If you have sparse pastures and no irrigation, there are a few factors to consider. Consider your stocking density (number of horses per acre), soil type, forage variety, and utilization of a sacrifice lot and supplemental feeding of conserved forage. While it sounds like you have a lot of acreage, you will need fewer horses per acre if your pasture cannot support the horses nutritionally due to annual rain fall or irrigation, soil type, and forage variety. The best way to maintain all pastures is to utilize a sacrifice lot. This is a space that is supplied with shelter, water, conserved forage (usually hay), and enough room to safely move and socially interact. Horses are then kept in the sacrifice lot when the pasture is too wet, forage is too short, during drought periods, etc. For more information about setting up pastures, utilizing sacrifice or dry lots, and pasture management visit Extension Horses or contact your county or state equine extension professional.  

Thank you for the information, Susan.  We are currently using a sacrifice lot.  Is there a particular kind of grass seed that would be worth using while the horses are off the pasture?  Our area is typically pretty dry so I don't want to waste money on seed that won't do well here.

Seedling dryland pastures is challenging at best. Often the most successful approach is to seed during the dormant season which would typically be mid to late November through early March in your area. Drilling the seed is generally better than broadcasting it on the ground and then trying to cover it with soil using a harrow but it will work to some degree if you can't find someone to drill the seed. This approach takes advantage of winter and early spring moisture to get the seeds germinated and established before the hot conditions typical of late June and July. Another option is to seed in mid July if your area typically gets monsoonal rains during July and August. You will want to keep the horses off of the seeded area until the plants are well established which can take time (often at least a full growing season). Because of that, you might want to consider seeding only part of your area at a time and electric fencing the horses out of that area. I have done a couple of recent webinars on dryland pasture management that might be useful: https://youtu.be/7-6FNOM8_S0 and https://youtu.be/XzrqXPdgBXY. We have also put together a Colorado Forage Guide that has lots of useful information in it: https://sam.extension.colostate.edu/wp-content/uploads/sites/2/2016/07/forage-guide.pdf. After looking through some of this information, let me know if you have further questions. You can contact me through my regular email if that is easier: <personal data hidden>.