According to the Weed Science Society of America, herbicide tolerance is the inherent ability of a species to survive and reproduce after herbicide treatment, meaning that there was no selection or genetic manipulation to make the plant tolerant; it is naturally tolerant.

Examples:

• Shattercane cannot be controlled by 2,4-D, because it is naturally tolerant
• Vines such as morningglory and bindweed are not controlled well by glyphosate

Weed Shift
Weed shift is a natural increase in a weed species due to natural tolerance to the primary herbicide used for weed control and/or elimination of competition from other weed species controlled by the primary herbicide.

Examples:

• Increases of annual grass species were noted in corn with use of 2,4-D for broadleaf control
• A University of Missouri study showed that waterhemp populations doubled to more than 65 plants per square meter from 1999 to 2000, and increased nearly 15-fold from 2001 to 2002 after repeated glyphosate applications — one in soybeans, two in corn