1.   Evolution of Herbicide Resistant Weeds

Weeds will eventually adapt and circumvent most control mechanisms. Herbicideresistant weeds are a well-established aspect of weed control for many herbicide classes including ACCase and ALS inhibitors, dinitroanilines, triazines and other PSII inhibitors. The occurrence of herbicide resistant weeds is dependent on the herbicide program, the weed species present, and the other crop management practices a farmer employs.

Herbicide resistance usually evolves in only one or two weed species in an area, even though a much larger number of weeds are exposed to the same herbicide selection intensity. Nonetheless, weed resistance to herbicides currently affects hundreds of thousands of fields and the most widely used herbicides (Heap, 2005). According to a recent survey, more than 290 types of herbicide resistant weeds are present in agricultural fields around the world (www.weedscience.org/in.asp). Resistant weeds often increase the cost of crop production and limit the effectiveness of herbicides that can be used and the crops that can be grown. Despite these challenges, farmers have used a variety of approaches to limit the impact of weeds on crop productivity. 

Developing new varieties of herbicide resistant crops may improve a farmer’s weed control options, but having access to new varieties will never completely eliminate the problem of weeds evolving resistance to herbicides (Heap and LeBaron, 2001; Owen, 2001).However, by using a wide variety of weed control options (including proper use of herbicides), a farmer can delay the development of weeds resistant to herbicides.

The capacity of weeds to become resistant to an herbicide depends, first and foremost, on the existence of individual weeds (biotypes) that happen to have resistance genes that enable them to both survive and reproduce when exposed to the herbicide. The herbicide itself does not directly cause the genetic change that imparts resistance; the resistance trait appears randomly in different populations of different weed species. It is difficult to predict exactly which weed species will contain biotypes that are resistant to specific herbicides.

A weed species’ inherent rate of mutation is a key factor for predicting new occurrences of resistant weeds, but this rate is difficult to quantify (Shane-Friesen and Hall, 2004; Gressel, 2006). Actual mutation rates have not been measured for any weed under field conditions. Saari et al. (1994) estimated for Arabidopsis that one mutation in a billion may lead to a gene variant that confers herbicide resistance. Other models that researchers use to study resistance evolution are based on a much higher mutation rate of one in a million (Cavan and Moss, 2001). Even a low estimate of mutation frequency can give a high probability of a resistant biotype occurring, because weeds produce vast numbers of seeds each year. Weed seed banks can be as high as 50,000 seeds/m², but even a modest 1,000 seeds/m² represents 10,000,000 seeds/ha.

To date, the resistance gene in all cases in which a weed has become resistant is the result of the weed’s inherent ability of generating gene variability. No weed has become resistant by acquiring resistance genes from herbicide resistant crops. Scientists are able to determine the source of the resistance gene by studying the molecular basis of the resistance. Weeds have evolved biochemically unique resistance strategies not found in crops. Even so, it is still possible for weeds to acquire resistance genes from crops that are close relatives. This is especially true if the resistant phenotype is provided by a single gene, as it is often the case with crops developed through biotechnology. For a discussion of the potential of weed herbicide resistance acquired through gene flow, see Science Narrative ‘Gene Flow via Pollination‘ on this website.

The spread of a resistance phenotype, which leads to a weed population that is not susceptible to the herbicide, depends primarily upon the exposure to the herbicide that the weed is able to tolerate. When an herbicide is applied, most of the susceptible weeds die, while the resistant weeds survive, mature, and produce seed. Even though they may still be few in number, repeated application of the same herbicide continues to increase the proportion of resistant weeds in the population. Thus, when growers say that their "weeds have become resistant," they really mean that the population of resistant weed biotypes, which formerly existed at low numbers, has increased.

In the absence of exposure to the respective herbicide, resistant weed biotypes may be maintained in the populations at low frequencies, if the new resistance phenotype does not significantly reduce the weed’s fitness. If, however, the new resistance gene carries a significant fitness cost, the resistant gene’s frequency will decrease over time in the absence of any selection for the gene, i.e., application of the herbicide. For example, many weed biotypes that evolved resistance to atrazine have lower vigor and fitness and cannot compete with biotypes that are susceptible to atrazine (Stowe and Holt, 1988), because the resistance trait is associated with a less efficient photosynthetic system.

The existence of herbicide resistant weeds is not necessarily an economic or ecological problem. It is an economic problem only if:

• the herbicide the weed tolerates is an economically desirable option, and
• few herbicide options can be used in the crop(s) where the resistant weed occurs.

Herbicide resistance can become an ecological problem if the resistant weed biotype replaces the non-resistant biotype in the weed population. Even then, the shift to an herbicide resistant population of weeds has ecological consequence only if the resistant population cannot be controlled with other herbicides or other control practices. This is rarely the case. Many hundreds of cases of resistant weeds have been documented worldwide, but only under exceptional circumstances has resistance become a limiting factor for crop production. For example in some locations in Australia, biotypes of rigid ryegrass (Lolium rigidum) are resistant to many herbicides in several different classes (Heap, 2005). 

In spite of the evolution of herbicide resistance in weed populations, farmers in the U.S. who grow corn and soybeans continue to have many herbicides and agricultural management options for weed control. Even so, growers must always be concerned about herbicide sustainability and the economic consequences of losing an herbicide due to the evolution of resistant weeds. This is especially the case if the most difficult to control weeds become resistant to more than one herbicide, which is known as cross-resistance.  With very high re-registration costs for older herbicides and high development costs for new herbicides, farmers cannot assume they will continue to have access to new herbicides that replace those that have lost their value.

2.   Weed Spectrum Shifts

The composition and density of weeds in cropland will change in response to weed control practices, whether or not herbicides are used. A few individual weed plants that are partially resistant or difficult to control with certain practices will survive and reproduce, eventually leading to weed populations that are more difficult to control and a need for the grower to change weed control measures and/or herbicides. The ability to use a greater array of herbicide options with herbicide resistant crops should help to minimize shifts in weed spectrum that occur and will provide more opportunities for effective control if shifts occur.

Not all cases of weed shifts that are driven by herbicide use can be explained by herbicide selective pressure that favors weeds with a genetically based biochemical capacity to survive exposure. Weeds with delayed emergence and slower development are able to avoid exposure to the herbicide (Hilgenfield et al., 2004). Therefore, application of herbicides can favor weed species that emerge late or can select for biotypes within a species that are capable of delaying emergence. In a study with a range of weed species, ivyleaf morning glory (Ipomoea hederacea), and shattercane (Sorghum bicolor) were able to avoid exposure to glyphosate applications due to delayed emergence