A weed which is extremely resistant to herbicides especially one created by the transfer of genes from genetically modified crops into wild plant are titled as Superweeds.
- Superweeds an emerging threat
Herbicides use in agriculture to control the superweeds but now a time superweeds have gained resistance against these herbicides. Weeds like marestail also known as horseweed and Palmer amaranth also known as pigweed are growing rapidly and creating many problems for the farmer community.
Farmers use much more amount of herbicides like 2,4-D and Roundup. These herbicides have many harmful effects on the human health, environment and the use of over doses of these products causing many disadvantages.
Overall landscape’s biodiversity is under the threat of these superweeds. There are abundant amount of natural fauna and flora which are very beneficial for human kind with various aspects but now they are passing with the threats of herbicides which are using for the superweeds.
When we use much amount of herbicides which cause minimum effects on the targeted weeds or species but cause adverse lowering in population of other beneficial flora and fauna.
Herbicides resistant genetically engineered (GE) crops such as Roundup Ready Corn and Soybeans planting are consider the cause of the development of superweeds. The purpose of genetically engineered crops were to reduce the use of herbicides.
But they allow for the application of high concentration of powerful herbicides. However over the years genetically engineered crops have opposite effect. Active ingredient present in the Roundup is glyphosate which is increased by 26% (81.2 million pounds) between year 2001 – 2010.
Where does herbicide resistance come from?
There are two things which we think to cause the arose-up of superweeds:
First one is resistance to herbicides is a natural phenomenon found widely among plants and animals. Such type of resistance depends on specific aspects of the biology of an organism. These aspects are controlled by genes depending on their DNA sequences.
There are many things that change the DNA sequence of a gene. They are called “mutagens” and they include radioactivity, exposure to sunlight ,natural chemicals in the environment (e.g. alcohol), heat, salinity and more.
Over many generations these mutagens have led to wide genetic differences between and among different plants and animals. These differences make specific weeds more susceptible to certain herbicides than others.
Such genetic differences impact the way an animal or plant can process a chemical it encounters when eating or drinking for example.
Through mutagenesis and natural selection some plants and animals have acquired genes that protect them against control measures like pesticides or herbicides making them resistant.
Second is that most of the scientists or people think that Roundup created superweeds.
Now, we will discuss two different aspects of the emergence of superweeds.
- Superweeds defeated Roundup
Many non-farm press think that superweeds are created by Roundup which is totally misleading and wrong definition. The truth is that Roundup did not create superweeds but rather superweeds defeated Roundup. The great example of this is Palmer amaranth as it was a superweeds long before Roundup or any herbicide for that matter was used.
The pigweeds in general and Palmer amaranth in particular have long been fellow travelers with man. They evolved in flood plains where spring and summer floods remove all the existing vegetation. They emerge and grow very quickly and produce an abundance of seed that can float and are very competitive if the water is limited as it often is in sandy areas of flood plains.
Now replace a flood removing all vegetation with tillage or even Roundup and you have essentially the same environment. It is no wonder the pigweeds and Palmer amaranth have gone on to cause agriculture so many headaches.
The developing resistance to herbicide particularly Roundup came long after Palmer amaranth evolved into a superweed.
- Monsanto responsible for emergence of superweeds
In the mid 1990s Monsanto first introduce its line of “Roundup Ready” seeds. These crops include corn, soybeans, cotton, canola, alfalfa, and sugar beets are genetically engineered to be immune to the company’s Roundup herbicide (glyphosate). This convenient system enabled farmers to plant these seeds and later spray fields with Roundup to kill any weeds that might compete with the crops.
The seeds were expensive but in the early days farmers enthusiastically adopted them because they saved time and made weed control easier. This system was heralded as an environmental breakthrough. Using it was supposed to make farming safer because Roundup was widely thought to be more effective than other common herbicides and not as toxic less total herbicide would be needed.
Advocates claimed that Roundup would reduce soil loss through erosion given that farmers would not need to till their fields as much to control weeds. For several years Monsanto’s system did seem to work as intended. But after a temporary reduction herbicide use on U.S. farms has increased dramatically because of growing weed resistance.
Roundup given that other chemical agents also have to be employed overall pesticide use is an estimated 404 million pounds greater than if Roundup Ready crops had not been planted. Farmers costs are rising and the short-term benefit of reduced soil erosion is being reversed because farmers facing resistant weeds often find they need to till again.
As a result of this heavy use weeds showing resistance to glyphosate began appearing in fields. A survey revealed that almost 50 percent of surveyed farms were infested with glyphosate resistant weeds and the rate of these weeds spread has been increasing.
Twenty four species of weed are now glyphosate resistant (International Survey of Herbicide Resistant Weeds 2013). The worst cases are in the southeastern United States where a reported 92 percent of cotton and soybean fields are infested as a result of Roundup Ready crops (Fraser 2013).
- How Resistance Occurs
Glyphosate resistant weeds have arisen largely because the overuse of this single herbicide designed to make weed control easy in fields of crops genetically engineered to resist it also permits those rare weeds with naturally occurring resistance genes to generate offspring.
Thus while glyphosate kills the weeds that do not contain the resistance genes it allows the resistant weeds to flourish and spread. Pesticide resistance is not new. But the rapid spread of glyphosate resistant superweeds today is the result of a “perfect storm” of three practices that have accelerated resistance problems (Mortensen et al. 2012).
- Monoculture. Most pests including many weeds prefer some crops over others. By growing large swaths of the same crop in the same place year after year a practice known as monoculture.
- Overreliance on a single herbicide. Resistance has also been accelerated by heavy reliance on glyphosate alone. This herbicide is popular with farmers because it is relatively inexpensive kills a broad spectrum of weeds often controls larger weeds better than many other herbicides and is easy to apply. Glyphosate resistant crops give farmers the convenient option of spraying directly onto the crop rather than having to apply herbicide to the soil. As a result many farmers have come to rely exclusively on glyphosate. And whereas weed populations treated with a variety of herbicides are less likely to develop resistance because different herbicides act by different molecular mechanisms and very few individual plants carry genes that can defeat more than one chemical weeds treated only with glyphosate have quickly become resistant.
- Neglect of other weed control measures. Many farmers have all but abandoned nonchemical weed control methods even though sophisticated agricultural techniques such as crop rotation can control weeds without excessive dependence on herbicides and reduce the likelihood that resistance will develop. Other nonchemical methods include the use of cover crops conservation tillage that does not facilitate erosion and ways of planting crops that enhance their competitiveness with weeds. Some crops or crop varieties may also produce substances that suppress weeds a phenomenon known as allelopathy.
But the temporary convenience of herbicide resistant crops has led farmers to neglect the use of these other methods. Moreover federal farm and bio-fuels policies that favour just a few crops have entrenched monocultures and essentially encouraged chemical based approaches to control weed.
The Real Solution: The Science of Agro-ecology
Recent studies have shown that use of herbicide could be reduced by more than 90 percent while maintaining or increasing yields and net farmer profits through practices based on the principles of ecological science that reduce weed numbers and growth. These practices include:
- Crop rotation (alternating crops from year to year).
- The use of cover crops and mulches.
- Judicious tillage.
- Taking advantage of the weed suppressive chemicals produced by some crops and crop varieties.
- Even the use of composted livestock manure and crop residues rather than synthetic fertilizers can help to control some weeds.
As these methods generally release nutrients more slowly which can favor the growth of larger seeded crops over small seeded weeds.
These agro-ecological methods have other important benefits such as
- Increased soil fertility.
- Water holding capacity.
- Reduced emissions of water pollutants and global warming gases
- Enhancement of habitat for pollinators and other beneficial organisms.
- And when small amounts of herbicides are used in the context of bio-diverse agro-ecology based systems weeds are much less likely to develop resistance because selective pressure is greatly decreased.
What we should do
Despite their promise agro-ecological practices have been held back by farm policies and research agendas that favor monoculture as well as a lack of information and technical support for farmers who want to change their methods.
To encourage the adoption of these healthier practices we should take following actions:
- Support organic farming and those who want to transition to organic farming with research, certification, cost-sharing, and marketing programs. (Organic farming serves as a “test kitchen” for integrated weed management practices that can be broadly applied to conventional farm systems.)
- Support multi-disciplinary research on integrated weed management strategies and educate farmers in their use.
- Bring together scientists, industry, farmers, and public interest groups to formulate plans preventing or containing the development of herbicide-resistant weeds and make the approval of new herbicide tolerant crops conditional on the implementation of such plans.
- Fund and carry out long-term research to breed crop varieties and cover crops that compete with and control weeds more effectively.
Authors: Muhammad Sajjad*, Muhammad Umar Hameed
(University of Agriculture Faisalabad)