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When label says 4 inches, it means 4 inches

Soybean Pest Beat: Catching weeds small is difficult but worth the effort.

March 31, 2020

2 Min Read
lambsquarters weed in soybean field
CATCH AT RIGHT HEIGHT: Don’t let this lambsquarters plant get too tall before you spray if it is representative of most weeds in the field. It’s already on the radar as pushing the height limit for some herbicide labels.

Do you know how hard it is to spray when weeds are no taller than 4 inches? Yet, that’s what many labels say and the experts harp on. That’s just not real life. Realistically, how big can weeds get and yet I can take them down without losing yield in soybeans?

The Indiana certified crop advisers panel answering this question includes Gene Flaningam, Flaningam Ag Consulting, Vincennes; Greg Kneubuhler, G&K Concepts Inc., Harlan; and Bryan Overstreet, Jasper County Extension ag educator.

Flaningam: Early-season weed control during the first four to six weeks of soybean development is very crucial. Severe weed infestations can result in 35% or more yield losses in soybeans. Controlling these weeds while they’re small is very critical for optimal herbicide performance. Escapes and large weeds are hard to control, and lead to developing herbicide resistance and crop failures.

Kneubuhler: Unfortunately, there are several herbicides that contain that kind of height restriction on their label. I think glyphosate has made us all passive and lazy in terms of weed management. That is largely to blame for the weed resistance we now all fight today to varying degrees. That is also the reason why new herbicide-tolerant technologies are on the market.

Yes, we all push labels from time to time, but as a CCA, I must adhere to the label as best as possible. To me, if you find yourself struggling to make herbicide applications within the recommended window, then maybe it’s time to reevaluate your herbicide program to fit your operation. In this day and age, there are many options available to us that should give us flexibility. While I believe labels tend to be conservative and do include a margin of error, the prudent thing to do is adhere to the guidelines.

Overstreet: I understand it is hard to get to all the weeds before they are 4 inches tall, but there are many reasons for this recommendation. The main reason is that once they get to that point, they are harder to kill. There will be smaller weeds underneath the canopy that your spray won’t cover and will miss. The more you miss or just ding with the herbicide, the faster it can lead to resistance. Also, by the time they have reached 4 inches tall, they’re already impacting your yield, in most cases.

I recommend using a residual preemergent herbicide early to slow down the emergence of the weeds and give you more time to prepare to spray that postemergence treatment. By doing so, you should also have fewer weeds to control with a post application.

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