Do Iowa Counties Have Sufficient Crop Land to Meet Manure Management Plans Requirements Without Exporting Manure?

Iowa is known for its significant agricultural production, including crops like corn and soybeans and livestock such as hogs, cattle, and eggs. However, given the size of the Iowa livestock industry, a common question becomes whether there is too much manure. In large part, manure from these livestock operations is often used as a nutrient source for crops, so people are asking two questions at its core. 1. Is there enough crop ground to utilize produced manure and 2. Are farmers taking credit for this manure and reducing fertilizer purchases?

Whether Iowa counties have sufficient cropland to utilize all the nutrients from manure without exporting it depends on various factors, including:

Livestock Density: Areas with high concentrations of livestock will generate more manure and, therefore, have more manure nutrients.

Cropland Availability: The amount of available cropland, its proximity to livestock operations, and the productivity and nutrient need determine the feasibility of using all manure locally.

Manure Management Practices: Effective manure management practices can maintain more nutrients in the manure, but they require more land to use the manure nutrients. Manure practices a farmer picks could be influenced by the amount of manure they need. With that said, we want to pick practices that conserve manure nutrients because they increase the circularity of agricultural systems.

Within this work, we will look at these concerns in several ways. The first two are at a state level. This work is built off Andersen and Pepple's (2017) A County-Level Assessment of Manure Nutrient Availability Relative to Crop Nutrient Capacity in Iowa: Spatial and Temporal Trends. Within that work, I defined algorithms for using Census of Agriculture data to estimate livestock populations and from this both manure nutrient excretion and available manure nutrients for land application, with the former being an estimate of what is excreted by the livestock, and the latter being an estimate manure nutrient recovered for land application and corrected per Iowa State suggestions for nutrient availability and application losses. I also estimated a state level of nutrient needs using the Census of Agriculture production statistics and the USDA Crop Nutrient Removal Database for evaluating the nutrient content of the harvested material.

A summary of nitrogen and phosphorus comparisons between manure (excreted and available for crop use) and crop capacity is provided in Figures 1 and 2. Crop capacity focuses on corn, corn silage, soybean (for phosphorus only), hay and haylage (phosphorus only), and small grain. It represents the amount of nutrients estimated to be harvested and removed, not the amount to support the crop. As such, it is a low estimate of nutrient requirement. Pastureland nutrient needs were not considered, though, for animals estimated to be on pasture (beef cows), only a fraction of the manure was estimated to be recovered, with the remaining being on pasture at approximately nutrient need. Manure production system, nutrient excretion, and availability were estimated based on production practices standard from 2000-2025. As a result, manure estimates earlier in history (predating approximately 1990) may not be as representative as animals may have spent more time on pasture (especially dairy cows), or other production styles (open lot pigs) may have been more prevalent. 

Figure 1.  Comparison of crop nitrogen need and both manure nitrogen excretion and manure nitrogen retained and available to support crop production.

Figure 2.  Comparison of crop phosphorus need and both manure phosphorus excretion and manure phosphorus estimated to be retained and available to support crop production.

Results indicate nitrogen excretion with livestock manures has returned to levels last seen in 1970. Today, a more significant amount of this nitrogen is estimated to be retained and available to help support crop production. In at least part, this represents a shift from cattle systems (which, given the open lot nature, generally had higher ammonia volatilization losses than current swine systems and lower nitrogen availability due to differences in ration). Manure phosphorus levels have also returned to levels seen in the 1950s-1970s. Available phosphorus mirrors excretion due to limited means of nutrient loss during storage.

Over the same period, crop nutrient needs have significantly increased due to significant crop yields per acre increase.

Overall, while Iowa has a significant amount of cropland and livestock, the balance between manure production and cropland capacity varies by region and depends on the specific practices employed by farmers and regulators to manage nutrient cycling effectively. To understand these results and contextualize them, I look at two other variables: the percent of N or P that is excreted, recovered, and available to be used as a crop fertilizer. In general, this estimate has been trending up and I currently estimate it at around 60% of N and 80% of P. These estimates are slightly low, as they aren’t crediting N and P deposited on pasture in grazing systems where the nutrients could be used. The other factor I look at is what percent of nutrient need is supplied by livestock manures. In 2022, this is about 38% of the N and 30% of the P. Again, this represents the crop removal rate, not the nutrients required to support crop nutrient production.

I estimate this is sufficient nitrogen to supply between 4.1 and 4.8 million acres of corn production (if all the manure was applied to the ground for corn production). In 2022, Iowa had about 12.9 million acres planted to corn, so manure should account for 32-38% of all nitrogen fertilizer use in Iowa. In the fall of 2021, the National Agricultural Statistics Service collected nitrogen fertilizer use for corn in the Agricultural Resource Management Survey. They reported 87% of Iowa corn acres received fertilizer (presumably, the other 13% were manure only). Manure should be about 1/3 of the nitrogen fertilizer use; 13% sounds too low. But many acres would get some of their fertility from manure and be supplemented with commercial fertilizer, so that data doesn’t tell the whole story. The survey estimated the total commercial N fertilizer applied to corn at 834,650 tons of N. I estimated 393,000 tons of N from manure. Based on these figures, manure was at 32% of the nitrogen fertilizer supplied by the state. For phosphorus, the ARM survey reported that 49% of corn acres received phosphorus fertilizer (presumably, the other 51% were either manure only or had high-testing soils that didn’t need additional P applied). In this case, the estimate is that there were 201,500 tons of P from fertilizer. I estimated 89,000 tons of P from manure, making manure about 30% of the P applied in the state, in agreement with the phosphorus budget proposed earlier.

I also like to look at this data on a county level. Again, I’ll be using my estimate of crop nutrient removal and comparing that against the amount of manure I estimate to be produced, retained, and available for crop production within that county. While the work assumes no manure is moved from one county to another and gives a low level of crop nutrient need, but still serves as a helpful indicator of nutrient budgets. In general, we see a continuation of the trends we’ve been seeing; some counties are getting more manure-rich, and others continue to get a smaller fraction of their nitrogen and phosphorus needs from manures. Again, this figure shouldn’t indicate whether we have sufficient land for manure but more an indicator of potential areas where giving a closer look makes sense. In particular, you could question plenty of assumptions – the percent of manure I’m collecting on cow-calf farms and the type of storage. While probably reasonable for the state, I think these assumptions might have outsized effects in this area. Specifically, the southern region of Iowa may be more likely to choose lagoon manure storage because of both location and associated lagoon performance (a warmer area of the state), and many of the more extensive swine facilities in this area may be related to gestation-farrowing operations or nursery farms.

Figure 3. County level comparisons of crop nitrogen removal with harvest (excluding legumes and hay) as compared to amount of manure estimated to be available to support crop production in the county. Example, the dark green counties indicate that less than 10% of the nitrogen removed in the harvested fraction of crops could be supplied by livestock manures.

Figure 4. County level comparisons of crop phosphorus removal with harvest as compared to amount of manure estimated to be available to support crop production in the county. Example, the dark green counties indicate that less than 10% of the phosphorus removed in the harvested fraction of crops could be supplied by livestock manures.

I also wanted to look at this another way: if we use my estimates of manure production and available for land application, would it be possible for all the manure within a county to be used within that county in compliance with manure management plans (and for ease I’m going to assume all manure would require manure management plans). To do this, I obtained corn, corn silage, soybean, and alfalfa acres from the USDA Census of Ag at the county level. I got hay production numbers from the Census of Ag and divided them by hay acres to get a yield value. I then used Appendix A from the Iowa Manure Management Plan form to get estimated yields of corn and estimated corn silage yield (Assumed 70% moisture and a harvest index of 0.5). The Nitrogen Use Factor for corn (weighted based on the county being considered), corn silage, and hay were obtained. Only three counties, Lyon, Washington, and Clark, couldn’t use all the manure produced on the corn acres available in their count, but each had sufficient acres if land was considered.

Several factors could be contributing to this:

1. I could be making flawed assumptions about the types of manure systems used (more lagoon systems instead of deep pits, for example).

2. Some manure is being applied to pasture land, which I didn’t consider in the analysis.

3. Some manure is going to hay or soybean.

These three considerations alone would take care of any issues. However, it could also be that some of the manure within these counties is exported from the county.

The final piece of the puzzle is understanding how farmers are valuing the manure and trying to breakdown the commercial fertilizer and manure at a county level. Unfortunately, at least for today I’m out of space, and as of yet, having trouble finding county level nitrogen fertilizer data.

People, Pigs, and Poop

 

Recently, there was a little exercise for how much swine poop there is in Iowa and turning it into a pyramid. In that exercise, they (Raygun – I didn’t fall out of my chair or roll my eyes. I was excited, conversations about manure are welcomed) calculated about 85 billion pounds of pig poop per year – I won’t dispute that number though I calculate a slightly higher amount. I’ll even add in cattle and poultry and estimate 85 million tons of livestock poop annually.

But how much human poop is there? Iowa has about 3.2 million people in it. A person poops about 175 grams per day or 0.38 lbs. This is about what was assumed when they compared humans to pigs, but unfortunately, that’s not the pig number they were using; the pig number includes urine and wash water, too. As it should, because, in the name of water quality and manure management, we would also manage that component. Humans generally make about 0.37 gallons of urine daily, so another 3 pounds of material. So, humans are up to 3.4 lbs of “manure” a day, not that 0.38 lbs.

I mean, if we are going to talk “poop,” we probably want an apples-to-apples, or a poop-to-poop comparison, don’t we? But here is where it gets complicated – for my livestock manure numbers, I include wash water volumes – because we manage it like manure, and we should view this wash water like manure. I’m glad we do. But does that mean for humans we should include our “wash water” as well? That would include the water you use when you flush a toilet, shower, or do dishes. In developed countries like the US, the average person generates about 80-100 gallons daily. Let’s go with 90 gallons a day, or 750 pounds a day. Extrapolate this to a year, and you get 438 million tons of human wastewater! Or about five times what we generate from livestock production.

So, let’s play the game of how much poop is there?

So, for a pig, we have about 10 pounds a day, about 10% of this is solids material, and I’m going to say that fecal material is about 50% moisture, so a pig excretes about 2 pounds of feces a day, 8 pounds of urine. Throw in wash water used at the site, and we are at 10.8 lb/day. So, what’s the comparison now?

Table 1. Comparison of human and pig related “manure” and wastewater generation.

	Human	Pig
Feces	0.38	2	lb/day
Poop (urine + feces)	3.4	10	lb/day
Wastewater	750	10.8	lb/day
Population	3,200,000	30,500,000	million
Feces	221,920	11,132,500	tons/yr
Poop (urine + feces)	1,985,600	55,662,500	tons/yr
Wastewater	438,000,000	60,115,500	tons/yr

 How we choose to manage wastewater greatly influences the question of what characteristics are important for me to know about that wastewater. Alternatively, the characteristics of the wastewater greatly affect how I’d choose to manage the wastewater. All that to say, a simple volume comparison isn’t enough; we have to dig deeper. What does this tell us? Humans send more “manure” to wastewater treatment systems in Iowa than livestock would, but the animal manure would have more feces in it. But this, at its heart, is why we choose to manage human and livestock manures so differently. If you have a lot of water, not much stuff in it, and are far from cropland, treatment and discharge makes sense. If you are managing volume to be smaller and get higher nutrient concentrations in it, then making decisions to use that material to replace fertilizer makes more sense.

So, how should we think about wastewater and characterize it? There is more to it than this, but if we want to keep it simple, we should start with four parameters.

Total volume, chemical oxygen demand (COD), nitrogen, and phosphorus. Why these four? Because, at their core, they tell me a lot about how poop could impact the environment. How much are we dealing with, what’s the immediate impact to water (chemical oxygen demand), and what is the potential for eutrophication (nitrogen and phosphorus).

Alright, let’s look at chemical oxygen demand. For untreated municipal wastewater the COD/five-day biochemical oxygen demand (BOD5) ratio is about 2. Why am I using this ratio? BOD5 is a much more common measure of wastewater strength (great history to this measurement, and it comes from London and the Thames River – basically because it took five days for the sewage they dumped in the river to make it to the ocean). So, what’s the BOD5 of municipal wastewater? It depends, but a good average number is around 220 mg/L. I’ve also included N and P in human wastewater and what I estimate is excreted by a pig for comparison (Table 2).

Table 2. Estimated COD, N, and P in human and swine wastewaters.

	Human	Pig
Wastewater	750	10.8	lb/day
COD Concentration	440	84,500	mg/L
N Concentration	40	8400	mg/L
P Concentration	8	1360	mg/L
COD Mass	192,165	5,065,142	tons/yr
N Mass	17,470	503,517	tons/yr
P Mass	3,494	81,522	tons/yr

 How do we try to turn this into water quality impacts? Quantifying impacts is difficult, it requires us to make assumptions about how treatment and utilization impacts COD, N, and P movement and losses to water quality. With municipal wastewater, we typically treat and then discharge. To quantify what may be making it to a stream, we have to estimate the percent removal with treatment and then quantify where it ends up. For COD, hopefully, around 90% will be removed, and this will be mostly converting material into CO2 (70%) and municipal solids (20%).  The municipal solids would then be land applied. However, land application is highly effective at COD removal and preventing it from entering water, so we’ll say 0.05% is lost from the land applied fraction.

In terms of nutrients, it gets a little more complicated and depends on the treatment system being used. For phosphorus, hopefully 50% of the P ends up in the municipal solids (which are land applied) and 50% are discharged after treatment. Of those land applied, again it depends on the management practices used, but assuming good phosphorus management, probably only 0.5% of the P land applied moves with water from the land application area. For animal manures, we will use the 0.5% for all phosphorus as it should all be land applied. In the case of nitrogen, ultimate fate is again harder because it is very much dependent on if the wastewater treatment method employed. Still, for a working version of what is happening, we’ll go with 30% is denitrified, 40% is nitrified and discharged, and 30% is recovered in the wastewater sludge and land applied (assume 20% of N is lost during storage before land application). Assuming that it is land applied as a fertilizer, we’ll go with 20% of the nitrogen is lost after land application. With manure, I’m going to assume 20% is volatilized during manure storage and lost to the environment and that, again, 20% of the nitrogen that is land applied is lost. I’m providing these results in Table 3 to show an estimated N loss.

Table 3. Estimated impact on the environment from human and pig manure after treatment for human wastewater and land application as a fertilizer for pig manure.

	Human	Pig
COD	19,236	2,533	ton/yr
N	8,875	181,266	ton/yr
P	1,756	408	ton/yr

 Where does that leave us? Swine manure probably is having more impact on the environment than human wastewater in Iowa. At least in part this is due to the vast differences in populations of pigs and people. I’ll give you one more table, COD, N, and P estimated to be released to the environment, but on a per person and per pig basis.

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

	Human	Pig
COD	12	0.2	lb/person(pig space)-year
N	6	12	lb/person(pig space)-year
P	1	0.03	lb/person(pig space)-year

 I want to do this one more time (table 5). What happens if we say that where we were applying manure would have received fertilizer anyway. Well, assuming the manure is being managed like a fertilizer, nitrogen and phosphorus losses from that acre would be similar. That is, the losses are driven by land use, and not directly by manure (and we can, and should in the future have a discussion on if manure is being managed as well as commercial fertilizer, and how to continue to improve our management of both).

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

	Human	Pig
COD	12	0.2	lb/person(pig space)-year
N	5	12	lb/person(pig space)-year
P	1	0.03	lb/person(pig space)-year

 Each method, municipal treatment for human wastewater and storage and land application of manure for livestock, has its pros and cons. If we were to treat pig manure like human waste, does water quality get better? For COD and P, I don’t think so; in fact, we probably add more of each to Iowa water ways using this method. If we treat N like human waste – it’s complicated and depends greatly on the amount of N that goes into denitrification, but unless there was a land use change associated with no longer having manure as fertilizer, we’d still get some of the losses with the use of commercial fertilizer on those crop acres that we get right now when we use manure.

The system is complicated. We need to continue to innovate to reduce N volatilization losses from storage. Specifically, these volatilization losses, are what make nitrogen losses from manure greater per pig than per person. We need to continue to develop improved nitrogen utilization practices and nitrogen fertilizer recommendations tailored to each year, location, and growing season so we can do better utilizing manure nutrients and lessen impact on water quality. The conversation is difficult, and hopefully, that comes through, that it is more than a pyramid of poop.