Crop and herbicide rotation effects on weed population dynamics and the characterization of imidazolinone-resistant smooth pigweed (Amaranthus hybridus)

Shifts in weed populations to herbicide-resistant biotypes are occurring more frequently. In two adjacent field studies from 1991 through 1994, crop rotations and herbicide programs affected control and densities of common lambsquarters, common ragweed, smooth pigweed, redroot pigweed, jimsonweed, goosegrass, stinkgrass, large crabgrass, smooth crabgrass, fall panicum, and yellow nutsedge. Generally, the continuous use of the same herbicide or herbicides with similar selectivities resulted in proliferation of tolerant weed species. Corn, tomato, and soybean yields were affected mostly by crop rotations, rainfall, and weed control. Herbicide rotations or combinations must include herbicides that are efficacious on the target weed species to preclude weed shifts.

Approximately 5 million smooth pigweed plants in Painter, VA were treated with imazethapyr or nicosulfuron from 1992 to 1994, and no ALS-inhibitor-resistant plants were identified. Smooth pigweed in Marion, MD and Oak Hall, VA, and livid amaranth in Warren County, NJ were reportedly escaping control from imazaquin or imazethapyr. In greenhouse studies, control of smooth pigweed from Marion and Oak Hall was 3 to 18% by 560 or 1120 g ai/ha imazaquin. Control of smooth pigweed from Painter was 81% by 70 g ai/ha imazethapyr. Control of livid amaranth from New Jersey was 8 to 15% by 560 g/ha imazethapyr.

Field, greenhouse, and laboratory studies were conducted on Marion [resistant (R) biotype] and Painter [susceptible (S) biotype] smooth pigweed to characterize herbicide resistance in the R biotype. The R biotype was resistant at high levels to imazaquin and imazethapyr, and was cross-resistant at low levels to rmsulfuron and chlorimuron in the greenhouse. Both biotypes were equally susceptible to ASC-67040, CGA-152005, flumiclorac, halosulfuron, lactofen, metribuzin, nicosulfuron, pendimethalin, primisulfuron, pyrithiobac, and thifensulfuron in field or greenhouse studies. ALS enzyme assays confirmed target site-based resistance to imazaquin, imazethapyr, and rimsulfuron but not to chlorimuron in the R biotype. Metabolism of ¹⁴C-chlorimuron was more rapid in the R than in the S biotype which may explain the low level of whole plant resistance to chlorimuron in the R biotype. The occurrence of herbicide resistance and patterns of cross-resistance in weeds cannot be predicted.