There have been a number of threads that discuss reproduction:
http://forum.barrowdowns.com/showthr...t=reproduction
http://forum.barrowdowns.com/showthr...t=reproduction
http://forum.barrowdowns.com/showthr...t=reproduction
These are just a few. Sorry, I never learned to substitute text in a url link.
Since Tolkien considered orcs to be corrupted or "muitated" elves (and later considered that they might be corrupted men) the biology of reproduction would be identical to human biology.
Considering the social interaction needed to implement such breeding, I personally like to think of orc-women as being similar to the aggresiveness of Klingon women from the
Star Trek universe. That's just me, though. I don't believe Tolkien ever really expressed himself on this subject.
As for Saruman keeping slaves for breeding, it's possible, I guess, but there is no reason to assume that an orc/human mating would produce a hybrid, incapable of producing their own progeny. Saruman could easily have found existing orc/human half-orcs somewhere in the mountains, perhaps, and mated them to produce more. I believe the Uruk-hai were just a different breed of Orcs, not a mix. I could be wrong, though