or close to maturity and fully developed by the time they are treated with a harvest aid, complete termination of the plant is unlikely. Follow-up application before a fallow period or a successive winter crop will typically be required.
Harvest aid use in Oklahoma soybean production can be a very valuable tool. Increased harvest efficiency, aid in weed management, and evening out field maturity are all benefits that can be gained from this practice. However, growers have to know the limitations of the practice. Increased seed dry down and complete termination of adult weed stands cannot be consistently done with harvest aids. The success of using harvest aids as an in-season management tool for soybeans has been done in part with good early-season control and adequate fallow season management.
For further information, please contact your local county educator.