As stated previously, you need to make the disconnect at BOTH ends. You don't want someone in the future to be energizing those lines again from the other end, even if they are disconnected on your end.

I am also questioning if in fact the cable is actually in "Conduit" the entire 600' to the other barn? Often, (more often than not) this cable is direct burial and the conduit you see above ground on both ends only goes a couple feet down below surface and then ends. If this is the case, you will play H**L trying to "pull" this cable. Gonna need a BIGGER truck to pull that one!