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.

Thinking about animal production and sustainability

Last week I had the opportunity to give some freshman agricultural engineering students a tour of a beef feedlot near Ames. I enjoy doing this as I often get lots of interesting questions and discussion about animal production. There is great variety in the student’s agricultural backgrounds. Some grew up on animal farms while others have never seen an animal production facility before; this adds a degree of difficulty to discussing the topic, as you have to keep both audiences interested and engaged.

This means it’s important to cover the basics, i.e., what are we trying to accomplish at these farms, what are potential issues we might have, and what are the basic principles behind what we are doing. We are providing the animals with food, water, and protection to keep them healthy, happy, and growing quickly to make a product… meat, eggs, milk, or fiber. This makes sense as a first cut approach to thinking about the farm and discussing about design for animal needs, but to me the real excitement is a discussion on nutrient cycling at animal farms, specifically talking about the manure.

Nutrients are constantly cycling on farms and how we manage these cycles controls how effective we are at achieving our production goals while limiting environmental impact (of course mother nature plays a pretty big role in the nutrient cycling as well). At animal production facilities our goal is to take nutrients (in the form of animal feed ingredients) and change them into a more valuable form, specifically we are trying to convert them into tasty proteins. What we really want is to get as many of those nutrients as possible incorporated into our animals and then to sell them, unfortunately, only about 10-30% of the nitrogen and phosphorus we feed to animals ends up in the end products (meat, milk, or eggs). The remaining portion of these nutrients is excreted in manure.

Because of these relatively low protein conversion efficiencies (10-30%) many scientific arguments suggest that we need to eat less meat to promote sustainability. However, what many of these arguments fail to consider is that recycling of animal manure in integrated farms (farms with both crop and animal production) keeps a substantial share of the N, P, and K excreted by the animals circulating within agro-ecosystems, and in doings, means those proteins can still be used. This means our actual protein conversion efficiency can be substantially higher than those calculated just by calculating what percent of the nitrogen fed to an animal ends up in its body.

For example if I was to make a guess for swine farms in Iowa, I’d say about 20% of the N we feed ends up in the pig, 15% ends up lost as ammonia volatilization during manure storage, and the remaining 65% gets land applied to serve as a crop fertilizer for next year’s crop. This means that we are really looking at a protein conversion efficiency a lot closer to 85% that the 10-30% we might originally suggest. Of course, it gets a little more complicated as the crop will use not all of the nitrogen we apply to our field; some will be lost during the growing season. As a first guess I’d say about 20% of the applied N will be lost (though it depends on the production practices, crop rotation, soil and weather conditions during the growing season, as well as numerous other factors), but even including this in the calculation we still would estimate a conversion efficiency of around 75%.

Getting our students to start think about these issues, understanding the challanges facing production agricultural, and giving them an opportunity to get their hands a little dirty are crucial steps in continuing to move agriculture forward and getting the next generation of Agvocates ready to raise to the challenges ahead.

How do you determine N, P, & K needs for your farm?

 This may seem like a very basic question, and you are probably thinking I've been doing this for years, I think I have this part of the equation figured out (or perhaps the more you've learned the less certain you are on this part). In either case, I encourage you to keep reading, because while I agree this topic is fundamental it is anything but basic.

When I think about this topic there are two concepts that immediately spring (season joke for those of you counting at home) to mind: mass balance & dose-response. You might have different names for them, but since I’m trained as an engineer I've had them ingrained in me. However, the important point isn't what we call them, but what they tell us and what they mean.

So what is the “mass balance” approach? In the mass balance approach we often use and equation like:

In – Out = Change in Storage

There are a few ways to conceptualize this, but to me the easiest analogy was always the haymow (if I grew up on a swine farm it might have been a gain bin, or if I was a small child growing up on the farm today perhaps I’d think of a commodity shed). For those of you not versed in hay making the haymow refers to that place where you store hay, particularly the upper part of the barn (alternatively called hay loft). So how does this analogy work? Well if “ins” are greater than “outs” we’d be accumulating more hay in the barn. This would happen every summer, we’d make hay and stack it in the barn. We’d still feed some hay every night, but thankfully we were making it at a faster rate than we were using it. Come winter time the opposite would occur, we’d be making zero hay (since the world was frozen and snow covered), but we’d still be feeding some to the cows every day. Clearly the goal was to build up a big enough surplus in the summer so that our supply would last us until the next in-flow event (harvest).

So, what’s the analogy to this in nutrient management? I’m going to talk about it from the perspective of phosphorus, but it would work for any other nutrient as well. So in our hay example I used the “mow” as the place where we might have a change in storage, in the our agricultural system this will be our field, specifically from just above the soil surface to the depth our crop roots (I tend to picture about 4 feet deep, but it depends on the crop – Christmas trees root vastly differently than corn, just like alfalfa might root drastically different than soybean). We also have ins and outs, i.e., phosphorus is added to and removed from our field. Additions of phosphorus could include adding manure an putting on commercial phosphorus fertilizers (MAP, DAP, triple phosphate). Outs are things like the removal of phosphorus in the grain and crop residues we harvest, the loss of soil particles and attached phosphorus with erosion, or any dissolved phosphorus that flows from the field in surface runoff (this could also include phosphorus that moves vertically below the root zone of our crop as water leaches through the soil, but usually this is a really small amount in Iowa. A common management strategy would then be to try to balance the inflows and outflows of phosphorus from our field so that our soil stays like it was. That is we keep our inputs and outputs in balance so that we are neither gaining or losing phosphorus and hopefully our soil productivity remains the same.

An alternative way to view this is through the concept of dose-response. Dose-response refers to the relationship between the amount of a substance that an organism receives (the dose) and the effects on the organism (the response). In this approach the idea is to figure out how much response you might get from applying some dose. To get an idea of this one let’s think about the “do your chores” analogy. A parent might offer some economic incentive (an allowance) to try to achieve a desired response (for example unloading clean dishes from the dishwasher). In this case the parent would try to pick the lowest amount that would motivate their child to perform the task. The parent could of course pay more than this amount, but they don't receive any additional benefit from it.

In an agricultural system this would be akin to developing a curve that would predict the relative increase in yield that would be achieved by adding a certain amount of some nutrient. Again it will work on a variety of nutrients, but the one I’m going to pick as an example is nitrogen (because the work has all been done for me – see the maximum return to nitrogen tool @ http://extension.agron.iastate.edu/soilfertility/nrate.aspx). I’ve given an example of what this curve looks like for Iowa (at least on average) for corn following soybean. Just like the parenting analogy, our goal here would be the same – to provide just enough of what we are dosing (nitrogen addition) to maximize our response (I’m showing yield in the graph below, but we are doing it to maximize profit from our acre of crop), so supplying just enough nitrogen so that the next bushel of corn the extra nitrogen we add to our yield, will just pay for the costs of putting that little bit more fertilizer on.

So where does that leave us? Well, I’ve introduced two concepts for you to think about here: The mass balance approach and the dose-response approach. In the next couple posts I'll try to give more detail about both for the cases of nitrogen, phosphorus, and maybe even potassium. I won’t promise any answers about which one is right for your farm, but hopefully it will get you thinking about it in a new way and I’ll give you some insight into my thoughts.

PS - If you are already itching for more information on this topic I actually introduced a comparison between the mass balance approach and the dose-response in the blog post Iowa manure management plans: Yield goal versus maximum return to nitrogen (which you can find here: http://themanurescoop.blogspot.com/2015/01/iowa-manure-management-plans-yield-goal.html.