you have to find all the cowgirls and get them to come to the farm, then Bell asks you if you want to invite bulls; as for how to find them:
one will be at the bar in the slums, after you buy all her stuff and sleep she's at the strip club, after you sleep again she will be in the top middle of slums (where the hatch leads); the other cowgirls are one in front of the science building in the city and one in Transylvania before the graveyard gate (after you beat the Manor quest); the bulls are one in the gym and one in the nightclub bathroom in the middle left of the city and one is in the capsule hotel in the underground mall