The best converison for farm animals comes from pigs,with a ratio of around 2:1. That high ratio is possible because pigs are able to be fed high quality food such as grains, meat and oils.
Grass eaters such as cows have lousy converison ratios because grass is a very low quality feed. Cattle can be fed hihger quality food such as grain but unlike pigs they haven't evolved to deal with such a diet and they can't metabolise it as efficiently.
Let's not forget that broilers "meat chickens" have the same conversion rate as do swine. Additionally, they have a faster return on money invested. It all depends on what one is looking for, their interests and overall goals.
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Grass eaters such as cows have lousy converison ratios because grass is a very low quality feed. Cattle can be fed hihger quality food such as grain but unlike pigs they haven't evolved to deal with such a diet and they can't metabolise it as efficiently.