Cows are pretty smart and self-aware animals (e.g. they are ashamed if they are seen by their peers during milk extraction). I'm not sure that they'll take an aggressive stance against the collars if it's not used abusively. On the contrary, they might be more content if they are both being able to roam outside and be warned of the limits without being irritated so much.

AFAIK, cows are not aggressive animals unless they're physically attacked or threatened.