Agronomic practices (mechanization, herbicides, new crops and crop varieties) and an
increase in the size and structure of land holdings can change the weed species present
(interspecific population shifts), as well as the genotypes of a species present
(intraspecific population shifts), in a local agroecosystem.
Inter-specific Population Shifts
Many weeds have undergone very large extensions of the ranges (e.g. foxtails) they thrive in while others formerly widespread have all but disappeared.
Before WWII and the use of herbicides the weeds in Iowa corn fields were different than they are today. At that earlier time broadleaf weeds were considered the major weed problem (farmers grouped plants in their fields into "weeds" and "grasses"). With the introduction of 2,4-D, selective weed control selected for the weeds that herbicides didn't kill. 2,4-D killed the broadleaf weeds in most cases, but did nothing to the grasses. All the foxtails and other grasses suddenly became much larger weed problems than before. Giant foxtail was largely unknown to midwestern growers before 1950. Since that time the same population shift story has been repeated over and over again. In the last 10-20 years the appearance of wild proso millet and woolly cupgrass, weeds largely unknown previously, have spread across the countryside.
Weed populations in a crop field differ from those on periphery (Barrett, 1983); conditions in a field often are less complex than in adjacent areas.
Intra-specific Population Shifts
Selection for weedy adaptations within variants of a species population. Just as herbicides selected and shifted species populations, herbicides and other forces have caused shifts within populations of one species to better adapted variants.
Non-herbicide Population Shifts
Crop Mimicry in Barnyardgrass.
Barnyardgrass has a variety (E. crus-galli (L.) Beauv. var. oryzicola (Vasing) Ohwi.) that mimics cultivated rice so well it escapes handweeding. Its growth habit and appearance are more similar to rice than some of rice's closer relatives. The main selective force has been intensive handweeding
practices in the rice crop of Asia.
Early Flowering in Wild Oat.
Population shifts to early flowering forms of wild oat (Avena fatua) and Arabidopsis thaliani were selected for by changes in weed seed cleaning and crop harvesting practices.
Dwarf populations of Aethusa cynapium and Torilis japonicum in cereal crops were selected by the introduction of harvesting equipment. The equipment selected against the taller variants, but the dwarf variants thrived.
Herbicide Induced Population Shifts
Life Cycle Shifts
Populations of summer annuals subjected to herbicide selection pressure has resulted in a population shift to winter annual life cycle or to seed that germinated much later in the season in England.
Shift to Lower Biodiversity.
Herbicide use can also decrease the diversity of genotypes present in a weed species population in California.
Herbicide Resistance Genotye Shifts: Altered Herbicide Target Variants.
Herbicide resistance can be conferred in a plant by many mechanisms. These resistance mechanisms include a change in the target site (usually a protein) that the herbicide binds to, causing death or injury. Below is a partial list of some of these type of population shifts caused by herbicide use that kills the susceptible variants and allows the resistant survivor variants to thrive. Altered herbicide binding (target) site variants:
Herbicide Resistance Genotye Shifts: Enhanced herbicide metabolism variants
Herbicide resistance can also be conferred on variants within a species that detoxify herbicides more rapidly than their susceptible bretheren. Repeated herbicide applications kill off the less able and leave the enhanced detoxifying variants to thrive.