Just a couple of first hand accounts on alfalfa , poplar, cows and trees.
I have grown alfalfa most of my life. As a feed when I had cattle, as a crop when I sold bales and notably as a rotational crop in grain production. Alfalfa is a nitrogen fixer. It takes nitrogen from the air and converts it into a form that can be used by plants. It's root system also breaks through hard pan and mellows up soil for multiple years. It along with other legumes and beneficial plants can be part of a plow / graze down system for organic production. ( which we currently do )
On poplar. They are a pretty good firewood. No, not as good as oak or ash but properly cured and STACKED it's good wood. We currently burn almost exclusivly spruce as many of the spruce planted by my father and uncle in the 30s or maybe even late 20s have timed out and are dieing. I'd much rather burn poplar but we need to clean up the dead spruce. One of the good things about planting poplar is I dont believe any of the host of diseases and bugs have attacked them yet. Dutch elm disease got our elms and now the Emerald Ash bug will devastate the ash. ( including all the ones plated as windbreaks ). Poplar are a resilient, fast growing , tree. There was a push to plant marginal land to trees for pulp and paper a few years ago and it was poplar that was being planted.
On cows and trees : They certainly can clean out all the young trees out of a bush, depending on the number of cows and the size ( acres ) of the bush.. There was a study done in our area a few years ago that showed the most environmentally beneficial thing to do with our riparian zones was to fence the cattle out but allow them to come in and graze off the grasses a couple of times a year. That way there is less damage to trees, and soil but less grasses on the clear spots that build up a thatch and send phosphates down stream.
Edit : Dammit all that cow/ poplar talk has dredged up a memory. I can't stop myself from telling it .I had a cow, " Missy " that had a real talent for walking down poplar trees. She would take a poplar up to 3 inches in diameter ,maybe 15 ft tall , push on it then get it between her front legs on her brisket and walk it down till she got to leaves. The crazy part was no other cow could do it quite like her but they knew that she could and would gather around when she started doing it so they could join in the feast. You wouldn't want a herd with her ability.