Half an acre may seem like a lot of space but if there are 30 hens on there constantly, month on month, year on year, then the ground will become "chicken sick" over time and they will reinfect themselves very quickly after worming if not given a change of land to break the parasite reproduction cycle. If this is the only land available to you it might be worth considering dividing it in half and moving the hens from one half to the other every 3 months.