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FAQ's - Weed Control
FAQ's - Herbicides
FAQ's - Herbicide Resistant Crops
FAQ's - Evolution of Herbicide Resistant Weeds
Weed Control
Why do growers use herbicides?
Growers must control weeds. Weeds compete with crops for moisture, nutrient, growing space and sunlight. In the absence of weed control, the average crop losses for U.S. corn, soybean and cotton growers would be approximately 65%, 74% and 94%, respectively.
In addition to causing significant crop losses, weeds make crop harvesting more difficult. If the harvested crop is contaminated with weed seeds, growers who save harvested seed grains to plant the next season will sow weed seeds along with crop seeds. In addition, the quality of the food and feed derived from the crop is lower if weed seeds contaminate harvested grains.
What is the annual cost of weeds to U.S. agriculture?
The cost of weed control in U.S. agriculture is greater than the combined cost of controlling insects, plant pathogens and nematodes. On average, the total cost of weeds to U.S. agriculture exceeds $12 billion/year: $ 8 billion in control costs plus $4 billion in lost yields. For example, U.S. cotton growers spend approximately $200 million/year on weed control, but still lose 15% of the cotton crop, which has an average value of $600 million/year.
Of the $8 billion spent annually on weed control in the U.S., approximately $ 2 billion is for herbicides. Three crops account for 75% of the $4 billion in annual crop losses and the $2 billion spent each year on herbicides - corn, cotton and soybeans. Globally, $10 billion is spent on herbicides.
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In addition to herbicides, what weed control methods do growers use?
Most farmers in developing countries have relied primarily on hand weeding and hoeing. Labor costs and average farm sizes make manual weed control impossible for most U.S. growers. For example, in the early 1990s the cost of manual weed control for lettuce growers in Florida was $200/acre, compared to an average cost of $15-20/acre for herbicides at that time. Manual weed control is being used less often in developing countries, because laborers are leaving rural areas for better paying jobs in urban areas.
Other weed control techniques U.S. growers use on a regular basis are cultivation, crop rotation, cover crops, mowing, mulching, planting geometry and planting time to give crops the competitive edge over weeds. In a few instances, biocontrol agents, such as plant pathogens and insect herbivores are used to manage weeds.
What is Integrated Weed Management?
Integrated Weed Management (IWM) is modeled after the more familiar Integrated Pest Management (IPM) used to control insects and plant pathogens. Both IWM and IPM are based on ecological and evolutionary principles. The ultimate objective is to achieve cost effective control while reducing inputs of synthetic pesticides.
A fundamental concept in IWM is implementation of control measures based on economic thresholds. Crop plants can withstand some competitive pressure from weeds without a significant decrease in crop yield. Weed scientists have established the maximum number of various weed species that can be tolerated before significantly decreasing yields – the economic threshold. Growers are encouraged to wait until the weed reaches the economic threshold before implementing control measures. In addition, IWM relies on using a variety of control measures to slow the evolution of resistance to a single control measure. Therefore, IWM is maximized when growers have access to the widest possible array of weed control tools.
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Herbicides
What are herbicides?
Herbicides are synthetic chemicals that control weeds. Some naturally-occurring chemicals, such as copper sulfate, also have the potential to control weeds, but the term herbicide is reserved for chemically synthesized compounds.
How does one herbicide differ from another?
Herbicides differ from each other in many ways. The spectrum of weeds controlled by an herbicide can be quite broad or narrow. Broad spectrum, or non-selective, herbicides control a wide variety of weeds. Selective herbicides control only certain weeds. Typically herbicides that control grasses (monocots) do not control broadleaf weeds (dicots), and vice versa. The concept of selectivity applies to crop plants as well. In order to use an herbicide on a crop, the crop must not be harmed. Therefore, crops are selectively resistant to some herbicides but not others.
Herbicides have different modes of action, although members of the same herbicide class share a mode of action. At the physiological level, the various herbicides control plants by inhibiting photosynthesis, mimicking plant growth regulators, blocking amino acid synthesis, accumulating nitrogenous wastes or inhibiting cell elongation and cell division. In many cases, the physiological effects can be traced to the herbicide binding to an essential plant protein, such as an enzyme or cell receptor, which is known as the target or active site of the herbicide.
Herbicides also show extreme variation in environmental persistence and potential impact. Some are degraded within days; others persist for more than a year. Those characteristics, as well as the amount that must be applied to the plant or incorporated into the soil, determine the differences in herbicide effectiveness and their environmental footprints.
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How do differences in herbicides affect a grower’s options for weed control?
Decisions about weed management may well be the most complex decision that growers must make, because each weed control option has trade-offs and affects the feasibility of using other options.
The first constraint a grower faces is herbicide spectrum. A grower almost always has a number of weed species that must be controlled. Therefore the broader the spectrum, the better for cost effective weed control, because the grower can use fewer herbicides. However, herbicides with the broadest spectra also may injure crop plants.
Spectrum differences also affect a grower’s freedom to use the “wait and see” approach advocated by Integrated Weed Management. Because of their past experiences, most growers assume they will have a weed problem caused by both grass and broadleaf weeds. Once the crop emerges, they are limited to using a selective herbicide that does not harm the crop (post-emergence). Unfortunately, the selectivity also means it will only control certain weeds and not others. Therefore, before the crop is planted or emerges, growers often use non-selective herbicides (pre-plant and pre-emergence herbicides) as insurance against the weeds that cannot be controlled with herbicides once the crop emerges.
Persistence, in conjunction with herbicide spectrum, plays a role in determining the grower’s crop rotation options. For example, a herbicide used in a soybean field may negatively affect corn yield the next growing season, provided the herbicide persists in the soil and corn is susceptible to this particular herbicide.
Finally, an herbicide’s mode of action can also limit a grower’s options if weeds have evolved resistance to herbicides with a similar mode of action.
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What are the benefits of using herbicides?
In addition to providing a cost effective means of weed control, the use of herbicides increase yield, makes the crop easier to harvest, allows the grower the spend less time in the field and promotes less tillage less in weed management. A decrease in tillage significantly decreases soil erosion, which is an environmental concern, associated with agriculture.
What are the limitations of using herbicides?
In addition to the limitations caused by herbicide spectrum and carry over discussed above, if applied inappropriately, herbicides can damage crops. In addition, herbicide drift can harm neighboring crops.
As is true of all agricultural pest control strategies, weeds can evolve to develop resistance to herbicides. In addition, the continued use of herbicides will cause the spectrum of weeds in a field to shift to those that are more difficult to control with herbicides.
Herbicide Resistant Crops
Are herbicide resistant crops new?
No. Farmers in the U.S. began growing herbicide resistant plants in the 1940s, soon after the first synthetic herbicide was developed. The use of herbicides and crops that could resist them increased markedly in the 1960s and 70s. By 1995, herbicide resistant crops were grown on virtually all acres of major commodity crops.
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How are herbicide resistant crops developed?
Researchers have used a variety of methods to discover and develop herbicides and the crops that can resist them.
They began, in the 1940’s, by identifying synthetic chemicals that controlled weeds but did not harm crops. Virtually all crop plants have some endogenous resistance to the toxic effect of certain chemicals, because different plant groups, such as monocots and dicots, have significant biochemical differences. Organic chemists generated hundreds of molecules, sprayed them on weeds, and retained those that suppressed weeds. They then tested the herbicidal chemicals on crops. Promising candidates were subjected to a variety of tests required for regulatory approval: toxicity to various animals, environmental fate, environmental metabolism and agronomic performance. This method led to the discovery, regulatory approval and registration by the U.S. Environmental Protection Agency of over 100 herbicides.
In the early days of herbicide discovery and development, researchers discarded the chemicals that harmed both weeds and crops and those that degraded rapidly. Eventually, however, they realized broad spectrum herbicides, especially those with short life spans, could be useful in certain situations, such as to control the many species of weed that emerge before crop plants.
After a number of decades, discovering new herbicides became increasingly difficult and conducting the requisite tests for commercial approval increasingly expensive and stringent. To maximize the usefulness of the approved herbicides, crop developers turned their attention to broadening the selectivity of the crop plants. They needed to discover new sources of genetically based herbicide resistance that could be bred into crops; alternatively, they needed to generate new genes for herbicide resistance. Both strategies had very few successes. They found almost no genetic variation for herbicide resistance in crop cultivars or their wild relatives. Chemical mutagenesis, breeding and screening for herbicide resistance led to only a handful of cultivars with slightly broader resistance.
Breakthroughs in modern biotechnology have provided crop developers with a new set of tools to broaden the number and types of herbicides that crop plants can tolerate. Especially important are the molecular techniques that remove the boundaries to gene exchange. Useful genes for herbicide resistance found in other organisms, such as microbes, can now be incorporated into crop plants.
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How do herbicide resistant plants survive exposure to herbicides?
The precise molecular mechanism of resistance varies with different plants, but, in general, plants resist herbicides in one of three ways: deactivating the herbicide by chemically altering it; changing the structure of the target site of the herbicide so that the plant is no longer sensitive; avoiding the herbicide by not absorbing it or, if absorbed, by compartmentalizing it away from its target site.
These three basic mechanisms of resistance can also be enhanced with other changes. For example, decreasing plant sensitivity by providing it with a new gene that encodes an insensitive form of the target site does not eliminate the plant’s fully functional target site. Providing the plant with many copies of the new gene, however, increases the proportion of insensitive target sites to the plant’s sensitive target sites. Therefore this increases the probability the herbicide will encounter a target that it cannot affect and normal plant function will be maintained.
Why are soil microbes often a source of herbicide resistance genes?
Soil microbes have been exposed to herbicides for more than 50 years. Such selection pressures typically lead to the evolution of genetically–based resistance mechanisms. It is reasonable to think that the prevalence of herbicide resistance genes in soil microorganisms evolved from such selection. Microbes and plants share many biochemical pathways, so normal physiological functions in microbes will also rely on molecules that are the target sites of herbicides.
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Evolution of Herbicide Resistant Weeds
Can a susceptible weed become resistant to an herbicide?
Yes. All pests that growers must control in agricultural systems have the capacity to become resistant to whatever tactic is used to control them.
The first herbicide resistant weed appeared in the 1960s. Within a decade after the first documented case of the evolution of a resistant biotype, certain populations in more than 30 weed species had evolved resistance to at least one herbicide of 15 classes of herbicide. By 1995, that number had increased to more than 100, and every class of herbicide was no longer effective against populations of at least one weed species.
What factors influence the rate of evolution of resistant weeds?
The first factor is the rate at which a gene for resistance spontaneously appears in the population. Weeds have a short life cycle and produce many offspring. Both traits predispose them to a higher probability of resistance genes appearing in the population when compared to species with long life spans and few offspring.
Another important factor influencing the rate of resistance evolution is the level of selection pressure imposed by the herbicide. If an herbicide is used repeatedly and is the sole weed control strategy, resistance will evolve much more rapidly than if the grower uses a variety of different tactics, such as cultivations and crop rotation, and varies the classes of herbicides the weed population encounters.
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Can weeds become resistant through gene flow from the herbicide tolerant crop to the weed?
To date all instances of weeds becoming resistant have resulted from the weed evolving its own biochemical mechanism and not by acquiring genes for resistance from the crop. Nonetheless, in certain circumstances it would be possible for herbicide resistance genes to flow from the crop to weeds. The most important variable affecting gene flow is the degree of relatedness between the crop and the weed, because gene flow is only possible if close relatives are growing near the crop. As a result the possibility of gene flow depends first and foremost on the presence of wild, weedy relatives. For example, gene flow from maize to wild relatives is only possible in certain areas of Central America.
The occurrence of gene flow does not constitute an ecological or economic problem in and of itself. Additional factors play essential roles in determining the impact of gene flow. For a complete discussion of this issue, please see the Science Knowledge Topic, Gene Flow from Crops to Wild Relatives.
How can growers slow the evolution of herbicide resistance in weed populations?
In addition to using other control tactics in addition to herbicides, growers can use herbicides with two different modes of action. This tactic is most effective if the two herbicides are applied simultaneously, not sequentially. However, the freedom to use the strategy of simultaneous application of two distinctly different herbicides relies on the crop plant being resistant to both. The tools of modern biotechnology improve the prospect for developing crops resistant to herbicides with diverse modes of action. For example, researchers are developing crops with dual resistance to herbicides that have very different modes of action: glyphosate and the group of herbicides known as ALS herbicides. The ALS herbicides include herbicide classes such as the sulfonlyureas and imidazolines.
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