From 1996 to 2006, there were 231 documented incursions onto U.S. soil by the Mexican military - on average, twice per month.
In other words, once every two weeks, the Mexican military purposefully crosses into the United States onto public and private land. Granted, these are only the cases with video or photographic documentation, and only cases that the Executive branch wasn't able to squelch.
The coyotes and drug smugglers with their AKs are bad enough, but if you're able to lay down enough fire to overwhelm them, you have a fairly high chance they'll radio the Mexican military. Under orders from the cartel, they'll roll out with shiny H&K G3s, maybe some new FX-05s, or even a Ma Deuce or two and put themselves between you and the smugglers while they make their way back to Mexican soil. All this and more has been documented with video or photographic evidence, along with the first-hand testimony of federal and local law enforcement officers.
They've done it to the Border Patrol. They've done it to LEOs in Arizona. And recently, they've done it to American citizens on private land. If I lived within sight of the border, I'd have an accurized .308 caliber semiautomatic (probably an AR variant) with all the spare mags I could find, with a high-fidelity night vision scope. I'd also have a really good night-vision spotting scope that mates to a digital video camera with excellent zoom capability. If you didn't record it, it never happened.
And if Homeland Security gets it before Lou Dobbs, it never happened.
For walking around your own property, I wouldn't ever go out without a .45 on the hip and something like an M1 carbine or even an AR. Perimeter security would depend on how close I live to the border, whether my property is part of the smugglers' regular stomping grounds, and whether sparsely suburban landscape or truly acres of ranch. Intel / recon / surveillance is your friend. Spend time watching the border day and night to assess the threat, if any. Talk with neighboring property owners to the east and west, and trade information on recent incursions by either coyotes, smugglers, or the Mexican army.
By the way: if this is the first time you've heard any of this, ask yourself why.