I'm with u boys all the way, do not agree with dairy herds being in all year and the type of intensive dairy that do it also tend to have those big holstien type cows, bloody horrible looking things, can just about hear the bones rattling as they walk, compared to the more old fashioned Holstien type cows of yester year different breed of cow all together, far longer lasting and bull calves have a value.
Just thought if there is some indoor herds in would be possible to rule out or in certain types off transmission, use them as some sort of control, i obviously don't know anyone down there like u boys, but have heard plenty off horror stories about people with closed heards testing positive. If a closed herd is becoming infected or reinfected after a long time there must be an outside vector, ie wildlife, the zero graze herds could possibly prove that if they were staying clear it really is the wildlife.
Always more info to back the cull (or possibly not as the sase may be)