That has the problem of having to grow entire corn plants just to get the starch in their seeds. A more resource-efficient alternative is cellulose digestion, because one can use the entire plants instead of some easily-digestible parts of them. But that has the problem is that cellulose digestion is much more difficult than starch digestion, but some people are working on using some organisms' cellulose-digesting enzymes for that.
A bigger problem is that
cellulose is fundamentally a string of glucose (sugar). And that's all you get out of it when it's digested. It's calories but it's hardly a nutritious meal by itself. The organism doing the breaking-down can provide some nutrients, but is it a balanced diet? Doubtful. Live on nothing but bread for a while (which is functionally the same mix) and see how you do.
The main reason to research efficiently breaking down cellulose is for energy production that doesn't involve burning the raw material, which is not as clean as burning an alcohol byproduct.
But the real limiting factor for crops (and remember, to make all this cellulose you have to grow a whole lot of crops) is that the amount of arable land (that is, which
can be used for crops) worldwide is actually rather small. Here's a handy chart:
http://data.worldbank.org/indicator/AG.LND.ARBL.ZS/countries?display=map
This amounts to an average across all nations of just over 10% of the world's land surface. However, some parts of the world have essentially none, others considerably more (the U.S. among them).
Switching entirely to plant-based agriculture would do several things:
It would require a great deal better food distribution than we have today, since currently most of these arable-land-poor countries survive mainly via animal husbandry, and one of the big problems is access -- since the more rugged the land, the more likely it is to only support animal agriculture. (And a great deal more of the world is rugged than most city folks realize.)
A great deal of the world simply doesn't have the water for crops, even if everything else was good. Perhaps a third more land would be usable with a degree or so of global warming (eg. much of the Sahara Desert, which greens during warm periods) and with massive desalination projects and irrigation pipelines (such as Saudi Arabia can afford, but they are not a typical economy). Who is going to pay for all this? Still, even at best-case, this would not be enough to feed everyone. See below.
It would require even more intensive use of fertilizer, which to get a decent crop requires somewhere around 50 pounds per acre per year (and that's even with the soil the U.S. is blessed with, which thanks to geology is far better than the average worldwide). And it would double our use of natural gas. Right now the world gets about half its nitrogen fertilizer (which after water is THE limiting factor on crop production per acre) as a byproduct of animal operations. But the shortfall is made up from an industrial process that uses a lot of natural gas, and there are no alternatives that can make enough fertilizer to fill the gap. Functionally, elimination of animal agriculture would also cause the loss of about half the world's crops -- mostly in developing nations, since they don't have the industry to make up the shortfall (even if they have the natural gas). And remember, for subsistence farmers, manure is free, but manufactured fertilizer has to be purchased. Who is going to pay for all this?
Further, loss of animal agriculture leads to desertification, especially in areas that are already too short of water to support crops. Don't believe me?
Listen to Allan Savory, who has the results to back this up. Grasslands evolved to be grazed, and it doesn't matter which animal does the grazing. (This also squares with my personal observation. Where I lived in the desert, so long as the big flocks of sheep came through a few times a year, we had grass and wildflowers. When the sheep stopped coming, within two years we had weeds and dirt.) Remember that the vast majority of the world's animal ag does NOT involve a feedlot.
Incidentally,
research at the University of Arizona found that numbers of desert tortoise are directly proportional to the number of cattle present. The more cattle, the more tortoises. Why? Because the primary diet of the desert tortoise is the dung of large animals. They thrive best on cow shit.
For those worrying about direct land impact, animal ag, properly applied, doesn't do much that nature doesn't, since all these lands evolved to be grazed. But crop ag functionally destroys the land as a habitat for anything else (plant or animal), turns that land into a monoculture, and use of harvesting machinery is estimated to kill up to half a million small animals per acre (yes, really. Mostly mice and rabbits.)
But perhaps the most salient for the world today: Plant ag alone would, per several estimates I've seen, support a max of about one billion people worldwide. Who gets to choose which 6 billion people are eliminated?
