Concrete Lot Slope

I need to replace the concrete where we winter feed our cows and would like to know what degree slope can I get by with without being being too steep for the cattle to slip.  I'm replacing it because it is almost flat and holds water instead of it draining.  Also wanting to construct a manure holding area to push the manure and keep the concrete clean when spreading cannot be done.  Any ideas of what you have or seen?

I helped build some feedlot pens that had 4% slope on the bunk aprons.