One of the foundational arguments for animal farming is that livestock provide free, organic manure - manure that even vegans need if they want their foods to be organically produced. The argument is especially key among advocates of organic animal farming. While I support humane organic animal farms (not all are particularly humane, but they exist and I've visited some of them) as an alternative to factory farms, I don't accept the argument about manure.
For one thing, nitrogen-rich cover crops can be used as "green manure." That may or may not be as efficient as tapping into the brown manure of a mixed farming system. But no one can argue that we already are sitting on a vast reserve of untapped animal manure. Our own.
A story last week in Time Magazine is a fresh (no crude wordplay intended) update on the status of the "humanure" alternative, with the emphasis on its use in urban gardening. There's no reason, however, why modest technological innovations couldn't efficiently convey vast quantities of humanure from town to country in order to reduce agriculture's dependence both on artificial fertilizers and on the manure of captive food animals.
In my humanure-powered vegan utopia, all those farm animals we've loved since childhood still exist. Indeed they thrive as never before. That's because we breed them in small numbers on no-kill farms where we can visit and appreciate them alive, not dead, and even enjoy their milk, eggs, wool and skins, because now we come by these animal products nonviolently instead of in the abusive and murderous ways we did before.