Grass-fed beef has a patina of good enviro-ethic stewardship to it. Without grains there is little or no need to turn soil and with it cause problems like of erosion and fertilizer run-off from tilling. It's a good story, but as with any product as varied and variable as grass-fed beef reality is a mixed bag. There are about as many ways to raise grass-fed beef as there are to skin a cat. It can be done on low fertility hillside perennial pastures. It can be done on set-stocked over-grazed lawn-like swards. It can be done on high fertility arable land growing only annuals. It can be done with hay or other stored forages. It can be done with a mix of these strategies.
It can even be done in a feedlot situation (and is!) much to the chagrin of those of us in the eco-ag movement who seek to produce high quality food in a sustainable manner. If the ration used in a feedlot doesn't have grains in it the beef from said steers can sell as "grass-fed".
My personal inclination is toward managing for perennials, especially since a lot of my land is comprised of hillsides and prone to erosion. But I don't want to be dogmatic about only using perennials and I recognize that appropriate use of annual grasses can work well in my farming operation.
Since I now have pigs in my life and one of their great joys is turning sod, I had bare dirt to contend with this year on a scale not seen in my previous 6 years farming. One of the experiments I ran this year was broadcasting sorghum-sudan grass where the pigs did some rooting. My hope was to both cover the bare soil more quickly than mother nature would if left to her own devices and to provide some high quality grazing for my cattle during the summer and early fall. Sorghum-sudan is a relative of corn that grows like crazy once it gets established. It likes heat and can grow well with 1/2 the available water of many cool season perennials. This is to say it hits a growth peak in the height of summer when cool season perennial grasses slow down from heat and water stress.
The germination and establishment of the sorghum-sudan grass left a lot to be desired, but considering the fact that I forwent all the usual steps one ought to take to improve the odds of growing a good stand of grasses, i.e. tilling evenly, discing, and seeding with a mechanical drill, I can't complain. I think I got my money's worth from the $40 worth of seed I spread even it I would have liked to have more plants to show for it. This is what the patch looked like a few days ago when I turned two finishing steers into it for its second graze down of the year.
Annuals like sorghum-sudan and pearl millet have substantial potential for farms like mine that strive to produce the highest quality meat. Both of are annuals that direct their full growth potential into the current year and don't try to sock any energy away for early spring regrowth. Most of the carbohydrates the plant produces remain above ground where ruminants can eat them, and as we ought to all know by now, carbohydrates are the engine that drives fat deposition. And fat is where flavor resides, so for a delicious full flavored cut of beef the animal must be gaining weight during the days leading up to slaughter.
Finally, if you're really interested in the topic of planting annuals and grazing them off, my friend Dave over at Wrong Direction Farm has two posts here and here about a millet experiment he ran this year.