Bowman, Turnips are easy to grow, as long as you get timely rains. You are correct, they should be planted late July-early Aug, they are slow to get started but when they do take off they grow very fast. With the soil you have, you could probably get by with just nitrogen added before you plant. They can use as much as 150lbs of urea per acre but just add what you can. It is best to broadcast the seed and lightly cover with a drag or pack the soil after sowing. They should be sowed at 3 to 6 lbs per acre, thinning is not necessary, the tops contain the most nutrition and the deer should assist in any thinning
A good mix is 3lbs of Purple Top Turnip and 3lbs of Dwarf Essex Rape.
Here is a few that is sold in those pretty commercial bags.
Tyfon Forage Brassica, Appin Forage Turnip, Barkant Forage Turnip, Pasja Hybrid Brassica, Barnapoli Rape seed, Bonar Rape seed.
Most of these can be purchased in one pound qauntities for $2 to 3.50 a pound.
Let me know if you need a on-line source.