Good advice, Tyler! I think I will try to use the natural features of their pasture to achieve my goals, instead of investing in expensive and time-consuming fencing - especially since it's not my property. The pasture is in a pretty wild state with bushes and trees throughout the pasture. The terrain is varied with areas of poor, rocky soil, shady trees, sparse grass, pines and bushes, so not the typical pasture that most of us picture as an ideal home for our horses. However, this rugged set-up is actually a more natural environment for horses, versus a "well managed" field of lush green grass, and I want to keep these wonderful natural elements in their pasture. I spent all day yesterday cutting away low hanging branches and the horses now have a clear walkway around the perimeter of their pasture, along with other trails that connect to various clearings or shady spots in the pasture. I am going to set up a sort of "Paddock Paradise" system in the pasture. This system was created by Jaime Jackson, a barefoot trimmer in Southern California who spent time observing the behaviors of wild horses on BLM land. Hay is spread along "tracks" throughout the pasture, which encourages the horses to walk around and "forage" for food. This simulates their natural foraging behavior and is a much healthier way to feed horses, as their stomachs are designed to accommodate small and frequent meals, versus a couple scheduled meal times a day. It also encourages the horse to move, which keeps the horse conditioned, builds better hooves and provides mental stimulation for the other 20 hours of the day, when I'm not there to feed or work the horses.
I am considering hanging a hay bag right in the middle of a big patch of the horehound to see what happens. The plants haven't flowered yet, so they don't have a bunch of stickers at this point, and I'm wondering if I could employ my horses to trample the horehound as they eat! However, I'm not totally dead set on eradicating the horehound just yet. I moved into this pasture just a week ago, and I want to see what kind of role the horehound plays before I come in, brim full of hubris, trying to change everything around. The horehound is only a nuisance for me, the human, who has to pick burrs out of the horses' manes and tails. The horses don't seem to mind it and some of the other plants seem to like to grow near it because they stabilize the soil with their roots. The big patch of horehound in their pasture grows under the cover of a giant oak tree on the downward side of a gentle slope. Beyond the cover of the oak, there are other plants that prefer more sun that bees also happen to love - maybe if I could transplant these around the perimeter of the horehound patch, or otherwise encourage them to grow there, they will provide a type of barrier that prevents the horses from getting into the horehound.
Since I've only been there a week, I've yet to see how the weather and seasons change the pasture, but I think it will take some observation and reflection to develop a good pasture management plan. It's a fun and interesting experiment, and I'm excited to see what happens!