Manuresheds: Advancing nutrient recycling in US agriculture

2.1. Data aggregation and processing

NuGIS is a tabular and spatial database developed by the International Plant Nutrition Institute (IPNI) for the US (IPNI, 2012). We downloaded tabular, county-level data for the year 2012 and performed unit conversions to build our own data table comprising the following variables in tonnes per county: manure P, N; fertilizer P, N applied to farmland; P, N removed by crops, and N fixation by crops (details in supplement 1.1). The manure nutrient variables represented nutrient available for land spreading or other uses after accounting for losses from collection, spillage, volatilization (N only), and denitrification (N only) (called “recoverable” by IPNI). The nutrient removal estimates comprised 21 common crops including forages, and their production values at harvest in 2012. In total, NuGIS provided values of zero or greater for manure, fertilizer, or crop removal nutrients for 3068 counties in the contiguous US, with 3060 counties having at least one positive value for one or more of the variables. These data were used to classify counties as sources or sinks, and to calculate the surplus manure nutrient or sink “strength” of the counties for manureshed delineation.

The county-level manure estimates from NuGIS represented all confined livestock per county. For our manureshed delineation, we sought to quantify the respective contributions of the confined hog, poultry, dairy, and beef cattle industries. To do so, we obtained county-level inventory and sales data from the 2012 United States Census of Agriculture for seven livestock types (confined dairy cows, beef cattle on feed, broilers, layers, pullets, turkeys, and hogs) (USDA-NASS, 2014); used coefficients and equations provided by Kellogg et al. (2014) to calculate tonnes of manure P and N produced by each livestock type; and aggregated the manure P and N production to the level of industry (hog, poultry, dairy, beef) for each county (details in supplement 1.2). Like the NuGIS data, these industry-specific manure nutrient estimates represented nutrient available for use after accounting for losses from collection, spillage, volatilization (N only), and denitrification (N only) (referred to as “recoverable” by Kellogg et al., 2014). In total 3052 counties were represented in the sales and inventory data downloaded from NASS. After we removed counties because their entries were undisclosed by USDA-NASS to protect producer anonymity, 2990 counties with positive, non-zero values for at least one industry remained for analysis. Of the 2990 counties, 2225 had >12 animal units (AU) of confined livestock in one or more livestock type – these were the counties for which industry-specific manure P and N were calculated (supplement 1.2).

We joined our NuGIS data table and industry-specific manure nutrient data table by unique Federal Information Processing Standard (FIPS) codes, using county names and codes recognized by the US Census Bureau as of 2012 as the definitive authority (Walker, 2019). The US Census Bureau recognized 3109 counties in the contiguous US, more than were reported in NuGIS and the downloaded Census of Agriculture data items. “Extra” counties were assigned to have zero generalized and industry-specific manure, fertilizer, and crop removal (41 counties were assigned all zeroes in the case of NuGIS; 119 counties were assigned all zeroes in the case of industry-specific manure P and N).

With regard to just manure P and N, NuGIS reported positive values for 3032 counties, whereas positive, industry-specific values were estimated for only 2225 counties. The discrepancy was due to methodological differences between IPNI (2012) and Kellogg et al. (2014), especially concerning non-disclosed data in the Census of Agriculture and the threshold at which to convert AU per county into manure nutrient estimates (supplement 1.2). As a quality-control check, we quantified the correlation of the sums of manure P and N from all industries per county and the general manure P and N derived from NuGIS per county – both on the basis of 3109 counties. Pearson’s correlation was 0.9 (p < .001) for both P and N, and we deemed that using the datasets together was acceptable for our analytical purposes.

Data wrangling, county classification, manureshed delineation, and associated mapping described below were conducted in R 3.6.0 using the tidyverse set of packages (R Core Team, 2018; Wickham et al., 2019).

2.2. County classification based on nutrient source and sink potential

Counties in which manure nutrients exceeded crop nutrient removal were first classified as sources (step 1 in Table 1). Next, counties with <500 ha of cropland that were not sources were excluded from further classification (step 2). Counties in which crop removal exceeded nutrients applied in manure and fertilizer were then classified as potential sinks for nutrients (step 3), as were counties in which fertilizer was applied at rates exceeding crop removal (step 4). The latter sink category (step 4) may reflect fertilizer rate adjustments due to lesser nutrient use efficiency, including build-up to maintain soil fertility recommendations (Raun and Johnson, 1999; Macnac et al., 2012). The remaining counties – where neither manure nor fertilizer alone exceeded cropland removal, yet cropland removal was exceeded by the sum of the two exceeded it cropland removal – were identified as candidates for within-county nutrient transfers. In those counties, the management focus would be transferring the manure nutrients to be used in place of fertilizer nutrients within the counties, with less focus on inter-county nutrient export or import.

The rules described above included a variety of assumptions, some of which were different for P and N. For both P and N, it was assumed that the manure nutrient was spread for recycling on all harvested cropland and grazingland per county. Whereas all P applied in manure and fertilizer was assumed to be assimilated and removed by the harvest during the course of 2012, only 50% of the N applied was assumed to be assimilated and removed (sensu Lassaletta et al., 2014). The lesser efficiency of N use by crops reflects N losses by volatilization, leaching, immobilization, and other pathways. Although N utilization varies with soils, management, climate, and scale of inference, average N use efficiencies for the US are often in the range of 30–80% (e.g., Lassaletta et al., 2014; Swaney et al., 2018). To capture the central tendency, a factor of 0.5 was applied to the manure and fertilizer terms for N in mathematical statements of Table 1. We evaluated the sensitivity of our results to this factor by comparing outcomes when 0.3, 0.5, or 0.8 were applied (supplement 2.2). Differences in conclusions were minor, so we deemed the factor of 0.5 to be appropriate for our ultimate purpose of delineating manuresheds.

Another assumption for both P and N was that counties with <500 ha of total cropland should be excluded as potential sinks or candidates for within-county transfers. This decision reflects our experience and observation that investments in manure import and spreading are rare in counties with negligible extents of cropland. The exclusion decision was further supported by a known data anomaly in the NuGIS database, in which fertilizer estimates in several counties with negligible cropland greatly exceeded expected rates (supplement 2.3).

After classifying the 3109 counties, we mapped them in terms of quantiles in each class (after MacDonald et al., 2011), to illustrate their degree of surplus manure nutrient (source counties) or their sink “strength” (sink counties). Sink strength was calculated as either the amount of nutrient in deficit or the amount of commercial fertilizer in excess of crop removal. We also calculated the mean, standard deviation, and N:P ratio of the nutrients in the county-level manure, fertilizer, and crop removal within the county classes. N:P ratios ultimately informed the decision to delineate manuresheds only on the basis of manure P.

We evaluated the accuracy of our data sources and county classification by comparing the quantile maps and summary statistics against local knowledge and published data about typical P and N balances (e.g., Pierzynski and Logan, 1993). We encountered a few surprises for some counties, but as we found reasonable explanations for them (supplement 2.3), we used the classes and nutrient balances as the basis for the manureshed analyses described next.

2.3. Delineating manuresheds

We delineated four manuresheds as representative examples of major confined livestock industries in the United States: poultry, hog, dairy, and beef. Manuresheds were delineated only on the basis of P source and sink strength, because statistical analyses of our county classes revealed that N-based redistribution of manure surplus from source to sink would over-apply P relative to crop demand (section 3.1). Relatedly, with the exception of one county, all source counties for manure N were also source counties for manure P, and thus it was assumed that calculating manuresheds on the basis of P would also encompass manure N redistribution.

We selected source areas for the manuresheds by first identifying assemblages of counties that comprised the largest available count of neighboring P source counties dominated by one livestock industry (hog, poultry, dairy, or beef) and then expanding the boundaries to include all adjacent P source counties. Industry dominance within each assemblage was assessed using the relative proportion of manure P produced by each industry, per our county-level estimates derived from the 2012 Census of Agriculture (USDA-NASS, 2014; Kellogg et al., 2014). Due to methodological discrepancies between the NuGIS and Census of Agriculture data sets explained above, we had industry-specific estimates for about 90% of our source counties.

We performed cluster analysis to assess the cohesiveness of livestock industries in the delineated source areas (supplement 3.1), for purposes of accurately describing the composition of the source areas. In addition, we sought to verify that each selected assemblage of source counties had sink counties in its vicinity. To do so, we quantified, for each county nationwide, the spatial autocorrelation between county-level manure P surplus and county-level sink strength for P, with respect to ten nearest neighbor counties (Getis and Ord, 1992; supplement 3.2).

Once satisfied that our selected assemblages of source counties were adequately cohesive in livestock industry composition and were locally spatially correlated with sink counties, we conducted the manureshed analyses. First we unioned each assemblage into a source area polygon. Manure P surplus (in tonnes) was summed across all counties in each source area polygon. Next, Euclidean distance was calculated between the centroid of each sink county nationwide and the nearest edge location of each source area polygon (Pebesma, 2018). We then cumulatively summed the sink strength of sink counties at progressively longer distances from each source area edge. The set of nearest sink counties with a cumulative sink strength that equaled the surplus manure nutrient in the source area was designated as the set of “requisite” sink counties to assimilate the surplus from the source area.

Manuresheds were named for their general geographic location and their dominant and – if present – subdominant livestock industries. We calculated summary statistics for each manureshed: the tally of county types within the manureshed (count of counties), the relative contribution of livestock industries to the total manure P in the source area (percent), the total wet weight of manure produced in the source area (tonnes), and the average minimum distance required to travel from source area counties to requisite sink counties for manure nutrient recycling, in terms of Euclidean distances between source and sink county centroids (kilometers).

3.1. County classification

Manure source counties were identified in 40 of the 48 contiguous United States, with a substantial concentration in the southern part of the country (Fig. 3; data available at Spiegal et al., 2020). Source counties for manure P greatly outnumbered those for manure N, with counts of 390 for manure P (~13% of all counties) and 100 for manure N (~3% of all counties). Ninety-nine of the 100 source counties for manure N were also source counties for manure P, the sole exception being Clark County, Nevada, which was a candidate for within-county transfers for P, but a source county for manure N. Clark County cropland was dominated by alfalfa hay production in 2012 (Fig. 1a), such that county-level N-fixation rendered manure N to be in excess of the assimilative capacity of county cropland [0.5 * 87 t manure N – (297 t N removed by crops – 267 t N fixation) = 13.5 t manure N surplus] (step 1 in Table 1 for N).

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Fig. 3. Counties classified with respect to manure nutrient source and sink potential for a) phosphorus (P) and b) nitrogen (N) in 2012. Each class’s shade of color (lighter vs. darker) represents a split at the median value of the class. See supplement 2.1 for maps in terms of kilograms of nutrient per hectare of harvested cropland per county.

One hundred counties with <500 ha cropland were excluded from the pool of counties to be classified as sinks or candidates for within-county transfers of P from manure stocks to fertilizer stocks; 106 were excluded for N. The discrepancy between P and N consisted of six counties with <500 ha cropland that were source counties for manure P but not for manure N (Alpine County, California; Clayton County, Georgia; Dekalb County, Georgia; Duke County, Massachusetts; Philadelphia County, Pennsylvania; and Storey County, Nevada).

Counties representing net nutrient sinks outnumbered manure nutrient sources by 6-fold for P and 28-fold for N. Arkansas and Georgia were among the states with the greatest proportion of source counties with surpluses of both P and N. Notably, in Tennessee, a state located between Arkansas and Georgia, the majority of nutrient sink counties were classified as sinks for both P and N – highlighting an interstate opportunity for manure nutrient recycling. All of the counties in Montana and Wyoming were potential sinks for P, due mainly to P deficits and fertilizer surpluses, respectively. For N, all of the counties in five states were potential sinks, either due to a crop N deficit or a fertilizer N surplus: New Hampshire, North Dakota, Utah, Vermont, and Wyoming.

Overall, 449 more counties were classified as potential sinks for N than for P. N deficit counties were more numerous than P deficit counties (2181 for N vs. 1642 for P). However, counties that were sinks due to a surplus of annual fertilizer application were roughly comparable for the two nutrients, with 675 counties for P and 585 counties for N. The sinks due to fertilizer surplus were clustered in the Midwest, but they were also distributed in the Southeast (e.g., Florida, Alabama and North Carolina) and the West (e.g., Texas, California, Oregon and Wyoming).

The stoichiometry of nutrients in manure has been identified as a key factor leading to the accumulation of P in farmland soils (e.g., Sharpley et al., 2013). Indeed, the N:P removed by crops greatly exceeded the N:P in manures in all county classes (Table 2). In strong contrast, commercial fertilizer nutrients tended to possess N:P at least four-fold greater than manures in the same counties, and nearly two times more than the N:P removed by crops, likely reflecting fertilizer rate adjustments related to low use efficiencies for N and “build-up/maintain” philosophies for P.

3.2. Manuresheds

Three of four manuresheds contained secondary livestock industries that contributed, on average, at least 20% of the manure P in their source areas: Carolina Poultry and Hog, Puget Sound Dairy and Poultry, and Southern Plains Beef and Dairy (Fig. 4; Table 3; data available at Spiegal et al., 2020). Within the fourth, Interior Highlands Poultry manureshed, there were only minor contributions from other industries. Among all manuresheds, substantial differences existed in the transport distances required to recycle surplus manure P from source areas in sink counties (Fig. 4a; Table 3). The average minimum distance ranged from 147 ± 51 km (Southern Plains Beef and Dairy manureshed) to 368 ± 140 km (Carolina Poultry and Hog manureshed).

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Fig. 4. Manuresheds in the contiguous United States with a) source counties in browns and sink counties required to assimilate the manure phosphorus (P) surplus from the source counties in grays; and all county classes in the manuresheds for b) dairy and poultry, c) beef and dairy, d) poultry, and e) poultry and hogs.

Table 3. Characteristics of manuresheds delineated for manure phosphorus (P) produced by US confined livestock industries in 2012.

^a Wet weights derived from the 2012 United States Census of Agriculture (USDA-NASS, 2014) and coefficients and equations from Kellogg et al. (2014). Manure P and surplus manure P derived from NuGIS (IPNI, 2012). Estimates account for losses from collection, spillage, volatilization (N only), and denitrification (N only).

^b Mean contribution of industry per county in the source area, derived from the 2012 United States Census of Agriculture (USDA-NASS, 2014) and coefficients and equations from Kellogg et al. (2014).

^c Average Euclidean distance between the centroids of source counties and centroids of requisite sink counties in each manureshed.

The most extensive manureshed was the Carolina Poultry and Hog manureshed, with necessary transport distances extending well into the mid-Atlantic and southeastern United States (Fig. 4a). This manureshed spanned 512 counties in total (Fig. 4e): 36 in the source area, 297 requisite sinks, and 179 other counties within the matrix of requisite sinks (40 candidates for within-county transfers, 88 source counties for manure P that did not fit the definition for the source area in the manureshed, and 51 counties that were excluded from classification due to minimal croplands, many in West Virginia and Kentucky). Thus, the large size of the Carolina manureshed can be explained in part by the geometries of its counties: as in the other three manuresheds (Fig. 4b–d), the centroids of the requisite sink counties in the Carolina manureshed were, by design, as close as possible to the source area edge; however, the perimeters of the requisite sink counties had irregular geometries, so other classes of counties were interspersed between them. The scope of the Carolina manureshed also reflects a combination of factors related specifically to production in the region: the nationally significant concentration of hogs in the area (Fig. 1b), the surfeit of nutrients generated by hog farms in eastern North Carolina and by poultry farms throughout the greater source area (39,566 t surplus P from 24 million tonnes of wet weight; Table 3), the large extent of forested land without the assimilative capacity of croplands for manure nutrients, and the relatively low crop nutrient demand of many Appalachian counties (light shading of the “P deficit” quantiles in Fig. 3e).

Extending from the Ozarks region of southern Missouri through western Arkansas to the Gulf Coastal Plain of northern Louisiana, the Interior Highlands manureshed (Fig. 4d) now encompasses one of the largest concentrations of broilers nationwide (>1.1 billion birds), trailing broiler production in only Georgia (~1.4 billion birds) and roughly equivalent to Alabama (Fig. 1b; USDA-NASS, 2019). Despite the large extent of the Interior Highlands source area, we estimated that its 32,176 t of surplus manure P could be assimilated by crops in a relatively tight radius of sink counties, dominated by croplands along the Mississippi Delta region (Fig. 4d).

At 29 counties total, the Southern Plains Beef and Dairy manureshed contained the fewest counties of all the manuresheds, with only 8 source counties, 14 sink counties required to assimilate the surplus from the sources, and 7 other counties interspersed in the matrix of requisite sink counties (Table 3, Fig. 4c). The relatively small size of this manureshed is a function of the high assimilative capacity of croplands on farms surrounding the beef and dairy feedlots of the region, as well as the low concentration of P per unit mass of beef cattle manure compared with that produced by other types of livestock (Kleinman et al., 2005; Kellogg et al., 2014). Indeed, several counties in Texas that support high-volume beef feedyards did not even have a surplus of manure P. For instance, Potter County, Texas, which contains high-profile beef feeding operations around the city of Amarillo, was not classified as a source county, but instead as a candidate for within-county transfers of P.

The Puget Sound Dairy and Poultry of Washington state was not the most extreme in terms of surplus P from dairy or poultry manure (e.g., the county of San Bernadino in southern California alone had approximately 3080 t of dairy manure P in excess of the county’s cropland assimilative capacity), but it did comprise the highest count of adjacent source counties for dairy manure P. Notably, in the region around the Puget Sound, the surplus of 1628 t of manure P from dairy and poultry, in addition to smaller beef and hog industries, could be absorbed by crops produced in 12 neighboring sink counties in Washington and Oregon (Fig. 4b).

4.1. Manuresheds: Tools to advance nutrient recycling in US agriculture

The manuresheds presented here elucidate factors critical to advancing nutrient recycling in four major livestock industries that have inherently different structures. The vertical integration of poultry and hog production organizes feed and animal production management within single institutions, providing a decision-making structure that may be conducive to implementing manureshed recycling. For instance, Tyson Foods recently announced sustainable nutrient goals for their feed producers that could be tied to a manureshed framework (Tyson Foods Inc., 2018). Consolidation within the dairy industry and organization around regional cooperatives and processing facilities also provide opportunities for the aggregate, industry response implicit in a manureshed. Beef production, in contrast, includes diffuse production stages (cow-calf) in which nutrients are largely returned to land by grazing animals, although the later stages of production (backgrounding, finishing) offer the concentration of animals and connection to transportation infrastructure required for wholesale manure transport (Drouillard, 2018).

Nutrient density, a key issue affecting practical transport to distant operations, is particularly limiting in manuresheds circumscribing dairy and hog feeding operations. In general, high-liquid manures are considered to have a practical transport limit of <10 km (Bartelt and Bland, 2007; Hadrich et al., 2010), making solid-separation a critical step in converting liquid manures into transportable components. Conversely, solids such as poultry litter can be moved much farther (Herron et al., 2012). Ratios of N:P are another critical consideration for both liquid and solid manures (Kleinman et al., 2005). For manures with lesser N:P ratios (e.g., dry poultry manures where considerable ammoniacal N has been lost in storage and handling), the application of manure to meet crop N requirements accelerates the accumulation of P in soils relative to fertilizers and other manure sources. Here we use our four example manuresheds to briefly review the opportunities and constraints for optimal manure use and recycling in the four major confined livestock industries of the United States.

4.2. A need for systemic adjustment

The simplistic goal of balancing manure nutrients among counties belies the complexity of implementing the manureshed vision. For the concept to advance nutrient recycling, changes would be required in farm, market, and regulatory systems, by a multitude of actors, on cascading geographic scales and levels of social organization (Reganold et al., 2011). For confined animal producers, comprehensive manure management strategies are required that reflect the value of the resource as well as its liabilities, and the interaction of many farm management decisions on manure yield and quality (Kleinman et al., 2020). Without innovations in storage, dewatering, and nutrient recovery, export of manure nutrients will not expand. However, the capacity to oversee nutrient export cannot reside with the producer alone and will require, by necessity, external entities whose business is manure translocation (e.g., AgMap, 2019). Similarly, transformation is required in grain and forage production, which has come to rely on commercial fertilizers instead of manure nutrients. While some nutrient recovery systems do produce concentrated fertilizers from manures, those such as struvite are rarely used in modern production systems. Sometimes, the effort may be in dispelling perceptions that manure will adversely interfere with production. Other times, cropping systems may need to be adjusted to account for differences between manures and commercial fertilizers. No single approach can prevail, as all have tradeoffs.

Arrangements among farmers with excess manure nutrients and farmers who could use them are a form of collaborative adaptive management (Klerkx et al., 2010). Research from Denmark, where redistribution of manure nutrients is regulated nationally to address water quality concerns, shows that most collaborative manure managers knew each other socially or professionally before they established their manure transfer arrangements (Asai et al., 2014). Some producer arrangements are already underway in the United States. For instance, truckers from western Pennsylvania haul low-grade hay to the mushroom industry in the eastern part of the state, and they pick up poultry litter for the return trip west. This “back-hauling” effectively exports poultry litter nutrients from the sensitive Chesapeake Bay watershed in the east, and several small companies specialize in this service (AgMap, 2019). Characterizing social networks that enable such manuresheds for the United States is a new field of research which may ultimately help to advance collaborative adaptive manure management and identify new pathways for niche marketing by the collaborative manure managers.

These new frontiers of farm- and community-level manure management will require incentivization. For instance, forgiveness on toll roads for back-hauling poultry litter from eastern to western Pennsylvania (as discussed above) could improve cost effectiveness. Further incentivization may come in the form of collective marketing through a vertical integrator (e.g., Tyson poultry and pork, Niman Ranch beef and pork, Organic Valley dairy), so that producers engaging in manureshed approaches are adequately compensated for putative or quantifiable societal benefits (Honeyman et al., 2006). Incentivization through regulation is already occurring in different ways across the country. In Minnesota, for example, permitting requires that each dairy have an adequate land base for the manure it produces (Minnesota Pollution Control Agency, 2019). In Pennsylvania, farms with surplus manure nutrients are mandated to export those nutrients, and the exported nutrients are to be land-applied to balance the needs of importing operations (Pennsylvania Nutrient Management Program, 2017). The state of Maryland has adopted a similar approach, while also incentivizing farther transport distances (Maryland Department of Agriculture, 2016). The Nutrient Surplus Areas (NSAs) designated by Arkansas (Arkansas Natural Resources Commission, 2010) promote a manureshed approach. While these state-based regulatory infrastructures are promising for advancing nutrient use on a case-by-case basis, a standardized, nationwide regulatory infrastructure as seen in other nations is not yet in place (e.g., Asai et al., 2014). The interplay of management planning and regional planning is a frontier that can actualize aspirational manureshed visions. Herein we provide critical data that can help inform such developments.