I always calculate slope on farms i review with 2' contour lines. Some fields are difficult since i need the average slope, but the field is rolling hills. I usally take several slopes and take the average. I want to have a way to do this in ArcMap 10. Most farms have 6 - 10 fields and every field needs its individual slope (in degree) and slope length.
I assume you are using DEMs of the area. I would start by obtaining or creating an outline shapefile of each farm field in question. Use Slope to calculate the slope of the entire farm. Then use Zonal Statistics (or Zonal Statistics as Table) with MEAN as the statistic type, using the field outline shapes as the in_zone_data. Each field will need a unique identifier. This will calculate the average slope for each given area.