What other inputs? Sunlight and water? Grass is an efficient way of harvesting solar energy. If you're talking about a growth cycle which doesn't involve animal fertilizer inputs, you've got to have a huge external energy source involved to manufacture fertilizer, so that's a pretty big cost of not raising animals on available pastureland.

That, and don't even get me started on methane emissions, which is a handy way to deflect discussions about CO2, but not a great way to actually do anything productive (look up radiative forcing and the relative atmospheric lifetimes of greenhouse gases).