Why is cows' feces not grass?

Cows' feces is not grass because it is a byproduct of digestion. When cows eat grass, their digestive systems break down the plant material through fermentation and microbial action, extracting nutrients. The remaining waste, which includes undigested fibers, bacteria, and other substances, is excreted as feces. Thus, while grass is the input, the feces is a transformed output resulting from the complex digestive process.