Every year, the researchers at PennState Extension are asked about how late is too late to spray herbicides to control weeds such as marestail, Palmer amaranth, waterhemp, burcucumber and others. As usual, the answer depends on the product being sprayed. Most products have cutoff restrictions based on crop growth stage and/or days before harvest. The herbicide guidelines must be followed, whether using "high-boy" sprayers, aerial or drone sprayers, or some other contraption.
In most cases, the crops are currently in "no-man's land" because most are beyond the maximum growth stage for a standard broadcast application and are not yet mature enough for a harvest aid. Also, keep in mind that late applied herbicides are usually not as effective since the weeds are well beyond the recommended stage of growth for best control.
Furthermore, be aware of special application restrictions if the crops are being used as a forage. Illegal herbicide residues can result if specific application timing and other label guidelines are not followed. Also, consider the amount of crop damage and potential yield loss from sprayer wheel tracks when spraying this late in the season. Below are some basic guidelines for selected POST soybean and corn herbicides that could potentially be used for later season applications.
- Assure II/Targa – apply at least 80 days before harvest.
- Basagran – after emergence; soybean is tolerant to Basagran at all stages of growth.
- Cadet – Preplant through full flowering and at least 60 days prior to harvest. Classic (or Synchrony) – at least one trifoliate leaf up to 60 days before maturity.
- Cobra – apply POST up to 45 days before harvest.
- Enlist One – can be applied in Enlist E3 soybean varieties only from emerge to no later than R1 and do not apply within 50 days of harvest.
- FirstRate – 1st soybean trifoliate through 50% flowering (R1) and at least 65 days before grain harvest.
- Glyphosate (Roundup Ready or glyphosate-resistant varieties only) — emergence through flowering. Can be used as a harvest aid at least 7 days before harvest and when soybean pods have set and have lost all green color and 80-90% of leaves have dropped.
- Harmony SG – after 1st trifoliate has expanded fully and no later than 60 days before harvest.
- Liberty 280 (Liberty Link or varieties than include glufosinate-resistant traits such as Enlist E3, XtendFlex, and LLGT27) – emergence through bloom (R1) and at least 45 days before harvest.
- Pursuit and Raptor – PRE to before bloom and 85 days before harvest.
- Reflex or Flexstar (or Flexstar GT with glyphosate) – PRE to within 45 days of harvest. Select/clethodim – apply at least 60 days before harvest.
- Ultra Blazer - at least one to two trifoliate leaves up to 50 days before harvest.
Very few corn herbicides allow applications up to 48 inches or taller. Most prohibit applications after corn reaches V8 or 36 inches tall.
- Basagran — No restrictions Cadet – V2 until corn is 48 inches tall or prior to tasseling.
- Impact/Armezon — Can be applied up to 45 days prior to harvest.