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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, fields should be scouted to accurately determine the growth stage of the crop and the degree of variability that may exist across the field. Weed species that are present and their height should also be recorded. 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
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.
For POST applications to Genuity® Roundup Ready 2 Yield® soybean, consider applying Roundup PowerMAX® herbicide and Cobra®. Also, tank-mixing a residual product that is safe for in-crop use such as Warrant® Herbicide, is recommended. Warrant Herbicide may be applied POST before soybeans reach the flowering (R2) growth stage. POST applications of Roundup PowerMAX® herbicide can be made through flowering (R2) growth stage. The most restrictive label directions apply when considering tank-mix combinations.
For POST applications to Roundup Ready 2 Xtend® soybean, consider applying XtendiMax® herbicide with VaporGrip® Technology plus a label approved glyphosate tank-mix product. Products containing the potassium salt (e.g. K salt) formulation of glyphosate such as Roundup PowerMAX® herbicide, Roundup PowerMAX® II herbicide, and Roundup WeatherMAX® herbicide are approved tank-mix partners. Monsanto believes that tank mixes of dicamba with potassium salt formulations of glyphosate have lower volatility potential than tank mixes with IPA (isopropylamine) and DMA (diammonium) salts of glyphosate. XtendiMax herbicide with VaporGrip Technology can be applied from emergence up to including beginning bloom (R1) in Roundup Ready 2 Xtend soybean.
Also, the labeled glyphosate options and some other tank-mix partners will require the use of SPECIFIC approved drift reducing adjuvants (DRAs) when tank-mixed with XtendiMax herbicide with VaporGrip Technology. Do not use ammonium sulfate or ammonium-based additives with XtendiMax herbicide with VaporGrip Technology.
Information concerning current tank-mix partners, drift reduction adjuvants (DRAs), and nozzles/operating pressures approved for use with XtendiMax herbicide with VaporGrip Technology are available at www.xtendimaxapplicationrequirements.com/. Additional information is also available at www.roundupreadyxtend.com/. Famers are encouraged to check the websites for the most up-to-date information. Always read and follow all pesticide label requirements.
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 05/27/17. 170530095458