Documented Weed Shifts
Part 2, Section 1: Pest Management
WEED SPECIES SHIFTS
Documented Weed Shifts
Weed shifts have occurred throughout history as people have modified cropping systems and the environment for the production of desired crops. These shifts can be immediate or short-term fluctuations in response to transient changes in production systems, or they may represent long-term changes. Shifts occur within weed populations, weed communities, or concurrently within both, in response to single or multiple changes in agricultural systems. Weed shifts have occurred in response to changes in tillage, irrigation, fertility, crop rotation, and herbicide use practices. Examples of weed shifts that have occurred in recent history include the following:
Community shift in response to herbicide use. In the Corn Belt and winter wheat areas of the western United States, changes in weed communities were noted within 10 years of the introduction of 2,4-D for the control of broadleaf weeds. In corn, summer annual grass species increased as broadleaf species were controlled. In wheat, winter annual grass species replaced broadleaf species as the predominant troublesome species.
Community shift in response to tillage change. Changes from conventional to reduced tillage systems often cause weed community shifts that include increases in summer annual grasses and small-seeded summer annual broadleaves, winter annual, biennial, and perennial species, and decreases in large-seeded summer annual species.
Community shift as a result of new localized or long-distance introductions. Common lambsquarters, a weed believed to be native to Europe and Asia, is now found throughout much of the United States. In much of Pennsylvania, common lambsquarters has become predominant in the weed community. The shift occurred because the species grows aggressively, is difficult to control, and is a prolific seed producer. Long-distance dispersal has also resulted in the introduction of many noxious weeds to the United States, some of which have caused weed community shifts (including field bindweed in the western plains, leafy spurge in rangeland, and multiflora rose in pasture).
Population biotype shifts in response to herbicide use (herbicide-resistant populations). In the mid-west, in many populations of common waterhemp (pigweed species), biotypes differed in susceptibility to ALS-inhibiting herbicides. With recurrent spraying of ALS-inhibiting herbicides, populations shifted from susceptible to highly resistant biotypes. (Note: A concurrent shift in many weed communities also occurred. Other species were controlled with these highly effective herbicides, and the waterhemp numbers increased as a proportion of the entire weed community). Other ALS-resistant pigweed species have also developed in several areas of the United States, including the northeast. Most recently, glyphosate resistant weeds such as horseweed (marestail) and pigweed species are a problem in different regions of the U.S. as a direct result of glyphosate use in herbicide resistant crops.