One of the most important things to decide when you are managing your horse farm is what exactly are you managing for? Are you hoping to supplement your horse’s feed, looking for enrichment for them, or just hoping to get by without too much effort?
When you are trying to decide on a seed combination for your fields it is important to choose forages that will not cause problems down the road. If you plan to have a small broodmare band you should be careful to use an endophyte free grass to avoid pregnancy problems later on.
Much of the information you will find about grazing is geared toward meat or milk producing animals, and consequently the emphasis is placed on getting the most nutritional value for your buck. The grass combinations for this end may not be ideal if your primary reason for having your horse on pasture enrichment. If nutritional value is secondary to allowing the horse to move and graze, maintaining as much as possible their natural lifestyle you will be looking at very different forage combinations.
If your horse has insulin resistance or tends to founder easily you have additional challenges, because you want to encourage your horse to move as much as possible but you need to be very careful about the amounts and types of grasses or legumes they consume and even when they consume them. In general grasses have low sugar levels in the early morning or at night but it can vary depending on how low the temperatures go. Until temperatures have dropped long enough to assure the grass is dormant, avoid letting your horse on grass if there is frost in the morning.
But this too can vary depending on the type of grass and the degree to which the field has been stressed owing to drought or overgrazing. You can have the forage in your field tested to give you a more accurate picture of the sugars in the forage.
These two articles give a good summary of how different grasses react to the weather and how sugars migrate in grasses during the day, how to test your forage and how to interpret the results.