Scientists say they're still concerned protesters will try to destroy a trial of genetically modified wheat in Hertfordshire.
About 200 activists from the group Take the Flour Back held a demonstration near the Rothamsted Research site at Harpenden this weekend.
The protesters marched towards the trial crop but were held back by a heavy police presence.
Despite two arrests, the protest passed by peacefully. But there is still a high level of security at the scientific research site.
The wheat has been engineered to produce a chemical that scares off aphids - a common wheat pest. Scientists believe it could be the future of pest control. The crop is now undergoing open-air trials, which are tightly controlled within a large buffer zone. But protesters still believe pollen from the wheat could contaminate the outside environment.
At Sunday's demonstration, one protester told ITV Anglia:
The trial is part of a 5-year study. Scientists say the open air trials are necessary and today pleaded with protesters to engage in dialogue, not destruction.