It's just something to think about, the real miniatures might be a hassle if there's nothing around to breed them to.
Also the miniatures, and to a lesser extent some of the smaller breeds, can have some odd genetic problems associated with dwarfism.
PS..don't forget that the real old breeds were triple purpose...beef, milk and draught...those breeds can also make excellent draught animals / small oxen and could be a real help if you're doing larger scale gardening.
If you have small cows or especially heifers be cautious about the bull you breed them to so as to avoid difficult calving
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