Vary by site -- there would be a great difference between a free stall and open lot dairy. I don't think it would be appropriate to pick a number out of the sky.
Roofs, pavement, sidewalks, etc. are impervious. if you're trying to come up with a Curve Number for the drainage area, you calculate a Weighted Curve Number based on the Impervious areas (typically CN 98) and the Hydrologic Soil Group (A B C or D) of the soils, along with the ground cover type (bare ground, grass, woods, etc) and condition (poor, fair, good, etc.)
See instructions for TR-55 or TR-20 for calculating Weighted Curve Number.
A CAFO should be analyzed very carefully. If it is covered that is one consideration, but open feed lots are not that pervious in terms of runoff. Highly compacted soils will be everywhere and with already high saturation rates it can be very deceptive. I would treat the whole area like a very impremeable surface and be as detailed as possible. Most CAFOs have a lot of wash off, drain off systems in place so make sure you undersand how and where those are going.
