What did they do to Candy's old dog in the story Of Mice and Men?

Candy's old sheepdog is a very important part of the
story.  He is symbolic of Candy, and Lennie for that matter.  That is, something that
"ain't no good to hisself no more", something to be put out of its
misery.

So in the bunkhouse after work one evening, Carlson
convinces Candy to let him take the sheepdog up the hill and shoot it.  Slim says he can
have one of his pups as a replacement, but it's very difficult for Candy to give up the
dog.  Right there in front of the other field hands, he feels he has to, but instantly
regrets it.

Later in the story, Candy says how he wishes
that, when he ain't no good no more, someone would shoot him.  Instead, he says, they'll
"can" him, and he won't have any place to go.

It seems like
a minor part of the story at first, but actually it's a very important one to
Steinbeck's overall message.