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Shallow Burial with Carbon for Swine Mortality Carcass Disposal

John McMaine1,2,*, Robert C. Thaler3, Gary Flory4, Amy M. Schmidt5, Morghan Hurst1

Published in Journal of the ASABE 68(3): 477-487 (doi: 10.13031/ja.15462). Copyright 2025 American Society of Agricultural and Biological Engineers.

1 Agricultural and Biosystems Engineering, South Dakota State University, Brookings, South Dakota, USA.

2 Biosystems and Agricultural Engineering, University of Kentucky, Lexington, Kentucky, USA.

3 Animal Science, College of Agriculture and Biological Sciences, South Dakota State University, Brookings, South Dakota, USA.

4 Virginia Department of Environmental Quality, Richmond, Virginia, USA.

5 Biological Systems Engineering and Animal Science, University of Nebraska, Lincoln, Nebraska, USA.

* Correspondence: john.mcmaine@uky.edu

The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/

Submitted for review on 16 November 2022 as manuscript number PAFS 15462; approved for publication as a Research Article by Associate Editor Dr. Sheryll Jerez and Community Editor Dr. Shafiqur Rahman of the Plant, Animal, & Facility Systems Community of ASABE on 29 August 2023.

Citation: McMaine, J., Thaler, R. C., Flory, G., Schmidt, A. M. & Hurst, M. (2025). Shallow burial with carbon for swine mortality carcass disposal. J. ASABE, 68(3), 477-487. https://doi.org/10.13031/ja.15462

Highlights

ABSTRACT. The advance of highly transmissible and deadly animal diseases, such as African Swine Fever virus (ASFV) and foot-and-mouth disease (FMD), presents a critical need for mass mortality disposal that does not further transmit the disease or contaminate water resources. Additionally, the decomposed carcasses should not be an environmental hazard but rather something that can be returned to and benefit the soil. Above-ground burial (AGB), also called Shallow Burial with Carbon by the United States Department of Agriculture (USDA), is a method of mortality disposal where carcasses are placed on top of a carbon source (wood chips, wood shavings, corn stalks, etc.) that have been placed in a shallow trench. The spoil of the trench is then placed on top of the carcasses, creating a low mound. This study was a meso-scale carcass disposal trial (200 carcasses in four trenches) to determine differences in potential risk to groundwater contamination from nitrate-N, E. coli, and survival of Seneca Valley Virus (SVV) utilizing two carbon sources (corn stalks and wood shavings) and two burial times (June and November). The results showed that carcasses buried above the wood shavings carbon source demonstrated less risk for nitrate-N loss, while corn stalks demonstrated higher nitrate-N movement below the burial trench. E. coli results were highly variable, with a range from 0-18,000 colony forming units per 100 mL (CFU/100mL) with no statistically significant difference between the two carbon sources or three well depths. SVV was present in the .15 m (6 inch) and .46 m (18 inch) wells but not in the .91 m (36 inch) well, with the corn stalk carbon source having lower numbers than the wood chips. The SVV material detected was either on parts of the virus or inactive virus, and there was no risk for new disease transmissions. These results show us that AGB is a viable way to dispose of carcasses, but care must be taken to minimize risk to vulnerable aquifers.

Keywords.Above-ground burial, African Swine Fever, carcass, E. Coli, Corn stalks, leaching, Nitrate-N, Seneca Valley Virus, Wood shavings.

In 2018 and 2019, the African Swine Fever virus (ASFV) spread through much of China’s swine herd, resulting in significant mortality and economic losses. The characterization of the outbreak in China resulted in an average fatality rate of 64%, with outbreaks occurring in many of China’s provinces (Liu et al., 2019). According to economic models, this resulted in a 9-34% decrease in global pork production (Mason-D’Croz et al., 2020). As evidenced by outbreaks in eastern Russia in 2017 potentially leading to outbreaks closer to the Chinese border and eventually into China (Zhou et al., 2018), ASFV has demonstrated the ability to be transmitted across expansive distances. Limiting offsite transport of pigs and products was identified as a keyway to reduce ASFV transmission and should be included in mortality disposal plans (Liu et al., 2019).

A review of animal mortality management systems identified risks associated with some common mortality disposal methods, such as rendering, which poses risks of transportation of carcasses and spread of disease; burial, which poses risks of contamination of soil and groundwater; and incineration, which poses risks of aerosolizing infectious particles (Costa and Akdeniz, 2019). Composting is a preferred method due to increased temperatures that are high enough to inactivate viruses and increase the rate of carcass decomposition (Won et al., 2016). However, composting requires higher maintenance than a “fix it and forget it” approach such as burial. Above-ground burial presents a disposal method that reduces the risk of soil and groundwater contamination but allows effective disposal without long-term maintenance requirements.

Above-ground burial has been evaluated on a small-scale in Virginia, North Carolina, and Texas, with promising results (Flory and Peer, 2021). A larger scale project with 100 sows was conducted in Oklahoma (Ebling et al., 2022) and analyzed several factors, including swine pox virus inactivation, insect activity, and nutrient migration, with results showing inactivation of the swine pox virus after 11 days. An ongoing study in Vietnam is looking at the inactivation of the African Swine Fever virus, and preliminary results suggest that the method effectively inactivates ASFV (Flory and Peer, 2021). This method was successfully implemented in Tunisia (Flory et al., 2017) to manage sheep infected with foot-and-mouth disease (FMD) and to dispose of 3,000 sows and 8,000 piglets killed in a barn fire in Minnesota in 2021 (Flory, 2022).

This study sought to evaluate a meso-scale carcass disposal trial (200 carcasses in four trenches) to determine differences in potential risk to groundwater contamination, microbial activity, and survival of Seneca Valley Virus utilizing two carbon sources (corn stalks and wood shavings) and two burial times (June and November of 2019).

Materials and Methods

Burial at scale was important to further prove the aboveground burial concept. Two carbon sources (corn stalks and wood shavings) and two burial times (June and November of 2019) were compared. Carcass temperature was measured to assess microbial activity within each trench. Water samples were also collected from monitoring wells and analyzed for E. coli, nitrate-N, and Seneca Valley Virus (SVV). It should be noted that while SVV is a commonly used surrogate for foot-and-mouth disease (FMD) (Mason et al., 2022), caution should be exercised when comparing SVV transport and fate to those of ASF, for which there are limited or no real surrogates since ASF is the only member of the Asfarviridae family (Dixon et al., 2013).

Experimental Setup

A total of five trenches were excavated to a depth of 0.5m (1.5 ft), a width of 2.4 m (8 ft), and a length of 18.3 m (60 ft). Two pits were excavated in June 2019, and two pits were excavated in November 2019 to test the effect of season on burial. Precipitation following the burial of pigs in the June trial exceeded the monthly averages (table 1), and water was present in most wells during most sampling events through January 2020. Normal to dry precipitation and cold conditions began in January 2020 and persisted for the duration of the study through November 2020 (table 1). All four of these trenches were monitored for temperature within the trench and leachate below the trench. One additional trench was excavated in August 2019 to compare microbial activity in opened and intact carcasses. The temperature was measured in the August trench. Carcass temperature was compared to a control bare soil temperature (20 cm or 8 inch depth) measured at the Brookings Station of the Mesonet at SD State (South Dakota Mesonet, South Dakota State University, 2023) located three km from the site. Precipitation and air temperature were also measured at the Brookings Mesonet station. For the excavation of all trenches, a toothless bucket was used on the excavator, and the bottom of each trench was scraped to seal open pores and decrease hydraulic conductivity. A soil core was taken at the site of the trenches to a depth of 101.6 cm (40 in) and analyzed every 25.4 cm (10 in) for particle size using the hydrometer method as described in Ashworth et al., 2001. At a depth of 0-76.2 cm (0-30 in), the soil type was found to be sandy clay loam, which changed to sandy loam at 76.2-101.6 cm (30-40 in).

Table 1. Meteorological conditions during project period. Monthly and annual totals are presented along with the 30 year averages. Monthly average data were retrieved from the National Oceanic and Atmospheric Administration, National Centers for Environmental Information online database (NOAA NCEI, 2023). Precipitation and air temperature were measured at the Brookings, SD, station of the Mesonet at SDState, located 3 km away (South Dakota Mesonet, South Dakota State University, 2023).
JanFebMarAprMayJunJulAugSepOctNovDecAnnual
Precipitation
(mm)		20191922691051448617087182501728977
2020111215207679102444224387469
Monthly Average1113275588109928279502015641
Temperature
(°C)		2019-12-15-5611192219175-2-75
2020-10-815122123221552-47
Monthly Average-11-9-2713192220158-1-96

To measure leaching potential and contaminant transport risk below the trenches, well nests, consisting of three wells dug to depths of 15 cm, 91.4 cm, and 45.7cm (6, 36, and 18 in, respectively), were installed along the centerline of the trench at 4.6 m (15 ft), 9.1 m (30 ft), and 13.7 m (45 ft) (25%, 50%, and 75% of the length, respectively). These three well nests (or nine wells) were installed in four of the five trenches (leachate was not monitored in the August 2019 burial). Wells were made of 5 cm (2 inches) PVC pipe and screened using an oscillating saw blade (thickness of 0.8 mm). Slits were approximately 2.5 cm (1 in) wide, spaced every 1.3 cm (0.5 in) on four “sides” of the pipe. Slits were made up to 15.24 cm (6 inches) from the bottom of the well, and the well was capped at the bottom. Wells were dug with a 7.62 cm (3 inches) auger bit to the depth desired, the well was inserted, and sand was added around the well screen to minimize soil intrusion to the wells. After the sand was added, bentonite clay mixed with water was added by hand around the top of the well and bottom of the trench, to seal the well screen and prevent preferential flow from short circuiting flow from the surface down the pipe to the well.

After the wells were installed in the bottom of the trench, a carbon source was placed over the top of them. For this experiment, 50.8 cm (20 in) of corn stalks were installed in one trench excavated in June and one trench in November, and 50.8 cm (20 in) of wood shavings were installed in one trench excavated in June and one trench in November (fig. 1). These two sources of carbon were chosen due to their potential availability if a mass mortality event should occur.

Figure 1. Excavated trenches showing corn stalks (left) and wood shavings (right). Pipes sticking out of the ground mark well nests at three different depths. The tallest pipe in each nest is the shallowest (15 cm), the middle and shortest pipe is the deepest well (91.4 cm), and the middle height pipe is the well at 45.7 cm.

Once the carbon source was placed in the trenches, carcasses that were transportation mortalities from a commercial packing plant and weighed approximately 131.5 kg (290 lbs.) and nursery pigs challenged with SVV, only buried in the June burial trials, weighing 11.3 kg (25 lbs.) were placed on top of the carbon source. The SVV nursery carcasses were buried with approval from the IACUC. The 12 nursery carcasses, in the June burial pits, and 44 market weight carcasses (per trench) were placed so each of them was touching each other on all sides, with the SVV carcasses placed 1m (3 feet) from the wells. The initial load of market weight carcasses was split along the underside of the pig from throat to tail at the packing plant prior to placement. Since open carcasses were different from the original experimental design and less likely to be the same to how carcasses would be disposed of in an emergency mass mortality event, one trench contained both split and intact swine carcasses. The temperatures in the opened vs. intact carcasses were compared as a proxy for measuring of biologic activity. To measure the temperatures of each trench, three probes were installed in each of the four main experimental trenches for replication purposes (fig. 2). For the trench where both opened and intact carcasses were compared, probes were taped on the fore flank under one of the front shoulders. A total of eight probes (two in each treatment) were used to measure differences in opened and intact carcasses.

Figure 2. Swine carcasses placed in trench on top of carbon source (corn stalks in picture). Grey conduit extending from trench contain temperature probes placed inside the swine carcasses. White pipes extending upward are well nests to collect leachate samples.

With the carcasses and temperature probes installed, the trench was then covered, so a mound of soil was formed on top of the carcasses with the excavated spoil (fig. 3). This soil would gradually settle in the trench as the carcasses started to decompose. Cattle panels were installed around the June trenches to discourage wildlife from scavenging the carcasses, and a game camera was installed to measure wildlife activity. Since no wildlife activity was recorded for the June burial, cattle panels were not installed for the November burial.

Water Sampling

Water samples were collected monthly (for the duration of a year) using a portable peristaltic pump. The collection end of the tubing had a coarse screen, mesh size 14, to prevent large sediment from entering the pump tubing while allowing most small sediment to enter. Using the portable peristaltic pump, the collection end of the tubing was inserted into the 5 cm (2 in) PVC pipe until it reached the bottom of the well, and the pump was turned on. Approximately 24 hours before the collection of the aliquots, all the water was removed from the wells to ensure that the water being collected from the well was current pore water and not stagnant water. After approximately 24 hours from the purge of water, the wells were then ready for sampling. This was done by again inserting the collection end of the tubing into the 5 cm (2 in) PVC pipe until reaching the bottom and allowing one system volume (volume available in pump tubing) of water to be pumped through and disposed of before each sample was collected. If sufficient water was available, four separate aliquots per well were collected into sterile bottles for E. coli and SVV and into acid washed bottles for nitrate-N. The aliquots were tested for nitrate-N concentration, E. coli concentration, and the presence of SVV. If SVV was present, one aliquot was used for bioassay to see if pigs exposed to the water sample contracted SVV. Samples were analyzed within 24 hours of collection for E. coli and SVV and either analyzed or frozen within 48 hours for nitrate-N.

Figure 3. Completed trenches with a layer of excavated spoil replaced over the swine carcasses.

E. coli

For E. coli, water samples were filtered through a sterile, 0.45-µm-membrane filter. Filtered samples were plated in triplicate at dilutions of 1 and 10 mL on modified mTEC agar, placed in a water bath at 35 ± 0.5°C for 2 ± 0.5 h, and incubated at 44.5 ± 0.2°C for 22 ± 0.5 h. Concentrations of E. coli were determined as the mean number of colony-forming units (CFUs) among triplicates of each sample using only plates containing 30 to 300 CFU. One to 10 E. coli isolates were streaked on tryptic soy ager (TSA) plates and subjected to phenotypic antibiotic susceptibility testing using a modified Kirby-Bauer method (Bauer et al., 1966; CLSI, 2011). The E. coli concentration was analyzed at the lab operated by the Water Resources Institute at South Dakota State University.

Seneca Valley Virus (SVV)

SVV Bioassay

A bioassay trial was conducted to see if the SVV PCR-positive tissue, carbon source, and water samples were infective. Twenty-four 21-day old, weaned piglets were placed in raised decks that were 1.6 x 2.1 m (5 x 7 ft) in the Animal Research Wing. Four rooms were utilized, with two decks per room and three pigs per deck. Pigs within the same deck were on the same experimental treatment. The negative control pigs were in one room, the water-treatment pigs in a second room, the tissue treatment pigs in the third room, and the carbon treatment pigs in the fourth room. There were solid dividers between decks in a room. Piglets were fed a Phase 1 pelleted nursery diet for the 21-day trial. There was a 7-day acclimation period pre-challenge. On day 7, ½ ml of blood and oral and fecal swabs were taken from every pig. On day 8, all pigs received a 2 ml oral inoculation of SVV PCR-positive samples via oral feeding. For the water sample treatments, 2 ml of water were given directly to the piglets as inoculation. For the tissue and carbon source samples, those samples were mixed with 10 ml of water for 1 hr., and then 2 ml aliquots of that fluid were given to each piglet as inoculation. On days 11, 13, 14, 15, 18, and 22, oral and fecal swabs were taken from every pig for SVV analysis. On day 22, ½ ml of blood was also obtained from every pig for SVV analysis as well. When an animal had two positive SVV results, it was euthanized. All other pigs were euthanized on day 22 after all samples had been obtained.

SVV Water Sample Analysis

SVV was analyzed at the Animal Disease Research and Diagnostic Laboratory at South Dakota State University. SVV was analyzed using standard polymerase chain reaction methods. Seneca Valley Virus was detected via PCR in the water samples, but only from the 15.2 cm and 45.7 cm wells (table 2). No SVV was detected from water from the 91.4 cm wells. Also, only one water sample from the corn stalk pit (45.7 cm) tested positive for SVV, while 10 samples over 11 months from the wood chop pit tested positive for SVV. It should be noted that SVV was detected in the July 2019 through November 1, 2019 testing, but not through the May 2020 sampling. The Seneca Valley Virus was detected in the carcasses of the challenged feeder pigs, the carbon source, and the surrounding water. However, the SVV concentrations were lower in the carcasses in the corn stalks versus the wood chips. PCR testing only determined the presence of SVV; the state of the virus as an alive virus, a dead virus, or fragments of the virus was unknown. Thus, bioassay challenges of SVV-free weaned pigs with the PCR-positive material was conducted. None of the animals seroconverted.

Nitrate Nitrogen

Nitrate-N concentration was analyzed with a SEAL AQ2 discrete analyzer (SEAL Analytical, Inc., Mequon, WI) using the EPA-114-A-Rev 9 Method, which is a cadmium coil reduction followed by a sulfanilamide reaction in the presence of N-(1-naphthylethylenediamine) dihydrochloride (USEPA, 1993). Duplicates were run for all samples and rerun if the difference was greater than 10%. The average value was taken if the difference was less than 10%.

Table 2. Seneca Valley Virus Polymerase Chain Reaction detection from monitoring wells. WC refers to samples collected from wells in a woodchip trench, while CS refers to samples collected from wells in a corn stalk trench. In row two, the letter identifier is the well nest (A is 4.6 m from one edge, B is 9.1 m from one edge, or in the middle, C is 13.7 m from one edge), while the number is the depth (6 inches or 152 mm, 18 inches or 457 mm, 36 inches or 914 mm). Depth is not listed in incremental order but in the order in which the wells were dug. Date is noted as MM-DD-YY. ND refers to no detection, and SVV refers to a detection of Seneca Valley Virus. If a sample identifier is blank, there was no water present to collect a sample.[a]
Wood ChipsCorn Stalks
DateA6A36A18B6B36B18C6C36C18D6D36D18E6E36E18F6F36F18
7-19-19SVVNDNDNDNDSVV
8-21-19NDNDNDNDNDNDSVVNDNDNDNDNDNDNDNDNDNDND
9-17-19NDNDNDSVVNDSVVSVVNDNDNDNDNDNDNDNDNDNDND
10-17-19NDNDNDSVVNDSVVSVVNDNDNDNDNDNDNDNDNDNDND
11-1-19NDNDNDSVVNDNDSVVNDNDNDNDNDNDNDNDNDNDND
11-19-19NDNDNDNDNDNDNDNDNDNDNDNDNDNDNDNDNDND
1-23-20NDNDNDNDNDNDND
3-2-20NDNDND
4-1-20NDNDND
5-1-20NDNDNDND

    [a] ND = Not Detected, SVV = PCR-positive for Seneca Valley Virus, WC = Wood Chips, and CS = Corn Stalks.

Carcasses and Carcasses Decomposition

Market weight carcasses weighing 131.5 kg (290 lbs.) from a commercial packing plant were used for the experiment, as well as 24 nursery pigs weighing 11.3 kg (25 lbs.) that had been challenged with SVV. Market weight mortalities from transport losses were transported to Brookings and placed in a single layer in the pits. There were 44 market weight mortalities in each pit. Three days before the carcasses were placed on the pit in the June trial pits, twenty-four 50 lb. pigs were taken to the SDSU (South Dakota State University) Veterinary Science Department and challenged with SVV. When viremia was high, the pigs were euthanized and taken to the research site. Each SVV-infected pig was wrapped in plastic bird netting and placed in the pit next to the market weight mortalities (12 SVV mortalities per trench). The SVV mortalities were placed in three locations throughout each trench and were within 1 m (3 ft) of the test wells. For four consecutive months post-burial, three SVV mortalities were dug up and cerebral and intestinal tissues collected for SVV analysis.

Temperature Monitoring

Temperature differences among treatments were used as a proxy to determine differences in microbial activity. Correlations between heat output and microbial activity have been demonstrated in past research (Alef et al., 1988; Sparling, 1983). Temperature probes (Model 107, Campbell Scientific, Logan, UT) were inserted into carcasses using sharpened electrical conduit. Three probes were installed in each of the four main experimental trenches (June vs. November burial and wood shaving vs. corn stalks). In the trench comparing opened and intact carcasses, four probes were installed per treatment, and probes were taped on the fore flank of carcasses under one of the front shoulders. The probe location was selected to cover the full trench at regular intervals, roughly the same spacing as the monitoring wells, but at least two meters from the well nests so their presence did not bias measurements. Care was taken so the probes remained set while the excavated soil was being placed on top of the carcasses. All probes were run through an electrical conduit to one of two site data loggers (CR 1000X, Campbell Scientific, Logan, UT). The temperature was measured every minute and averaged over every hour. Data was downloaded every month.

Game Camera Monitoring

A M-80BLX GameSpy trail camera was installed to monitor for predator or scavenger intrusion to the trenches and disturbance of the carcasses. The camera was placed 27 m away from the trenches, with a view that captured all trenches. The camera was motion activated and an AA battery operated with an 8 GB SD memory card. The pictures were downloaded once a month. Some activity was likely not caught due to limited visibility conditions.

Data Analysis Methods

The data were tested for normality using the Anderson-Darling test. The data were determined to be non-normal, so hypothesis testing was performed using the 1-sided Mann-Whitney test for the median value of each sample date. Water sample concentrations (E. coli and nitrate) were averaged for each treatment (cornstalks vs. wood shavings and June burial vs. November burial) for each date and compared in pairs were compared across treatments. SVV was compared qualitatively since the samples were analyzed using PCR, which did not show presence but not concentration. Temperature data was compared using a 1-sample Wilcoxon test, again pairing the mean value across replications for each hour of the day for each treatment (cornstalks vs. wood shavings; June burial vs. November burial; and intact vs. opened carcasses). All data analysis was performed using Minitab (Minitab LLC, State College, PA).

Results and Discussion

Results

E. coli

As expected with a broad-spectrum biological measurement, E. coli concentrations were highly variable. Concentrations ranged from 0 to almost 18,000 colony forming units per 100 mL (CFU/100mL). One influence on the results was the abundance of moisture in 2019. According to the National Oceanic and Atmospheric Administration (NOAA) website (US-NOAA, 2006) (table 1), Brookings County received 330 mm (13 inches) of precipitation above the normal annual average. No statistical difference between carbon source treatments or well depth was observed. As expected, concentrations were low during the cold months as biological activity went dormant, and subsequently increased again in June and July. No samples were analyzed for E. coli in April or May due to lab shutdowns from COVID-19. E. coli concentration must be measured within 24 hours of collection, so samples could not be preserved and analyzed later.

Sample concentrations exceeded recreation water quality designated use standards of 235 to 575 CFU/100mL. Concentrations were not statistically different, due in part to limited statistical power from fewer samples as well as high variability among samples. Numerically, the highest concentrations in initial sampling events tended to be at shallower depths, and some spikes in later sampling events were in deeper wells (fig. 4). This numerical difference in concentration was surmised to indicate some downward migration or colonization of E. coli at deeper depths.

In comparison, these values were well below some benchmarks, such as swine manure (770,000 CFU/100mL, Pappas et al., 2008), but well above tile drain concentrations that had swine manure applied at recommended rates (82 CFU/100 mL, Pappas et al., 2008). E. coli is found in the intestines of animals and humans and covers a broad range of types, including ones that are dangerous as well as ones that are beneficial to gut health. While the high concentrations found in the sampling wells do not necessarily translate directly to human health risk, these concentrations would likely cause gastrointestinal illness (Vanden Esschert et al., 2020). The temperature conditions necessary to eliminate live bacteria for a sufficient time to reduce E. coli concentrations were not achieved.

While direct comparisons could not be made due to the limited number of samples across treatments, high concentrations were found in the deepest wells for the summer burial only after the first month and remained below 4,000 CFU/100 mL after July 2019. Comparatively, remarkably high concentrations were present in the deepest wells during the summer of 2020 for the winter burial carcasses. While not a direct statistical comparison, this suggested that winter burial may be more susceptible to E. coli movement due to delayed carcass decomposition.

Figure 4. Concentration of E. coli compared to the carbon source (corn stalks or wood shavings) and depth of the well (0.15,.46, or.91 m) from July 2019 to July 2020.

Seneca Valley Virus

Seneca Valley Virus was detected in the water samples, but only from the 15.24 cm (6 in) and 45.7 cm (18 in) wells. No SVV was detected in the water from the 91.4 cm (36 in) wells. Also, only one water sample from the corn stalk pit (45.7 cm or 18 in) tested positive for SVV, while 10 samples over 11 months from the wood chop pit tested positive for SVV. It should be noted that SVV was detected in July 2019 through November 1, 2019, testing, but not through the May 2020 sampling. Fourteen days after challenging the weaned pigs with PCR-positive water samples, none of the animals seroconverted, so the SVV material detected was either on parts of the virus or an inactive virus, and there was no risk for new disease transmissions.

Nitrate Nitrogen

Initial well nitrate-N concentrations were relatively high, especially for corn stalks, with several concentrations over 10 ppm and one greater than 55 ppm. Nitrate-N concentrations were 10 ppm or less for wood shavings for every sample and remained around or below 5 ppm for all samples two months after burial and beyond. Normal to dry precipitation and cold conditions began in January 2020 and persisted for the duration of the study through November 2020 (table 1). This led to few or no samples being available from shallow well depths in December, January, February, April, and June, and no more samples being collected after June 2020. According to the results of the statistical test, concentrations under corn stalks were significantly (P = 0.002) greater than concentrations under wood chips. There was no statistical difference in mean nitrate-N concentration between measurement depths of 15.24 cm and 45.7 cm (6 and 18 in), but the mean concentration in 91.4 cm (36 in) wells was greater than the concentration in 45.7 cm (18 in) wells. This may be due in part to reduced statistical power from a smaller sample size (few or no samples from 15.24 cm and 45.7 cm (6 or 18 in) for the later sampling dates).

Precipitation following the burial of pigs in the June trial exceeded the monthly averages (table 1), and water was present in most wells during most sampling events during this period. While nitrate-N concentrations decreased over time, with concentrations in January and beyond less than 10 ppm, it is possible that continued carcass decomposition coupled with downward water movement could lead to some additional nitrate movement. Above-average precipitation coupled with fresh carcasses in the three-month period following the establishment of burial trenches likely created a worst-case scenario for nitrate nitrogen leaching. This was indicated by high nitrate-N concentrations in leachate from multiple wells beneath corn stalks during June through September (fig. 5). Even in worst-case conditions, the consistently low concentrations present in wells beneath wood shavings indicate that it is not likely that high nitrate-N loading will be an issue with a wood shavings carbon source.

Figure 5. Nitrate-N concentration by carbon source (corn stalks or wood) and depth of well (0.15, 0.46, and 0.91 m) from June 2019 to June 2020.

Temperature

Temperature was measured using temperature probes inserted into three carcasses per pit at the time of burial to assess biological activity and potential decomposition. Temperatures were compared hourly, and all treatments were statistically different from each other. Temperatures in pits constructed using wood shavings were significantly (P < 0.001) higher by 0.9°C than those in pits using corn stalks. Both were significantly (P < 0.001) higher than the air temperature by 5.6 and 4.7°C for wood shaving and corn stover, respectively. Similarly, when compared to soil temperature, wood shaving and corn stover demonstrated an average increase (P < 0.001) of 1.8 and 2.5°C, respectively (fig. 6). Higher temperatures were maintained for longer into the winter in the burial pit established in June as compared to the pit established in November. Overall, the November burial exhibited a significantly higher temperature (1.17°C, P < 0.001) compared to the June burial for wood shavings, while it showed a lower temperature (0.68°C, P < 0.001) for cornstalks (fig. 7).

The November burial was significantly higher overall than the air and 20.3 cm (8 in) soil temperature, but there was less difference from ground temperature through the winter. This is expected since microbial activity decreases at cooler temperatures.

Carcasses were required to be opened in the June burial. To test the effect of opening the carcasses, a separate trench was dug in August to compare the opened and intact carcasses. Interestingly, temperatures in the pits receiving intact carcasses were significantly (P < 0.001) higher by 0.5°C than in those receiving opened carcasses. While statistically significant, the relatively small difference in median temperature between the two approaches nonetheless indicated similar microbial activity and decomposition in both intact and opened carcasses (fig. 8).

Discussion

Above-ground burial (AGB) presents both advantages and disadvantages over other mortality disposal methods. Compared to grinding and composting, farmers were much more likely to have both equipment and materials on-site that could be used for AGB disposal. Since all equipment and animals stay on-site, contamination risk is dramatically reduced (Liu et al., 2019). Grinding operations for mass mortality operations are typically performed by moving an industrial grinder from farm to farm or bringing the culled animals to a central location. Both options move equipment or animals off the farm and present a risk of disease spread. However, carcass breakdown with above-ground burial does not reach as high of a temperature and has been shown to occur more slowly than with composting (Kalbasi et al., 2005).

From a water quality perspective, there were four mechanisms preventing downward migration of pollutants and possible groundwater contamination: inactivation of disease due to elevated temperatures (Kalbasi et al., 2005); physical and chemical filtration of soluble and particulate forms of bacteria and nutrients (Berge et al., 2009); competition from beneficial bacteria, which reduced the number of harmful bacteria (Berge et al., 2009); and nitrogen cycling due to the presence of anaerobic conditions, carbon, and denitrifying bacteria (Bednarik et al., 2014).

Figure 6. Comparison of temperatures between the trenches with wood shavings, the trenches with cornstalks, and the 20 cm (8 in) control depth soil temperature over the course of the experiment.
Figure 7. Comparison of the temperature between the June burial, November burial, and 20 cm (8 in) control depth soil temperature over the course of the experiment.

Carcass temperatures were significantly higher than a roughly equivalent-depth soil temperature, which indicated that there was significant microbial activity within the trenches. However, carcass temperatures were not shown to exceed those required for the termination of certain diseases (Wilkinson, 2007). Composting utilizes high temperatures (>55°C) to inactivate disease (Kalbasi et al., 2005; Pepin et al., 2021), but the trenches were not shown to reach similar temperatures seen in composting. Very wet conditions after the initial burial created a worst-case scenario since rainfall events prevented the ideal moisture conditions needed for composting. While SVV detections were limited and never at levels that were able to transmit the disease to living pigs in the bioassay, E. coli concentrations in later samples indicate that E. coli was able to colonize and remain viable or continue to source from the decomposing carcasses. While pathogen inactivation was not achieved through high temperatures, mesophilic bacteria fostered in the AGB environment could misrepresent the true mechanism of pathogen inactivation. Realistically, pathogen inactivation in the AGB environment is more likely due to a combination of predation by microbial populations, a shift in pH, and the absence of a living host (Lepesteur, 2022).

Figure 8. Comparison of the temperature between intact swine carcasses, opened swine carcasses, and the 20 cm (8 in) control depth soil temperature over the course of the experiment.

Overall, results indicate that care should be taken to prevent the leaching of E. coli to sensitive aquifers. While summer burial using wood shavings exhibits less risk for E. coli transport in leachate numerically (not statistically significant), all treatments demonstrated high concentrations at the deepest well depth 91.4 cm (36 inch) below the bottom of the trench (approximately 152.4 cm below the carcasses). To mitigate the risk of groundwater contamination with E. coli, the use of AGB is not recommended in areas with sandy or loamy soils with high hydraulic conductivity, a high-water table, or a sensitive aquifer. When possible, the establishment of AGB pits during the summer is preferable to jumpstart decomposition prior to any significant downward water movement through the soil.

Overall, nitrate-N concentrations in leachate were lower than results from other previous research trials, which ranged from 39.7 mg/L to 403 mg L-1 (Chowdhury et al., 2019). Nitrate-N was shown not to create elevated risk under the wood shaving burial but was shown to create a concern under corn stalks. The difference was attributed to hydraulic retention time or carbon availability. While not measured directly, the movement of nitrate-N was possibly due to the hydraulic conductivity of the corn stalks compared to the wood shavings. Previous research of denitrifying woodchip bioreactors has demonstrated that hydraulic conductivity plays a significant role for nitrate transformation and removal (Duncan, 2022). While this parameter was not directly measured, the wood shavings were uniform in shape and packed more evenly than the corn stalks when carcasses were laid on top. This difference in shape was assumed to lead to increased macropores, or pathways, where water could quickly move through the corn stalks but not the wood shavings. For flow through woodchip bioreactor media, differences in saturated hydraulic conductivity were observed for woodchips compared to corn stover and barley straw (Feyereisen and Christianson, 2015). Bioreactors typically reduce nitrate concentration by 45% or more (Christianson et al., 2021).

With 50 carcasses buried in each trench, a 2,400-head finishing barn would require about 48 trenches of this size. Allowing for a 3 m (10 ft) buffer on all sides of each trench, 48 trenches would occupy about 0.93 ha of ground. For most farming operations in the Midwestern US, access to this amount of land would be feasible.

Conclusions

Above-ground burial was demonstrated to be a viable option for carcass disposal that uses equipment and materials commonly available on most farming operations. This method prevents the spread of disease by keeping all carcasses and equipment on the farm. Above-ground burial effectively leads to carcass breakdown, even under wet conditions, as were present in the first part of the study. While wood shavings effectively limited nitrate-N movement downward into the soil profile, E. coli was present at high concentrations during the summer months for the duration of the study. Care should be taken that above-ground burial should not be used over sensitive aquifers or over soils with high hydraulic conductivity. In the case of the need for mass mortality disposal, suitable soil and water table surveys should be conducted so mortalities can be buried without an extensive site evaluation during an emergency. A fully scale burial of a 2,400-head finishing barn would require about 0.93 ha of ground.

Additional research is required to evaluate performance with other soil types and under different weather conditions to further optimize this approach. Additional factors that must be considered are whether carcasses could be stacked, the long-term effects, the viability of above-ground burial on soil, and the rehabilitation of that ground into farmland.

Acknowledgments

Thank you to Minnesota Pork, the South Dakota Pork Producers, the Iowa Pork Producers Association, the Nebraska Pork Producers Association, and the South Dakota Soybean Association for their financial support of this project.

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Nomenclature

SVV = Seneca Valley Virus

AGB = Above-Ground Burial

ASFV = African Swine Fever virus