One key feature of a composting system is a way to use or to dispose of the compost. Some people find that they can give away or even sell compost to local gardeners and farmers. My plan from the beginning was to spread it on my own pasture because the whole point of composting for me was self-sufficiency.
When to spread:
You can spread your aerated compost safely as soon as it has heated up enough to kill weed seeds and parasites (monitor your temps so you know when this happens!). The longer it is able to "cure," the better it will be; it will start to look less like manure and stall waste and more like uniform, dark, crumbly mulch. I never really have time to let this happen so I spread one bin as soon as the second fills up and I need the first available again.
Because the compost gets so hot, in theory there's no reason you can't spread it on pasture that your horses are currently grazing (unlike fresh, un-composted manure, which can present health risks). Personally, I still prefer to spread it in sections of the pasture that are currently resting. (Although I only have 3 acres of pasture, it's divided into three sections for rotational grazing; see more about cross-fencing here and the benefits of rotation here). I try to spread compost on a section that I have just started resting, so it has some time to work into the soil before the horses are back on it.
Keep in mind that it's bad for the environment (and wasteful of your precious compost) to spread before heavy rains or snowmelt, or when the ground is frozen. The compost will run off and the nutrients will end up in the nearest lake or river, fertilizing algae blooms instead of your grass. Winter spreading is even against the law in some places! (Penn State offers some winter spreading best practices that you might want to review.) So, if you are going to compost find out what your local regulations are and come up with a plan for the winter (like giving it away or stockpiling it until spring).
How to spread (equipment):
Really the only efficient way to spread large quantities of compost is a manure spreader. If you have a lot of time and/or patience and/or muscles, you might devise some other method like forking it out of the bed of a pickup truck or filling your tractor bucket 1,000 times, but I don't...so I bought a manure spreader.
There are two basic types of dry manure spreaders, ground-driven and PTO-driven. Ground-driven basically means that the rotation of the wheels powers the spreading action. PTO is powered by your tractor's engine. A quick Google search will reveal that people have lots of opinions on which is best. I initially leaned towards ground-driven because they're somewhat less expensive and seem to have less mechanical complexity (I thought they might be less likely to break/require service). However, with a PTO spreader you can control the spreading rate better and ground-driven spreaders may tear up the ground more or take more passes to empty. You can also empty a PTO spreader at a standstill (for example, if you want/need to stockpile your compost before spreading). Ultimately I decided to go with a PTO spreader.
My original plan was to buy a used spreader. There are a ton of them out there that you can find at local dealers or online (try https://www.tractorhouse.com). Unfortunately, I hit a snag. Most of the used manure spreaders out there are on the large side, and my 23-horsepower Kubota didn't have the power for them. I looked high and low for several months but could not find a used spreader small enough for my tractor to pull and yet large enough to handle my compost in a reasonable number of loads. Ultimately, I had to give up and buy a new spreader, which made the whole composting project much more expensive than I had planned (I had already bought the O2 Compost system and built the bins at this point, so there was no turning back).
I chose a 50-cubic-foot PTO-drive ABI, which was delivered to my farm on a flatbed truck. It was all shiny and new and poop-free! (For about a week...) Here is it's inaugural load:
This spreader cost around $4,500 including delivery, which is much more than I had originally intended to spend. The (very old, as in 30+ years in some cases) used spreaders I saw locally were plentiful in the $1,000-2,000 range but sadly, I would need to upgrade my tractor. ABI's 25- to 65-cubic-foot spreaders range from about $3,000 to $5,000.
To fill the spreader, I park it near the bins and use the front loader to dump the compost in. At a certain point, my front loader can't reach anymore or I start interfering with the aeration pipes, so I have to start using a shovel. It's back-breaking work that is even less pleasant in the summer.
My ABI is easy to operate but doesn't spread super evenly. I'd say about 50-70% of the compost just drops out the end gate, directly behind the spreader. The rest is spun by the beaters and flies off to the sides.
The spreading process is easily the most labor-intensive phase of the composting process, but it can bring benefits to your land by returning nutrients to it.