Steven M. Brown, The University of Georgia, P.O. Box 748, Tifton, GA 31794
Although genetically modified cotton was first commercialized in 1995 with bromoxynil-tolerant (BXN) cotton, the more recognized introduction occurred in 1996 with widespread planting of Bt (Bollgard) cotton. In 1997, glyphosate-tolerant (Roundup Ready, RR) cotton was introduced; thereafter, cultivars that included both Bt and RR genes became available. Adoption rate reflects perceived value and performance. In 2005, U.S. cotton producers planted 14 million acres, with 83 percent devoted to transgenic varieties. In 13 of 17 states, transgenic cotton exceeded 90 percent of the total. Transgenic technologies have revolutionized insect and weed management. Bt cotton has provided superb control of tobacco budworm (Heliothis virescens) and good to excellent control of corn earworm (Helicoverpa zea), insects that have long been primary pests in many areas. Bt technology has reduced overall insecticide use, which has elevated challenges with bug pests (plant bugs and stink bugs). RR technology has significantly reduced residual herbicide use, simplified weed management, facilitated expansion of conservation tillage, and reduced labor requirements. Sustained use of RR cotton has contributed to shifts in weed problems and the development of glyphosate-resistant biotypes, notably horseweed (Conyza canadensis) and Palmer amaranth (Amaranthus palmeri). Occasionally, excessive fruit shed and other problems have occurred, presumably associated with the lack of reproductive tolerance conferred by RR genes or the lack of adaptability in some varieties. Conversion to transgenic varieties initially coincided with concerns of a national yield plateau, though recent record yields have dispelled criticism. Likewise, incorporation of transgenes into existing varieties failed to advance fiber quality, creating problems with fiber length (staple) and density (micronaire) and elicited complaints from a modernizing textile industry. Success of RR technology has limited development of new herbicide chemistry for cotton. Additional transgenic traits, including glufosinate tolerance, two-gene Bt (Bollgard II and Widestrike), and Roundup Ready Flex, have recently been introduced.