ABSTRACT

Herbicide resistance is increasingly significant problem. Seedling johnson- grass, goosegrass, and Palmer amaranth resistance to dinitroaniline (DNA) herbicides, cocklebur resistance to DSMA and MSMA, and johnsongrass resistance to fluazifop and quizalofop have been observed in one or more states. Although resistance to cotton herbicides is not yet a widespread problem, failure to recognize the potential seriousness of the problem and to react accordingly could lead to a disastrous situation. Herbicide resistance is an evolutionary response to selection pressure by herbicides. Resistance will evolve only if genes conferring resistance to a particular herbicide are present within the population and only if sufficient selection pressure is exerted on the biotypes possessing those genes. Resistance to cotton herbicides has evolved only in areas with a monoculture of cotton where herbicides with the same mechanism of action were used continuously.

Growers have two options in managing herbicide- resistant weeds. They can either avoid evolution of resistance by implementing an integrated management strategy aimed at reducing selection pressure by individual herbicides or they can deal with the problem once it develops. Avoiding resistance evolution is clearly the better option for the industry as a whole, and it is probably the better option for individual growers.

A management strategy to avoid evolution of resistance should consist of several tactics. Use of short- residual herbicides and use of economic thresholds as aids in determining the need for postemergence applications are often cited as potential components of a management strategy to avoid resistance. The concept with each of these strategies is to obtain the minimum control needed to avoid economic losses while allowing some susceptible weeds to produce seed. Seed production by susceptible biotypes will allow gene flow and fitness processes to interact to maintain susceptibility in the population. While both tactics can help delay or avoid evolution of resistance, they are of more value in crops such as corn or soybeans than in cotton. Because of its poor competitiveness with weeds during the first third of the growing season and the potentially adverse effects of low weed populations on lint quality, cotton needs to be relatively weed free for most of the season.

Tactics in a resistance avoidance strategy for cotton should include crop rotation and the accompanying a relatively new but rotation of herbicides having different mechanisms of action, use of tank mixtures or sequential applications of herbicides with different mechanisms of action, use of alternative application methods and herbicides such as glyphosate and wipers or rope wicks where applicable, cultivation, and agronomic practices which encourage good stand establishment and early-season cotton growth which in turn allows for more effective cultivation and postemergence-directed herbicide applications. Of these tactics, crop rotation and the accompanying rotation of herbicides with different mechanisms of action offer the greatest potential to avoid problems with resistance evolution. It is significant to note that resistance has never evolved where an integrated system including crop and herbicide rotation has been followed.

In situations where resistance already is a problem, a grower typically will switch to another herbicide which controls the resistant biotype, assuming such an option exists, and continues producing monoculture cotton. While this practice can be of short-term benefit, it is not a long-term solution because the grower has not addressed the root cause of the problem.

Where resistance is a problem, the grower needs to take steps to avoid spreading seed of the resistant biotype to other fields or farms. Additionally, he needs to implement an integrated management strategy consisting of rotation of herbicide mechanisms of action, use of tank mixes or sequential applications of herbicides with different mechanisms of action, and cultivation. In most situations, crop rotation will be necessary. He also needs to obtain excellent control of the resistant biotype for several years in order to reduce its population in the seed bank. New herbicides expected to be registered for use on cotton in the near future include clomazone (Command) , MON 13200, and DPX-PE350 (Staple). clomazone and DPX-PE350 represent new mechanisms of action in the cotton herbicide market. Clomazone will help in managing DNA-resistant goosegrass and seedling johnsongrass while DPX-PE350 will help in managing DSMA/MSMA-resistant cocklebur and DNA-resistant Palmer amaranth. MON 13200, an effective preemergence herbicide for annual grasses, will not help in managing any of the currently resistant weed biotypes. MON 13200 is not effective on DNA-resistant goosegrass or seedling johnsongrass because its mechanism of action is similar to that of the DNA herbicides. Additionally, it does not provide adequate control of Amaranthus species or cocklebur.