ARC-CO Improvement Act This bill amends the Agricultural Act of 2014 to authorize the Department of Agriculture (USDA) to make certain adjustments to county yields used to calculate actual crop revenue under the Agriculture Risk Coverage-County Level program (ARC-CO) to reduce disparities between neighboring and nearby counties with similar soils and climate that cannot be explained by a localized weather event. (Under ARC-CO, farmers receive payments when the actual county crop revenue of a covered commodity is less than the ARC-CO guarantee for the covered commodity.) In the case of county coverage, USDA must: (1) prioritize the use of yields from sources that provide the greatest geographic coverage of county-level data from the same source; and (2) use the yield, benchmark revenue, and payment rate based on the county of the physical location of the farm. With respect to a farm that crosses county boundaries, USDA must: (1) split the base acres on a pro rata basis based on the relative quantity of cropland in each county, and (2) calculate any payments on the same pro rata basis.