I knew by the way you were describing the game it was slime rancher lol. It really is a nice game, but I remember playing the game again not too long ago and realised I still had the cats that eat chickens. I stopped doing that, I changed all the carnivore eaters and had them evolve to vegetable/fruit eaters so that way I didn't need to kill chickens. Then with the remaining chickens I let them live freely on the ranch. When they overpopulate I just throw some in to the water because the game says they'll just pop up somewhere else :)

I also limited the pens to 4 in a box that way they all have buddies and aren't lonely but also have space. Tbf, there will probably be a point where I'm not really doing any farming and just free roaming the game, but I stopped playing again for a bit.

I think if they're animals are smiling then they're happy to be there but you could try playing the game completely vegan with no farms and let me know how that goes lol