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Biotechnology Products: Plants that Tolerate Herbicides

What are herbicide-tolerant plants, and where are they found?

Herbicide tolerant plants are plants whose growth and development are not significantly affected by herbicides used on the weeds growing around them. Farmers use a number of these plants.

All plants have a natural ability to tolerate some specific herbicides. Some, like various herbicide tolerant canolas, have been genetically modified to tolerate herbicides. That is, scientists have developed new strains of plants that are not affected by a specific herbicide.

Herbicide tolerance in plants is not new. For many years, scientists and farmers have known that herbicide tolerance can be transferred from one plant to another through crossbreeding, in farmed crops and in wild plants. People have been observing, studying, and managing the transfer of herbicide tolerance for a long time-long before the techniques of modern biotechnology were used to genetically modify plants to have these characteristics. And scientists continue to study this.

Why have genetically modified herbicide tolerant plants been developed?

These types of plants were developed to help farmers control weeds that compete with crops for soil, space, water, and sunlight.

Farmers who choose to plant a crop that tolerates a specific herbicide can use that herbicide to spray their fields. This eliminates weeds without risking their crops, thereby helping farmers to produce higher yields of their crops. The herbicide destroys the weeds in the field, but the herbicide-tolerant plant is not affected.

Farmers may also choose to use herbicide-tolerant plants for environmental reasons. Using herbicide-tolerant plants may mean releasing fewer herbicides into the environment, or using herbicides that are less harmful to the environment (such as those that break down quickly). It can also mean that the farmer doesn't have to come in direct contact with the herbicide, as happens when it is sprayed several times on a field.

How does a plant become tolerant to a herbicide?

Some plants are naturally tolerant to a specific herbicide, while others develop this tolerance in the evolutionary process of adapting to their environment (sometimes plants develop a tolerance to a specific herbicide because the herbicide was not used properly for weed management). Other herbicide-tolerant plants were developed through biotechnology.

Have concerns been raised about herbicide-tolerant plants?

Yes. Some people have raised concerns that herbicide-tolerant plants will breed with other plants and weeds, creating weeds that are hard to control. Some have dubbed these "superweeds". If a herbicide-tolerant plant successfully breeds with another plant that is considered a weed, it could create a version of the weed that is resistant to regular doses of that particular herbicide. Agricultural experts, including those at the Canadian Food Inspection Agency (CFIA), concur that this is possible.

However, the term "superweed" suggests that the weed can not be controlled in any way. Scientific research and historical practices in agriculture indicate that this is not the case.

First, traits can be transferred only to closely related plants. Several of Canada's major crops (such as corn, tomatoes, potatoes, and soybeans) don't have weedy relatives in Canada that they could transfer their traits to, so there is no potential for hard to control weeds to develop from these crops.

Second, regarding those crops that do have "weedy" relatives, there are many other ways of controlling weeds that become tolerant to herbicides. Farmers have traditionally used a number of methods to manage weeds that have become herbicide-tolerant, whether naturally or through genetic modification. For example, they may:

  • use a different herbicide to control the weeds
  • till the land immediately before they plant their seed
  • use herbicide mixtures to treat fields
  • rotate the herbicides they use
  • use non-chemical weed control methods such as silage and green manure
  • rotate the crops planted in a field
  • swath at the optimum crop stage

As a further precaution, the CFIA requires developers who are granted approval for new herbicide tolerant plants to have a herbicide tolerant crop stewardship plan, that details steps farmers can take to minimize the chances of weeds becoming herbicide resistant. They are also working with industry to develop similar stewardship plans for plants that have already been approved.

Others have raised concerns about what happens if a plant becomes tolerant to more than one herbicide.1 This can happen when herbicide-tolerant plants breed together. The offspring (new plants) may be tolerant to more than one herbicide. The new plants have combined characteristics from each of the parent plants.

This process is called "gene stacking". Gene stacking is not unique to plants developed through biotechnology. Field plants may become tolerant to more than one herbicide if the herbicides are not used properly to manage weeds. Farmers use the methods listed above to manage these sort of weeds, as well.

Concerns have also been raised that herbicide-tolerant plants will take over other cultivated areas or natural habitats. Studies have shown that herbicide-tolerant plants cause no more problems than do other plants-unless that specific herbicide is the only one used to get rid of weeds and other competing plants.

Studies have also shown that domesticated plants, such as crop plants, do not survive well without human care. This means that a herbicide-tolerant plant that grows outside a farmer's field has little chance of surviving or breeding with other plants. In 2001, a 10-year study was published that looked at four types of genetically modified crop plants. It found that the plants did not survive well in the wild and are no more likely to invade non-farm habitats than their unmodified counterparts. The study, by M. J. Crawley et al, called " Biotechnology: Transgenic crops in natural habitats can be found in the journal Nature, volume 409, pages 682-683 (2001).

What is the Government of Canada doing to study the potential impacts of herbicide-tolerant plants?

Scientists at the CFIA are studying the effects that herbicide-tolerant plants will have on the environment over time. In particular, they are considering whether there should be a limit on the number of new characteristics that can appear in each new crop plant developed through biotechnology. They will also decide whether the government should set any additional requirements for managing biotechnology-derived plants, above what are already in place.

To study herbicide-tolerant plants, the CFIA works within the scientific community across Canada. This includes academics and agronomic specialists. The agency also sponsors studies on herbicide-tolerance. As well, the CFIA consults with other agencies of the federal government, such as Health Canada's Pest Management Regulatory Agency, which regulates pest control products, including all types of herbicides.

The Government of Canada is committed to increasing your knowledge about herbicide tolerance and its effects. It is also committed to managing advances in biotechnology so that our environment and our health and safety are protected.

1 There have been some reports of canola that is tolerant to three different types of herbicides. This is very rare.