A Survey of Residential Nutrient Behavior. World Almanac data on total US cars. American Horse Council 1996 estimate of horses in the US was 6.9 million about half of which are in racing or showing and do not leave home. This puts about one quarter of horses potentially on the trails. Winters are difficult to ride on trails, so most riding occurs between April and November.
Many more people use trails than horses.
For example, in San Mateo County, the human population is nearly 700,000 compared to 4,000 horses. While all citizens and all horses may not be trail users, the horse subset that uses trails is probably very small. We believe that such small numbers of horses on urban trails and the brief time spent on them constitutes very little true environmental risk in terms of volume or contents of horse excrement.