Donnie K. Miller, Carol Pinnell-Alison, Bill J. Williams, Steve Kelly and Donna R. Lee

Discovery and introduction of aryloxyphenoxy- propionate and cyclohexanedione selective postemergence grass herbicides (graminicides) in the late 1970s and early 1980s gave producers a highly effective means for over-the-top control of most annual grasses and perennial grasses, such as johnsongrass, in cotton and soybean fields. Effective johnsongrass control programs in these crops include initial over-the-top application of a graminicide with follow-up applications to control escaped plants, regrowth or populations that emerge after initial treatment. Continued reliance on these herbicides to control johnsongrass, as well as multiple applications in a single growing season, has brought about increased selection pressure for resistant populations.

Reduced johnsongrass control following treatment with these herbicides under ideal growing conditions was initially reported in the early 1990s in neighboring states. Greenhouse studies confirmed resistance to labeled rates of these herbicides when compared to plants collected from fields with no history of graminicide use. An increasing number of control failures, often following multiple graminicide applications and not attributable to poor environmental conditions or herbicide misapplication, has been reported in Northeast Louisiana. Documentation of johnsongrass resistance to graminicides is needed to alert producers to resistant populations and to provide options for control.


Greenhouse Study

A greenhouse study was conducted in 1999 to determine the extent of graminicide resistance in a suspected resistant johnsongrass population and to evaluate alternative control options. The suspected resistant population was located in a field in Franklin Parish with a history of repeated graminicide use. Johnsongrass rhizomes were collected and planted in the greenhouse at the LSU AgCenter’s Northeast Research Station. Rhizomes also were collected from a susceptible population at the station, in which no graminicide had been used for 15 years. Plants were allowed to reach approximately 15 inches in height before herbicide application. Both the resistant plants and the susceptible plants were treated with five different graminicides—Assure II, Fusilade DX, Fusion, Select and Poast Plus. Resistant plants were treated with one (1x), two (2x) or four (4x) times the labeled rates. The susceptible plants were treated with only the labeled rate (1x) of each graminicide. The labeled rates used were 10 ounces per acre (Assure II), 12 ounces per acre (Fusilade DX and Fusion), 8 ounces per acre (Select) and 36 ounces per acre (Poast Plus).

Alternative herbicides—Liberty, Roundup Ultra, Touchdown and Accent—were applied to resistant plants to evaluate possible control alternatives, should resistance be determined. The rates were 28 ounces per acre (Liberty), 32 ounces per acre (Roundup Ultra), 26 ounces per acre (Touchdown) and 0.67 ounce per acre (Accent). Nontreated resistant and susceptible controls were included. A visual assessment of control, based on plant chlorosis (yellowing) and growth reduction, was made 14 and 28 days after treatment. Plant dry weight was determined following the 28-day visual assessment by harvesting all above-ground biomass per plant and drying for seven days.

Assessment

At the 14-day assessment, 4x rates of both Assure II and Fusion provided 20 percent control of resistant plants. Fusilade DX provided 18 percent control, and Poast Plus provided 64 percent control. This is compared to at least 94 percent control of susceptible plants. In contrast, Select at 1x, 2x and 4x rates provided 27 percent, 84 percent and 81 percent control, respectively, of resistant plants compared to 96 percent control of susceptible plants. Liberty, Roundup Ultra, Touchdown and Accent controlled resistant plants 95 percent, 83 percent, 95 percent and 94 percent, respectively.

At the 28-day assessment, 4x rates of Assure II, Fusilade DX and Poast Plus controlled resistant plants no more than 28 percent compared with at least 98 percent for susceptible plants. Poast Plus at the 1x and 2x rates provided no more than 25 percent control of resistant plants while 4x rate resulted in 85 percent control. Select at the labeled rate resulted in 18 percent control of resistant plants, whereas a 2x rate controlled 95 percent of the johnsongrass. Select and Poast Plus at labeled rates resulted in complete control of susceptible plants. Alternative herbicides evaluated controlled resistant plants at least 94 percent.

With the exception of the 2x and 4x rates of Select and the 4x rate of Poast Plus, resistant plants treated with all graminicide rates produced significantly more above-ground dry weight than treated susceptible plants. Nontreated resistant above-ground dry weight was 45 percent higher than that of nontreated susceptible plants, indicating increased vigor in suspected resistant johnsongrass plants.

A high level of graminicide resistance was observed in this study with a 2x rate of Select and a 4x rate of Poast Plus needed to achieve acceptable control of resistant plants. These herbicides are members of the cyclohexanedione family. In contrast, Assure II, Fusilade DX and Fusion, members of the aryloxyphenoxypropionate family, provided less than 30 percent control of resistant plants.

Although members of different herbicide families, the graminicides evaluated in this study act on the same enzyme system in grass species. Any alteration to this enzyme, which is the basis of most reported cases of graminicide resistance, can render a grass species cross-resistant to all graminicides. The fact that herbicides representing both two families did not control the johnsongrass at labeled rates in our study shows that johnsongrass was indeed cross-resistant. Therefore, producers are cautioned that switching to another graminicide following a control failure, when environmental conditions and misapplication can be eliminated as factors, can prove costly and ineffective. Based on this research, Liberty, Roundup Ultra, Touchdown and Accent are viable control options when a graminicide-resistant johnsongrass population is suspected.