Glyphosate Resistant Weeds in South America: an Historic Perspective

R. A. VIDAL, R. DE-PRADO, J. P. RUIZ-SANTAELLA, M. VILA-AIUB, M. DE-LA-VEGA

San Antonio, Texas (USA)

Congreso; Weed Science Society of America; 2007

During the past five years, glyphosate resistant weeds (GRW) have been documented in South America for five weed species. Classical dose-response curves were used to compare known susceptible against the suspected resistant biotypes in all reported cases. The first GR weed registered in the literature was Lolium multiflorum (italian ryegrass), found in orchards in central Chile in 2001. Later, biotypes from the same weed species were documented as GRW in annual cropping systems in Chile. In southern Brazil, italian ryegrass biotypes resistant to glyphosate were discovered in apple orchards in 2004. In the same year, new GRW biotypes of the same species were located in no-tillage soybean cropping systems, also in the state of Rio Grande do Sul (RGS). In 2005 and 2006, almost simultaneous, but independent, discovery of GRW biotypes from the species Conyza bonariensis and Conyza canadensis (horseweed, marestail) were confirmed resistant in the south and central part of Brazil (states of RGS and São Paulo). In São Paulo state, GR marestail biotypes have been discovered in citrus orchards. In RGS, marestail biotypes have been diagnosed GRW in areas under the no-till system for aproximately 15 years, with winter-crop/soybean monocrop, from which glyphosate-resistant soybeans were cropped during the last 4-5 years. In 2006, an Euphorbia heterophylla (wild poinsettia) biotype was reported GRW in the state of RGS in a similar cropping systems as described for marestail. In the northwest part of Argentina, Sorghum halepense (Johnsongrass) biotypes were diagnosed as GRW in 2005, and were found in areas cultivated with glyphosate-resistant soybeans after about 8 years. The mechanisms of resistance for most of these biotypes are under scientific investigation, but are unlikely to be target site-type resistance because of low level of resistance. The resistant mechanisms may involve processes which are coded by several quantitative trait loci. This may explain the selection of GRW biotypes faster than earlier expected through simulation methods.