Greenpaws has the best answer; one of my dogs eats grass all the time, the other one practically never does. The one that likes it doesn't throw it up unless I let her eat too much. And I never let her eat grass in a yard that might have been chemically treated; just my own yard and a nearby field that's not treated with insecticides or herbicides.
It is quite normal for your Cocker Spaniel to eat grass, since it is a common practice for most dogs. It is however, not healthy because the dogs often vomit after eating grass. You can try training the dog not to eat grass or watch its movements when the grass in your compound has been sprayed with chemicals.