Value of Preemergence Herbicides Demonstrated

Friday, May 5, 2023

Nothing demonstrates the value of a preemergence herbicide quite like a missed postemergence ONLY application. In 2022, we did just that, intentionally. The products we will talk about in this article are for educational purposes only; they are not intended to be an endorsement of success or failure. Many product choices are available and can be adapted to your needs. The full set of 2022 corn weed management data is available, but in this article, we’ll focus on just two: 1) a delayed POST only application, and 2) a PRE followed by a POST.

Corn weed management trial

In treatment 19 of the 2022 corn weed management trial, we intentionally delayed a POST only application of Acuron GT + Atrazine. There are numerous scenarios that could cause a delay in a POST application, and I am sure we have all experienced many of them. Unsuitable weather and field conditions, equipment failures, custom applicator delays and the list could go on...

First things first, let’s look at how weedy things were at the time of our first POST I application. Plots were planted on May 19th, PREs went on May 23rd, the first (POST I) application was on June 9th, the second (POST II) application was June 17th. Photo 1 is of the weedy check on June 7th.

Treatment 4 is an example of a PRE followed by POST I application. It consisted of 36 fl oz/a Acuron Flexi PRE followed by (fb) 36 fl oz/a Acuron Flexi + 32 fl oz/a Roundup PowerMAX. Photo 2 shows what the weeds looked like prior to the POST I application on June 9th.

Treatment 19 is an example of when a POST only application gets delayed. Treatment 19 consisted of a POST II application of 3.75 pt/a of Acuron GT + 16 fl oz/a atrazine. Photo 3 shows what the weeds looked like ahead of the June 17th POST II application.

The Treatment 19 picture shows the great potential for weed populations to develop when a PRE is not used and the POST application is delayed; at this point there is substantial crop weed competition occurring. Ten days after the POST II application (June 27th), we can see dead weeds in treatment 19 but if you look closely, you can see just how stressed the crop is at this point. We’ve also included photos of the weedy check and Treatment 4 taken on June 27 for comparison:

Source: umn.edu