Part 2, Section 1: Pest Management
Pest Management
WEED SPECIES SHIFTS
Production practices that influence shifts
Effectiveness of weed management program. The effectiveness of the overall weed management program will be the single most important factor affecting weed shifts. If the program is not managing all weed species, species that are not effectively controlled will increase. Farmer diligence and ability to make adjustments in a weed management program will directly influence the occurrence of weed shifts.
Speed of weed shift occurrence. The speed at which weed shifts occur will depend on various factors. The ability to prevent the introduction of new weeds to an area, the buffering capacity of the soil seed bank, weed characteristics described previously, and the farmer's ability to quickly adjust a weed management strategy when a shift is first observed will all influence the speed at which a shift occurs -or if a shift occurs at all. With poor management, a field could shift predominant weed species very quickly. For instance, a single application of Roundup early in a field with late-emerging large or smooth crabgrass could result in a severe problem with these annual grasses or other similar late-emerging weeds.
Interface areas. A cropped field will likely include places where no herbicide is applied (skips) or where reduced rates are applied (field edges, places with a lack of overlap). These areas may or may not influence weed shifts. In some cases, these areas may allow genotypes that were not subjected to the selection pressure to survive and reproduce. However, these areas may be so small that they have a negligible effect on overall field effects.