While the term “road apples” may conjure up a pleasant image of apple trees growing by the side of a bucolic country road, the truth is in fact much uglier. This term is used in regional American slang to refer to horse manure, a substance which is only distantly related to apples. Written evidence suggests that people started referring to horse dung as road apples around the mid 20th century.
The term references the fact that the manure of a healthy horse does look sort of like a small apple, in the sense that it is round. When horses were used as a major mode of transportation, they deposited their manure along the way, and one could be forgiven for mistaking a pile of horse poop for a pile of apples when glancing at it from a distance. At one time, horse droppings littered many roads around the world; in some countries today, horses are required to wear special bags to catch their droppings when they are out on the road, to prevent messes.
Now that you know what road apples really are, it might help to know about some of their uses. Horse manure is a great material for mulching and fertilizing, and it can also be used as fuel or as a source of building material. Horse manure has historically been mixed with mud and clay to make adobe, for example, since it is basically perfectly shredded straw.
When road apples were allowed to fall where they might, some people made a profession of collecting them for their various uses. Many cities promoted this, allowing people to profit while also clearing the roads to make them more pleasant for everyone. In areas where there was a surplus for manure or no immediate use for it, things could get rather disgusting. The streets of 19th century New York City, for example, were sometimes thick with horse manure, among numerous other unsavory substances, making travel a perilous proposition.
The term “road apples” has also been used to refer to actors on tour. In this sense, it is presumably a reference to the manure generated by the actor's horses as he or she travels on the road, although it could also be a disparaging comment about acting ability.
You may spot road apples now and then, especially if you live in a rural area. Many riders take shortcuts across minimally trafficked roads, and their horses may on occasion leave a deposit along the way. Road apples can also be seen on heavily trafficked riding trails and paths, and they are of course a ubiquitous feature of the stable and barnyard.