To make this work the best thing to do is when ur horses need harvesting in the Nursery Barn put all of one kind in there to produce that baby.
IE if you put all Chocolate Calfs in your nursery barn it will produce the holstein cow.
List of Foal and Calf Produces:
You can find out any others by simply mousing over your Foal or Calf in your Nursery Barn.
Chocolate Calf produces Holstein Cow
Longhorn Calf produces Holstein Cow
Kelly Green Calf produces Chocolate Cow
Percheron Foal produces Cream Draft Horse
Purple Pony Foal produces Light Blue Pony
Clydesdale Foal produces Cream Draft Horse
White Foal produces Black Horse
Pinto Foal produces Black Horse
Green Calf produces Chocolate Calf
Pink Patch Calf produces Chocolate Calf
Neapolitan Calf produces Chocolate Calf
Fan Calf produces Chocolate Cow
Pink Pony Foal produces Light Blue Pony
Regular Calf produces Holstein Cow
Light Blue Pony Foal produces Light Blue Pony
Brown Pony Foal produces Light Blue Pony
Black Foal produces Black Horse
Tuscan Calf produces ???
Wild Mustang Foal produces ??? – Currently the Wild Mustang cannot be put in the stables