Wednesday, 30 January 2013

GM CROPS PART 3: What have GM crops done so far?

Other articles in this series:
Part 1: What are GM crops?
Part 2: Why do we need to improve our crops? What's wrong with the way they are now?

What have GM crops done so far?

Farmers have been growing GM crops since 1994 and they are now grown in 29 different countries. Most of the GM crops currently being grown are modified in one of three ways - they are either herbicide tolerant, insect resistant, or both. This article will explain what these things are and what they have achieved. I will then briefly introduce some of the other GM crops that have been grown.

If you like graphs and want to know more about the history and current status of GM crops around the world, I thoroughly recommend these slides.

Herbicide tolerant crops
When crops are grown in fields they have to compete with other plants (weeds) for the things that they need (water, soil nutrients, sunlight). This is a big problem for farmers because it means that the crops produce less food than they would if there were no weeds competing with them.

Farmers have a number of methods for dealing with weeds. Before the crop is planted, the field can be ploughed to destroy weeds. Once the crop is growing, the farmer can go into the field and remove weeds by hand. The general word used to describe mechanical methods such as these is 'tillage'. Another option is to spray the field with herbicides (chemicals that kill plants). Some herbicides are 'specific', meaning that they only kill certain types of plant, so the crop itself is unharmed.

However, all of these methods have their downsides. Tillage disturbs the soil, which causes greenhouse gasses to be released and results in soil being removed by the wind and rain. It is also time consuming and requires a lot of work. Herbicides pollute the environment, and the use of specific herbicides means that many different herbicides have to be used to make sure that all the different types of weed are killed. Also, herbicides are expensive, especially if you have to buy many different specific ones. As I mentioned in part II, many of the world's poorest farmers cannot afford herbicides.

Not all herbicides are specific. There are some that kill almost all types of plant. These are known as 'broad-spectrum' herbicides. Of course, the problem with using them is that they would also kill the crop. This is where GM comes in. Some plants and some bacteria have genes that tell them how to survive the broad-spectrum herbicides. These genes can be transferred to a crop, making it resistant to the herbicide. GM crops that have been created in this way are known as 'herbicide tolerant crops'. A farmer who grows herbicide-tolerant crops can spray their fields with broad-spectrum herbicides safe in the knowledge that the crop will be unharmed, while almost all weeds will be killed.

Farmers have been growing herbicide-tolerant crops for about 16 years now and it has had three main impacts:

Firstly, it has meant that weeds are killed more effectively. This means that more food is produced per unit of land and farmers no longer have to use mechanical methods such as ploughing which release greenhouse gasses and lead to soil degradation.

Secondly, it has caused farmers to switch from using many different specific herbicides to using just one of the broad-spectrum herbicides. Not only does this save the farmer money, it also benefits the environment because the commonly used broad-spectrum herbicides are less damaging to the environment than most specific ones.

Thirdly, the use of broad-spectrum herbicides means that less herbicide has to be used, because one spray of herbicide kills almost all of the weeds. Research shows that in the 12 years from 1996 to 2008, the use of HT crops caused a 5% reduction in the amount of herbicide used worldwide on cotton, soybean, maize and canola. This benefits the farmer because they spend less money on herbicide and less time spraying it, and the environment because less herbicide is released.

Insect resistant crops
Another problem that farmers face is insects eating their crops. One way to deal with this is to spray crops with insecticides (chemicals that kill insects).

However, there are a number of downsides to this approach. Many insects (such as the corn borer) are able to dig their way inside plants and lay their eggs there. Since insecticides are only sprayed onto the outside of the plant, the insects on the inside are not affected. Also, insecticides can be washed away by rain (which can lead to contamination of rivers). Also, although it is usually only one or two types of insects that are eating the crop, many insecticides are very general and kill lots of different types of insects (including natural enemies of the target insects). Insecticides are generally quite expensive and many of the world's poorest farmers cannot afford them. Poor farmers who can afford them often spray them onto the crops by hand. This means that the farmer comes into contact with a large amount of insecticide and poisonings are common in countries such as India and China.

There is a soil bacterium known as 'Bt' which has a special set of genes. Each of these genes tells it how to make one particular protein, and each of these proteins is poisonous to one specific type of insect. It is possible to take one (or more) of these genes and transfer it into a crop plant. This results in a 'Bt crop', which is able to produce its own insect poison. So far, Bt genes have mostly been used in cotton and maize and it has had many benefits.

Since the insect poison is constantly produced by the plant itself throughout the whole of the plant, there is no risk of it being washed away and it works inside the plant as well as outside. This means that the plant is better protected so more cotton is produced per field1. Secondly, since each Bt protein is only poisonous to one particular type of insect, there is no harm to other insects2 and there is no harm to the people who eat it or the farmers who grow it. Thirdly, there is no need for the farmer to buy or spray insecticide. This saves the farmer money3, prevents farmer poisonings4, and reduces the amount of insecticide released into the environment5.

What other GM crops are being grown?
Almost all of the GM crops that are currently being grown are plants that have been modified in one or both of the ways discussed above. However, there are a few other GM crops being grown in relatively small amounts. For a full list, see the slides I mentioned in the introduction, but here I will just mention one type that I think is quite interesting: the virus-resistant crops.

Just like animals, plants can be affected by viruses. It is possible to protect plants from viruses by transferring a harmless gene from the virus to the plant. This tells the plant how to produce a harmless protein that the virus usually produces. The plant's immune system then trains itself to recognise the protein, so that if it is attacked by the virus the plant will be able to recognise it quickly and destroy it. Papaya that has been modified in this way is grown in the USA and has been credited with saving the Hawaiian papaya industry.

PART 4: What could GM crops do in the future? (coming soon)
There are a lot of new GM crops that have been developed and are going to start being grown in the next few years. Most of these are very different to the ones currently being grown and often address different problems. Part 4 will introduce you to these new GM crops and what they could achieve.

Herbicide-tolerant crops
Bt crops
The impacts that GM crops have had so far

1 Huang and co-workers, 2002 (table 3)↩(return to article)

2 Lu and co-workers, 2012 analysed twenty years worth of data on the number of different types of insects in fields in northern China. They found that the introduction of Bt cotton over the last 16 years has led to a decrease in the number of aphids (which eat crops) and an increase in the number of spiders, ladybirds and lacewings (which eat aphids). They found that the presence of these aphid-eaters also protects nearby fields of non-GM cotton.↩(return to article)

3 Huang and co-workers, 2002 (table 5)↩(return to article)

4 Huang and co-workers, 2002 (table 6)↩(return to article)

5 Huang and co-workers, 2002 (table 4)↩(return to article)

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