Once a mandatory evacuation order has been issued, or the sustained winds reach 35 mph, horse trailers and commercial vehicles are discouraged in gaining access to the highways. This makes it imperative for a horse owner to evacuate earlier than most residents.  As per the LSU School of Veterinary Medicine,  It is important to evacuate your horses a sufficient distance from the coast and a good general guideline is north of Interstate 10, preferably north of Alexandria. January to May would be good months to prepare this plan. Arrange to leave a minimum of 72 hours before the arrival of the storm.  Visit their page for other important information you should include in your equine evacuation plan.  Find additional tips here Department of Agriculture & Forestry and here Louisiana State Animal Response Team.