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Soybean herbicide harvest aids can serve multiple purposes. In some regions, harvest aids are used to desiccate the green tissue (pods, leaves, and stems) on physiologically mature soybean plants to help promote harvest efficiency.1 Harvest aids can also be used to help desiccate weeds that may interfere with harvest operations. The potential to provide weed control benefits or reduce weed seed viability is highly dependant on the herbicide used, weed size, and application timing.
Things to consider prior to the selection and application of a harvest aid product:1
The commonly used harvest aid products have specific instructions regarding application timing (Table 1). Once soybean plants reach the R6 growth stage it is a good time to begin sampling fields in preparation for a harvest aid application. In general, it is advisable to wait until the plants have reached the R7 growth stage, which is when rapid leaf yellowing begins and one or more pods reach the mature brown or tan color.2 Once seeds turn yellow, they begin to separate from the white membrane of the pod, indicating that seed filling is reaching completion and an application may be considered. Application of a product prior to label instructions can reduce seed yield potential or quality. To determine the growth stage of a soybean field, randomly collect pods from the top 1/3 of plants throughout the field and examine the pod and seeds to determine if all of them meet the criteria for an application as specified by the product label.
Gramoxone® SL, Sharpen® powered by Kixor®, Aim® EW, and Clarity® can be used as harvest aids in soybean (Table 1). Each product has unique attributes and products differ in speed of activity, efficacy, pre-harvest intervals, and re-cropping restrictions. Please check product labels prior to use for specific recommendations and precautions.
Spray coverage is essential to the success of contact herbicides. Gramoxone SL is a contact herbicides, which can provide the quickest desiccation of soybean plants and should be applied in at least 20 gallons of water/acre for good coverage.
Translocated herbicides, like Sharpen or Clarity, are taken up and move through the plant requiring more time for more complete activity. A tank mixtures may provide the best combination of crop desiccation and weed control. Spray adjuvants are typically recommended with harvest aid products to improve coverage and efficacy.
Low densities of weeds have been shown to reduce harvest efficiency. A research study showed that the presence of broadleaf weed densities (selected to be below a yield loss threshold) at harvest reduced cylinder and combine speed.5 The amount of foreign material nearly doubled, while damaged soybean seed and soybean seed moisture increased, compared to the weed-free control plots.
Pre-harvest herbicides most likely will not prevent weed seed production, but some research has demonstrated a reduction in weed seed production and seed viability.3,4 Keep in mind that results can be influenced by the type of herbicide used, weed species, and application timing in relation to weed seed development.5 Pre-harvest herbicides that are translocated within weed species have the potential to reduce seed viability when applied during the initial seed set stage of weed growth. Precise herbicide application timing is required to have any effect on weed seed viability. In most cases, the application timing may not coincide with label restrictions for application to the crop to prevent crop seed injury or herbicide residues in crop seed. Weed competition should be addressed early in the season to protect crop yield potential and reduce weed seed production. In situations where crop growth or canopy closure is delayed and weeds continue growth late in the season, pre-harvest herbicide applications may be used to help increase harvest efficiency.
The application of certain herbicides prior to harvest can be used to help manage late-season weeds that could interfere with harvest efficiency. Additionally, these products can be used to help desiccate aboveground green soybean tissues to also improve harvest efficiency. The type of herbicide used, weed species present, and application timing are all factors that can affect harvesting efficiency.