They shut the gate.

A committee would be assigned to access the potential risk factor for each cow and evaluate the economic loss that might be incurred by a distant farmer if  cows in their jurisdiction are allowed free motion.

Then, after each cow has had a GRF* assigned, a transmitter would be attached with its own digital code, and a drone would be flown (by a rotating crew of four) to keep constant tab on the cow's location.

In the case of a cow that roams beyond the committee-set perimeter a hearing would be set up to consider possible extended circumstances, with, of course, an opportunity for judicial review.

Then, if the lawyers for the cow union and the CDC agree, the cow would be asked to meet the new roaming limitations.

Any cows that refused to abide by the restrictions (or showed poor judgement) would be reported back to the CDC committee for further action.
 

*Geophysical Roamability Factor