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Sponsored: Risk and Reward: Late Herbicide Application

Human nature tends to make us push the limits on most things and believe it or not, farmers are human! So, it’s only natural that when a herbicide label reads “Do not apply after R2” it still sometimes get applied at R3. Why do we have these cutoff restrictions and what happens when we don’t always follow them to a T? Spraying past the designated cutoffs can cause phytotoxicity or destruction of plant tissue, which in turn limits the amount of photosynthates that plant can produce. The production of photosynthates dictates how many flowers/pods will be kept or shed, and ultimately decides what the yield potential of that plant will be. According to the Purdue University Corn & Soybean Field Guide, two pigweeds in a 10 in. section of a row can cost up to 50 percent in yield loss. So, what’s the right thing to do? I believe scouting crops, making timely herbicide applications when weeds are small enough to control, and mixing a residual component to your herbicide are the best solutions.

Let’s consider a few other things while we are talking about late season implications.

Have you ever considered how many bushels you lose by driving a sprayer through the crop later in the year?

Studies conducted by Purdue University Extension have estimated that yield losses of 1.3, 1.9 and 2.5 percent can be expected if using 120, 90, and 60-foot spray boom, respectively. However, this same study also found that if the sprayer tracks are made at or before R1, the soybeans have time to compensate for the extra space and there is little to no yield loss associated.

What about plant back restrictions? Because of the cool, wet spring we’ve had this year, many soybeans are either being replanted or going in the ground later than we’d like. It’s important to consider that many soybean herbicides have a lengthy plant back restriction to corn. For example, Reflex®/Flexstar® (fomesafen) has a 10-month plant back interval to corn and 18 months for sorghum. Classic® (chlorimuron) is another popular soybean herbicide used to control cocklebur and lambsquarters, and also carries a 10-month plant back interval to corn. There are also preharvest intervels (PHI) associated with herbicides that could influence the quality and marketability of your crop. A popular herbicide to help control grasses in soybeans is Select Max® (clethodim). This herbicide has a 60-day PHI associated with it.

Most herbicide labels are very lengthy and technical, but is important to reference them before making an application to be sure you are within the defi ned parameters for that chemical. Spraying within the designated time frame not only gives us the best chance to avoid crop injury, but what is written on that label is also the law. If you have any questions about your herbicide program, please reach out to your local agronomist or herbicide specialist.

Austin Scott, CCA | Field Agronomist and Herbicide Specialist 

For more Agronomic News from Austin Scott, certified crop adviser, please visit his Agronomy Page on


Reflex® and Flexstar® are registered trademarks of a Syngenta Group Company. Classic® is a registered trademark of E.I. DuPont de Nemours & Company. Select Max® is a registered trademark of Valent U.S.A. Corporation

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