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Post Herbicide Considerations: Growth Stages and Hot Weather

Post Herbicide Considerations: Growth Stages and Hot Weather

By Dwight Lingenfelter

Corn and soybean crops are approaching maximum growth stages for post herbicide application. Also, hot weather is causing its own set of challenges.

As the growing season progresses, crops are growing rapidly. As a reminder, many herbicides have application restrictions related to the crop stage of growth, as well as if the crop is being harvested for silage/forage/grazing or grain. Here are a few things to consider.

Post applications

Depending on your part of the state, many of the corn and soybean crops are getting close to or beyond the growth stage for a legal post herbicide application. Some corn herbicides (e.g., Armezon Pro, Callisto, glyphosate, Halex GT, Impact, Status, and others) can be applied broadcast, over-the-top up to corn that is 30 inches tall. However as the corn gets taller, more herbicide is intercepted by the crop leaves and thus not deposited onto the weeds, causing reduced control and potentially more crop injury since the herbicide can get concentrated into the corn whorl. Once the corn grows beyond 30 inches tall, it is best to consider using drop nozzles for better deposition of the spray solution to the target weeds below and less onto the crop. Certain herbicides (Accent Q, Capreno, DiFlexx, glyphosate, Liberty, etc.) can be applied to corn that is 30 to 36 inches tall but with the use of drop nozzles. Few herbicides allow applications up to 48 inches or taller. For more details see Table 2.2-15  in the 2021-22 Penn State Agronomy Guide. Many of the typical soybean post herbicides vary in their application time frames. Products such as Classic, Reflex, clethodim and others have a rather wide window for application; however, products like glyphosate, Pursuit, FirstRate, Liberty, Enlist, and a few others must not be applied beyond the flowering stage. For more details see Table 2.4-14  in the 2021-2022 Penn State Agronomy Guide. Don’t confuse late post applications and harvest aid applications. Not all herbicides can be applied up to harvest. There are only several herbicides that can be applied as a harvest aid (e.g., 2,4-D, dicamba, glyphosate, Gramoxone, Aim and a few others) and these must be sprayed within a certain time period when the crop is mature and ready to harvest.

Harvest intervals

Several herbicides have restrictions when harvesting corn or soybean for silage or forage. Harvest restrictions are based on the potential for illegal herbicide residues in the feed or forage. Although not generally a problem, early harvested corn may fall under some of these restrictions. Some pre corn herbicides such as atrazine, Acuron, Anthem, and Sharpen have intervals from 60 to 90 days; while others pre herbicides can be less than 45 days. For example, post applied products such as Steadfast Q and Resolve Q have a 30 day, Status a 32 day, Liberty a 70 day, Roundup a 50 day, and Impact/Armezon a 45 day harvest restriction for silage following herbicide application. For more details see Table 2.2-18  in the 2021-2022 Penn State Agronomy Guide. Many of the pre- and post-applied soybean herbicides are not labeled for soybean forage but some that do include BroadAxe/Authority Elite-30 days, Boundary and metribuzin-40 days, FirstRate-25 days, and Liberty-45 days, Enlist One/Duo-56 days, and Engenia/Xtendimax-7-14 days. Fortunately, in-crop applications of Roundup have only a 14-day harvest restriction, while other glyphosate products may vary regarding their harvest restrictions. Another consideration is herbicide application and time frame to grain harvest. Some products require the application to occur at least 30 days prior to harvest (Basagran); others 45-50 days (Cobra, Liberty, Ultra Blazer); 60-65 days (Anthem Maxx, Cadet, Classic, FirstRate, Harmony SG, Select, Synchrony XP); while others require 80-85 days (Assure II, Pursuit, Raptor). For more details see Table 2.4-14  in the 2021-2022 Penn State Agronomy Guide.

Hot weather and weed control

With much of the region in drought, there is some concern about the success in trying to control certain weeds under these dry conditions.

Certainly, annual plants become much more “tolerant" with maturity, dry weather, and persistent warm temperatures. Large, drought-stricken annuals are harder to kill. With perennial weeds, the effect of drought is less clear. Cool season perennials including Canada thistle, quackgrass, and dandelion will certainly go into a summer dormancy period when dry warm weather persists. If possible, they should not be treated with an herbicide until actively growing. Cool season perennials mimic the same growth cycle as your lawn; active in the late spring and early summer followed by a slow period and then a rebound in later summer and early fall. Once the “heat" of summer has passed and assuming they have relatively healthy green leaves, then an effective systemic herbicide should work well.

Some things to consider as we experience dry weather, and the weeds continue to grow:

  • Increase the herbicide rate if the label allows and make applications at the most favorable time for increased control. Make applications in the morning when the weeds are most active.
  • Apply herbicides to smaller weeds or wait a few days to spray if rainfall is in the forecast.
  • The post grass herbicides (Assure II, Select/clethodim, Poast, etc.) tend to be one of the most susceptible groups to decreased efficacy in dry conditions followed by the ALS-inhibitors (Resolve, Permit, Raptor, etc.). Contact herbicides (Reflex, Liberty, Gramoxone, etc.) are generally less affected by drought stress, but be sure to increase carrier volume to achieve good coverage. 
  • Think about adjuvants. You may need to use a higher rate or switch to MSO (methylated seed oil) or COC (crop oil concentrate) if they are allowed which can increase herbicide uptake and improve control. However, remember oil-based adjuvants can also increase the potential for crop injury. Sometimes there is a fine line between controlling the weeds and injuring the crop.
  • Even though weeds may be more “tolerant" during times of drought, crop injury can still be a concern. Since crops are stressed during hot weather, it is more difficult for them to detoxify the herbicides, in addition, leaf burn can occur if too many different pesticide formulations and adjuvants are added to the tank. EC formulations tend to cause the most crop injury.
  • Hot temperatures, >85°F, drastically increase volatilization of many plant growth regulator (group 4) herbicides such as dicamba and 2,4-D, so use with caution or choose another product.

Be cautious of herbicide drift

As herbicides such as glyphosate and glufosinate can be difficult to find on the market this season, more are resorting to the use of “good-old" products and tank-mixes that include PGR herbicides (dicamba, 2,4-D, etc.). These types of herbicides can cause problems outside the field boundaries. To help reduce drift, use drift reducing nozzles such as venturi or air-inductions (AI) style tips. Most manufactures now make these tips as extended range models in order to reduce the spray pressure down to 15 psi. Also, consider the difference between particle drift and vapor drift. Particle drift occurs when small droplets actually move and deposit onto leaf surfaces (this can be prevented by choice of nozzle, pressure, spray volume, application time, etc.). Vapor drift is related to the function of the herbicide formulation (e.g., ester vs. amine) and does not matter what kind of tip or pressure is used. Many factors, including environmental issues play a part in the overall potential for herbicides to volatilize. Temperature and relative humidity are important for volatility – high temperatures and very low relative humidity usually cause much more volatility. Though volatility is important, off-target movement of herbicides is also due to small spray droplet drift and/or temperature inversions. However, keep in mind, if using herbicides that require good spray coverage (e.g., Liberty, Gramoxone, Cadet, Cobra, Reflex, etc.), AI tips may not be the best option unless certain adjustments are made to allow for better coverage including, higher spray volume, pressure, and boom height.

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