What are the montana laws pertaining to fence boundary placement in agricultural areas?

i own a 50 acre parcel next to an 80 acre parcel. the neighbors have split thier property in to two 40 acre pieces and are building a new boundary fence. their new placement is going to be on my side of the existing fence by about a foot. this would not matter to me except i will no longer be able to take my 4 wheeler between the fence and my irrigation mainline to irrigate. what can i do to dispute this? i have been told that if there is an existing boundary that has been undisputed for so many years then that is where it is supposed to be. is this true?