2006 Glyphosate-Resistant Horseweed Map

2006 San Joaquin Valley Glyphosate-Resistant Horseweed Survey

Spatial distribution of glyphosate-resistant and glyphosate susceptible horseweed in the San Joaquin Valley of California in a survey conducted in late summer 2006.  Five individuals were tested at 61 locations centered around Fresno, CA.  Many resistant plants were identified in Fresno, Kings, and Tulare counties where collections were primarily from roadsides, canal banks, or in orchards and vineyards where glyphosate is commonly used.  Conversely, fewer resistant plants were identified in Stanislaus, Merced, and Madera counties, where horseweed is less common and most collections were from rights-of-way, natural areas, or near pastures.