The NFO (National Farmer's Organization?) tried to unionize farmers in the 1960's. It made headlines, but it didn't have much impact.
Few farms sell directly to consumers. Most farms sell raw inputs that are widely available from so many sources they are considered commodities.
To actually start to control prices you need to get down to eight or fewer suppliers so it's easier to cooperate i.e.:
air lines (still consolidating),
meat packing companies,
and grain companies.