Texas: Don’t Forget Residual Herbicides for Resistant Weeds

With continuing El Nino conditions this year, the moisture outlook for the 2016 crop is still favorable. Keep in mind that good growing conditions also favor the growth and development of our weed species as well. Our primary concern this season, as it has been for the past few years, is with glyphosate-resistant Palmer amaranth and common waterhemp (you may know these as pigweed or carelessweed).

Currently, resistant Palmer amaranth has nearly become a statewide issue, while significant populations of resistant common waterhemp exist across large portions of central and southeast Texas.

The backbone of our current management recommendation for these (and most other weeds) is the inclusion of residual preplant incorporated or preemergence herbicides. When we include an effective residual herbicide in our weed control program, we take a large amount of pressure off of glyphosate and other herbicides used postemergence.

The key point here is that an effective weed control program should be centered upon these soil residual herbicides. Postemergence herbicides such as Roundup, Liberty (in LibertyLink crops), and others should only be viewed as a means to control weed “escapes” that make it through the residual herbicides.

As the season progresses and these postemergence herbicides are applied, the inclusion of a soil residual tank-mix partner should be seriously considered, as it will extend your protection from later-emerging weeds. This is especially true when a wetter than average season is expected and multiple flushes of weeds are possible throughout the season.

In order to realize the benefit from these residual herbicides, it is important to remember that these materials MUST be incorporated into the soil in order for them to work. The herbicide must be present in the zone of the soil where weed seeds are germinating. If a proper incorporation does not take place, the herbicide will simply sit on the soil surface (often degrading rapidly due to sunlight), and weeds will germinate and emerge safely from below.

Mechanical incorporation through shallow tillage (typically to a depth of 2 to 3 inches) is a highly effective method of incorporation. This is particularly important with the “yellow” herbicides such as Prowl and Treflan. When mechanically incorporating, it is important to uniformly distribute the herbicide into the soil to avoid “streaking” of the herbicide.

This is best achieved by making two tillage passes in different directions. The depth of incorporation is also critical – the herbicide only needs to be mixed to the depth of germinating weed seeds.

More On Herbicide Resistance

If incorporated too deeply, the herbicide will be “diluted” in the soil and poor weed control will be likely. For example, if you have incorporated to twice the required depth, you have effectively reduced the herbicide rate by 50% in the zone where weed seeds are germinating.

Power-driven incorporation tools will place the typically place the herbicide as deep as the machine is running, while implements such as a tandem disk or field cultivator will place the herbicide to approximately one-half the depth of tillage. Keep in mind that mechanical incorporation may not be recommended for some herbicides, so read the product label for instructions about recommended incorporation methods.

Many residual herbicides may be incorporated by rainfall or irrigation, and it is the first rainfall or irrigation that determines the depth of these herbicides. In these situations, we are relying upon water to move the herbicide into the soil, thus the quantity of water is critical for getting the herbicide deep enough. In dryland production, if a significant rain (0.75 in. or more) is not predicted within 7 days after application, a mechanical incorporation may be required.