by Dwight Lingenfelter

Every year, we are asked 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, the crop, the setting and the weeds.

Most products have cutoff restrictions based on crop growth stage or days before harvest. Whether using highboy sprayers, aerial or drone sprayers, or some other contraption, the herbicide guidelines must be followed.

In most cases, crops are currently in “no-man’s land” because most are beyond the maximum growth stage for a standard broadcast application and are not mature enough for a harvest aid. Also, keep in mind that late-applied herbicides are usually not as effective because the weeds are well beyond the recommended stage of growth for best control.

Most labels require that weeds be sprayed before they reach 6 inches tall. 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 late in the season.

Soybean herbicides

Here are guidelines for selected postemergence soybean herbicides that could potentially be used for late-season applications:

Assure II/Targa. Apply at least 80 days before harvest.

Basagran. After emergence, soybeans are tolerant to Basagran at all growth stages.

Cadet. Preplant through full flowering, and at least 60 days before 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 soybeans from emergence to R1. Do not apply within 50 days of harvest.

FirstRate. First 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 seven days before harvest and when soybean pods have set and have lost all green color, and 80% to 90% of leaves have dropped.

Harmony SG. After first 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). Emergence to before soybean bloom (R1) and at least 70 days before harvest of grain or forage.

Pursuit and Raptor. Preemergence to before bloom and 85 days before harvest.

Reflex or Flexstar (or Flexstar GT with glyphosate). Preemergence 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.

Corn herbicides

Very few corn herbicides allow applications to corn once it reaches 48 inches tall. Most prohibit applications after corn reaches V8 or 36 inches tall.

Here are guidelines for selected postemergence corn herbicides that could potentially be used for late-season applications:

Basagran. No restrictions.

Cadet. V2 until corn is 48 inches tall or prior to tasseling.

Impact/Armezon. Apply postemergence up to V8 growth stage or 45 days before corn harvest, whichever is more restrictive.

Glyphosate (Roundup Ready or glyphosate-resistant varieties only). Emergence through 48 inches tall. Can be used as a harvest aid at least seven days before harvest and when the grain has 35% or less moisture, and after maximum kernel fill is complete and fully mature.

Don’t let weeds set seed

In other fields that are between crops or are fallow, now is the time to think about controlling summer annual weeds before seed set.

Preventing seed production is important for driving down the weed seed bank and reducing the need for weed control inputs. It is easy to prevent weed seed production following a cereal grain like wheat, barley or oats — as well as some vegetable crops such as sweet corn or snap beans.

Proper timing of the control practice is essential to prevent seed production. To prevent seed production, fields can be sprayed with an effective herbicide or mowed once or twice. For example, for giant foxtail, spray the field before mid-September to greatly reduced seed production.

On the other hand, pigweed can begin to produce mature seed by mid-August. Therefore, it is best to check weed seed heads to determine at what stage they are flowering and mature, and time control practices accordingly.

Glyphosate is particularly effective at stopping grass growth and reproduction. The plant growth regulators (2,4-D and dicamba) would probably be a better choice for broadleaf weeds.

Taking the time to prevent seed production this year can make a big difference in future years. In most cases, up to 80% of annual weeds in the field are the result of last year’s seeds.

Furthermore, many perennial broadleaves are evident in small grain stubble. The challenge with perennial weeds this time of year is the fact they are still in the vegetative and reproductive phases. Therefore, most of the plant sugars are not being significantly transported to the roots, and an herbicide application right now will likely only affect top growth.

One consideration would be to mow those fields soon to prevent seed production and allow regrowth to occur. Then, apply an effective systemic herbicide (glyphosate, 2,4-D, dicamba) in late September or early October so the herbicide will be transported to the roots for more effective control.

Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension.