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Weeds emerging with the soybean crop can cause a greater reduction in yield potential compared to weeds emerging after crop establishment. University research indicates that weeds emerging with the crop should be removed by the V2 (second trifoliolate) or V3 (third trifoliolate) soybean growth stage when weeds are less than 4 inches tall, to minimize yield loss.1 Weeds can reduce soybean yield by 1% per day if left uncontrolled after the V2 to V3 growth stage.1 As control of these weeds is delayed, the yield lost to weed competition increases.
Before applying POST herbicides, scout fields to accurately determine the growth stage of the crop and the degree of variability that may exist across the field. Also, record weed species that are present and their height. Most herbicide labels define the soybean growth stage and maximum weed size for application, in order to prevent crop injury and facilitate good weed control.1
For soybean POST applications, consider applying Roundup PowerMAX® and Cobra®. Also, tank-mixing a safe for in-crop use overlapping residual product, such as Warrant® Herbicide, is recommended.
Avoid making POST applications during abnormally cool or dry, droughty weather since weeds may not be actively growing under these conditions. Delaying application until weeds resume active growth may result in improved control. Herbicide penetration and activity are usually improved when the temperature and relative humidity is high, resulting in better control, but possibly increasing potential for crop injury.2
The use of an adjuvant such as a surfactant, crop-oil concentrate (COC), methylated seed oil (MSO), or fertilizer solution is often recommended on product labels to improve spray coverage and herbicide uptake. Weed control may improve with the use of adjuvants, but crop injury may increase. Always follow product label instructions regarding the use of additives.
1 Hartzler, B. 2003. Is your weed management program reducing your economic return? Iowa State University. http://www.weeds.iastate.edu/mgmt/2003/economics.shtml. 2 Loux, M.M., Doohan, D., Dobbels, A.F., Johnson, W.G., Young, B.G., Legleiter, T.R., and Hager, A. 2016. Weed control guide for Ohio, Indiana and Illinois. The Ohio State University. https://u.osu.edu/. Web sources verified 06/9/16. 160608172747