Ah,ah,ah,ah,
Colorado law.
Says that if a fence is necessary to be built on a property line that each party has to pay their half of it, and have to maintian their half of it. So, yes, you are right on both counts, the law would say that a fence had to be built, and you would have to pay for half of building and maintaining it.
I think you’re looking at this quite nicely. I buy the buffs and you have to pay half to fence them out while I would have to pay for the other half of building and maintaining it.
It’s nice when neighbors step up and pay half the fencing costs of their ranching neighbor. In Texas, I had to pay all the costs.