The classic definition of a weed is a plant out of place. Johnsongrass, I was taught in the state of New York (and all ag schools, being state schools, teach chiefly their states' ag), was a weed; then I went to Texas and found it a forage. So if you're farming weeds, I suppose they really aren't weeds anymore: you must have decided that whatever's in your field IS in place, no matter what it does or how it got there. In a way this is like rangeland management: you're accepting the plants that are there, seldom doing much to help them grow but never doing anything to drive them right out of business, moving your livestock along before they overgraze the area. Only here I'm not talking about raising animals but the plants themselves. Animals make sense where land can't be practically tilled, where the few plants that can survive on it can't be turned into row crops, where the only way to harvest such riches as there are is with things that can walk around great expanses of stony terrain and turn pulp into protein. But can the plants alone be made to make sense in such circumstances?
I had imagined (really, fantasized) many alternatives to the above, in most cases postulating that the farmer of weeds wasn't harvesting: that what was being done was stewarding a purposely messy preserve, where useful parasites or insects or birds or suites of DNA and RNA could occasionally move out or be found out - carefully collected while the field was otherwise left in convenient peace. But that was all just too vague, even for my tastes. So I went back to the idea of almost-real farming: kudzu, say, would invade your land, and you'd decide not to inquire too closely as to how it got there (or worry, ha-ha, about whether it was going to come back next year) and just basically mine it. What you'd be mining it for would probably be just cellulose, the only thing you can count on all vascular plants to produce in large quantities.
So, the "lightweight agronomic calculator" here calculates as follows:
Not this webpage's source code, though. The model is capacious, so I've put it on its own page.