Over on Reddit user BigDaddyBioShock has revealed his resource friendly Smart Breeder 2.0 for Minecraft and has posted a gallery on imgur detailing how it all works.
Described as an "Upgraded, more efficient smart breeder," BigDaddyBioShock shows a simple Minecraft layout commenting "The pathway is made of upside down half slabs, to prevent the baby animals from spawning on the path."
Once you've leashed an animal you can then have it climb ladders and if needed push it towards the fence post you want to attach it to. Once you've done this you can then repeat the process with another animal. At this point your animals will be ready to breed and by the time you've walked between your animals mating them they should have breeded plenty of new animals which fall into the pit below giving you a ready made food source. It's all very efficient if a bit cruel.
For full details on how to work it check out imgur. Perhaps someone should tell Peta just in the hope they have another outburst similar to the fuss they kicked up about whaling and Assassin's Creed 4: Black Flags which earned a hilarious response from Ubisoft.