There are lots of ways to automate the breeding and slaughtering of animals in Minecraft. I tend to like using a ComputerCraft Turtle and a Thaumcraft Brain-in-a-Jar on the slaughtering end of things for two reasons: 1) I can turn it off easily, and 2) unlike vanilla slaughtering methods, this method preserves the experience orbs that drop from the disemboweled bellies of my unfortunate, definitely-not-free-range animals. I know the Ars Magica 2 Sigil of Butchery drops the experience orbs, as well, as do Thaumcraft golems (I’m pretty certain), but I like ComputerCraft for this job. But prior to the slaughtering process there is a wide variety of ways to accomplish the other tasks involved in an automated breeding/slaughtering system (i.e. breeding, separating out the babies from the breeding stock or separating out the adults for a slaughtering stock, collecting drops). In a past build I created a ridiculously efficient cow farm that used Railcraft for everything except the slaughtering that filled up Factorization barrels with leather and beef in a surprisingly short period of time. In subsequent games, however, I’ve tried to find a less OP balance that I can leave on all the time and that will produce what I need and maybe a tad more. What I’ve put together in my Magriculture game with my wife, at least for cow and sheep slaughtering, is very close to achieving that balance.
Obviously, there are ways to streamline what I have done here. The slaughtering pen certainly need not be as spacious as I have made it, nor is there any compelling reason for the cows and sheep to be separated, other than certain obsessive tendencies on my part and a firm moral stance against inter-special relations (as Bill Murray prophetically warned: “Human sacrifice! Dogs and cats living together! Mass hysteria!”). I may at some point replace the Sigil of Progeny with a Thaumcraft Lamp of Fertility just to see if there is a performance difference, but the fact is that the system as I currently have it produces more than I need but not an obscene amount more than I need. Nor does it burden the server with entities the way my Railcraft system did. The number of entities at any given moment is kept pretty low.
One final note: Ars Magic 2 by itself provides an easy way to do all of this with its Sigils of Progeny and Butchery and its magic Broom (to collect drops), but not only is setting two Sigils on a Sigil Stand too simple to be satisfying to me, it’s also just a little slower than I want, because the Sigil of Butchery only leaves two adults for breeding. I guess that’s a game-balancing mechanic: if you want production to be faster, you would need to build multiple Sigils (which are actually mildly expensive) or do something a little more creative than just put two cows and two sheep in a pen next to a Sigil Stand. Another reason I like my system is the use of multiple mods rather than just one. Because why not be complicated when you could be simple?