Josh McGinty (joshua.mcginty@ag.tamu.edu), Paul Baumann, and Pete Dotray
Department of Soil & Crop Sciences

You’ve all seen the horror stories about glyphosate (Roundup) resistant pigweed infestations in the Southeastern United States. Texas farmers largely dodged the bullet until 2005, when reports of resistance started to trickle in. Texas A&M AgriLife Extension and Research first confirmed glyphosate resistant common waterhemp in Central and Southeast Texas. This was followed by the confirmation of resistant Palmer amaranth on the Southern High Plains in 2010.

Since that time, these researchers have conducted numerous field studies to provide recommendations for managing glyphosate resistant weeds as these populations have become more widespread. Given the fact that a single waterhemp or Palmer amaranth plant can shed 500,000 to 1 million seed, one weed left in the field is too many. What follows are recommendations for managing these weeds early in the season.

Often, the most competitive weeds are those that emerge prior to, or at the same time as the crop. These weeds are quite effective at competing for the same consumable environmental resources such as water, nutrients, and sunlight that the crop seedling needs for healthy growth. This is particularly important in cotton, which tends to have a slower growth rate as a seedling compared to other crops. This early-season competition is also highly detrimental to sorghum and corn seedlings as well. If these weeds are not controlled, significant yield losses can be expected. By controlling early-season weed infestations, the yield potential of the crop is protected. Additionally, emerged weeds are easier to control with post-emergence herbicides at this point. Later in the season, weeds can become “hardened-off” due to hot, dry conditions, and will be more difficult to control.

Prior to planting, emerged weeds should be controlled with preplant burndown herbicides or tillage. Due to widespread problems with glyphosate resistant weeds in Texas, consider using a tank mix partner when applying glyphosate as a burndown, especially if the field has a history of glyphosate resistant weeds. Adding a soil residual tank-mix partner to burndown applications will provide some insurance against early season weed competition from later emerging weeds. Remember that rainfall, irrigation, or mechanical incorporation is required to move residual herbicides into the soil and “activate” them. When using soil residual products, keep in mind the planting interval and crop rotation restrictions of the product(s) used, if planting intentions change. Also, the plant-back residual activity may not start until after an inch of rainfall or irrigation.

As planting time approaches, the application of residual herbicides prior to, or at planting is critical. Figures 1 and 2 below show the vast difference in weed density between an non-treated cotton plot and one that has received a pre-plant application of a residual herbicide. Ideally, these applications will overlap the pre-emergence weed control provided by pre-plant burndown applications that included a soil residual herbicide. This will extend pre-emergence weed control longer into the season, and lessen the pressure placed on post-emergence herbicides such as glyphosate, thus reducing the risk for developing glyphosate-resistant weed problems. If these programs still don’t adequately control waterhemp or Palmer amaranth, there are many options for managing them with post-emergence (POST) products. The key to POST herbicide efficacy is treatment timing; most applications will require treatment of pigweeds species less than four inches in height. Many times, farmers have assumed that failures in control are due to application errors, and will follow up with another application of glyphosate. By the time that application has had a chance to work yet still doesn’t control the weeds; it’s too late for treatment with any other post-emergence herbicide. Because of these problems, it has become more important than ever to diligently scout fields for weed escapes and treat them with alternative products. Please understand, we are not recommending the elimination of glyphosate from your herbicide program, it is still very effective on many of our weed species. However, if glyphosate resistant weeds are a possibility on your farm, partner glyphosate with other pre-plant, pre-emergence, and post-emergence herbicides.

Josh McGinty
Assistant Professor & Extension Agronomist
Corpus Christi, TX
jmcginty@tamu.edu

Figure 1. Non-treated cotton plot.