Been planting annual mixes for the past 9 years down here in GA, experimenting with different blends. Most of my annual acreage is clean till that I oscillate between summer and winter annuals, so take my recommendations with that consideration. I've tried overseeding bahia/bermuda pastures and haven't had much success. My organic matter levels are between 1-2.5% mostly. I forage develop heifers and bulls for sale, and will give young pairs access sometimes as well.
The past couple years my Pearl Millet has been more susceptible to some form of rust, so I'm thinking of increasing Sorghum Sudan, which previously had been susceptible to the Sugar Cane Aphid, and decreasing Pearl Millet. This will be my mix next year.
BMR Sorghum Sudan 10lbs
Tif Leaf 3 Pearl Millet. 8lbs
Sunn Hemp 8lbs
Red Ripper Pea. 4lbs
Okra 1lb
Sunflower 1lb
Crabgrass 1lb
The Okra and Sunflower are there more for soil health as diversity than making a heavy grazing contribution. When you get your diversity up above 6-7 species, according to Dr. Christine Jones, Nature's entourage effect is more effective. The only issue with the crabgrass is it is easy to bury. I set my drill at 1/2" and don't worry if I bury some. My crabgrass comes on at the second or third grazing. (I'm in south Ga)
The SS, PM, and SH do the heavy lifting. They are aggressive out of the gate. You can substitute Iron and Clay for Red Ripper pea, but my sources tell me the Red Ripper is a little bushier and stands up to grazing pressure better. If you are further south or have been experiencing the sugar cane aphid (haven't seen the bugger in 6 years myself), then do 12lbs PM, and 6 lbs SS instead....or no SS if the outbreaks have been bad enough. That aphid can wipe you out.