Horses used in Arizona to help patrol the U.S.-Mexico border, are on a special diet so they don't wreak havoc with the environmentally sensitive dessert.
If horses are fed their usual hay, their droppings can introduce alfalfa and unfamiliar weeds into dessert ecosystem. To avoid that problem, and leave less of an impact, the horses are fed a fortified, commercially prepared pellet.
Besides horse waste, the Border Patrol must deal with piles of empty water bottles and brush fires sparked by migrant cooking fires.