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Early-season weed scenarios and observations

Early-season weed scenarios and observations

Compared to same period in 2013, early-season weed control this growing season "more problematic." Palmer amaranth becoming tolerant to Gramoxone?

It’s early June and planting in Tennessee is winding down.

What a difference a year makes! This time, last year, we were trying to finish corn planting, just getting into cotton planting and had hardly turned the tap on soybean planting. The results of that late planting was mixed as the soybeans yielded well, the corn also yielded well --though it took forever to get it to dry -- and the cotton, in many cases, ran out of growing season. 

As in 2013, we will have another relatively large soybean crop due in part to excessive winter kill of wheat on later sowed fields and a wet late March and April that had some planting intentions move from corn to mainly soybean.

From a weed control perspective we saw more pre-emergence herbicide injury in soybean and cotton back 2013 than we did this spring. That is not to say we had no injury this year. We did, particularly for cotton and soybean just trying to emerge the week of May 22 in very cold and wet soil conditions. 

We were going to have some struggling cotton and soybean stands with that weather even with no herbicide present. Young seedlings emerging with activated herbicides through cold and wet soil conditions magnified the problem. However, the soybean and cotton that emerged early or later than that week established stands in good order.

The jury is still out on the weed control effectiveness we are experiencing this year compared to 2013. Weed control has started out to be more problematic than what we experienced last spring. Most of May, it was often too windy to spray which resulted in late burndown on large Palmer amaranth in some cases. In other situations cotton and soybean planters were running without the sprayer backing them up with a pre-applied herbicide. 

Those late burndown applications often resulted in inconsistent burndown of large Palmer amaranth with Gramoxone. This occurred even at high rates and in some cases mixed with a PSII inhibitor like metribuzin or Caparol.


Some have asked if Palmer is becoming tolerant to Gramoxone. I think the answer is not that Palmer has developed resistance to Gramoxone but that spray coverage is almost impossible in heavy populations of large Palmer amaranth. A follow-up application in most cases did a good job taking out the regrowth, but no one wanted to have to spray twice.

The lack of pre-applied herbicides this year on substantial acres is one reason we experienced less pre-applied herbicide injury. On the other hand, we had pigweed emerging with the cotton and soybean. A number of calls have been from folks who were unable to spray pre’s asking about applying their first post application on cotton and soybeans in the cotyledon stage. In my mind this is the only way to go as pigweed will be too large to control if the post applications are delayed to first-leaf cotton or V1 soybean. Once cotton or soybean are out of the crook stage they are relatively safe to spray with either a PPO herbicide for soybean or Liberty in cotton.

Hopefully these early Post applications will make up for the lack of a pre-applied herbicide being applied. It seems like every winter we spend hours making our weed management plan and then most any given spring we have to develop a new plan on the fly.

I believe a famous boxer once said that “everyone has a plan until they get punched in the face.” Hopefully, our plan developed on the fly will be a good counter punch.

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