Bigger Pigs: More Manure and Impact on Facility Design

I was sent a question today that asked “why are more swine farmers hauling in spring and fall, why is this occurring, and what is a good number to use for sizing your manure storage?” So a nice multi-part question.

This is a great question for many reasons. At its surface, it seems easy, but I enjoyed it because it made me critically think about data I look at all the time. In particular, he had a follow-up question that said, how is this impacted by bigger finish weights (like 285 to 315 lbs.) This made me think of a blog article I read from Mike Brumm on the “Impact of heavier Sale Weights on Facilities.” In this post, he talked about space requirements for bigger pigs (no surprise bigger pigs grow better with more space) but didn’t touch on the manure topic, so what does bigger pigs mean for manure design (note – I’ll be thinking about mostly deep-pits, but the same ideas work for other storages).

So let’s start with some facts.
1. A recent survey of swine producers around the Midwestern US (lots of Iowa farms) conducted by the University of Minnesota suggested that about 50% of our farmers were applying once per year and 50% were hauling twice per year.
2. Increasing fiber in the diet (an example being DDGS) increases fecal output from the pig. An inclusion rate of 30-40% in DDGS increase fecal output by 5-10% .
3. Switching from nipple or cup waters reduces water consumption and potentially excess water reaching the manure storage, probably by roughly 5-10%.
4. Pig finish weights have tended to be increasing with 280 to 300 lb finish weights becoming more common.

So, that’s about it for cold, hard facts… what’s it mean and what else do we have to work with. So we know more farmers are pumping more often; is this because they have to because of storage constraints, a desires to apply more manure in the spring closer to planting time, issues with fall weather and getting the manure applied, or some combination of all three. From the survey we can’t tell, but I tend to think it all these factors coming together.

We also have competing factors for DDGS inclusion in the diet and improvements in watering systems to reduce consumption, I’m going to call them a wash, but at some farms they certainly could be making a difference. Finally, we know that pigs are getting and it seems intuitive that bigger pigs will make more manure per day.

So how do we typically estimate manure production from pigs, i.e., what’s your number? Well, in these my old go-to references on this have been the ASABE Manure Production and Characteristics standard, Midwest Plan Services (I use the Livestock Waste Facilities Handbook), or an extension publication from your local land grant university, but do these numbers still apply with bigger pigs?

Some colleagues and I recently had a project were we collected manure samples from swine manure deep-pits at right around 60 farms for a little over a year. As part of this study every month we measured the depth of manure in the storage pit, which allowed us to calculate manure accumulation (or production since we were catching all the manure) rates. There were a variety of diets being fed, the pigs were at various growth stages, some barns were wean-to-finish, some were grow-to-finish, and they had some different types of watering systems… in short, they were real production barns.

We found that on average manure was accumulating at a rate of 1.3 gallons of manure per pig per day (including any washwater used on the site). This was only a little above the ASABE manure production standard rate which suggests about 1.2 gallons per pig per day, so it seems reasonable. However, we saw lots of variation between barns, though they average 1.3 gallons per pig per day, the standard deviation was 0.4 gallons per pig per day. This means there is a lot of variation between barns, and in particular barns have a 25% chance of making more than 1.5 gallons per pig per day.

Now for the fun part, how will bigger pigs impact manure production? You’d think there would be lots of data floating around about how manure production changes with pig size, but it turns out that’s not typically how we collect the data. Its normally collected as amount of manure produced over a finishing cycle. So to get an estimate of manure production I used a couple of techniques. The first was looking back at some old manure production standards we use to provide an estimate of manure production per 1000 lbs of animal mass; using this information we can estimate the manure production rate for lots of different pig sizes. For the second approach, I used a phone-a-friend and to Brian Kerr, an animal scientist from USDA ARS who is my go to contact for swine diet stuff. He used some feeding trial data and the MWPS data and prorated it so that manure production didn’t increase quite as quickly in larger pigs as he thought the constant method over-predicted excretion from lager pigs. So these two methods are shown in the figure below. We expect actual manure production to fall somewhere between those two lines.

Figure 1. Swine manure production as a function of body weight.

Alright, now the fun part, to actually use this we need a swine growth model. I picked one I thought looked about right (shown below) and started with some math. Remember, what we wanted to know was how larger finish weights were impacting our manure number. What I’m showing in figure 3, is the number I’d pig for different finish weights. What this data suggests is that be switching from 250 lb. finish weights to 300 lb. finish weights would increase average manure production during the finish cycle by 13-17%, a pretty sizable increase. However, the actually increase in manure production is larger than this, by raising pigs to 300 pounds instead of 250 pounds would increase manure production by around 16 to 21%.

Figure 2. Body weight as a function of days on feed.

Figure 3. Average manure production rate based on finish weight.

 So where does this leave us? So for a grow-finish operation finishing at 285 lbs my average manure production number is right around 1.1 gallons per head per day, at 300 lbs it is right around 1.15 gallons per head per day. At this time I still use 1.2 gallons per head per day. For a wean finish operation I'd bring that number down to about 1.0 gallons per head per day. The truth is estimating actual manure amounts gets a little more complicated - do you double stock during the wean phase? When we start selling from barn how quickly is it empty? Is there down time between turns? Beyond that it seems like bigger pigs should lead to a bit more manure, but water and feeder systems and barn washing techniques probably still have as big of impact on manure amount variability that it is hard to see the impact of bigger pigs.