FrontierVille added something new for you to collect in the game -- manure. You can find all of the, uh, varietals while harvesting animals. Once you've completed the collection, you get a single Saltpeter for future use.
The five poo varietals (and how to find them) include:
Poultry Puddin' (harvested from chickens, geese)
Piggy Poop (harvested from pigs)
Brush Brownie (harvested from sheep, goats)
Prarie Pile (harvested from mules, horses)
Meadow Muffin (harvested from cows, oxen)
Once you trade these in you'll get a Saltpeter (you'll need this to make fireworks!), which looks like this: