The aliens should be nice when mutilating a poor farmers cattle and fill the cow back up with candy. Quoting: Anonymous Coward 597618
Maybe they are filling the cows up with something.
The cattle mutilations could be the American government recovering some monstrosity, after using the cows as an womb.
The damage to the cow might be extensive after a cow version of a c section, at the very least it would leave a scar, leading to questions and even cause for suspisicion of the base. So if the cow gets "mutilated"; the farmer is believed to be one of those whack jobs that believes in them UFOs with little green martian critters in them.
No one would ever X-ray a cow to see what it was carrying, the cost would be too prohibitive to a farmer. He'd think the cow was pregnant, he might not know why or he would probably think someone left a fence open.
Some of these critters might be too large for a "missing" person to carry to term.
So even though the American government might want to use a person to get better feedback on the status of the critter, a human surogate might not be the most cost effective or desirable option, due to size and weight related possible complications.
A cow would complain a lot less too!