There are a couple of expansions I think could help, as well as some strategery. First, expansions:
The River II tends to at least break the board up into 2 or 3
big farms, if not lots of smaller ones.
Abbey & Mayor adds a Barn piece that can really change the way
fields are played.
I think the dragon from The Princess & the Dragon might play in
here as well, but I haven't ever actually played that expansion, and
it doesn't appeal to me generally.
Inns & Cathedrals can really beef up city and road scoring,
rendering farms less important.
Other expansions could also help simply by adding tiles. Since more players means fewer tiles per player, adding more tiles can hold off the farm rush a bit. Adding other scoring mechanisms also helps de-emphasize farms.
Now for strategy:
Push for big cities. In general, my friends and I tend to fight over big cities, with some scoring over 30 points. Cities score more points this way than as field consumers, so if you exclusively focused on this while your friends exclusively focused on fields, you'd win every time. In reality, putting meeples on fields too early in the game tends to cripple players from playing on cities and cloisters, so take advantage and "share" their cities once they are out of meeples to fight back.
Make little fields pockets. If you spot an area where there are going to be three or four cities, try to isolate it and own it with just one farmer. This happens all the time in games I play, and is really annoying to be left out of.
Toward the end of the game, it's natural for everybody to start going for the big fields, but picking the right moment to really start pushing for this is part of strategy, so just enjoy it. If you are playing with 4 or 5 players, you have many fewer tiles per player than with 2 or 3, so farm rushing happens earlier. One possible way to take advantage is to just try to share in lots of features, rather than winning all of them. If you share every other players big cities or farms, you will do better than if you try to hoard the features to yourself. Obviously this is not true in a 2-player game, and not as true in a 3-player game.