When you plan to have 100 new head of cattle/hogs/sheep a year and they are all due in the spring, then you need barn space for all the new animals when they are due. normally you just take 100 head to the butcher and all is well.... Not so much anymore, now you have 100 babies and they need food/barn space and the older the old ones get the tougher the meat gets, so you want to keep the youngins over the older ones.