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.

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.