Are you hoping to overcome drought by planting a summer annual grass? What type of summer annual will you plant? This can be confusing because there are five different types of summer annual forage grasses you can plant: sudangrass, forage sorghum (also known as cane or sorgo), foxtail millet, sorghum-sudan hybrids, and pearl millet. You should base your choice on how you are going to use the grass because each grass has its strengths and weaknesses. 
Are you wanting a pasture? If so, you should plant either sudangrass or pearl millet. These grasses regrow rapidly, are leafy, and are less dangerous from prussic acid poisoning than other summer annual grasses.
Are you wanting to hay or green chop the grasses? If so, you should use sorghum-sudan hybrids or pearl millet because these grasses yield well and have good feed value when they are cut two or three times. If your soil composition is more sandy, foxtail millet is a better choice for a summer hay crop. Foxtail millet drys faster and is only good for one cutting, but it can handle dry sandy soil. Another option is cane hay. It is grown in many areas and can produce a lot of tonnage, but its feed value is not as good as the hybrids or millets.
Are you planning on chopping silage? If so, you should plant forage sorghums because of their high grain production. They can not be beat for their tonnage or feed value that they produce.
Picking a summer annual grass is not so confusing, you just need to pick a grass that is best adapted to what you plan on using it for.