Things to Consider for a Successful Postemergence Herbicide Application

By Liz Stahl and Phyllis Bongard et.al

Recent warmer temperatures and adequate moisture have set the stage for rapid weed growth in the coming weeks. Timely and effective herbicide applications will carry much of the weight in most weed management programs, since very few weeds were present at the time of pre-plant tillage this year due to the cold late spring.

Waterhemp is an early-emerging weed . Consult
the interactive weed seedling quiz at the end of this article
for comparisons of seedling identification characteristics
of waterhemp, redroot pigweed and other weed seedlings.
Photo: Dr. Tom Peters

Scout fields now for weed emergence, regardless if a preemergence (PRE) herbicide was applied or not. In much of southern Minnesota, delayed planting along with variable precipitation may have delayed the placement and activation of PRE herbicides. Weeds that emerged prior to PRE activation may require a pass with a rotary hoe or POST herbicide application to be controlled.

Corn yield can be affected by weeds as early as two weeks after crop emergence, especially when the weeds you are dealing with are very competitive weed species (e.g. giant ragweed) or when weed densities are high. Timely weed control is key in preventing yield loss and preventing weed escapes from ultimately producing seed and replenishing the weed seedbank.

Target applications before weeds exceed 3 to 4 inches in height

Figure 1.Herbicide applied on giant ragweed when it was too large. This plant had many growing points that were able to recover after the herbicide application.

Weeds larger than 3 to 4 inches are more likely to survive a herbicide application, especially when conditions are less than ideal during application (e.g. issues with rate, coverage, temperature, time of day). Larger weeds have more growing points than smaller seedlings, which helps them tolerate and escape an herbicide application (Figure 1). Herbicide coverage and uptake can also be reduced with larger weeds.

Weeds can grow quickly once emerged. When weather is favorable, waterhemp can grow one inch per day and can reach 4 inches in height very quickly. Therefore, early-season scouting is necessary, so that the weeds can be sprayed with postemergence herbicides before they reach a 4-inch height. Figure 2 demonstrates how quickly waterhemp and giant ragweed can exceed a height where control is more reliable.

Figure 2. Giant ragweed and waterhemp growth over time. Plants were measured and monitored at Rochester, MN. The beginning date is the emergence date.

Follow herbicide application cutoffs and guidelines

Always check the herbicide label for specific guidelines before application. Postemergence (POST) cutoff timing for some of the more common herbicide-resistant technologies include:

• Roundup Ready 2 Xtend^® and XtendFlex^® soybeans:
  • XtendiMax^®, Engenia^®, and Tavium^® - No application shall be made south of Interstate 94 after June 12, 2022. North of Interstate 94, use is prohibited after June 30, 2022. No application shall be made if the air temperature of the field at the time of application is over 85°F or if the National Weather Service’s forecasted high temperature for the nearest available location for the day exceeds 85°F. Temperature must be recorded at the start of the application.
• Enlist^® corn and soybeans:
  • Enlist One^® and Enlist Duo^® - Apply when weeds are small (less than 6 inches) and corn is no larger than V8 growth stage or 30 inches tall, whichever occurs first. Use drop nozzles for corn 30 to 48 inches in height to avoid spraying into the whorl. Do not apply within 30 days of forage harvest.
  • For soybeans, apply when weeds are small (less than 6 inches) and any time after emergence through the R1 stage. Two applications should have a minimum of a 12 day interval. Do not apply within 50 days of soybean harvest.
• Corn and soybeans containing Roundup Ready^® trait:
  • Glyphosate (e.g. Roundup PowerMax^®, Roundup PowerMax 3^®, and Durango^®, etc.): Apply before corn reaches V8 or 30 inches (for proper spray coverage and weed control, drop nozzles are recommended in 24 to 30 inch corn); For corn 30 to 48 inches in height drop nozzles should be used. Do not apply within 50 days of forage or grain harvest and do not make any applications less than 10 days apart.
  • For soybeans, apply from emergence (cracking) throughout flowering (R2 stage). Do not apply within 14 days soybean harvest.
• Corn and soybeans containing LibertyLink^® trait:
  • Liberty^® - Apply to small and actively growing weeds, targeting weeds less than 3 inches in height. Glufosinate is most effective when applied under warm, sunny, and humid conditions. Apply in a minimum spray volume of 15 gpa to ensure good spray coverage.
    • Apply Liberty^® from emergence through V6 stage of corn growth. Use drop nozzles for corn up to 36 inches in height. Do not apply Liberty within 60 days of harvesting corn forage and within 70 days of harvesting grain.
    • For soybeans, apply Liberty from emergence to bloom or R1 growth stage. Do not apply Liberty within 70 days of harvesting. Sequential applications must be at least 5 days apart.
• Be sure to check the herbicide label for application restrictions prior to use and follow the most restrictive guidelines for products in a tank-mix. Adjust application rates as needed depending on weed size and species, and follow label instructions regarding adjuvants, surfactants and oils, mixing order, allowable tank-mix partners, potential antagonism with tank-mix partners, etc.

  Herbicide resistance management and season-long weed control

  The prevalence of herbicide resistance continues to be a challenge. Recent resistance screening research at UMN confirmed that the waterhemp and giant ragweed populations resistant to ALS inhibitors (Group 2; Pursuit®, Classic®, and FirstRate®) and EPSPS inhibitors (Group 9; glyphosate products) are widespread in MN. Moreover, waterhemp populations resistant to PS II inhibitors (Group 5; atrazine), PPO inhibitors (Group 14; Flexstar®, and Cobra®) and HPPD inhibitors (Group 27; Callisto®, and Laudis®) are also confirmed. In populations where these traits are "stacked," postemergence options are limited.
  
  The herbicide diversification strategies developed by U of MN Extension and North Dakota State University are useful tools in the management of herbicide-resistant weeds. Use of preemergence herbicides, including the layering of preemergence herbicides around 30 days after planting is another key strategy. A multi-pronged approach that includes non-chemical strategies (e.g. expanded crop rotation, delayed planting, cultivation, hand weeding, prevention of seed production, use of cover crops) will greatly assist in long-term, herbicide-resistant weed management.

  Know your weeds

  Proper identification is a critical step in selecting the correct weed management tools and strategies. See the following University of MN Extension website for pictures and identifying characteristics of common annual grass, annual broadleaf, and perennial weeds in MN: https://extension.umn.edu/weed-management/weed-identification. Additional resources are listed following the quiz.

  Can you differentiate waterhemp from other weed seedlings? 

  Test your identification skills by taking our Weed ID quiz below or at: http://z.umn.edu/weedIDquiz!