Manure and Mortality Composting
Because of the small amount of grass acreage on our farm, concern about overuse of chemical dewormers and the need for a good organic fertilizer, all manure is regularly collected from our pastures, in addition to the stalls and corrals (Fig. 1). As you can imagine, this practice generates a large amount of waste for seven equines. The manure is not returned to the pastures as fertilizer, or used on our gardens and orchards, until it has been composted in an aerobic pile for at least 1-2 years. The composting process destroys internal parasites and most pathogens, and reduces the concentration of veterinary drugs to very low levels. The first part of this web page briefly discusses our composing program and the management of piles necessary to produce a good organic fertilizer.
Because of the small amount of grass acreage on our farm, concern about overuse of chemical dewormers and the need for a good organic fertilizer, all manure is regularly collected from our pastures, in addition to the stalls and corrals (Fig. 1). As you can imagine, this practice generates a large amount of waste for seven equines. The manure is not returned to the pastures as fertilizer, or used on our gardens and orchards, until it has been composted in an aerobic pile for at least 1-2 years. The composting process destroys internal parasites and most pathogens, and reduces the concentration of veterinary drugs to very low levels. The first part of this web page briefly discusses our composing program and the management of piles necessary to produce a good organic fertilizer.
Figure 1. Yes, I REALLY do pickup this stuff by hand! Pictured with my assistant Gus. Pasture scooping is only interrupted by the occasional “stick throwing”!
The second part examines the topic of mortality composting as an alternative to rendering, or burial of equine carcasses. Proper disposal of carcasses becomes increasingly important from an environmental perspective when diseased animals, or those euthanized by barbiturate overdose, are considered.
The proper management of animal wastes and carcasses is critical in karst limestone areas, such as The Mitchell Plain, which are more susceptible to pollution of groundwater resources.
Composting horse manure
There are many good sources of information on composting on the Web and in the scientific literature. One particularly good reference is the On-Farm Composting Handbook (NRAES-54) ©1992 by NRAES (Natural Resource, Agriculture, and Engineering Service). Refer to scanned sections of some of the chapters of the handbook on the Cornell Composing Page until the new edition becomes available electronically. Information presented on this page is also based, in part, on the publications and research programs of the Cornell Waste Management Institute and the Washington State University Puyallup, Organic Farming Systems and Nutrient Management Program. The following discussion assumes that you are familiar with the information in the scanned sections of Chapter 2 in the NRAES Handbook (“What happens during composting" and "Compost microorganisms"). If you are interested in a more general, less scientific introduction to composting horse manure refer to articles in online horse magazines, such as EQUUS.
The proper management of animal wastes and carcasses is critical in karst limestone areas, such as The Mitchell Plain, which are more susceptible to pollution of groundwater resources.
Composting horse manure
There are many good sources of information on composting on the Web and in the scientific literature. One particularly good reference is the On-Farm Composting Handbook (NRAES-54) ©1992 by NRAES (Natural Resource, Agriculture, and Engineering Service). Refer to scanned sections of some of the chapters of the handbook on the Cornell Composing Page until the new edition becomes available electronically. Information presented on this page is also based, in part, on the publications and research programs of the Cornell Waste Management Institute and the Washington State University Puyallup, Organic Farming Systems and Nutrient Management Program. The following discussion assumes that you are familiar with the information in the scanned sections of Chapter 2 in the NRAES Handbook (“What happens during composting" and "Compost microorganisms"). If you are interested in a more general, less scientific introduction to composting horse manure refer to articles in online horse magazines, such as EQUUS.
Our manure and bedding is composted in relatively small piles measuring about 4-5 feet in height (Fig. 2). The pile is constructed over a period of about 1-2 months with frequent turning (about once every 3-4 days). If the piles are too large the weight of the overlying material will compress the compost, decrease the porosity (bulk density) and limit the passage of air and oxygen through the pile, reducing the activity of aerobic microorganisms.
We have found that piles much smaller than those shown in Fig. 2 lack the critical mass and insulating properties necessary to achieve high internal temperatures (~ 60°C = 140°F). A typical thermometer for measuring temperatures within the compost pile is shown in Fig. 3.
If the piles are not turned, the lack of oxygen in the center of pile produces an anaerobic environment . These piles usually have a distinctive odor due to the release of ammonia and hydrogen sulfide gases and can form chemical substances that are toxic to plants.
We have found that piles much smaller than those shown in Fig. 2 lack the critical mass and insulating properties necessary to achieve high internal temperatures (~ 60°C = 140°F). A typical thermometer for measuring temperatures within the compost pile is shown in Fig. 3.
If the piles are not turned, the lack of oxygen in the center of pile produces an anaerobic environment . These piles usually have a distinctive odor due to the release of ammonia and hydrogen sulfide gases and can form chemical substances that are toxic to plants.
Figure 2. Compost piles in various stages of development. The wooden palette in the background is 4 feet in height. The pile in the center has reached maturity and is no longer generating heat on turning. Its darker color reflects the greater proportion of straw bedding to manure. The more reddish pile in the foreground was composted with wood pellets rather than straw bedding.
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Figure 3. Stainless steel composting thermometer with 3-foot probe. Dial is hermetically sealed to prevent fogging during temperature measurements.
Initially, the piles consisted primarily of straw bedding mixed with manure. In the more recent piles, wood pellets have replaced the straw bedding, which is reflected in the more reddish color of the compost. Straw bedding produces a dark, rich humus (Fig. 4) but wood pellets are less expensive, and require less maintenance and waste management. For more information on wood pellet bedding refer to the article " Wood Pellets as an Alternative Stall Bedding Material" by Peter Moon, P.E.
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Figure 4. Compost texture in darker pile after about 1 year. Small white particles are chips of limestone. Some of the plant material has not decomposed.
The Organic Farm Systems and Nutrient Management Program at Washington State University Puyallup (WSU-Puyallup) has done some interesting research on the carbon to nitrogen ratio (C:N) and moisture content (%) of horse manure with and without bedding. However, only paper and wood shavings bedding were considered and their results for horse manure without bedding are dependent on environment and diet. Therefore, we decided to collect analytical data specific to our Midwest horse farm.
Fresh horse manure (without bedding), wood pellet bedding, and finished compost were collected on our farm and analyzed for C:N ratio and moisture content (%) by A & L Great Lakes Laboratories in Ft. Wayne, Indiana (Manure Test Package MCNC) (Table 1).
The C:N ratio and moisture content of our fresh manure are similar to the average values reported by WSU-Puyallup (20.2:1; 75%). However, Total N% is ~70% lower than that determined in their study (0.466 vs. 1.59%). Our result is also lower than the average value (1.6%) and outside the range (1.4-2.3%) tabulated for horse manure (general) in the Appendix of the On-Farm Composting Handbook.
A composite manure sample was collected in the pastures during the morning over a period of about 1 hr and then sealed in a 1-gallon Ziploc storage bag before shipping. The diet for all horses during the previous day consisted of pasture plants (primarily fescue) and a small amount of orchard grass/alfalfa hay in the evening (last summer’s second cutting with about 30-50% alfalfa). Fescue grass has about the same nutrient value (e.g., crude protein and total digestible nutrients) as other common grass forages in this region, such as Kentucky bluegrass and orchard grass (RL Preston, 2011 Feed Composition Tables, beefmagazine.com, March 2011). Fresh alfalfa is the only forage with higher crude protein levels.
Two possible reasons for the lower total N% in our horse manure are: 1) As part of our parasite control program, we regularly remove manure from our pastures, which have not been fertilized with finished compost for several years. Without some type of fertilizer, N uptake from the soil is limited, and 2) Grains, or commercial feeds, are not included in the diet of our mustangs.
If you haven’t visited the other pages on this web site, you should know that our horses are not stabled and have 24/7 access to pasture. As a result, our bedding requirements are much less than those of the average horse owner. Most of our compost piles have a significantly higher proportion of horse manure to bedding.
The C:N ratio and moisture content of our manure are close to the optimal starting values (C:N ratio ~30, moisture content ~40-60%) commonly quoted in the literature for rapid composting. This is why horse manure without bedding, or with smaller amounts of bedding, makes good compost.
An initial C:N ratio of about 30:1 is optimal because the microbes use about 30 parts carbon for every 1 part nitrogen. If the C:N ratio is too high the nitrogen will be depleted, decreasing microbe populations and lowering the temperature in the compost pile. If the C:N ratio is too low, the nutrient value of the compost will be less because the excess nitrogen is released to the atmosphere as ammonia gas.
In our case, the addition of pellet bedding to the fresh manure will not increase the C:N ratio of the compost pile above the optimal value because the pellets analyzed have a low C:N ratio (41.4:1 w/o urine, 18.2:1 w/urine; Table 1) relative to other types of bedding (w/o urine), such as wheat straw (~127:1), sawdust (~442:1) and wood shavings (~641:1), and less pellet bedding is required because of its greater absorbency.
As a matter of curiosity, you might be wondering why these wood pellets have a C:N ratio an order of magnitude (10Xs) less than sawdust. After all, pellets are essentially compressed sawdust. According to the manufacturer, their pellet bedding is "produced from byproducts of renewable resources..." The lower C:N ratio is due to higher N content (1.306%, Table 1), which is anomalously high for both softwoods (N% range 0.04-0.23) and hardwoods (N% range 0.06-0.11).
Table 2 compares C:N ratios, Total N% and moisture content (%) of several different types of horse bedding available in Harrison County, Indiana (Analyses by A & L Great Lakes Laboratories, MCNC Manure Package).
A composite manure sample was collected in the pastures during the morning over a period of about 1 hr and then sealed in a 1-gallon Ziploc storage bag before shipping. The diet for all horses during the previous day consisted of pasture plants (primarily fescue) and a small amount of orchard grass/alfalfa hay in the evening (last summer’s second cutting with about 30-50% alfalfa). Fescue grass has about the same nutrient value (e.g., crude protein and total digestible nutrients) as other common grass forages in this region, such as Kentucky bluegrass and orchard grass (RL Preston, 2011 Feed Composition Tables, beefmagazine.com, March 2011). Fresh alfalfa is the only forage with higher crude protein levels.
Two possible reasons for the lower total N% in our horse manure are: 1) As part of our parasite control program, we regularly remove manure from our pastures, which have not been fertilized with finished compost for several years. Without some type of fertilizer, N uptake from the soil is limited, and 2) Grains, or commercial feeds, are not included in the diet of our mustangs.
If you haven’t visited the other pages on this web site, you should know that our horses are not stabled and have 24/7 access to pasture. As a result, our bedding requirements are much less than those of the average horse owner. Most of our compost piles have a significantly higher proportion of horse manure to bedding.
The C:N ratio and moisture content of our manure are close to the optimal starting values (C:N ratio ~30, moisture content ~40-60%) commonly quoted in the literature for rapid composting. This is why horse manure without bedding, or with smaller amounts of bedding, makes good compost.
An initial C:N ratio of about 30:1 is optimal because the microbes use about 30 parts carbon for every 1 part nitrogen. If the C:N ratio is too high the nitrogen will be depleted, decreasing microbe populations and lowering the temperature in the compost pile. If the C:N ratio is too low, the nutrient value of the compost will be less because the excess nitrogen is released to the atmosphere as ammonia gas.
In our case, the addition of pellet bedding to the fresh manure will not increase the C:N ratio of the compost pile above the optimal value because the pellets analyzed have a low C:N ratio (41.4:1 w/o urine, 18.2:1 w/urine; Table 1) relative to other types of bedding (w/o urine), such as wheat straw (~127:1), sawdust (~442:1) and wood shavings (~641:1), and less pellet bedding is required because of its greater absorbency.
As a matter of curiosity, you might be wondering why these wood pellets have a C:N ratio an order of magnitude (10Xs) less than sawdust. After all, pellets are essentially compressed sawdust. According to the manufacturer, their pellet bedding is "produced from byproducts of renewable resources..." The lower C:N ratio is due to higher N content (1.306%, Table 1), which is anomalously high for both softwoods (N% range 0.04-0.23) and hardwoods (N% range 0.06-0.11).
Table 2 compares C:N ratios, Total N% and moisture content (%) of several different types of horse bedding available in Harrison County, Indiana (Analyses by A & L Great Lakes Laboratories, MCNC Manure Package).
The data indicate that Pellets I contains nitrogen compounds not found in bedding produced from byproducts of pine logs. Possible N sources in recycled wood products include glues, resins and preservatives in composite materials, such as plywood, fiberboard and particle board. Studies have shown that modern chemicals present in composites will readily degrade in the environment (Composting of Composite Wood Products: Literature Review, Recycled Organics Unit, The University of New South Wales, Sydney, Australia, 2007). However, the rate of decomposition will be much higher if the recycled material (e.g., pellet bedding after use) is composted under the conditions discussed above. To my knowledge, there are no studies of possible health effects of these synthetic chemicals on stabled horses.
As mentioned above, one disadvantage of bedding produced from log byproducts (samples 2, 3 & 4, Table 2) is their high C:N ratio, which must be compensated for by the addition of high-N materials to the compost pile. For example, poultry (laying hens, C:N: 3-10:1), and sheep/goat (C:N:13-20:1) manures, or vegetable wastes (C:N: 11-13:1), can reduce the C:N ratio to values closer to ~30:1. You calculate the overall C:N ratio of your compost pile if you know the C:N ratio, mass and moisture content (%) of each material you would like to compost.
Based on C:N ratio only, straw is preferred over wood byproducts. However, conventional straw bedding is less absorbent and, therefore, significantly increases the initial volume of waste relative to wood pellets. An attractive alternative to long straw is a pelletized form called STREUfex, available in Harrison County through Longbottom and Hardsaw, Central, Indiana.
Komar et al. (2012) have investigated the effect of different types of bedding (i.e., hardwood pellet, shavings, long rye-straw and pelletized straw) on the physical and chemical properties of composted equine wastes. One interesting conclusion is that the optimum temperature required for microbial degradation was maintained for the longest period of time with straw bedding. The initial C:N ratios in compost piles containing the four different types of bedding (manure plus bedding) varied between 55 (long straw) and 88 (hardwood pellet). After composting, the piles with wood byproducts had greater C:N ratios (49 and 40 vs. 28 and 27) and less Total N% (1.05 and 1.22 vs. 1.72 and 1.55) than those with straw-based materials. The reduction in dry mass of the piles during composting was approximately the same for all bedding types. Based on the results of this study and the presence of N-contaminants in pellets produced from recycled wood byproducts, we are now using the STREUfex straw pellets.
The N content in our finished compost is slightly higher than that in the fresh manure (0.581 vs. 0.466%). The relatively low N content of the compost reflects, in part, the smaller amount of bedding in our manure piles. Total N%, which is determined by the Kjeldahl method, consists of organic N and inorganic ammonium and nitrate ion. The inorganic forms are immediately available to plants. In contrast, organic N must first be mineralized, a microbial process that releases ammonium bound in organic compounds, such as plant proteins and amino acids. The ammonium ion is subsequently altered by bacteria to form nitrate, which is readily assimilated by plants. This process occurs slowly over several years. As shown below, the nitrogen cycle can be used to describe the biological and chemical interactions between compost and soil (Fig. 5).
As mentioned above, one disadvantage of bedding produced from log byproducts (samples 2, 3 & 4, Table 2) is their high C:N ratio, which must be compensated for by the addition of high-N materials to the compost pile. For example, poultry (laying hens, C:N: 3-10:1), and sheep/goat (C:N:13-20:1) manures, or vegetable wastes (C:N: 11-13:1), can reduce the C:N ratio to values closer to ~30:1. You calculate the overall C:N ratio of your compost pile if you know the C:N ratio, mass and moisture content (%) of each material you would like to compost.
Based on C:N ratio only, straw is preferred over wood byproducts. However, conventional straw bedding is less absorbent and, therefore, significantly increases the initial volume of waste relative to wood pellets. An attractive alternative to long straw is a pelletized form called STREUfex, available in Harrison County through Longbottom and Hardsaw, Central, Indiana.
Komar et al. (2012) have investigated the effect of different types of bedding (i.e., hardwood pellet, shavings, long rye-straw and pelletized straw) on the physical and chemical properties of composted equine wastes. One interesting conclusion is that the optimum temperature required for microbial degradation was maintained for the longest period of time with straw bedding. The initial C:N ratios in compost piles containing the four different types of bedding (manure plus bedding) varied between 55 (long straw) and 88 (hardwood pellet). After composting, the piles with wood byproducts had greater C:N ratios (49 and 40 vs. 28 and 27) and less Total N% (1.05 and 1.22 vs. 1.72 and 1.55) than those with straw-based materials. The reduction in dry mass of the piles during composting was approximately the same for all bedding types. Based on the results of this study and the presence of N-contaminants in pellets produced from recycled wood byproducts, we are now using the STREUfex straw pellets.
The N content in our finished compost is slightly higher than that in the fresh manure (0.581 vs. 0.466%). The relatively low N content of the compost reflects, in part, the smaller amount of bedding in our manure piles. Total N%, which is determined by the Kjeldahl method, consists of organic N and inorganic ammonium and nitrate ion. The inorganic forms are immediately available to plants. In contrast, organic N must first be mineralized, a microbial process that releases ammonium bound in organic compounds, such as plant proteins and amino acids. The ammonium ion is subsequently altered by bacteria to form nitrate, which is readily assimilated by plants. This process occurs slowly over several years. As shown below, the nitrogen cycle can be used to describe the biological and chemical interactions between compost and soil (Fig. 5).
Figure 5. Schematic representation of the nitrogen cycle. Decomposers convert organic nitrogen in compost to inorganic ammonium ion by the process of mineralization, or ammonification. Nitrifying bacteria alter (oxidize) ammonium to nitrate ion, which is readily assimilated by soil plants. Figure from Wikimedia Commons.
The C:N ratio of our compost (15.2:1) is within the range of values (10:1-20:1) found in a quality product, i.e., a compost that releases nitrogen to the soil (~12 lbs N per ton compost). If this ratio is above ~20:1, nitrogen in the soil will be used by bacteria to continue decomposition of the wood or straw particles, depleting the soil of some nitrogen. During this time, the nitrogen will be immobilized by bacteria and unavailable to plants. This is the reason why the application of high-carbon (C:N >> ~30:1) fresh manures will create a nitrogen deficiency in the soil.
The fate of internal parasites, pathogens and veterinary drugs in your compost pile
One of the reasons for composting is to destroy all internal equine parasites in the manure before reusing it as a fertilizer. Hébert et al. (2010) have reported a substantial reduction in the viability of roundworm eggs (Parascaris equorum) in less than two hours at 55°C and 60°C. These temperatures can be easily achieved over a period of several days in our piles with frequent turning of the compost. Since we began composting, no roundworm eggs have been observed in fecal analyses of any of our mustang horses, including our young filly who was born on the farm.
It is also important to reduce pathogenic levels during composting to 1) protect soil and water resources from contamination by pathogen-containing fluids passing through the compost (leachates) into the environment, and 2) avoid infection of pastures and vegetable gardens fertilized with composted manure.
It is also important to reduce pathogenic levels during composting to 1) protect soil and water resources from contamination by pathogen-containing fluids passing through the compost (leachates) into the environment, and 2) avoid infection of pastures and vegetable gardens fertilized with composted manure.
A number of studies have shown that less hardy pathogens, such as Escherichia coli and Salmonella, can be eliminated at temperatures produced during the thermophilic stage (~ 40-70°C, or 105-170°F). What is the effect of composting on more resistant bacteria? Mycobacterium avium paratuberclosis is a very resistant bacterium that causes Johne’s (pronounced Joh-nees) disease in ruminants. Bonhotal et al. (2011) were unable to culture this bacterium after 5 days of thermophilic composting at temperatures greater than 55°C. Continued monitoring at thermophilic temperatures showed that the bacterium remained unculturable through 70 days.
Bonhotal et al. did not investigate the presence of Mycobacterium avium paratuberclosis in the matured compost. According to the authors, it is possible that this bacterium became dormant (state of low metabolic activity without cell division) in response to temperature-induced stress and could reappear in the mesophilic, or curing stage (~ 10-40°C, or 50-105°F), when the temperatures in the compost pile are more favorable.
Bonhotal et al. did not investigate the presence of Mycobacterium avium paratuberclosis in the matured compost. According to the authors, it is possible that this bacterium became dormant (state of low metabolic activity without cell division) in response to temperature-induced stress and could reappear in the mesophilic, or curing stage (~ 10-40°C, or 50-105°F), when the temperatures in the compost pile are more favorable.
There has been a limited amount of research on pathogens in horse manure. The few studies that have been conducted indicate very low levels of pathogenic organisms. One of the most cited papers involved an analysis by Derlet and Carlson (2002) of potential human pathogens collected from horse/mule samples along trails in the Sequoia and Yosemite National Parks. No Escherichia coli O157 or Salmonella were found in any of the samples.
Leptospira are zoonotic bacteria that can cause abortions in pregnant mares in late gestation. Twenty-four cases of equine leptospirosis abortion have been confirmed during the 2011-2012 season in Central Kentucky. The bacteria originate from the urine of wild animals and are transmitted by contact with the skin and mucous membranes of horses. Schwarz et al. (2010) have shown that composting of road-kill deer carcasses can reduce this pathogen to near zero levels, suggesting that careful composting of equine wastes can help prevent the spread of this pathogen in pastures and vegetable gardens.
There is one last undesirable element in horse manure that we need to eliminate - veterinary drugs! For a list of major drugs in terms of usage, refer to Table 1 in the article “Are Veterinary Medicines Causing Environmental Risks? ” The average horse owner is probably very familiar with some of the drugs on this list: antimicrobials or antibiotics, the non-steroidal anti-inflammatory drugs (NSAIDs) phenylbutazone (bute) and banamine, anthelmintics (dewormers), such as ivermectin, and the euthanasia drug sodium pentobarbital.
Leptospira are zoonotic bacteria that can cause abortions in pregnant mares in late gestation. Twenty-four cases of equine leptospirosis abortion have been confirmed during the 2011-2012 season in Central Kentucky. The bacteria originate from the urine of wild animals and are transmitted by contact with the skin and mucous membranes of horses. Schwarz et al. (2010) have shown that composting of road-kill deer carcasses can reduce this pathogen to near zero levels, suggesting that careful composting of equine wastes can help prevent the spread of this pathogen in pastures and vegetable gardens.
There is one last undesirable element in horse manure that we need to eliminate - veterinary drugs! For a list of major drugs in terms of usage, refer to Table 1 in the article “Are Veterinary Medicines Causing Environmental Risks? ” The average horse owner is probably very familiar with some of the drugs on this list: antimicrobials or antibiotics, the non-steroidal anti-inflammatory drugs (NSAIDs) phenylbutazone (bute) and banamine, anthelmintics (dewormers), such as ivermectin, and the euthanasia drug sodium pentobarbital.
Although the behavior of veterinary drugs in compost piles is poorly understood, a number of studies have shown that certain classes of drugs are degraded or reduced in concentration during composting.
In a study of four antibiotics, Dolliver et al. (2008) found that the concentrations of chlortetracycline, monensin and tylosin all declined in managed compost piles (frequent turning), with a reduction of >99% in chlortetracycline after 22-35 days. Sulfamethazine (SMZ), a very common antibiotic used in a variety of animals, including equines, showed no degradation during the same time period. The composted mixture consisted of turkey manure with aspen shavings and sunflower hulls for an overall C:N ratio of 13.3.
Most of the antibiotic dose given to animals (75%) is excreted in the urine and feces. The fact that there was no reduction in SMZ concentration in this particular study is troublesome because SMZ is not strongly adsorbed (attracted) to solids and is one of the more mobile antibiotics in the environment (Kwon 2011). The concern is that drug residues will infiltrate into soils and spread to the groundwater and surface streams where they can produce antibiotic-resistant strains of bacteria.
In a more recent study, Kim et al. (2011) found that the concentration of SMZ was reduced to very low levels in a compost mixture consisting of pig manure and sawdust. Composting the manure alone without an additional source of organic matter did not significantly affect the SMZ concentration, consistent with the study by Dolliver et al. (2008). The higher organic content (higher C:N ratio) of the manure/sawdust mixture resulted in higher composting temperatures and more possible binding sites for SMZ on particle surfaces (adsorption), which immobilized and degraded the drug. This study suggests that starting C:N ratios much lower than 30:1 are probably insufficient for reducing the concentration of some veterinary drugs, such as SMZ, during composting.
Ramaswamy et al. (2009) investigated the effect of composting on the degradation of salinomycin, an antibiotic used for coccidia infection in poultry and ruminants. Poultry manure was mixed with hay in 120 L plastic containers (C:N ratio = 25:1) and composted for 38 days. Temperatures in the compost mixture reached a maximum of ~ 65°C during the thermophilic stage and then cooled gradually to ~ 30°C after 38 days. The concentration of salinomycin was reduced by > 99% over this time period.
Ivermectin, doramectin (dectomax or doramax) and abamectin are different formulations of avermectins, or macrolytic lactones, common drugs used in domestic animals for the treatment of internal parasites. Unlike some veterinary drugs, avermectins have a high affinity for organic matter in the feces and soils. Doramectin, in particular, can persist in the environment for long periods of time. The problem is that avermectins are potent insecticides and can affect the life cycles of beneficial bugs if these drugs are not contained and degraded during composting. In one of the few studies on the degradation of avermectins during composting, Celestina et al. (2009) found that doramectin concentrations decreased by ~ 40% after 21 days at thermophilic temperatures in a composting reactor. Field investigations of compost piles under controlled conditions will be required to test the results of this laboratory experiment.
The breakdown of veterinary drugs during composting is discussed further in the next section on mortality composting.
In a study of four antibiotics, Dolliver et al. (2008) found that the concentrations of chlortetracycline, monensin and tylosin all declined in managed compost piles (frequent turning), with a reduction of >99% in chlortetracycline after 22-35 days. Sulfamethazine (SMZ), a very common antibiotic used in a variety of animals, including equines, showed no degradation during the same time period. The composted mixture consisted of turkey manure with aspen shavings and sunflower hulls for an overall C:N ratio of 13.3.
Most of the antibiotic dose given to animals (75%) is excreted in the urine and feces. The fact that there was no reduction in SMZ concentration in this particular study is troublesome because SMZ is not strongly adsorbed (attracted) to solids and is one of the more mobile antibiotics in the environment (Kwon 2011). The concern is that drug residues will infiltrate into soils and spread to the groundwater and surface streams where they can produce antibiotic-resistant strains of bacteria.
In a more recent study, Kim et al. (2011) found that the concentration of SMZ was reduced to very low levels in a compost mixture consisting of pig manure and sawdust. Composting the manure alone without an additional source of organic matter did not significantly affect the SMZ concentration, consistent with the study by Dolliver et al. (2008). The higher organic content (higher C:N ratio) of the manure/sawdust mixture resulted in higher composting temperatures and more possible binding sites for SMZ on particle surfaces (adsorption), which immobilized and degraded the drug. This study suggests that starting C:N ratios much lower than 30:1 are probably insufficient for reducing the concentration of some veterinary drugs, such as SMZ, during composting.
Ramaswamy et al. (2009) investigated the effect of composting on the degradation of salinomycin, an antibiotic used for coccidia infection in poultry and ruminants. Poultry manure was mixed with hay in 120 L plastic containers (C:N ratio = 25:1) and composted for 38 days. Temperatures in the compost mixture reached a maximum of ~ 65°C during the thermophilic stage and then cooled gradually to ~ 30°C after 38 days. The concentration of salinomycin was reduced by > 99% over this time period.
Ivermectin, doramectin (dectomax or doramax) and abamectin are different formulations of avermectins, or macrolytic lactones, common drugs used in domestic animals for the treatment of internal parasites. Unlike some veterinary drugs, avermectins have a high affinity for organic matter in the feces and soils. Doramectin, in particular, can persist in the environment for long periods of time. The problem is that avermectins are potent insecticides and can affect the life cycles of beneficial bugs if these drugs are not contained and degraded during composting. In one of the few studies on the degradation of avermectins during composting, Celestina et al. (2009) found that doramectin concentrations decreased by ~ 40% after 21 days at thermophilic temperatures in a composting reactor. Field investigations of compost piles under controlled conditions will be required to test the results of this laboratory experiment.
The breakdown of veterinary drugs during composting is discussed further in the next section on mortality composting.
Mortality composting: A better alternative for the environment
If you have been following the debate about the environmental effects of confined animal feeding operations (CAFOs), you probably know that composting is the accepted method for disposing of animal carcasses at these facilities. Although you may be opposed to CAFOs, this aspect of the operation (mortality composting), if done correctly, is an effective and environmentally responsible way to degrade both the pathogens and veterinary pharmaceuticals produced by decomposition of animal carcasses. There is some good scientific data to backup this claim based on recent field research conducted by the Waste Management Institute at Cornell University.
The composting process on our farm can best be described as rapid aerobic. Oxygen required by the microbes is supplied by frequent turning of the pile. In the case of mortality composting, it may not be feasible and it is not advisable, at least in the early stages of the composting process (first 4-6 months), to turn the pile. However, aerobic decomposition is still possible because the porosity of composting material (wood chips) that encases the carcass provides some natural aeration. This method is referred to as static passively aerated. The design of the pile is very simple and the materials needed to construct the pile are readily available at low cost. An illustrated fact sheet that describes the process and construction of the compost pile is available on the Cornell Waste Management Institute (CWMI) web site. Also included on the CWMI site is a horse specific fact sheet, "Being Prepared for the Worst: Develop a Health Care Plan for Your Horse “A Living Will For Your Horse”, which discusses composting as well as other disposal options. An accompanying DVD is also available. This YouTube video from Cornell Composting illustrates the methods recommended for natural rendering (composting) of horse carcasses. For those who reside in this area, first check with the Indiana State Board of Animal Health (BOAH) for policies on "Dead Animal Disposal".
The composting process on our farm can best be described as rapid aerobic. Oxygen required by the microbes is supplied by frequent turning of the pile. In the case of mortality composting, it may not be feasible and it is not advisable, at least in the early stages of the composting process (first 4-6 months), to turn the pile. However, aerobic decomposition is still possible because the porosity of composting material (wood chips) that encases the carcass provides some natural aeration. This method is referred to as static passively aerated. The design of the pile is very simple and the materials needed to construct the pile are readily available at low cost. An illustrated fact sheet that describes the process and construction of the compost pile is available on the Cornell Waste Management Institute (CWMI) web site. Also included on the CWMI site is a horse specific fact sheet, "Being Prepared for the Worst: Develop a Health Care Plan for Your Horse “A Living Will For Your Horse”, which discusses composting as well as other disposal options. An accompanying DVD is also available. This YouTube video from Cornell Composting illustrates the methods recommended for natural rendering (composting) of horse carcasses. For those who reside in this area, first check with the Indiana State Board of Animal Health (BOAH) for policies on "Dead Animal Disposal".
The CWMI pile design is very effective in eliminating or reducing the levels of pathogens and veterinary drugs. As noted above, pathogens in compost piles containing road-kill deer were reduced to near-zero levels in a relatively short period of time. Experimental piles containing a carcass of an euthanized horse were also monitored over time to determine the changes in the concentration of NSAIDs (phenylbutazone or "bute") and sodium pentobarbital during the composting process (Schwarz et al. 2013). Compost samples and leachate, collected at the bottom of the pile below the wood chip layer, were analyzed chemically. The concentration of both drugs was reduced to the extent (non-detectable to very low levels) that the matured compost was suitable as a soil fertilizer. The reduction in concentration was most significant during the mesophilic, or curing stage, when fungi and actinomycetes play a more important role in the biodecompostion. This result should not be so surprising because cured compost has been commonly used to reduce concentrations of organic pollutants in soils and aquatic environments (a process called compost bioremediation).
The results on pentobarbital are consistent with the earlier study by Cottle et al. (2010), in which drug residues were observed to range from 0.008 to 3.16 parts per million (ppm) (1 ppm = 1 mg solid per 1000 ml solution) after 180 days in compost piles containing carcasses of euthanized equines.
The results on pentobarbital are consistent with the earlier study by Cottle et al. (2010), in which drug residues were observed to range from 0.008 to 3.16 parts per million (ppm) (1 ppm = 1 mg solid per 1000 ml solution) after 180 days in compost piles containing carcasses of euthanized equines.
Finally, in terms of environmental risks, how does mortality composting compare to burial as a disposal method for horses euthanized by barbiturate overdose? Due to the lack of oxygen, animal carcasses undergo anaerobic as opposed to aerobic decomposition during burial. This process occurs over a longer period of time at mesophilic temperatures.
If the carcass is buried at a depth of 4 feet, as required by Indiana Code 15-17-11-20, the soil layer at this depth will be less absorbent than the 2-foot wood chip base used in composting. Wood chips can be added to the bottom of the burial pit but the porosity of this layer will soon be reduced due to the weight of the overlying carcass and soil.
The pentobarbital dosage for euthanasia of an average horse is quite high. The minimum recommended dosage is 100 mg/kg (1 kg = 2.2 lbs). For a 1000-lb horse dosed at 100 mg/kg, 45,455 mg of sodium pentobarbital, or about 114 ml at a concentration of 400 mg/ml, are required. Based on its physicochemical properties, sodium pentobarbital tends to form anions (ions with a negative charge). This means that it is not strongly attracted to the negatively charged surfaces of clays and organics in soils and is very mobile.
In karst terrains, such as those in the Mitchell Plain in southern Indiana, the solution cavities and fractures in limestone rocks can be a direct conduit to the groundwater without filtration. If sodium pentobarbital is not degraded, or altered after burial, this drug could migrate through the clay-rich soils to the groundwater (and cavern systems) where it can persist for long periods of time (Peschka et al. 2006).
Some recent research at Middle Tennessee State University by Dr. Mary Farone has identified bacteria that show enhanced growth on barbiturate media, suggesting that sodium pentobarbital might be biodegraded under aerobic conditions during soil burial (see page 30 of Powerpoint Presentation and Final Report of MTSU Project, pages 17-19, 2011). This is consistent with the work by Schwarz et al. (2013), who found that barbiturate levels in horse liver samples buried in "loose soil" were undetectable (< 10 parts per billion (ppb)) after 83 days.
In addition to barbiturates, one must also consider the fate of pathogens during burial. Although more research is needed in this area, it is generally thought that pathogens from decaying animal carcasses are not eliminated at mesophilic temperatures but rather contained by the soil for indefinite periods of time. This is the reason that mass burial of diseased animal carcasses normally requires constant monitoring, in some cases for 20 or more years. In addition, the rate of decay of soft tissue during burial is significantly slower than that obtained by composting. By analogy, it takes more than 10 years for an unembalmed human body not buried within a coffin to skeletonize. Refer to the review by the National Agricultural Biosecurity Center Consortium at Kansas State University for more information on carcass burial.
One final note: Scavenging of the animal carcass can be a problem if the carcass is not buried deep enough. During the initial stages of decomposition, when scavenging is most likely, the carcass will contain the highest concentrations of sodium pentobarbital, which is fatal to domestic animals and wildlife, particularly raptors. If the carcass is composted, the high thermophilic temperatures will discourage scavenging of the pile.
If the carcass is buried at a depth of 4 feet, as required by Indiana Code 15-17-11-20, the soil layer at this depth will be less absorbent than the 2-foot wood chip base used in composting. Wood chips can be added to the bottom of the burial pit but the porosity of this layer will soon be reduced due to the weight of the overlying carcass and soil.
The pentobarbital dosage for euthanasia of an average horse is quite high. The minimum recommended dosage is 100 mg/kg (1 kg = 2.2 lbs). For a 1000-lb horse dosed at 100 mg/kg, 45,455 mg of sodium pentobarbital, or about 114 ml at a concentration of 400 mg/ml, are required. Based on its physicochemical properties, sodium pentobarbital tends to form anions (ions with a negative charge). This means that it is not strongly attracted to the negatively charged surfaces of clays and organics in soils and is very mobile.
In karst terrains, such as those in the Mitchell Plain in southern Indiana, the solution cavities and fractures in limestone rocks can be a direct conduit to the groundwater without filtration. If sodium pentobarbital is not degraded, or altered after burial, this drug could migrate through the clay-rich soils to the groundwater (and cavern systems) where it can persist for long periods of time (Peschka et al. 2006).
Some recent research at Middle Tennessee State University by Dr. Mary Farone has identified bacteria that show enhanced growth on barbiturate media, suggesting that sodium pentobarbital might be biodegraded under aerobic conditions during soil burial (see page 30 of Powerpoint Presentation and Final Report of MTSU Project, pages 17-19, 2011). This is consistent with the work by Schwarz et al. (2013), who found that barbiturate levels in horse liver samples buried in "loose soil" were undetectable (< 10 parts per billion (ppb)) after 83 days.
In addition to barbiturates, one must also consider the fate of pathogens during burial. Although more research is needed in this area, it is generally thought that pathogens from decaying animal carcasses are not eliminated at mesophilic temperatures but rather contained by the soil for indefinite periods of time. This is the reason that mass burial of diseased animal carcasses normally requires constant monitoring, in some cases for 20 or more years. In addition, the rate of decay of soft tissue during burial is significantly slower than that obtained by composting. By analogy, it takes more than 10 years for an unembalmed human body not buried within a coffin to skeletonize. Refer to the review by the National Agricultural Biosecurity Center Consortium at Kansas State University for more information on carcass burial.
One final note: Scavenging of the animal carcass can be a problem if the carcass is not buried deep enough. During the initial stages of decomposition, when scavenging is most likely, the carcass will contain the highest concentrations of sodium pentobarbital, which is fatal to domestic animals and wildlife, particularly raptors. If the carcass is composted, the high thermophilic temperatures will discourage scavenging of the pile.
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