The popularity of leisure horses has steadily increased. In many areas, the available acreage for pasture is inadequate for year-round grazing. This leads to overgrazing and poor performance and aesthetic appearance of your land, as well the problem of weed encroachment. Horses are natural grazers, but most pleasure horses do not need 24/7 access to high-quality pasture forage. The trade-off for many landowners is to create some good pastures and a small “sacrifice” area where horses are fed, watered, and sheltered and where they can exercise or “loaf.” Once or twice a day, they can be turned out on pasture for exercise and grazing, but the bulk of their nutrition comes in the form of hay, which is basically provided year round. There are many excellent resources for pasture management, rotational grazing, weed control, and other topics. Specifically for noxious weeds, we recommend that small acreage landowners buy hay that has been field-inspected and certified to be “weed seed free.” Weed-free forages and feeds are now required for horses and livestock that are used in the backcountry on public lands in many states. This hay will ensure that you do not have recurring infestations of noxious and many common weeds on your property. http://agr.state.mt.us/weedpest/nwsff.asp
http://animalrangeextension.montana.edu/Articles/Forage/Fall/horses-weeds.htm