In the original Colonization, instead of Horses being produced like Cigars and etc. they used to reproduce automatically (it wasn't needed to employ any colonists in their production) at a rate of 4% without a Stable, and 8% with one, as long as you had Food left to produce Horses after feeding your colonists. I find the way they are produced in vanilla Civ4Col (Stables requires Horses to be built, and afterwards you can produce Horses from Food like you would produce Rum from Sugar) to be too bland, specially when compared to the original system. So I made this little modcomp.
I didn't want to do away with the employment of colonists in Horse production though, so instead of making the Horses reproduce automatically, they now reproduce at X%, where X would be the normal output for their production. For example, the iProfessionOutput field of BUILDING_STABLE is 2, so a Free Colonist employed in a BUILDING_STABLE would increase the amount of Horses by 2% every turn. If there are Horses in storage, the minimum Horse production will always be 1. If there aren't Horses in storage, they can't be bred at all.
Which yields function according to this system is regulated by the bLivestock field in CIV4YieldInfos.xml.
Livestock Breeding Modcomp 2016-10-05
Livestock Breeding Modcomp