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Application Reminders for Roundup® Brand Glyphosate Agricultural Herbicide

Posted 8/9/2016
Roundup® brand agricultural herbicides need to get on the plant, in the plant, and throughout the plant for good weed control.

Coverage, Absorption, and Translocation

Coverage is often compromised by the weed or crop canopy. Coverage can be improved by choosing the proper nozzles, adjusting the boom height, and spraying at an appropriate ground speed. Use of spray volumes that range from 10 to 20 gallon per acre generally provides good coverage on target weeds.

Weeds need to be actively growing for good absorption and translocation of the herbicide. Environmental conditions can affect absorption and translocation. Dry weather causes weeds to have thickened cuticles, which are harder for herbicides to penetrate. Dry weather can also increase dust that can bind with glyphosate, the active ingredient in Roundup agricultural herbicides, making it less available for absorption into the plant. Adjusting the rate and using proper additives can help under these conditions.

Translocation requires actively growing weeds with a good plumbing system, the xylem and phloem. Mechanical damage from tillage, planting or spray equipment can compromise the plumbing. Tillage that injures, but does not kill the weed can make them appear shorter because much of the plant is below the soil surface. Planters and drills can cause the same effect. Lack of weed control in sprayer wheel tracks can be due to restricted plumbing system of a plant and/or the presence of dust. Stem boring insects can also damage the plumbing, restricting translocation. When weeds injured by stem boring insects have been sprayed with a Roundup agricultural herbicide, the portion of the plant above insect damage should die. Below the insect damage, the weed often remains green and may regrow. Giant ragweed and marestail are two examples of weed species where this has occurred.

Use the Right Rate

The rate for the field should be determined by the largest or most difficult to control weed, not the most prevalent. The general recommendation for Roundup agricultural herbicides is to use 32 fl oz/acre and to spray when weeds are 4 inches or less. If the field has predominantly 3 to 4 inch velvetleaf, but also contains numerous 12 inch lambsquarters, the recommended rate would increase to 44 fl oz/acre due to the height of the lambsquarters.

Annual weeds that are older and more mature or hardened-off may require 44 fl oz/acre even if they are less than 12 inches tall. Environmental stress, such as dry weather, can cause weeds to be short for their age, requiring a higher rate for good control. Tough to control annual weeds like common and giant ragweed and perennial weeds generally require the higher rate of 44 fl oz/acre.

While weed characteristics such as species, size, and age are important to determine the right herbicide rate to use, there are crop restrictions that must be followed. For Roundup PowerMAX® herbicide, the maximum single in-crop rate for corn is 32 fl oz/acre; for soybean this rate is 44 fl oz/acre. Always check the herbicide label for application restrictions, and use full rates to help achieve complete control of existing weeds.

Ammonium Sulfate (AMS)

AMS conditions hard water and can be added to the tank at 8.5 to 17 pounds (1 to 2% by weight) of spray grade AMS or equivalent rate of liquid AMS product per 100 gallons spray solution. Although not always necessary, additional non-ionic surfactant can also be added to Roundup PowerMAX® herbicide at 1 to 2 quarts per 100 gallons spray solution to improve control.

Making Re-Treatments

When re-treatment is necessary, allow time for weeds to recover and resume growth. Use the right rate of a Roundup® agricultural herbicide, considering weeds are older, taller, and will probably be even more difficult to control. If sprayer wheel tracks were the problem, avoid the previous tracks. Weeds need to be actively growing for the best results.

Tank Mixtures

Labels for Roundup® agricultural herbicides include approved tank-mix partners. Combining Roundup agricultural herbicides with recommended tank-mix partners like dicamba can increase performance on certain weed species. Tank mixtures that could cause antagonism and reduce the effectiveness of Roundup agricultural herbicides should be avoided. Tank mixing with insecticides, fungicides, and nutrients or foliar fertilizers is generally not recommended.

Application Timing

The timing of weed control can affect yield potential. Weeds should be controlled prior to 4 inches to help maximize yield potential. Except for preharvest applications, DO NOT apply after R2 stage in Roundup Ready® soybeans, Roundup Ready 2 Yield® soybeans, and Roundup Ready 2 Xtend® soybeans or after V8 or 30 inch corn with Roundup Ready 2 Technology or Roundup Ready Corn 2 (without drop nozzles) (Table 1). For preharvest applications in soybeans, apply prior to harvest after pods have set and lost all green color. Follow all appropriate label instructions and restrictions including the time between preharvest application and soybean harvest.

Table 1. In-crop application guidelines.

Crop Maximum Single In-Crop Maximum Total In-Crop POST Application Timing Restrictions
Corn with Roundup Ready® 2 Technology or Roundup Ready® Corn 2 32 fl oz/acre 64 fl oz/acre
  • Can be applied up to 48 inch corn
  • Can be applied from emergence through V8 or until corn reaches 30 inches (free standing), whichever comes first
  • Drop nozzles are recommended for optimum spray coverage and weed control when corn is 24 to 30 inches
  • For corn heights of 30 to 48 inches, drop nozzles are required and should be kept below the crop canopy
Roundup Ready® soybeans, Roundup Ready 2 Yield® soybeans, and Roundup Ready 2 Xtend® soybeans 44 fl oz/acre 64 fl oz/acre
  • Can be applied from cracking through flowering (R2)
  • R2 ends when a 3/16 inch pod appears at one of the four uppermost nodes on the main stem with a fully developed leaf

Herbicide Mixing Order Matters…

  1. Tank should be filled 1/2 with water and gentle agitation started.
  2. Ammonium sulfate (AMS) should be added, allowing it to fully dissolve to tie up any hard water ions.
  3. Any dry formulations, suspensions, wettable powders, or flowables should be added, and should be agitated to fully dissolve any dry products.
  4. Drift reduction agents should be added.
  5. Water soluble formulations (liquids) should be added.
  6. Finally, Roundup® agricultural herbicides should be added. If using Roundup PowerMAX®, non-ionic surfactant can be added last.

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