I'm pretty sure Animae spells have a duration as you can either Creo or Muto them into existence. I think the only canon example of a magus who uses them is in Antagonists, so you'd need to check that book to see how they're written up.
Following the magically created animals leave behind a corpse if they've been fed for a year, does this mean the following item works?
The collar of the phantom flock
This leather collar has been opened as a greater item, it can hold (2 for leather x 2 for small item) 4 pawns of vis and so 40 levels of enchantment. When activated, it creates a flock of 10 sheep (group target, one of which is wearing the collar). Unless removed, the enchantment remains on continuously. If the sheep are grazed for a year, when the collar is taken off it will leave behind ten sheep corpses at next dawn/dusk. The collar can then be re-used.
Base 15 create a mammal, range touch, duration sun, target group, no size modifiers for sheep, +1 3 uses/day +3 environmental trigger (sunset/sunrise) so always on - CrAn44 total.
Birds, fish, reptiles and amphibians are a magnitude smaller - but most European examples of those are smaller than size 0, so sturgeon farming for the win (I don't think dolphins or small whales are really farmable, and it's a long way into Africa to find ostriches. I'm not sure I want to try crocodile or giant gold-digging ant farming)