By Joshua Bardwell – originally posted in September 2009 at Jack-Booted Liberal.
In my previous post on composting pig manure, I discussed the type of composting system Issa and I plan to use. A fundamental question for us in choosing a system was how much manure we could expect our forthcoming pigs to make. A system based on 55 gallon drums would be a hassle if the pigs could fill them in too short a time.
Issa found one site that stated that a 200 lb pig would produce about 13 lbs of manure a day. That’s all well and good, but unless you know the density of pig manure, it doesn’t tell you the volume you’ll need to contain it.
I found a site that stated that pigs would produce between 0.5 and 0.75 cubic feet of manure a day, per 1000 lbs of pig. For a 200 lb pig (close to market weight), that’s 0.1 to 0.15 cubic feet of manure. That gives us a density of about 87 to 130 lbs per cubic foot, or an average density of about 108 lbs per cubic foot.
A 55 gallon drum is about 7.3 cubic feet. If a 200 lb pig produces 0.1 to 0.15 cubic feet of manure a day, it will fill a 55 gallon drum in between 73 and 49 days. We plan to keep two pigs, so cut those numbers in half: 36.5 and 24.5 days. Granted, those numbers are for full-weight pigs. The pigs will produce less manure when they are smaller.
A 4-H site I found said that a healthy pig will gain approximately 1.6 lbs per day. According to that site, 50 lbs is a typical starter weight. Market weight is between 200 and 250 lbs. Based on the previously-given numbers, here’s a graph of the estimated total manure production of a pig from 50 lbs to 250 lbs:
The red line is the high estimate, based on 0.75 cubic feet of manure per 1000 lbs of pig, while the blue line is the low estimate, based on 0.5 cubic feet of manure per 1000 lbs of pig. You can see we end up with approximately 9 to 14 cubic feet of manure produced per pig. (For perspective, again, a 55 gallon drum is about 7.3 cubic feet.) This is estimated to weigh approximately 972 to 1512 lbs.
Here’s where things get a bit fuzzy. That manure is going to shrink down when it composts. A typical ratio given for compost shrinkage is 50% volume. That means we can expect to end up with approximately 4.5 to 7 cubic feet of compost, solely from pig manure. Bulk finished compost is estimated to weigh in the ball park of 800 lbs per cubic yard, or 30 lbs per cubic foot. That means our final compost will weigh only 135 to 210 lbs! That’s a heck of a lot of matter that left the system!
Realistically, though, the pig manure will not be the only input to the system. Pig manure has a high nitrogen content relative to carbon. This means that an appropriate quantity of high-carbon material must be added out to balance out the ratio in the final compost. The actual amount of material depends on the type of material added, and a consideration of this factor is beyond the scope of this post. The bottom line is that there’s going to be a bit more compost than the pig manure alone would produce.
The final question, then, is whether the two pigs we plan to keep will provide enough compost to fertilize the garden plot we plan to keep. I have estimated that the garden will start out at about 350 square feet and will probably grow from there. A common guideline is to till 1″ of compost over your entire garden. Based on this, a 350 square foot plot will require just about exactly 1 cubic foot of compost. Yeah, looks like we’re good to go, with compost to spare!
I have some water jugs that are almost exactly 1 cubic square foot. It’s hard to imagine that small amount of compost going over 350 square feet! Maybe it’s more compost than it looks like, though. After all, it’s certainly more water than it looks like!
Actual results will, of course, vary, but it’s still a fun thought exercise.
Editor’s Note: On the original post, a commenter said, “I think you made a miscalculation–I went to an online mulch calculator and it indicates that for 350 square feet you need 1.1 cubic YARDS not feet. That means you’ll need 27 cubic feet which means you’ll actually be a bit short on compost,” to which Joshua replied, “It’s true, as I discovered when I went to buy compost for the garden this year.”
For more information on raising pigs, these two books are both excellent sources – How to Raise Pigs and Storey’s Guide to Raising Pigs. I used both of them when I was getting started. Whether you’re raising pigs for pets, getting started raising a 4H pig, or going purely for yummy pork, either of these books will help you out in getting started.