1. Judging from the lack of pasture in your paddock, you have your animals in a high traffic condition. Meaning you probably don't have the horses stocked at 1 amu/ 10 acres. You may be closer to 1amu/acre. (I am not trying to slander, just quantify what I am saying) When you have high traffic conditions, the #1 concern in sowing a grass is its ability to withstand traffic! 90% of the grasses in Horse pasture mixes don't withstand traffic very well. Orchardgrass and Timothy are prime examples of poor grasses to sow in high traffic conditions. Bermudagrass is a good high traffic grass. The #1 high traffic grass that exists (to my knowledge) is Tall fescue (endophyte infected) The endophyte exists in a symbiotic relationship with the grass and is the reason it performs so well in adverse conditions. If you sow a tall fescue E- such as max-q, 1. you cannot tell when fescue e+ has crept into your field, 2. It doesn't perform any better than orchardgrass so what was the point, 3. It is expensive.
2. Horse pasture mixes are designed to make the seed salesman money.
3. The best thing you can do in high traffic situations for horse paddocks is 1. Lime, 2. Fertilize according to soil test, 3. Sow some fescue E+, 4. sow some red and white clover, 5. drill in small grains occasionally.