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Postemergence Herbicide Applications in Soybean

Posted 6/18/2018
Weeds can affect soybean yield potential by competing for light, water, and nutrients. The time in the growing season when soybean yield potential begins to be affected by weeds is called the critical period.

Apply at the Right Time

Weeds can affect soybean yield potential by competing for light, water and nutrients. The time in the growing season when soybean yield potential begins to be affected by weeds is called the critical period. This can vary depending on weed species and density, time of weed emergence in relation to soybean emergence, environmental conditions, and production practices. Roundup® brand glyphosate-only agricultural herbicides should be applied in Roundup Ready®, Roundup Ready 2 Yield®, and Roundup Ready 2 Xtend® soybeans when weeds are 4 inches or less in height for control prior to the critical period to help protect soybean yield potential and maximize profitability.  Where preemergence herbicides were applied at planting, the critical period is generally delayed to later in the season when postemergence herbicide may be necessary. Postemergence applications of Roundup® brand glyphosate-only agricultural herbicides can be made from soybean emergence (cracking) through flowering (Table 1).

Table 1. Maximum labeled rates for various application timings of Roundup® brand glyphosate-only agricultural herbicides in Roundup Ready®, Roundup Ready 2 Yield®, and Roundup Ready 2 Xtend® soybeans.

*Allow a minimum of 14 days between final application and harvest
Total of all in-crop applications from cracking through flowering (R2 stage soybeans) 64 fl oz/acre
Maximum rate for a single in-crop application 44 fl oz/acre
Maximum preharvest* application rate 22 fl oz/acre

Use Full Labeled Rates

In general, apply Roundup® brand glyphosate-only agricultural herbicides at 32 fl oz per acre in soybeans before weeds exceed 4 inches in height. If the application is delayed and weeds are larger in dense populations, apply a higher rate, up to 44 fl oz per acre (Table 2). Perennial weeds should be allowed to grow to at least 6 inches before spraying for best results.

Table 2. Recommended rates of Roundup® brand glyphosate-only agricultural herbicides for various weed heights.1

1For water carrier volumes between 16 and 40 gallons/acre
Weed Height Rate/acre
4 inches or less 32 fl oz/acre
4 to 12 inches 32 fl oz/acre
> 12 inches 44 fl oz/acre

A sequential application may be required to control new flushes or growth of new weeds. In general, apply 32 fl oz per acre of Roundup® brand glyphosate-only agricultural herbicides before weeds exceed 4 inches in height. A total maximum of 64 fl oz per acre of a Roundup® brand glyphosate-only agricultural herbicide can be applied in-crop when making sequential applications (Table 1).

Additives and Mixing Procedures

The addition of ammonium sulfate (AMS) may increase the performance of Roundup® brand glyphosate-only agricultural herbicides on annual and perennial weeds, particularly under hard water conditions, drought conditions, or when tank-mixed with certain residual herbicides. Use 8.5 to 17 pounds (1-2% by weight) of dry spray grade AMS or 2.5 to 5 gallons of liquid AMS per 100 gallons of spray solution. AMS should always be mixed first in the spray tank. Drift reduction additives, if used, should be added just prior to adding a Roundup® brand glyphostate-only agricultural herbicide. A non-ionic surfactant can also be added when using Roundup PowerMAX® herbicide, and should be added at the end of the mixing process.

Other Recommendations

When making the transition between corn and soybeans, spray application equipment should be thoroughly cleaned. Many corn herbicides can leave residues in equipment that could cause damage to soybean plants. Spray tanks, lines, booms, nozzles, and screens should be thorougly cleaned to remove these residues before making applications to a soybean crop.

Residual herbicides applied preplant or at planting can help to control weeds into the growing season, and provide more flexibility with postemergence herbicide application timing. Using preemergence residual herbicides plus planned postemergence herbicide applications is generally the best approach to avoid losses from early-season weeds and help protect soybean yield potential. Using residual herbicide in the postemergence application can also reduce the change for late-season flushes of weeds.

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Table 3. Recommendations for tank mixtures with Roundup® brand agricultural herbicides in Roundup Ready®, Roundup Ready 2 Yield®, and Roundup Ready 2 Xtend® soybeans.

Use full labeled rates of Roundup® brand glyphosate-only agricultural herbicides and follow label directions.
Tank Mix Partner Rate/Acre In-crop Application Restrictions Tough-to-control or Glyphosate-resistant Weed Situations
XtendiMax® herbicide with VaporGrip® Technology1 22 fl oz From emergence (cracking) up to and including beginning bloom (R1) broadleaves including Palmer amaranth, waterhemp, giant ragweed, and marestail (horseweed)
Warrant® Herbicide 3 pints V2 to V3 optimal timing, up to R2 grasses, common lambsquarters, pigweed, waterhemp, Palmer amaranth nightshade. WILL NOT CONTROL EMERGED WEEDS.
Warrant® Ultra Herbicide2 50 fl oz V2 to V3 optimal timing, up to R2 Grasses, common lambsquarters, common ragweed, giant ragweed, velvetleaf, Palmer amaranth, waterhemp, smartweed, morningglories
Select Max®3 6 to 12 fl oz Through R2 up to R3 volunteer corn, johnsongrass, Italian ryegrass.
Flexstar® 6 to 12 fl oz Through R2 up to R3 morningglories, common ragweed, giant ragweed, waterhemp, Palmer amaranth
Cobra® 10 to 12 fl oz Through R2 up to R3 common ragweed, giant ragweed, eastern black nightshade, kochia, waterhemp, Palmer amaranth.

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