The women, from the group Genetix Snowball, intended the action to be the start of a non-violent campaign.
All five were later released by police after the owners of the field, the biotechnology company Monsanto, declined to press charges.
Police arrested the five: Rowan Tilly, aged 40, from Hove, East Sussex, Kathryn Tulip, aged 39, from Oxford, Zoe Elford, aged 27, from Islington, north London, Melanie Jarman, aged 27, from Manchester and Jo Hamilton, aged 29, also from Manchester, in connection with inquiries into criminal damage.
Notice of protest
The action came after the group wrote to farmers in Berkshire, Buckinghamshire and Oxfordshire, who have leased test sites to agro-chemical companies, informing them of the action and inviting them to join in.
The five women wore protective clothing to dig up a sample of crops growing at the site and left them sealed in bags with the biohazard symbol.
Solicitor Ms Tulip, the first woman to be arrested, said: "I hope that my actions will encourage other ordinary people to join with us to take responsibility for stopping this technology from destroying our environment and endangering our food and our health."
A Thames Valley Police spokeswoman confirmed all five women were arrested and released without charge.
The group claims genetically-modified crops may cause "criminal damage" to conventional crops through cross-pollination.
No-one from Monsanto was available for comment.
The protest group is calling for a five-year moratorium on the release of genetically-modified crops into the environment pending research and public consultation on the experiments.
The latest campaign follows more than 20 attacks on sites this year by groups protesting at the introduction of genetically-modified crops.
Fifteen companies have more than 300 licensed test sites in Britain.
The Department of the Environment, Transport and the Regions said the Government was reviewing the effect of genetically modified crops on the environment.
It said Mr Watson's sweet corn crop was planted more than one mile from the maize and an advisory committee had said "the amount of cross-pollination is likely to be zero".
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