Ugh. My boy did the same today. The easy quick and long term solution is to just roach (shave/cut) their mane. I LIKE my horse's mane though, so step 1 was to pick out any/all loose burs I could with my fingers, and step 2 was to use some sort of product (show sheen, mineral or coconut oil, really it's whatever you like) generously and slowly and painstakingly comb them out from the bottom up. Once it's bur-free, you can put a neck-hood or sleazy on, braid it, or just wait for them to go roll around in the burrs again and start all over. Sigh.
we had a horse come here to be trained that had his mane/tail totally filled and balled up with burs! I used baby oil and alot of patience LOL I hand picked them all out over a matter of a week! I even found a piece of barb wire in his tail I had to cut the wire to get it out in pieces......
I just did this yesterday for my fiance's horse...I didn't use anything except my fingers, normally I have a brush & Mane N Tail detangler handy but yesterday we were hauling cows back home from the mountains so on a few minutes of downtime I decided to fix her hot mess of a mane lol I pull the burrs apart and they usually crumble into small pieces that can be easily brushed/combed out
thanks.. BABY Oil and Textured Garden Gloves did it. Unreal how great it worked. His front Bangs were one big Dreadlock with hundreds of them.. woven and knotted. Baby oiled and started at the bottom and they just started sliding out.. Took me 10 minutes to clear it. Spend over an hour last week just working on his mane and it wasnt near what this was. Thanks again.