According to Wikipedia [1], cattle have a population of 1.3 billion with a biomass of 156 million tons. Chickens have a population of 24 billion with a biomass of 14.4 million tons.

For reference, humans have a population of 7 billion, with a biomass of 105 million tons.

[1] http://en.wikipedia.org/wiki/Biomass_%28ecology%29#Terrestri...