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Growth Stage Cutoffs For Herbicide Applications In Corn And Soybeans

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After a delayed start to our planting season, we were able to plant both corn and soybean across the state in record or near-record time during May. This coincided with some timely rains and hot weather. This has allowed crops to emerge out of the ground quickly and rapidly progress through growth stages. We already have reports of knee high corn and rapidly growing soybeans across the state.

This article serves as a reminder for growth stage, height restrictions, and pre-harvest interval cutoffs for herbicide applications in both crops.

Corn

There are a number of corn fields that did not receive a preemergence herbicide before the crops emerged from the ground. Many of these preemergence herbicides can also be applied postemergence. Some of the earliest cutoff timings are 8-inch corn for broadcast applications of 2,4-D, and any atrazine must be applied before corn reaches 12-inches in height. If corn is 12 inches or taller, the atrazine must be left out of the tank. See table 8 in the weed control guide for height and growth stage restrictions of postemergence herbicides in corn. Table 8 is also included at the end of this article as well.

Soybeans

Even though we are still a few weeks away from the summer solstice, we are seeing soybeans starting to bloom in some locations that were able to plant in late April. Shawn Conley at the University of Wisconsin wrote a nice article explaining some causes behind this a few years ago. What this means for herbicide applications, is that the window of application for some postemergence herbicides has either ended, or is about to end in those fields that are entering reproductive stages.

We have heard reports that up to 65% of our soybean acres might be Enlist E3 which means it has the Roundup Ready, Liberty Link, and 2,4-D resistance trait. Up to 25% of our soybean acres could be Roundup Ready Xtendflex varieties, which means it has the Roundup Ready, Liberty Link, and dicamba resistance trait. The cutoff for glufosinate (Liberty, Cheetah, Interline, others) applications in Liberty-Link soybean is R1. In other words, once the soybeans are flowering, applying glufosinate is off-label. The cutoff for Engenia, FeXapan, and Xtendimax in Xtend soybeans is R1, or full flower or June 30 (federal label) or June 20 if you are in Indiana.

With these flowering soybeans still being small in many areas, it is imperative to include residual herbicides in these postemergence applications to help reduce weed pressure until crop canopy. See table 18 in the weed control guide for the pre-harvest intervals of soybean herbicides. More information on soybean crop growth stage cutoffs can be found in the soybean section of the weed control guide under the short narratives for each herbicide. Table 18 which shows the soybean preharvest intervals is also included in this article.

Written by: Bill Johnson and Marcelo Zimmer.

Source: Purdue Extension