So glad this thread took off, its awesome hearing everyone's suggestions :)
Ok, new ideas!
Having a vasectomy/neuter "Service for slaves", so female PCs don't continually get knocked up.
A new role in the farm, Stud. For 'alternate' milking :P Efficiency could be based off ball size and/or grade. Not sure about the morals in feeding the produce to your slaves, but that's for the farm manager to worry about.
Any effects caused by mutation should be available as a laboratory procedure. Fun as it is having a freaky mutant studying in the library just because i was determined to give her blue eyes.