If it frosted enough that your field has a heavy coating of frost, then it is time to treat your horses with a boticide dewormer. (If you have bots in your area) Bot worms, are red grub like worms that colonize the stomach lining, and are present wherever bot flies lay their yellow eggs on horses in late summer. Over years they can build up to large numbers in the stomach if the animals do not receive a “boticide” de-wormer after the first frost each year. The most common boticides are Ivermectin and Moxidectin.
If you have a chubby horse or a previously foundered horse you need to keep them off of it for 5-7 days. After a hard frost the growing season will be over for grass and it will not have much sugar until it is close to spring.
This type of a frost causes the grass to be full of the dangerous types of sugar. Once you have had a couple of frosts that kill the grass back, you should be fine until the grass begins to grow again in the Spring.