Technical Abstract: Conservation tillage reduces the physical movement of soil to the minimum required for crop establishment and production. Adoption of conservation tillage increased dramatically with the advent of transgenic, glyphosate-resistant crops that permitted in-season, over-the-top use of glyphosate, a broad-spectrum herbicide with very low mammalian toxicity and minimal potential for off-site movement in soil or water. Glyphosate-resistant crops are currently grown on approximately 70 million hectares (173 million acres) worldwide. The United States (US) has the most hectares of transgenic glyphosate-resistant cultivars, 45 million (99 million ac), and the greatest number of hectares in conservation tillage 46 million ha (114 million ac). The practice of conservation tillage is now threatened by the emergence and rapid spread of glyphosate-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.]. First identified in Georgia, it now has been reported in Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, and Tennessee. Another closely related dioecious amaranth, or pigweed, common waterhemp (Amaranthus rudis Sauer), has also developed resistance to glyphosate in Illinois, Iowa, Minnesota, and Missouri. Hundreds of thousands of conservation tillage hectares, some currently under United States Department of Agriculture, Natural Resources Conservation Service (USDA-NRCS) conservation program contracts, are at risk of being converted to higher-intensity tillage systems due to the inability to control these glyphosate-resistant species in conservation tillage systems. With the rapid spread of glyphosate-resistant amaranths, the hectares in conservation tillage potentially could decline without development of new, effective weed control strategies.