Horses are crucial to the U.S. Border Patrol's enforcement efforts, especially in remote and rugged terrain.

But what goes into the animals also comes out. And their droppings if they are eating hay can introduce alfalfa and unfamiliar weeds into the environmentally sensitive desert.

That is why the agency is feeding its horses a fortified, commercially prepared pellet so their waste does not disrupt nature's delicate balance.

Much of the Arizona border includes sensitive land -- for example, Organ Pipe Cactus National Monument; the Cabeza Prieta, Buenos Aires, San Bernardino and Leslie Canyon National Wildlife refuges; the Coronado National Memorial; and the San Pedro Riparian National Conservation Area. "Patrolling on horse is going to leave less of an impact, less of a footprint," said David Bemiller, public lands liaison for the Border Patrol's Tucson and Yuma sectors.

And, in mountainous places, "you simply can't drive an SUV," Bemiller said. "So it serves a dual purpose."

Horses have been an integral part of the Border Patrol since its inception in 1924, but automobiles and aircraft lessened the animals' role. In recent years, however, there has been renewed emphasis on horse patrols, particularly in the Tucson sector, which covers all but the westernmost part of Arizona's border with Mexico.

Nearly half the Border Patrol's 205 horses are in the Tucson sector. Since a mid-1990s crackdown in San Diego and El Paso, Tex., the sector has become the nation's busiest for illegal entries, with more than half of all such apprehensions coming along the Arizona border last year.

Horses are used elsewhere along the southwestern border, but the diet of those animals is strictly oats and hay. There is no need for the special feed, said El Paso sector spokesman Doug Mosier, because "we don't have the same ecologically challenged or sensitive areas."

The only other spot on the border where horses are fed the pellets is on federal lands near Jacumba, Calif., said spokesman Mario Villarreal, who is in the agency's Washington headquarters.

The decision to give horses special feed in Arizona was a preemptive one, Tucson Border Patrol spokesman Luis Garza said. Officials wanted to be sure an environmental problem would not develop.

"We consider refugees as more of a priority issue, but we do handle environmental issues," Bemiller said.

Besides horse waste, the Border Patrol must deal with piles of empty water bottles and brush fires sparked by migrants' cooking fires.

When the agency first discussed adding more horse patrols in Arizona's western deserts last year, "we suggested that was one way to minimize the impacts," said Curt McCasland, a biologist and assistant manager of the Cabeza Prieta refuge, an area on the Mexican border larger than Rhode Island. McCasland noted special feed is necessary, "because you don't know what you're getting in the hay."

Marco Mercado of the Border Patrol prepares Badger for the workday near Naco, Ariz. The agency's horses do not eat hay so that their droppings do not introduce alfalfa and nonnative weeds to the desert bordering Mexico.Feed pellets are given to horses near the environmentally sensitive area.