Replace the question mark with a number

A guy is trying to relax at home, but his wife keeps nagging him to mow the lawn. Finally, in between nags he blurts out, "Answer just this one question for me."
She pauses momentarily, and he takes this opportunity to say, "You know, a Deer, a Cow, and a Horse, all eat grass. But a Deer's excretions are pellets, while a Cow makes flat pies, and a Horse makes clumps...why is that?"
His wife says, "I don't know."
He replies, "Well then, how can you bring up the subject of the lawn, when it's obvious you don't know sh*t?"