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SUMMARY

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eturn to table of contents GLYPHOSATE RESISTANCE IN WEEDS by Mark Jeschke, Ph.D., Agronomy Information Manager HISTORY AND CURRENT STATUS • Glyphosate was introduced in the U.S. in 1976. • The first case of evolved resistance to glyphosate was confirmed in rigid ryegrass in Australia in 1996. • The first case of glyphosate resistance in the U.S. occurred in 2000 in horseweed (marestail) in Delaware. • To date, glyphosate resistance has been confirmed in 35 weed species worldwide, including 16 in North America. • Glyphosate resistant weed populations have been confirmed in 38 states and 4 provinces (Figure 1). HOW DO WEEDS BECOME HERBICIDE RESISTANT? • Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. • Herbicides do not induce resistance in weed species, rather they simply select for resistant individuals that naturally occur within the weed population. • Once a resistant plant has been selected, repeated use of a herbicide over multiple generations allows resistant plants to proliferate as susceptible plants are eliminated. Oregon Italian ryegrass Kochia California Annual bluegrass Hairy fleabane Horseweed Italian ryegrass Junglerice Palmer amaranth Rigid ryegrass Alberta Saskatchewan Manitoba Idaho Wyoming Colorado Kochia Montana Horseweed Kochia Russian thistle North Dakota Common ragweed Common waterhemp Kochia South Dakota Common ragweed Common waterhemp Horseweed Kochia Minnesota Common ragweed Common waterhemp Giant ragweed Iowa Common waterhemp Giant ragweed Horseweed Illinois Common waterhemp Horseweed Palmer amaranth Nebraska Common ragweed Common waterhemp Giant ragweed Horseweed Kochia Palmer amaranth Wisconsin Common waterhemp Giant ragweed Horseweed Palmer amaranth Indiana Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth West Virginia Horseweed Michigan Horseweed Palmer amaranth Ohio Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth Ontario Common ragweed Common waterhemp Giant ragweed Horseweed Pennsylvania Maryland Delaware Virginia Horseweed Palmer amaranth New Jersey Common ragweed Horseweed Palmer amaranth Arizona New Mexico Palmer amaranth Kansas Common ragweed Common waterhemp Giant ragweed Horseweed Kochia Palmer amaranth Texas Common waterhemp Palmer amaranth © 2016 PHII Oklahoma Common waterhemp Horseweed Kochia Missouri Annual bluegrass Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth Arkansas Common ragweed Common waterhemp Giant ragweed Horseweed Italian ryegrass Johnsongrass Palmer amaranth Mississippi Common ragweed Common waterhemp Giant ragweed Goosegrass Horseweed Italian ryegrass Johnsongrass Palmer amaranth Spiny amaranth Tennessee Annual bluegrass Common waterhemp Giant ragweed Goosegrass Horseweed Italian ryegrass Palmer amaranth Alabama Common ragweed Horseweed Palmer amaranth Florida Palmer amaranth Ragweed parthenium Kentucky Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth North Carolina Common ragweed Horseweed Italian ryegrass Palmer amaranth Georgia South Carolina Palmer amaranth Louisiana Common waterhemp Italian ryegrass Johnsongrass Palmer amaranth Glyphosate-Resistant Species 1 2 3 4 5 6 7 8 9 Figure 1. Confirmed cases of glyphosate resistance in North America as of fall 2016. 144

eturn to table of contents HERBICIDE FAILURE CAN BE RAPID • Weed resistance to continuous use of the same herbicide occurs on a logarithmic rate of seed increase. • The percentage of weeds in the population that are resistant to the herbicide gradually increases at an imperceptible rate and then makes a logarithmic jump to become more than half the weed population. • This is why fields typically go from adequate control (>90% control) to failure (

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