Why not just move all the horses to NYC? The people there would never even notice the smell.
Oddly enough, NYC's magnificently robust water supply system was built in the 19th Century to accommodate street cleaning. Every morning fire hydrants where opened to assist street sweepers in brushing all the horse manure into the storm sewers and out into the East and Hudson rivers, which washed it out to sea. Even the Twentieth Century's worst politicians couldn't destroy it.