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.