As you’ve read above, Roundup brand agricultural herbicide labels indicate applications cannot occur until pods have set and lost all green color. This coincides closely with the R8 growth stage (95% of the pods have lost green color and occurs about five to 10 days before the field is ready for harvest).2Glyphosate products are systemic. As a result, susceptible weeds and conventional soybean products (without glyphosate resistance) may not drop leaves for several days. Glyphosate-resistant soybean products are also not affected by glyphosate herbicides.For the best harvest aid effectiveness, tank mixes of labeled herbicides should be considered in accordance with label instructions. Table 1, below, provides information about Roundup brand agricultural herbicides including preharvest application timing, rates, days before harvesting after an application, and grazing or feeding restrictions.Labels for other herbicide products have similar restrictions which must be read for application requirements and may include contact herbicides which kill the tissue the herbicides land upon. The activity of non-systemic herbicides is often quicker than systemic alternatives. However, spray volume may need to be increased to help provide adequate plant coverage. These herbicides are likely to have activity on conventional and glyphosate resistant soybean products. For the best harvest aid effectiveness, reference tank mixes of labeled herbicides. The best practice is to begin sampling fields when soybean plants reach the R6 (full seed) growth stage. The R6 growth stage is characterized by a pod containing a green seed that fills the pod capacity at one of the four uppermost nodes on the main stem.3 Verification of growth stage can be confirmed by randomly collecting pods from the top one-third of plants throughout the field to determine pod coloration and seed development within the pods. The R7 growth stage is characterized by one main stem normal pod reaching its mature pod brown or tan color.3 When seeds turn yellow, they begin separating from the white membrane of the pod, indicating that seed filling is reaching completion and a harvest aid application may be considered as a solution.
When harvest time comes, improper weed management can cause a slew of problems on top of reduced yield. Did you know research at Mississippi State University showed that even low weed populations at harvest had the potential to reduce combine and cylinder speed? In addition, the amount of foreign material doubled, the percentage of damaged seed increased, and soybean seed moisture increased compared to weed-free control plots.4Many weeds that can be found in near-maturity soybean fields can have mature viable seeds. As a result, if applied preharvest herbicides cause adversely large weeds to die and drop leaves, then the seeds that drop to the ground or run through a combine can grow. Cleaning harvest equipment can also prevent the spread of unwanted weed seed from field to field. Weed competition should be addressed early in the season to help protect crop yield potential and beat back the spread of weeds.
1Heatherly, L.G. 2015. Using harvest aids for soybeans. Mississippi Soybean Promotion Board (MSPB). https://www.mssoy.org.2Davis, V.M. 2012. A review of glyphosate use for preharvest weed control. Integrated Pest and Crop Management. University of Wisconsin-Madison. https://ipcm.wisc.edu/blog/2012/09/a-review-of-glyphosate-use-for-preharvest-weed-control/.3Koger, T, Catchot, A., Allen, T., Zhang, L., Eubank, T., and Blessitt, B. 2010. Guide to soybean growth stages. P2588. Mississippi State University. https://www.mssoy.org/ .4Ellis, J.M., Shaw, D.R., and Barrentine, W.L. 1998. Soybean (Glycine max) seed quality and harvesting efficiency as affected by low weed densities. Weed Technology 12(1): 166 - 173. doi:10.1017/S0890037X00042743. Cambridge University Press. Web sites verified 9/2/21. 7014_S1