Genetically modified weed-resistant canola plants were cultivated in Canada, making it possible for farmers to apply herbicides freely to kill the weeds but not the canola plants. What went wrong with this scenario?

Answer: Canola plants which are weed resistant started to spread to the nearby farms which did not have canola plants.

When this happened they started to spread out rapidly outgrowing the other crops since traditional herbicides were not able to kill them

This means that they started affecting the actual plants which the other farmers had planted.

This affecting their production negatively.