Horticulture Forum: April 2001, Avoiding Herbicide Resistance

Combining herbicides with timely cultural management practices has significantly contributed to the overall aesthetic quality of turfgrasses. Yet over the years, these turfgrasses have developed a resistance to some herbicides.

HERBICIDE RESISTANCE HISTORY. Herbicide resistance has been slower to develop, or to manifest itself, than insecticide and fungicide resistance. Possible reasons include: a) weeds normally complete only one life cycle per year, b) weeds are not as mobile as insects and disease pathogens, c) crop rotations utilize different herbicide families and mechanical cultivation is routinely practiced in most crops or d) certain resistant weeds are less ecologically fit than their susceptible biotypes.

A common misconception is that continued use of the same herbicide causes a mutation to occur, which causes the weed to develop a herbicide resistance. However, herbicides do not cause mutations.

Research has shown that resistant individuals are naturally present at extremely low frequencies in a given population of weed species. Continued use of the same herbicide over a period of years controls the susceptible biotypes, but increases the population of resistant biotypes. The herbicide’s selection pressure is analogous to a plant breeder selecting biotypes that are resistant or tolerant to various types of imposed selection stresses (i.e. drought, mowing height, diseases, insects, etc.). Continued herbicide use for several consecutive years causes a herbicide-resistant population of weeds. However, this statement is true only if resistant individuals are naturally present on the site.

CASE HISTORY. In the mid-1980s, goosegrass resistance to dinitroaniline herbicides was reported in South Carolina.

A single application of oryzalin, prodiamine and pendimethalin at the maximum-labeled rate did not control the goosegrass. Sequential applications of these herbicides at the same rates also did not provide control. Additionally, dithiopyr did not control goosegrass as a single or sequential application, because pendimethalin, prodiamine and oryzalin are members of the dinitroaniline herbicide family and have the same basic modes-of-action. Though dithiopyr belongs to the pyridine herbicide family, it has a mode-of-action similar to dinitroaniline herbicides.

Single and sequential applications of oxadiazon provided approximately 90 percent goosegrass control in 1993 and 1994. Oxadiazon belongs to the oxadiazole herbicide family and has a mode-of-action totally different than the dinitroaniline herbicides and dithiopyr. Therefore, on sites where dinitroaniline or dithiopyr-resistant goosegrass is present, rotation to oxadiazon or other herbicides with different mode-of-action than dinitroanilines and dithiopyr is necessary to effectively control this biotype of goosegrass.

HERBICIDE ROTATION. Annual use of dinitroaniline herbicides for a period of several years contributed to the development of resistant goosegrass. Therefore, herbicide-resistant weeds can occur in turfgrasses, and turfgrass managers should implement an herbicide-resistant weed control strategy in their weed management plan.

Once resistance occurs, the only practical option is rotating an herbicide that has a different mode-of-action than the herbicide previously used.

Rotating to a different herbicide in the same chemical family is not effective, as members of the same family have the same mode-of-action. Also, increasing the rate of the herbicide is not an effective option, as true herbicide resistance is absolute and is not related to tolerance.

CONTROL STEPS. For several years, dinitroaniline herbicides have been widely used by turfgrass managers to effectively control goosegrass, crabgrass and other annual weeds. No one can accurately predict that resistant goosegrass will occur on every turfgrass site. If there are no resistant individuals in a given population of goosegrass, then the problem will not occur.

However, rather than take chances, turf managers should practice a basic principle of pest control, i.e. pesticide rotation. By following this basic principle, turfgrass managers can continue to depend on the effective, low-cost control that dinitroaniline herbicides have provided in the past.

Herbicide-resistant weeds are a true phenomenon. Factors that contribute to their development include: a) continued annual use of herbicides with similar modes-of-action, b) avoidance of herbicides with different modes-of-action, and c) allowing herbicide-resistant weeds to reseed.

Herbicide-resistant weeds are not a major problem in turfgrasses. However, they can become more widespread if turfgrass managers do not employ herbicide-resistant weed management strategies.

Management practices that discourage or prevent, herbicide-resistant weeds are: a) using herbicides with different modes-of-action, b) using tank-mix combinations of herbicides with different modes-of-action, c) controlling weeds that escape preemergence herbicide treatments with postemergence herbicides that have different modes-of-action, or d) preventing seed production by hand roguing.

Goosegrass resistance to the dinitroaniline herbicides and dithiopyr does not mean that it is time to push the "panic button." Nor does it mean that these herbicides are no longer effective. The dinitroasniline herbicide family can provide economical annual grass control in established turfgrasses.

However, herbicide-resistant weeds can become a problem in turfgrasses. Additionally, there is a natural tendency to continue to use pesticides that have been successful in the past. If turfgrass managers want to prevent widespread herbicide-resistance, then they should practice herbicide rotation.