The farm-scale evaluations (FSEs) of GM crops in the UK were the largest scientific experiment of their kind anywhere in the world. But they set out only to look at a narrow series of issues.
Which crops were tested?
Over three years, scientists tested three types of GM crops: maize, oilseed rape (spring and autumn varieties) and sugar beet. All the novel plants were tolerant to particular herbicides.
This means they can be sprayed and still prosper while all the weeds around them die. Proponents of GM have argued this can not only improve yields but can also be kinder to the environment because the different field-management practices involved - the timing and frequency of spraying - can give wildlife opportunities to thrive that present practices deny.
How were the tests planned?
Fields were planted at about 60 sites across the UK. Farmers planted the modified crops in blocks up to about 10 hectares (25 acres) in size in fields that also contained an equivalent non-GM crop. The GM plants were treated with their particular co-chemicals; the normal plants were managed in the usual way.
What did the scientists look for?
They wanted to measure the effect growing GM crops had on farmland wildlife. They looked at the range of vegetation growing in the trial fields and on their margins. They assessed the abundance of animal life - from slugs and snails to beetles and spiders. They also observed bees and butterflies, birds and small mammals.
What did the scientists find?
The results were mixed. The cultivation of GM rape and beet seemed to damage biodiversity. There were fewer insect groups, such as bees and butterflies recorded among these crops.
In contrast, growing GM maize was better for many groups of wildlife than conventional maize. There were more weeds in and around the biotech maize crops, more butterflies and bees around at certain times of the year, and more weed seeds.
What did the scientists not consider?
They have not assessed the health impacts of consuming GM products. They have not looked at the problems, benefits or economic impact growing GM crops will have for farmers.
The main trials have not looked at how GM traits might flow into the wider environment through pollen spread - although another team will report on this at a later date.
Also, the winter-rape data is not yet complete and will be reported on early next year.
What is the fuss over GM maize?
The comparison crop against which this GM fodder variety was tested was sprayed with atrazine, a powerful weedkiller that is about to be withdrawn by the EU.
Opponents of GM say this makes the maize results in the FSEs invalid, and that the bioengineered plant should be retested against comparison crops sprayed with chemicals that will stay in use.
The same scientists who carried out the FSEs say that although new, more-friendly sprays than atrazine will improve the performance of conventional maize, the GM variety will still be kinder to wildlife.
What happens next?
The government now has all the advice it asked for to make the decision on commercial planting. It has received an assessment of the economic benefits; it has undertaken a review of the science; it has held public consultations; and, crucially, it has the results in from the farm-scale evaluations. The decision will be a political one - but one based on the science, claim ministers.
Which crops are next?
Biotech companies have a range of trait-specific crops they would like to introduce. These plants include varieties that are toxic to insect pests and so should not suffer so much wastage or need as much chemical treatment. All these novel plants are likely to have to follow the FSE path if they want to be commercialised in the UK.
Remind me, exactly what are GM crops?
They are plants that have had genes added in the laboratory to give them very specific traits. This modification can, for example, produce crops that grow more favourably on poorer soils or have human proteins in their tissues that can be processed to make medicines.
Conventional breeding can shuffle genes between closely related plants but it cannot move genes between very distant relatives or incorporate genes from animals and bacteria. Only GM can do this.
This has led to maize that kills its main insect pest if it tries to eat the plant, and tobacco whose leaves can be used to treat stunted growth in children.