We've had the cows in the alfalfa/grass field for some time, but they were trimming it a little shorter than we liked. We have both hayfields electric-fenced. The way we keep the horses out is to string an electric wire up high enough for the cows to get under, but low enough that the horses would get shocked if they tried to go in. This has always worked great, with the exception of one of Bonnie's calves, who refused to ever go under the electric fence. While his mother feasted on the good stuff, he picked at bluegrass on the outside.
I asked Cliff if he had any idea why they will enter the other pasture but not this one and he had an excellent answer: The opening to the alfalfa (now closed) has fence or panels on either side, so it doesn't feel to the cows as though they are walking right through an electric fence.