The last 15 years can rightly be referred to as the "glyphosate era" of weed control. Glyphosate rapidly replaced other herbicides in soybean and by 2002 was used on 79% of soybean acres in the U.S. (Young 2006). A 2003 survey of Indiana soybean growers found that glyphosate was the only herbicide applied on 74% of glyphosate-resistant soybean acres (Johnson et al. 2007) and 65% of total soybean acres.
Adoption was slower in corn, but by 2010, glyphosate had become the most widely used herbicide in corn as well, with 66% of U.S. corn acres treated (USDA NASS 2011).
Many areas are now transitioning into a post-glyphosate era with glyphosate-resistant weeds now requiring additional or alternative management tools for satisfactory control. To date, glyphosate resistance has been confirmed in 24 weed species worldwide, including 14 in North America (Heap 2012). Glyphosate-resistant weed populations have been confirmed in 29 states and two Canadian provinces (Figure 2).
Problems with herbicide-resistant weed populations date back to well before the introduction of glyphosate-resistant crops in 1996. In fact, one of the reasons for the rapid adoption of glyphosate-resistant crops was as a solution to resistant weeds. At the time, glyphosate had been in use for over 20 years with no known cases of resistance evolution, which led some to doubt that resistance would ever develop. This doubt was reinforced by the relative scarcity of plant species expressing natural tolerance to glyphosate and the unlikelihood that the complex processes involved in creating glyphosate-resistant crops would be duplicated under field conditions. This outlook was short-lived, however, as cases of evolved resistance to glyphosate began to appear.
Despite the rapid increase in resistance cases over the last several years, glyphosate has a relatively low incidence of resistance evolution compared to many other herbicides. For example, the number of weed species with glyphosate resistance (24) is still relatively small compared to ALS inhibitor resistance (127) and triazine resistance (69). The total number of herbicide resistance cases worldwide has likely been reduced by the adoption of glyphosate-resistant crops because the herbicides replaced by glyphosate had a higher resistance risk in most cases. However, the unprecedented scale of glyphosate use increased its resistance issues. In addition, the high level of dependence upon glyphosate as a primary weed management tool across multiple crops makes the development and spread of resistant populations of particular concern.
Resistant weeds do not eliminate the usefulness of glyphosate as a herbicide. Glyphosate will continue to be an important and useful weed control tool for years to come, much like atrazine is still widely used in combination with other herbicides despite numerous cases of resistant populations (USDA NASS 2011). There are a small number of weed species, however, for which resistance to multiple herbicides now leaves growers with few viable options for control (Table 1).
|Multiple Herbicide Resistance - Definition
Resistance to several herbicides resulting from two or more distinct resistance mechanisms occurring in the same plant.
Table 1. Weed populations with multiple resistance to glyphosate and one or more other herbicide modes of action in the U.S. and Canada.
|Species||State||Modes of Action|
|Common Waterhemp||MO, IL¹||Glyphosate, ALS inhibitors, PPO inhibitors|
|IL||Glyphosate, ALS inhibitors|
|IL²||Glyphosate, ALS inhibitors, PPO inhibitors
inhibitors, photosystem II inhibitors
|IA||Glyphosate, ALS inhibitors, 4-HPPD inhibitors|
|Palmer Amaranth||GA, MS, TN||Glyphosate, ALS inhibitors|
|Giant Ragweed||OH, MN||Glyphosate, ALS inhibitors|
|Common Ragweed||OH||Glyphosate, ALS inhibitors|
|Horseweed||OH, MN||Glyphosate, ALS inhibitors|
|Kochia||AB||Glyphosate, ALS inhibitors|
¹ Hager 2011, ² Bell et al. 2009, all others Heap 2012.
The most troublesome multiple-resistant weeds for North American crop production are two pigweed species, common waterhemp and Palmer amaranth. Like corn and sorghum, pigweeds are C4 plants, making them very efficient at fixing carbon and well-adapted to high temperatures and intense sunlight. Pigweeds are also capable of producing greater than 500,000 seeds per plant and tend to germinate throughout the summer, making them difficult to manage in crops.
In contrast to other pigweed species, waterhemp and Palmer amaranth are dioecious (separate male and female plants). The resulting cross-pollination between plants can increase the genetic diversity of a population, which may favor development of herbicide resistance. Both species are very competitive with crops, particularly Palmer amaranth (Steckel 2007).
Common waterhemp resistant to glyphosate, ALS inhibitors, and PPO inhibitors is becoming increasingly common in Illinois (Hager 2011) and Missouri. A population resistant to these three modes of action plus photosystem II inhibitors (atrazine) has been documented in Illinois. In Iowa, a new type of resistance was added to common waterhemp's already impressive list when a population resistant to glyphosate, ALS inhibitors, and 4-HPPD inhibitors was discovered in 2011. Palmer amaranth resistant to glyphosate and ALS inhibitors has only been documented in three southern states so far, but will likely spread within the South as well as north into the Corn Belt (Hager 2005).
|LL - Contains the LibertyLink® gene for resistance to Liberty® herbicide.
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