Candy says that he knows that when he can no longer work, he will be treated just like his dog. What does he mean by this? How is Candy like his dog?

Candy is old and is missing a hand. This is a working ranch during the Great Depression. People or animals unable to work were a burden and hence expendable. Candy thinks that he will be let go as soon as he no longer can perform his duties as a swamper. He thinks he will eventually suffer a similar fate as his dog.