Willow Bottom-
I've been grass finishing under pivot in south georgia for the past 7 years without grain. If you are an excellent manager, you can finish high quality beef in 20-24 months without grain, if you wean a really good calf with the right genetics. Don't underestimate how important genetics are for finishing with a high percentage of forages, if you want to hit choice or better.
Grain makes your job a lot easier because it gives you a consistent gain-maker without having to surf the vagaries of plant species and maturity. Grain will also even out the flavor profile, which is good for some consumers, and will make your product more consistent, but you would be surprised how good (and how much some are willing to pay) for a properly grass finished animal. I said properly, S.D. There's probably more bad grass finished beef in people's freezers than good.
Ryegrass should be your backbone for finishing in spring. Crimson, Vetch, and arrow leaf clover work well in the sward here. Spring oats work well in the fall. I usually mix them with a small amount of a high quality brassica, such as T-raptor or Forage collard, but brassicas are extremely wet and will squirt right through them if they dominate the sward. They also have a potential to create off flavors. My summer annual mix involves BMR Millet, sunn hemp, cowpea, and crabgrass, although i've just establish Alfalfa in the Tift 85 pastures and are hoping to get the gains from it to minimize the summer annuals.
When you supplement with grain, be mindful of the energy-protein ratios, and make sure your mineral program compliments the feed and forage. Our forages in the south are usually lacking in energy, but often have plenty of protein. (except the summer perennials post-spring)
Feel free to PM me with any specific questions...