Pennsylvania Corn, Soybeans: Post Herbicide Applications – Considerations and 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

Many of the corn and soybean crops are getting beyond the growth stage for a legal post herbicide application. Some corn herbicides can be applied up to corn that is 48 inches tall but with the use of drop nozzles.

Many of the typical soybean post herbicides vary in their application timeframes. Products such as Classic, Reflex, clethodim, and others have a rather wide window for application; however, products like glyphosate, Pursuit, FirstRate, Engenia, Xtendimax, Liberty, Enlist, and a few others must not be applied beyond the flowering stage.

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

A number of 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 other 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.



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.

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 dry weather, maturity, 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, Select, 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.
  • 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.

Palmer amaranth and waterhemp ID

We are receiving some reports about Palmer amaranth or waterhemp invading and growing rapidly in fields across the state. Make sure to scout crops to determine what kind of pigweed species are in your fields. The sooner you ID them the easier it is to manage them. Many are confusing Palmer and waterhemp with redroot or another pigweed species.

Once Palmer or waterhemp reaches 4 inches tall they are very difficult to control. However, redroot and smooth pigweed tend to be easier to control. For best control, make sure to know what weed you are battling. For details on ID and management considerations, please refer to our Invasive Pigweeds website .

Source URL: https://extension.psu.edu/post-herbicide-applications-considerations-and-challenges