Why couldnt you?
You can breed dogs for the sole purpose of herding sheep.
You can breeds sleek race horses.
People have been doing this for hundreds of years.
It took hundreds of years to even domesticate dogs, and them to get them to the specific breeds they are now took several hundreds more (selecting traits, in breeding / line breeding, culling unwanted animals etc) Another thing to consider is maturity rates for dogs is two years, obviously you can use them for work before that age but most working dogs 1 yr old is the young your going to start them at. With humans you really would have wait until (as sick as this sounds) 7/8 yrs old before they would be useful and even then kid would burn out quick
So imo , forcing slaves to have more children probably did happen but not on a large scakryand with very little if any scientific motivation behind it