Human feces is the most efficient as far as poo to weight ratio, it is 6x more effective than small animal poo, and the ratio gets worse as you go up in poo size. Use a whip to gather poo quickly from around your tames.
On Ab, there are no Phiomias, so here's an alternative: If you go into the red mushrooms, you will poop... A lot. You can load all that poop on a dung beatle and get lots of fertilizer! It will give you more for bigger poop though.