Lucid Nagual wrote a horse pooping system, which included getting your shoes mucky, picking up poop and throwing it at people, and honestly, I thought it could use a serious overhaul.
new BaseCreature: FarmCow - this is just like your ordinary cow, but it produces dung on a timer (currently set to every 7 minutes, with first spawn occurring after 1 minute.) Lucid's system used wild horses, but I figure, if you're going to use this as fertilizer, it makes more sense to just get it from your cows around the farm.
new Item: DungShovel - when you double-click it, you can target a dung pile, then target an empty dung bucket to fill it up.
new Items: EmptyDungBucket - and FullDungBucket. When you double-click the full one, you can target some nearby Hay and it will produce a bucket of Fertilizer.
new Item: BucketOfFertilizer - Surprisingly, this little beauty does...yep, absolutely nothing. If you want this to be worth anything, you can add it to your farming system, or you can change the SBInfo on your Farmers so that they will buy it.
First you will need to modify your farm spawners so they produce the FarmCow type instead of the Cow. After 1 minute you will see the first fruits of your labors.
Next, you will need to make sure one or more of your vendors supplies an EmptyDungBucket and/or DungShovel. You could also just make them spawn laying around near a farm or in a house or barn somewhere.