My thread explains why other designs were scrapped and why you cannot just build a pit of dirt/stone/etc due to wall suffocation from pushing animals. Suffocation and glitching through fencing happens REGARDLESS of whether your farms are big or not. Saying we're not giving them enough space to roam or we're overcrowding, etc. etc. is moot as overcrowding only speeds up a behavior that would happen inevitably. Animals, sheep and chickens especially, will attempt to escape even if they are given generous areas to roam and this is why this issue remains today.
I've come to find you have to use a combination of everything including the lay of the land to trap mobs and animals successfully.
Though I hesitate to wager a 100% workaround on this issue I feel this is as close as one can get.
So using only fencing, only the lay of the land or an incorrect combination of the two will mean your animals will either suffocate, glitch out or a combination of both.
Mandy Wu
I realize this is not nearly as good as a fix by mojang but I did some testing in 1.7.2.
http://www.minecraftforum.net/topic/2598764-surv17xthe-end-of-skyblock/page__st__60#entry31713095
If it's a TLDR build this fence 2 blocks lower than the surrounding land. (pictured it's only one block lower for clarity)
http://i.imgur.com/s4VGErS.jpg
My thread explains why other designs were scrapped and why you cannot just build a pit of dirt/stone/etc due to wall suffocation from pushing animals. Suffocation and glitching through fencing happens REGARDLESS of whether your farms are big or not. Saying we're not giving them enough space to roam or we're overcrowding, etc. etc. is moot as overcrowding only speeds up a behavior that would happen inevitably. Animals, sheep and chickens especially, will attempt to escape even if they are given generous areas to roam and this is why this issue remains today.
I've come to find you have to use a combination of everything including the lay of the land to trap mobs and animals successfully.
Though I hesitate to wager a 100% workaround on this issue I feel this is as close as one can get.
So using only fencing, only the lay of the land or an incorrect combination of the two will mean your animals will either suffocate, glitch out or a combination of both.
Anyways hope this helps.