Palmer amaranth got the best of a lot of Tennessee farms this year.

For the last couple of summers around Aug. 1, I have written an article assessing the weed control season. We had a lot more issues this year than in 2012. Horseweed was more of a problem due mostly to later emergence events that escaped early burndowns. Italian ryegrass was also more of an issue, and it appears some of the true glyphosate-resistant version of this weed has become established in Tennessee. Glyphosate-resistant goosegrass is becoming more prevalent, which means we will have to employ more clethodim-type products to control both those glyphosate-resistant grass species in years to come.

The biggest weed issue in 2013 was, of course, glyphosate-resistant Palmer amaranth.

Palmer pigweed - it's all about timing

This time last year I thought we took a step forward in controlling Palmer amaranth. If anything we took a step back this year. We received a lot more calls, much like in 2011, on salvage-type situations, particularly in soybeans. Moreover, on trips across the state visiting farms we saw that we just have more pigweed in the field. There are four main reasons in my mind for pigweed getting the best of us in 2013.

The first is that Palmer amaranth is becoming more established in counties where it had only just gained a foothold last year. It came as a bad surprise to farmers who have heard about the weed but had not had to manage large populations on their farms until this year. It happens every time it moves into a new area — the “shock and awe” of how quickly it will grow and how in just a matter of days it becomes too large for herbicides to control. You can tell someone that Palmer amaranth can emerge and grow 8 inches in 13 days in the middle of June, but until someone actually tries to control it for the first time, it does not sink in that this weed has to be managed much more aggressively.

The second reason is we had a lot more rain this spring and summer than last year. In 2012, once an activated pre-applied herbicide stopped the first flush of pigweed, it was often too dry for follow-up Palmer emergence. With all the rain this spring, even the better pre-applied herbicides like Prefix, Envive, Authority MTZ, Fierce, Valor, etc., only lasted about three weeks instead of the four weeks to five weeks growers were often counting on. After they played out, we had repeated flushes of Palmer amaranth to contend with. Growers tell me they have learned that regardless of crop, a herbicide has to be sprayed about every two weeks until canopy to keep Palmer amaranth at bay.

The third reason was the large and high-yielding wheat crop. This is, of course, a good problem to have with much of the wheat in the state averaging in the high 80s and selling at a good price to boot. However, by the time growers were able to shake some labor away from wheat harvest to spray soybeans, and in some cases cotton, Palmer amaranth had broken through the pre-applied herbicide and was 8 inches to 12 inches tall.

In glufosinate-tolerant cotton, growers were able to salvage the crop with a couple of applications of Liberty over the top, followed with a hooded application. In Roundup Ready soybeans where the PPO herbicides like Flexstar, Rhythm, Cobra, Ultra Blazer, etc., will not control a Palmer pigweed much over 2 to 3 inches tall, we had few options.

 

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A fourth reason was due to the large increase in soybean acres this spring. Moreover, many of these soybeans were planted in about a three-week window. It is much harder to be timely with post-emergence applications when logistically there are more acres to cover and they all need to be sprayed at the same time.

The good news from this year is that it is raining now as I write. If this keeps up we will have good corn, cotton and soybean crops to go along with the great wheat crop. In fact, a wet high-yielding crop year is what we all want. It just happens to be an ideal environment for Palmer amaranth, which is not a coincidence.

lsteckel@utk.edu

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