A new PNAS Nexus study led by scientists from the USDA Agricultural Research Service (USDA-ARS) and the University of Illinois Urbana-Champaign takes a retrospective look at glyphosate efficacy after crops to withstand the herbicide were engineered. Amassing data from annual herbicide evaluation trials at land-grant universities across the U.S. and Canada, the research shows a significant and rapid decline in glyphosate control for all seven major weed species examined. Although glyphosate provided superior weed control in the early years, most of the weeds in the dataset showed signs of adaptation to the chemical in just two to three years. Within a decade, weeds were up to 31.6% less responsive to glyphosate, with further linear declines as time went on. Researchers noted that for plots in which a pre-emergence herbicide had been applied before glyphosate, loss of control was much smaller, with a maximum 4.4% decline.