Figure 1. The Mean Value of Ammonia in Aging Horse Manure Expressed in Parts Per Million
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Horse Manure Aging and Nutrient Content
and Implications for Trails
By Robert E Griswold
Rev May 2001
Abstract
Ammonia and nitrogen in horse manure volatilizes rapidly into the atmosphere when deposited on the ground. Most horse manure is deposited in paddocks or pastures. Unless manure is immediately turned into the soil or deposited directly in a stream, the risk of horse manure on trails providing nutrient enrichment to weeds or contaminating streams will be small. Simple management practices can avoid horse waste deposition in streams by trail horses. This is a companion piece to Horse Urine and Implications for Trails by EnviroHorse, and many of the same references apply.
Background
Some have speculated that horse manure (referred to here as the fecal component without urine) on trails can provide nutrient enrichment for weed seeds. The presence of ammonia in water bodies, even in very small amounts, has been shown to be harmful to some aquatic life, particularly salmonids. However, not all species are as susceptible to ammonia as salmonids. Cattle consistently defecate in stock ponds and these same ponds are often found to be the homes of red-legged frogs and tiger salamanders which are endangered species. 1 Ammonia is not ambient in aquatic environments. 2 Livestock excrement is coming under increasing scrutiny as a potential non-point source of nutrient enrichment from ammonia and nitrogen.
Nitrogen is needed to produce ammonia. In horses, nitrogen comes from the protein fraction of the horse's feed. Horses absorb protein in the form of amino acids directly into their small intestines. In the large intestine, some protein is readily converted into ammonia that is absorbed by the intestinal wall.3 The remaining ammonia that gets incorporated into manure comes from protein that is in the process of being converted to ammonia but has not yet been absorbed by the intestine.
An experiment was performed to determine the volatilization potential for horse manure left exposed to the environment on the surface of the ground.
Method
Manure was collected from three horses from June 5 to June 24, 1998. Daytime temperatures averaged 70 degrees F and nighttime temperatures averaged 50 degrees F. The days were mostly overcast with some sun.
Horses were placed and confined on clean rubber stall mats and kept under constant observation. Five manure samples were collected with a large scoop shovel immediately upon evacuation and placed on a sheet of polyethylene. 1st sample was placed in a sealed clean polyethylene container, then frozen at 0 degrees F by placing in the freezer section of a home refrigerator within 10 minutes of evacuation and kept frozen for transport to the laboratory. The other 4 samples were aged for the following intervals on the polyethylene sheet and exposed to outdoor ambient conditions to approximate the conditions found by small manure piles on the trail or in a pasture. The polyethylene sheet was used to prevent the effects of ground moisture or other variables from influencing the experimental design: 2). 12 hours later, 3). 24 hours later, 4). 1 week later, and 5). 19 days later. Subsequent samples were also frozen prior to transport to the Dana Analytical Laboratory at The University of California, Davis where analysis was performed.
The horses used in this study were members of the Morgan breed. They were similar to one another in size and use. Their weight was 430 kg +/- 10 kg (924 to 968 lb). Their estimated4 protein requirement was 564 grams per day. The estimated amount of protein fed per day was 617 grams total, with 495 grams (est5) from hay and 122 grams from concentrates (label declaration). The feed had a slight excess of protein (53 grams) in the daily ration. The impact of this excess on the nitrogen content of the manure was estimated to be approximately 0.016%
Results
The ammonia content decreased rapidly.
Figure 1. The Mean Value of Ammonia in Aging Horse Manure Expressed in Parts Per Million
Initially, the ammonia content varied considerably, but as the manure aged, this variability decreased markedly. See Fig 2.
Figure 2. Actual value of all samples taken, expressed as parts per million of ammonia
Conclusion
Among the many variables that will affect ammonia concentration in manure are diet, exercise, how the manure is handled once deposited, and the conditions in the environment where aging takes place, particularly sunlight and wind.
Aged horse manure left exposed on the ground for seven days does not contain sufficient nitrogen to be considered as a fertilizer, although it is considered a superior soil amendment.6
This study observed that the ammonia content in horse manure volatilizes rapidly. In seven days or less, the total ammonia had diminished to 14 ppm, a number that translates to one half an ounce of ammonia per ton of manure. The total Nitrogen content of all samples averaged 0.96%. The 1% residual nitrogen in the manure is consistent with the amount of nitrogen found in the fibrous residues of grasses on uncultivated ground7
This finding is consistent with those of Ajwa and Tabatabai, one of the few laboratory studies done specifically on horse manure. They found that total mineralization of horse manure (breakdown into CO2 and H2O) can occur within 21 days in composed scenarios with more than 50% of the total CO2 produced during the first 6 days. 8
This finding is consistent with information from the USDA Agricultural Research Service (ARS) Manure and Byproduct Utilization National Program Action Plan and Air Quality National Program. Rapid processes convert about 35% of the total organic nitrogen initially in manure to ammonia immediately. A total of 50-70% of the organic nitrogen can be converted to ammonia and volatilize into the atmosphere within a short time. 9
While this study sample is small, it confirms observations reported for years by those who are familiar with horse keeping and composting. It is supported by a plethora of literature, some of which is listed below under Other References.
Environmental Risk Assessment of Nutrient Enrichment to Weeds or Streams from Horse Manure on Trails
By Adda Quinn
Manure left in a loose heap in deposits on trails loses its nitrogen rapidly. 10 It is unlikely that decomposition of the small amount of horse manure left on trails provides any significant nutrient enrichment to the trail environment compared to either ambient detritus decomposition from trailside flora or atmospheric deposition. Horses primarily eliminate in their pastures and paddocks. 11 Horses generally are restricted to hardened trail treads or fire roads that do not provide attractive habitat for plant growth. Bacteriological and nutrient effects (on water bodies) are seldom detectable except next to stables. 12
In the arid Western United States drinking water is frequently stored in reservoirs and dams. These areas are oases for myriads of wildlife that can often be seen standing in the water, grazing on wetland flora, flocking on the water surface, and defecating in our drinking supplies. Entire forests of trees drop tons of leaves into these water bodies annually, the decomposition of which provides nutrient enrichment that sustains aquatic life. It is difficult to imagine how horse manure on trails can be considered worse than such ambient conditions when horses are not allowed the same unrestricted access to water bodies as wildlife and falling plant matter. Automobiles are a known non-point source of air borne NOx emissions. To put this into perspective, which should we be more worried about: nitrogen deposition from 33 million cars in California13 or nutrient enrichment for weeds from approximately 150,000 horses that occasionally frequent hardened trail systems in the state?14
Animal wastes should not be allowed in or on the margin of water bodies, and management practices exist that can help to keep this from occurring. An effective management practice is for trail riders to pause and allow the horse to relax prior to a water crossing. Defecation will be more apt to occur with a relaxed horse at rest.15
Horse manure may actually contain conditions initially detrimental to weed seed development. 16 Horse droppings have been known to suffocate or stunt plants underneath them. 17 Manure mingled with wood products has even been known to stunt plant growth through "induced nitrogen deficiency". 18 In general, such small quantities of horse manure that may be occasionally deposited on trails would be insignificant compared to the demand of plants, which must have the N converted by soil microbes into forms that can be up-taken at the root level.
The chances that an occasional pile of horse manure on a trail could provide much nutrient enrichment of any kind to plants is very low. 19
Disclaimer
EnviroHorse has prepared these materials for information purposes
only and are not legal advice. Subscribers and online readers should not act
upon this information without seeking professional counsel. Every attempt has been made to assure that the information contained in this publication is accurate. EnviroHorse assumes no responsibility and disclaims any liability for any injury or damage resulting from the use or effect of any product or information specified in this publication.
About the Authors
Bob Griswold has a B.A. from UCLA in Zoology/Chemistry with Post Grad courses in BioChemistry. He worked 3 years as a clinical lab chemist, 8 years as Chief Chemist at Star Kist Foods, and 6 years as a chemist in the food & cosmetic industries. He is a founding member and on the Board of Directors for EnviroHorse.
Adda Quinn was employed with a nationally known research institute for 21 years prior to her retirement. She has done research both on global climate change and contaminated soil and groundwater issues. As a trail advocate, she has provided research results in a variety of regulatory debates, both nationally and locally. She is a founding member of and on the Board of Directors for EnviroHorse. If you have any scientific studies that you think would be helpful, these papers are a work-in-progress and EnviroHorse would love to have copies of them. Please contact us at envirohorse@yahoo.com . If you have found these citations helpful, please consider a donation to EnviroHorse to help us find and sponsor more research. Papers are housed at www.californiastatehorsemen.com/envirohorse.htm
References Cited
1. Dr. Larry Gosselin, DVM, pers. comm. 4-10-01
2. Dr. Michael Rugg, Toxicologist, Calif. Dept. Fish and Game in personal communication to Adda Quinn and Alistair Bleifuss (RCD TAC), 1998.
3. Nutrient Requirements of Horses, 5th Ed. P8 http://bob.nap.edu/books .
4. Ibid. p.43
5. Ibid. pp. 50-66
6. 1% residual nitrogen is found in fertile soil. Generally speaking, the lowest nitrogen content found in products labeled as fertilizer is 4% per Griswold.
7. Rodale, J. I. Encyclopedia of Organic Gardening. 1975. p. 746
8. Ajwa, H. A. and Tabatabai, M. A. 1994. "Decomposition of different organic materials in soils". Biol. Fertil. Soils Vol. 18. Pp. 175-182.
9. http://www.nps.aars.usda.gov/ Appendix F to CNMP
10. New Hampshire Dept. of Ag and USDA. Good Neighbor Guide for Horse-Keeping: Manure Management. 1990.
11. "Horses spend most of their time in pastures or paddocks where the majority of their excrement is deposited, collected and managed. Horse manure is about 70-80% liquid and 20-30% solids." Personal communication with Dr. Deanne Meyer, UCDavis Manure Management Specialist, February 1997.
12. Williams, B. and L. Conway-Durver. "HORSE TRAILS IN ECOLOGICAL RESERVES" presented at Clemson University, 1998. Mr. Williams holds a BS Degree in Forestry from Clemson University. He worked for the U.S. Forest Service as a River Ranger on the Sumter National, organized the Association of Forest Service Employees for Environmental Ethics and is the Executive Director of the Chattooga River Watershed Coalition
13. Michael Benjamin, pers comm. 3-25-01, Air Resources Board Sacramento CA Planning and Technical Support Division. There were 132 Million cars in the entire US in 1998 according to the subsequent World Almanac.
14. American Horse Council 1996 estimate of horses in California at 640,000 about half of which are in racing or showing and do not leave home. About a quarter of the remainder are likely to be too old, or infirm to leave home (manure study results from San Mateo County January 2001). This puts about one quarter of horses potentially on the trails. The average trail horse in California, ridden by an employed owner from an urban area, would likely not be present on public trails more than 12 to 16 hours per week- a maximum of about 10% of a week. Winters are difficult to ride on trails, so most riding occurs between April and November. Many more people use trails than horses. For example, in San Mateo County, a supposed "horsey" jurisdiction, the human population is nearly 700,000 compared to 4,000 horses. While all citizens and all horses may not be trail users, the horse subset that uses trails is probably very small. We believe that such small numbers of horses on urban trails and the brief time spent on them constitutes very little true environmental risk in terms of volume or contents of horse excrement.
15. Toby Horst, Chairman BackCountry Horsemen, pers. comm. 2000.
16. Harmon, G., and F. Keim. 1934. "The Percentage and Viability of Weed Seeds recovered in the Feces of Farm Animals and Their Longevity when Buried in Manure." American Society of Agronomy Vol. 26. Pp. 762-767. In a study where barn animals were each fed 1000 weed seeds, chickens, sheep and horse guts seem to be able to destroy weed seeds more thoroughly than calves and hogs. An average of 14.2% of uninjured seeds was recovered, most within 48 hours of ingestion. Of these, only 6.7% were viable. The uninjured weed seeds were then buried in horse manure. At the end of one month, only velvet weed, bindweed, peppergrass, sweet bindweed and sweet clover were viable. The rest of the weeds were partially decomposed. Only the bindweed and sweet clover were viable at the end of the second month, but after 3 months only one weak bindweed seedling was obtained (1% viability). All other seeds were dead and had started to decompose at 4 months.
17. www.extension.umn.edu/distribution/naturalresources/components/7540_03.html Manure and Pasture Management for Recreational Horse Owners. Pg 1. Spreading Manure on a Few Acres.
18. www.magnumhorseproducts.com/education/soilnitro Horse Manure Management-Preventing a Soil Nitrogen Deficiency by Dr. Randall E. James Ohio State University Ext.
19. Dr. Ishwar Murarka is a nationally known Certified Professional Soil Scientist who holds a PhD in Soil Science from the University of Oregon. Dr. Murarka has 25 years of research with several prestigious national & private research institutions, over 100 publications, and has served on the USEPA Science Advisory Board for over 12 years. ishinc@ix.netcom.com (personal interview February 25, 2001) In the opinion of Dr. Murarka, inorganic fertilizers of ammonium nitrogen are considerably different than the organic forms of ammonium nitrogen found in horse manure. Horse manure requires treatment by composting or incorporation into soil to a depth of 9 inches to become inorganic and be bioavailable to plants. Without incorporation of manure into the soil subsurface where microbes could actively convert it to a bioavailable form, the chances that an occasional pile of horse manure on a trail could provide much nutrient enrichment to plants of any kind is very low. The organic and ammonium nitrogen from horse manure deposited on trails rapidly volatilizes when contact occurs with air, sun, rain, etc. In general, such small quantities of horse manure that may be occasionally deposited on trails would be insignificant compared to the demand of plants, which must have the N converted by soil microbes into forms that can be up-taken at the root level.
Other References
Provided by Adda Quinn in support of the Griswold study.
A. Definition of Terms from Managing Manure Nutrients at Concentrated Animal Feeding Operations. USEPA. EPA-821-R-01-017. Jan. 2001. Pp. A-1/5.
Ammonia nitrogen: The nitrogen component of the gas (NH3) released by the microbiological decay of plant and animal proteins, usually reported as NH3N.
Ammonia volatilization: The loss of ammonia gas to the atmosphere.
Denitrification: The chemical or biological reduction of nitrate (NO3) or nitrite (NO2) to gaseous nitrogen, either as molecular nitrogen (N2) or as an oxide of nitrogen (E.g., N20).
Nitrogen: A chemical element, commonly used in fertilizer as a nutrient, which is also a component of animal wastes.
Nitrogen fixation: The biological process by which elemental nitrogen is converted to organic or available nitrogen.
Nutrients Available in Manure (for Nitrogen) from Dev. Doc. For Prop. Revs to the NPDES Reg and ELGs for CAFOs. EPA-821-R-01-003. Jan. 2001. Pp. 8-144/146.
The EPA defines manure as the combination of fecal matter and urine from an animal. The total amount of N in manure is excreted in two forms. Urea, which rapidly hydrolyzes to ammonia, is the major N component of urine. Organic N, excreted in the feces, is a result of unutilized feed, microbial growth, and metabolism in the animal (Total N=NH3 (ammonia) + organic N). Before land application, inorganic N forms can be lost either to the atmosphere or into the soil profile, decreasing the nutrient value of manure in 3 ways: NH3 volatilized into the atmosphere; NO3 denitrifying and volatilizing into the atmosphere as N2; and NO3 (water-soluble nitrate) leached and carried down through the soil, where it is unavailable to plants and remain immobilized in the organic form. When manure is applied to the surface of land without incorporation into the soil, much of the inorganic N remains on the surface, is lost and will never be available to the plant.
B. www.agf.gov.bc.ca/resmgmt/fppa/pubs/environ/horse Environmental Guidelines for Horse Owners Ch. 8 Manure Management. Value of Nitrogen (N) in Manure. Nitrogen in manure exists in two forms: inorganic and organic. Inorganic nitrogen (ammonia) is the simple, soluble form that plants can use. After application to land, ammonia is converted by soil bacteria to a nitrate (NO3) form. Manure solids also contain nitrogen in the organic form. Until the organic matter is biologically decomposed in the soil, the nitrogen remains unavailable for plant use. Nitrogen in the organic form is desirable since it acts as a reserve in the soil and is slowly released for plant use. Horse manure looses much of the ammonia to the atmosphere (volatilization). Because of the lack of inorganic forms, the nitrogen compounds in horse manure must be decomposed by soil microbes before they are available to plants.
C. Winston Way "The Perishability and Profitability of Manure" www.eap.mcgill.ca/
Unless manure is appropriately stored, it will lose its liquid content and nutrients rapidly. Never leave manure on the ground exposed to sun and wind if you are interested in nutrient preservation. Manure must be either appropriately stored or incorporated into the soil as quickly as possible after deposition (within 1 hour) in order to preserve nutrients.
D. Manure Management in Small Farm Livestock Operations EM8649 May 1997, Godwin and Moore, Oregon State University. The nitrogen in fresh manure is in the form of ammonia and organic nitrogen. The organic nitrogen is in particles such as hay and grain (organic matter). In the soil, microorganisms convert the ammonia and organic nitrogen to nitrate (NO3) that is readily available to plants.
E. http://pasture.ecn.purdue.edu/ Dec 1992 Alan Sutton and Brad Joern
Growing plants can take up both nitrate and ammonium (NH4+) nitrogen from the soil, but manure contains mostly organic and ammonium nitrogen. These forms of nitrogen do not move through soils. However, some soil microorganisms convert these compounds to nitrate, the mobile form of nitrogen in soils.
F. http://hubcap.clemson.edu April 1996 Land Application of Animal Manure. The speed and extent of degradation are dependent on the type of manure, soil type, soil moisture and temperature and how well the manure is mixed with soil. Considerable quantities of ammonia/ammonium N occur in most manure. Most crops will uptake NH4+ as well as any other form of N but do not accumulate NH3 to any extent. Ammonia is readily lost to the atmosphere. As much as 15% of NH3 from surface applied manure can be lost each day …"
G. http://agri.gov.ns.ca/rs/greenplan/waste The Development of Weather-based Manure Spreading Index by Gordon, et al, Dec 1996. Approximately 50% of the N in manure is found in the liquid portion. Although soluble and therefore readily available for crop use, it is quite susceptible to loss through volatilization as NH3, particularly during the first few days after field application. Although estimates of NH3 emissions are still very uncertain, many studies have concluded that a great percentage of the N lost from manure occurs through volatilization of NH3 (Adriano et al, 1974; Lauer et al 1976, Hoff et al 1981, Beauchamp et al 1982 etc)
H. http://www.mda.state.mi.us/right2farm/manapp.htm The availability of N in manure for plant uptake will not be the same as highly soluble, fertilizer N. Therefore, total manure N cannot be substituted for that in fertilizers on a pound-for-pound basis, because a portion of the N is present in manure organic matter which must be decomposed, before mineral (inorganic) forms of N are available for plant uptake. (Assuming incorporation into the soil) the rate of decomposition (or mineralization) of manure organic matter will be less than 100% during the first year, and will vary depending on the type of manure and method of manure handlings and can take 2-4 years to become available for plants.
I. www.montana.edu Minimize Your Gamble When Applying Urea by Jim Bauder MSU Extension: It really cannot be over-emphasized that incorporating urea into soil will minimize loss. Do not allow manure to remain on the soil surface. Pennsylvania State University sources say that within a day of application, 66% of urea-nitrogen becomes ammonia-nitrogen. If not incorporated into the soil, significant nitrogen loss can occur…" DB Fowler Univ Of Saskatchewan emphasizes, "Once urea is well incorporated into soil, volatilization losses normally are minimal."
J. http://ceinfo.unh.edu/bmpnutr.htm plow down or disc manure within 2 days to avoid heavy losses of ammonia nitrogen
K. http://ohioline.ag.ohio-state.edu Ohio Livestock Manure and Wastewater Management guide Bulletin 604 "Manure should be incorporated into the soil as soon as possible after application".
L. "Incorporate animal wastes into the soil within 2 days of application, whenever possible." Managing Manure Nutrients at Concentrated Animal Feeding Operations. USEPA. EPA-821-R-01-017. Jan. 2001. Pg 38.