Entomologists for many years have dealt with insect resistance to insecticides. For weed scientists, however, weed resistance to herbicides is relatively new.

Herbicide resistance occurs when a weed population is able to survive a herbicide treatment that under normal use conditions would be controlled. The adage of “survival of the fittest” applies here, and, in fact, weed resistance to a herbicide is an example of accelerated evolution.

The process begins with just a few plants with the genetic capacity to survive the herbicide treatment. It is believed that these plants, which occur naturally in the population at a low level, are not a result of genetic mutation caused by the herbicide.

These inherently resistant plants when exposed to the same herbicide over several years produce seed, and over time the population slowly shifts such that the resistant weeds become dominant.

Since this process is slow, the producer may not notice the problem until large-scale weed control failures occur.

Roundup Ready crops introduced in 1996 allowed glyphosate herbicide to be applied to the crop without concern for injury. Because glyphosate was effective on most weeds and was economical, use of soil-applied herbicides at planting declined, and in many cases, weed control programs consisted of multiple applications of only glyphosate.

Even though the development of the Roundup Ready technology has greatly benefited weed management programs in the Mid-South, long-term use of glyphosate has selected for glyphosate-resistant weeds.

In Alabama, Arkansas, Georgia, Mississippi, Missouri, Tennessee and North Carolina, at least one weed species has been confirmed as being resistant to glyphosate.

Weeds resistant to glyphosate include Palmer amaranth, tall waterhemp, common ragweed, giant ragweed, horseweed, Italian ryegrass and johnsongrass.

In Louisiana it has always been our contention that it was not a matter of if we would see the problem, but when.

The delay in development of glyphosate-resistant weeds may be related to our cropping systems and weed control programs, which have included use of soil-applied herbicides at planting and combinations of glyphosate with other herbicides. The lag period has allowed us to learn from what has happened in other states and to aggressively educate producers as to potential problems and to promote weed control programs that prevent or delay development of glyphosate-resistant weeds.

LSU AgCenter weed scientists are evaluating several suspected cases of glyphosate-resistant johnsongrass, Palmer amaranth (pigweed) and tall waterhemp.

Daniel Stephenson has recently confirmed that Palmer amaranth collected from a cotton field in Concordia Parish is resistant to glyphosate, the first confirmed incidence of glyphosate resistance in the state.

The Palmer amaranth population is highly resistant to glyphosate, and to obtain 50 percent control would require a rate almost four times that of the glyphosate use rate. To obtain 90 percent control of the resistant population would require a glyphosate rate almost 23 times the use rate.

There is great concern as to the efficacy and cost of alternative control programs for Palmer amaranth and the ability to maintain crop yield potential.

LSU AgCenter weed scientists are actively involved in identifying weeds resistant not only to glyphosate but also to other herbicides used in our diverse cropping systems. Once resistance is confirmed, it will be essential that effective alternative control measures are identified and that research-based education programs are developed and delivered to our clientele.

 (This article was published in the summer 2010 issue of Louisiana Agriculture.)