Horses will eat anything and often prefer grazing over hay, I'm not familiar with some of those weeds as we don't really have them all here, but if they're harmful to horses you have to get rid of them, especially in such a small pasture. Some might need to be pulled up, others may need to be sprayed. Mown grass can also be bad for horses.
The best thing to do is to research the weeds and find the best method of dealing with them.
Thank you! I think my best option right now is taking a visit to the local riding stable where I used to take my lessons every week...they have immaculately kept pastures and I think I can get some good tips from them.
Today I've also started kind of a "scrapbook" for the weeds/various plants of our new pastures. I think it'd be a good idea to have proof of what each plant looks like up close, if I have it dried and pressed into a book with its name marked, if it's poisonous, and how to remove it. I'll be keeping myself busy with that for the next few days.
Thank you again!