A review of potential critical factors in horse keeping for anaerobic digestion of horse manure

Renewable and Sustainable Energy Reviews 65 (2016) 432–442

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

A review of potential critical factors in horse keeping for anaerobic digestion of horse manure Åsa Hadin n, Ola Eriksson, Karl Hillman Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, University of Gävle, Kungsbäcksvägen 47, SE-801 76 Gävle, Sweden

art ic l e i nf o

a b s t r a c t

Article history: Received 28 October 2015 Received in revised form 3 June 2016 Accepted 26 June 2016

Keeping horses causes environmental impacts through the whole chain from feed production to manure. According to national statistics, the number of horses in Sweden is currently 360,000 and is continuing to increase. This result in increasing amounts of horse manure that has to be managed and treated, which is currently done using practices that cause local, regional, and global environmental impacts. However, horse manure and its content of nutrients and organic material could be a useful fertiliser for arable land and a substrate for renewable energy production as biogas. The aim of the paper is to identify and describe potentially critical factors in horse keeping determining the amount (total mass) and characteristics (nutrient content and biodegradability) of horse manure, and thus the potential for anaerobic digestion. A systematic combining approach is used as a structural framework for reviewed relevant literature. All factors identified are expressed as discrete choices available to the horse keeper. In all, 12 different factors were identified: type and amount of feed, type and amount of bedding, mucking out regime, residence time outdoors, storage type and residence time of manure in storage, spreading and soil conditions, and transport distance and type of vehicle fuel used. Managing horses in terms of these factors is of vital importance in reducing the direct environmental impacts from horse keeping and in making horse manure attractive as a substrate for anaerobic digestion. The results are also relevant to environmental systems analysis, where numerical calculations are employed and different biogas system set-ups are compared to current and other treatments. In such assessments, the relevance and importance of the critical factors identified here and corresponding conditions can be examined and the most promising system set-up can be devised. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Horse manure Horse keeping Anaerobic digestion Nutrient recycling Systems perspective

Contents 1. 2. 3.

4. 5.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Feeding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Indoor housing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Outdoor keeping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Manure storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Fertilisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factor analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Horse manure as feedstock in anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Method used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Implementation of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. E-mail address: [email protected] (Å. Hadin).

http://dx.doi.org/10.1016/j.rser.2016.06.058 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440

1. Introduction Horses used for training, leisure and competition make a substantial contribution to social, economic, and environmental values in Western society. For example, people rehabilitate and develop useful skills with the help of horses and grazing horses help to maintain biodiversity [1–3]. Moreover, horse keeping and the equine sector contribute to GDP in many European Union countries [4,5]. Horse keeping has various environmental impacts through e.g. use of resources and emissions to air, soil, and water from different activities such as feeding, transport, housing, grazing, and outdoor paddocks. Negative environmental impacts associated with horse manure management include nutrient enrichment in soil and nutrient leaching from paddocks and stored horse manure [6]. Other environmental aspects include emissions to air from horse manure and bedding material [7]. Statistics on the number of horses in Sweden in 2010, based on predictions from a survey, show a total of 360,000, which represented a 10–20% increase on the number in 2004 [8]. Approximately 75% of Swedish horses are kept in close proximity to urban areas and about 17% of riding schools and trail-riding enterprises in Sweden report a lack of services for horse manure management [9]. Low rates of recovery and utilisation of nitrogen (N), phosphorus (P) and potassium (K) from livestock are a global problem [10]. Organic matter and recycled nutrients in manure are important for the structure and nutrient content of agricultural soil [11]. Horse manure has a natural content of nitrogen and phosphorus and if it is not used as fertiliser on farmland, natural cycles of nutrients are broken, increasing potential nutrient leaching and eutrophication and creating a need for chemical fertiliser [12,13]. Production of nitrogen fertiliser involves significant use of natural gas and generates emissions, which contributes to global warming [14]. Moreover, the current use of limited phosphate resources for phosphorus fertiliser is reported to be unsustainable [15]. Thus, by reducing consumption of chemical fertiliser through re-using manure, several problems could be mitigated. Previous Swedish and international studies have examined environmental aspects of horse manure management, horse manure in horse paddocks, and management of spent bedding material [16–20]. Economic and practical problems and aesthetic concerns associated with horse manure management are mentioned by several authors [17,19,21]. At the same time, there is increasing interest in utilising renewable energy from different types of organic waste to solve waste management problems and decrease use of fossil energy [18,22]. As part of these efforts, the biogas potential of horse manure and spent bedding material, which are regarded as waste problems for the horse industry, has been studied [18,19]. Biogas systems often lead to improvements in resource efficiency, energy recovery, and environmental impacts compared with existing waste handling and agricultural practices [23,24]. Previous research on horse manure and environmental impact is focused on different parts of horse keeping, but so far studies applying a systems perspective on horse manure management incorporating, comparing, and discussing the importance of different aspects is lacking. This study was aimed to shed some light on the first part, the manure production. Challenges in the following manure treatment including biogas systems, composting, incineration, etc. and environmental comparisons of these will be

covered in sequential papers. Thus this paper will contribute to the bigger picture and thus to conceptualisation of the problem. The objectives of the present study was to (1) review predominantly scientific literature on horse keeping and manure management, (2) structure the findings in a framework, and (3) identify and describe potentially critical factors affecting the amount (total weight) and characteristics (nutrient content and biodegradability) of horse manure. The factors covered involved horse management practices and their environmental impact. Doing this will increase the understanding of the underlying conditions in manure generation in using horse manure as a feedstock for combined energy recovery and nutrient recycling in anaerobic digestion.

2. Method The research approach was mainly based on a literature review for retrieving information and a systems perspective for structuring this information. Field observations were used to confirm findings in the literature and their influence on system design. The method can be described as a ‘systematic combining’ approach (Fig. 1), where multiple sources of data are used and theory and reality are matched during the research process as passive data are scrutinised and active data are discovered [25]. Literature on horse keeping, horse manure management, manure nutrient content, biogas potential, biodegradability, and environmental impact from horse manure was reviewed using these phrases as search criteria. Priority was given to relevant peer-reviewed scientific papers found in databases, e.g. ScienceDirect and Google Scholar, but also grey sources such as reports, official statistics, information issued by authorities and relevant authority websites were used. The framework for combining different types of information was guided by a systems perspective. This meant that horse keeping was viewed as a set of activities affecting horse manure and horse manure management. The system studied comprised all activities within horse keeping affecting horse manure, from feeding of horses to soil fertilisation, and relevant factors were identified using a life cycle approach in combination with the retrieved information. The identification process was performed by the authors by combining the framework from a specific paper and adding on knowledge about material and energy flows caused by the activities identified in literature. Judgements on the specific relevance of each issue were made by the authors supported by the literature information. The identified factors influence the amount and characteristics of horse manure relevant for anaerobic digestion and environmental impact. This means that other environmental impacts from equestrian sports or horse keeping in

Field observations MDR

MDR MDR

Systems perspective

Literature review

Factor analysis Fig. 1. The modified systematic combining approach used in this project (from [25]). Arrows represent matching, direction, and redirection (MDR).

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general, such as horse transport, water use, etc., and the biogas production process itself were not included. Information obtained in the literature review was analysed and allocated to different horse keeping activities. To avoid a study based exclusively on passive data, active data were collected through field observations to aid identification of activities and to check the feasibility of the results. The observations aimed to improve understanding of the current reality and to guide and redirect the literature review, mainly by confirming passive data obtained from the literature (hence the dashed line in Fig. 1). The field observations were made during participation in meetings and visits on-site and through personal communications (by telephone interviews and visits on-site) with horse keepers and experts working at local, regional, and national authorities and relevant organisations. In the final analysis, critical factors affecting amount and characteristics of horse manure were identified for each activity and corresponding direct environmental impacts were described. The quality of this work relies partly on the quality of the reviewed literature and also on the performance of the assessment that is the systematic review. To ensure the highest quality possible, scientific papers subject to peer-review were prioritised. But as there is significant gaps, grey literature together with field observations were also included. The assessment consists of a framework (described below) and expert's opinion (two of the authors are experts in systems analysis). The system studied was first defined in a previous study on livestock manure management by Oenema et al. [10]. It was then adapted to apply to horse manure management be defining the specific activities based on literature findings in combination with field observations (Fig. 2). The sections Literature findings and Factor analysis are organised according to these activities. The activities have been defined by the authors as an input/output process where the horse transforms feed into manure. Feed is the input and the output has a geographical aspect (indoor or outdoor) and a recovery aspect (storage and fertilisation). Using a life cycle approach, relevant transports for all these activities are also included. Aspects related to manure amount and characteristics, as well as environmental impact, are included whereas other factors affecting, e.g. economic importance (such as horse density, the number of horses at a manure collecting point, vicinity to biogas plants and arable land, etc.), are not accounted for.

3. Literature review Horse manure consists of faeces, urine, and used bedding material. The amount of horse manure produced depends on the size and age of the horse, with a horse producing more manure than a pony and an adult horse producing more than a foal. Whether the horse is, e.g., in light training or in competition affects the type and

amount of feed and the nutrient content of the feed (the ration), which determines the amount of manure and the content of nutrients in that manure. In this study, horse keeping was divided into six different activities (Fig. 2). Horse keeping emits greenhouse gases directly (e.g. enteric fermentation, decomposition of bedding material), indirectly (e.g. feed production, transports energy use) and through manure management (storage, composting) [29]. Manure and urine left on the ground indoors and outdoors cause leaching of nutrients [6]. Storing manure causes emissions to air, soil, and water [10,29,31] through the formation and release of ammonia (NH3), nitrous oxide (N2O), carbon dioxide (CO2), water vapour (H2O), and methane (CH4) [7,32]. In all manure management activities, energy is used and natural cycles are broken if manure is not used as a fertiliser on arable land (Fig. 2). Studied literature covers information about the six activities in the studied system, environmental aspects from the activities and their connection to horse manure amount and content and are summarised in Table 1. 3.1. Feeding The feeding practice (Fig. 2) refers to feed production and consumption, irrespective of place and time. Feedstuffs cause environmental impacts in their production and if spilled or refused. Overfeeding increases the nutrient content of the manure (excretion of nutrients see Section 3.5) and is an inefficient way of using scarce resources. Different amounts of different feedstuffs are combined in a ration, which is varied depending on the age and workload of the horse. A ration for a horse may include, e.g., forage, oat, beet-pulp, minerals, and straw [31]. Horses in Sweden are fed forage (roughage) such as silage, haylage, hay, straw, and lucerne, sometimes in different combinations [36]. Other common feedstuffs for horses are grain and pasture [35]. The grain can be barley, oats, beet pulp, maize, rice bran, soybean meal, soybean, or wheat [61]. Verhaar et al. [34] investigated feeding practices for horses used for equestrian sports in New Zealand and reported use of basic feed (e.g. oats and barley), concentrate feed (grains, premixes, muesli, or pelleted ration), pasture, and conserved forages. Hoffman et al. [33] review different surveys where horses were also fed dietary supplements, conserved vitamins, or mineral mixtures. A feed management study in the United States [37] found that 98% of horse keepers surveyed fed their horses mostly with hay and pasture, while grain (bulk, commercially bagged product, or whole grain) was used by 95%, other concentrates (e.g. wheat bran) by 41%, and dietary supplements by 43%. Sedentary horses (non-working, at maintenance) are generally fed with a high level forages ration in comparison to exercised horses, having ratios containing more concentrate diets and higher proportion of dry matter intake in relation to body weight [41]. Muhonen [36]

Emissions to air CO2, CH4, NH3, N2O, N2, NO, SO2, H2S, H2O…

Feeding

Indoor housing

Horse keeping Outdoor Manure keeping storage

Fertilisation

Transport

Emissions to subsoil, groundwater and surface water NO3-, NH3 NH4, K, Cl, SO4, PO4, Ca, Mg, Na, Cu, Zn… Fig. 2. Possible pathways for environmental impacts from horse keeping, with the focus on horse manure (modified from [10], with input from [6,20,26–30]).

Å. Hadin et al. / Renewable and Sustainable Energy Reviews 65 (2016) 432–442

Table 1 References on horse keeping activities, horse manure environmental aspects and horse manure amount and content used in this study. Activities Feeding [31,33–39] Indoor housing [7,9,16,37,44–52] Outdoor keeping [47,61–64] Manure storage [10,31,54,59,68] Fertilisation [9,69–72] Transport [9,70]

Environmental aspects

Manure amount, content

[26,27]

[38,40–43]

[7,46,50]

[18,19,21,53–60]

[6,20,29,61,65,66]

[10,20,45,67]

[26,29,31]

[21,31,68]

[73]

[31,41,69,73–75]

[27,29]

mentions that athletic horses could be fed twice the energy maintenance level in comparison to sedentary horses. A concern over limited data to calculate nutrient requirements of young foals and pregnant and lactation mares is announced by Tylutki [76], and that assumptions used in the studied computer model could lead to over- or underfeeding. A study of 11 horse farms by Harper et al. [39] identified overfeeding of horses with phosphorus by 185% and crude protein by 157% and concluded that reducing this overfeeding could reduce the nutrient output in the manure. Excess dietary phosphorus increases the excretion of phosphorus in faeces, compared with horses fed a low phosphorus diet [38]. Bott et al. [77] review literature and databases to assess potential environmental impact from horses and mention higher ammonia levels in stables with horses fed high protein diets. The review also mentions grasses having different crude protein concentration depending on season and maturity in forages plants; cold season grasses have higher levels of protein and immature forage plants have higher content than mature plants. In a survey in the United States by Westendorf et al. [37], 60% of the respondents did not have any support or plan for feeding and feed leftovers were discarded in the manure pile, left in field or put in a dumpster. Batch experiments indicate that the methane potential is higher for manure mixtures from horses fed hay than horses fed silage, while the latter gives a higher methane fraction in the biogas [55,56]. During the production of feeds, transport, and use of energy, resources (e.g. plastics), land, pesticides (herbicides, fungicides, and insecticides), and fertiliser (N, P, and K), gaseous emissions (CH4, CO2, N2O, CFC, NH3, NOX, SO2) and leaching of plant nutrients (NH3, NO−3 , PO4) occur [27]. Those authors examined the lifecycle environmental impact from feedstuffs used for dairy livestock from cradle to factory gate, where the end product was 1 kg feed produced. Forages were assumed to be consumed on the farm on which they were produced, including all processes until feeding. The results showed, e.g., that hay (grass) had less impact on climate change than baled silage (grass), while grain had a higher climate and eutrophication impact than roughage (hay and silage), but less impact on acidification [27]. Horses produce methane when digesting food. As non-ruminants, horses produce less methane than ruminants, e.g. dairy cows, through more intensive fermentation. IPCC has calculated an enteric fermentation factor for horses of 18 kg of CH4 per horse and year in cool temperate regions, although feed intake and feed composition affect the rate [26]. 3.2. Indoor housing In indoor housing (Fig. 2), amount and content of horse manure are affected by strategies for mucking and used bedding material.

435

Indoor housing for horses comprises box stalls, loose housing, tie stalls, or combinations of these. In box stalls, the horses are loose in individual boxes. In loose housing, groups of horses have free access to halls with deep-litter bedding and feeding, watering areas, and paddocks. In tie stalls, the horses are tethered, which is a system most often used for horses usually kept outdoors or working [44,45]. Depending on the horse housing system, horse manure is collected in different ways. The time horses spend in the stable, in combination with how mucking out is performed and whether manure is collected in paddocks, affects the amount of horse manure collected and the emissions from paddocks. A national survey on horse keeping in Sweden in 2010 found that 85% of horse keepers used box stalls, 25% used loose housing, and 5% used tie stalls [9]. The literature gives different figures on how much time horses spend in stables. For example, Fleming et al. [32] report that horses spend up to 23 h a day in the stable, while according to Westendorf et al. [37], 68% of horse keepers house their horses for less than 8 h a day, 20% for 8–12 h and 12% for more than 12 h a day. The manure left in the housing can be collected by spot cleaning of urine and faeces, cleaning of faeces or complete removal. Mucking out can be carried out each day or at longer intervals and adding bedding material and tidying up the bedding are parts of mucking out. The characteristics and amount of removed manure vary depending on the mucking out regime and the bedding material used [19]. Complete mucking out every day is highly material consuming, while removal of only faeces on a daily basis reduces material use [53]. Deep litter bedding in shelters for outdoor cattle may only be cleared out 1–3 times per year [48]. A Swedish study on a stable with loose housing and a deep litter straw bed reported one complete mucking out every 8 months and found that use of bedding material amounted to 1 m3 per month and horse [49]. Another important aspect of housing is the type of bedding material. Bedding material is used to collect and absorb faeces and urine in housing, creating dry, clean, and comfortable spaces for horses [31]. The water-binding capacity of the bedding material and how easy it is to separate clean and used bedding material affect the amount of produced horse manure. Bedding material for horses needs to be good in terms of ammonia absorption, generate low rates of airborne particles, be comfortable for the animal, degrade rapidly, and be easily incorporated into the soil. Type of bedding, together with the removal of manure, is also important to air quality and hygiene, as bedding material stabilises the air quality in the stable [7,16,32]. Types of bedding commonly used for horses are different types of straw, softwood shavings, wood chips, sawdust, peat, flax, straw pellets, wood shavings pellets, paper cuttings, and hemp (Table 2). A survey in the United States [37] found that 75% of horse keepers used wood products (wood shavings, chips, pelleted wood, sawdust), 24% used straw, and 1% used other bedding material (not explained). The use of more than one type of bedding material was also shown in a Swedish study, wherein about 70% of respondents used wood shavings, 40% used straw, and about 20% used peat [9]. According to Airaksinen et al. [54], the amount of bedding in the collected manure depends on how easy it is to separate bedding material from the manure. They found that long straw results in larger volumes than, e.g., a mix with peat and wood chips, resulting in the estimated amount of bedding manure removed being 19.5 m3 per horse (Table 2) and year for long straw and 9.1 m3 per horse and year when a mix of peat and wood chips were used. Wood chips alone were calculated to produce 12.4 m3 per horse and year. Separation of clean and dirty material proved more difficult when wood chips, shredded newspaper, and straw were used, in comparison with removing used peat and hemp bedding [54]. The same study showed that the content (kg/m3) of

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Table 2 Bedding material and amount of horse manure (bedding and horse dung) in studied literature. Bedding

References (bedding) Annual amount References horse manure (amount horse manure)

Straw

[7,16,18,46,51,78,79]

Softwood shavings Wood chips Peat Flax Straw pellets Wood shavings pellets Paper cuttings Hemp Sawdust Linen

[46,51]

19.5 m3 8–10 t

[54,65]

[37,54] [51] [79] [46,79] [7,79]

12.4 m3 9.8 m3

[54] [54]

[16,46,51] [46,51] [16,51,80] [46,51]

11.7 m3 9.1 m3

[54] [54]

phosphorus and potassium in horse manure with different bedding material increased after storage for most bedding materials studied (P not for peat and hemp, K not for peat). Soluble nitrogen was highest in peat-based bedding and lowest in hemp-based bedding, probably reflecting the ammonia absorption capacity in these bedding materials. General advice on storage capacity of horse manure issued by the Swedish Board of Agriculture cites a figure of 10 m3 (about 5 t) deep litter manure per horse and year [57]. This figure is based on standard values for amount of manure produced and storage duration and takes into account faeces and urine contributions, bedding material, composting losses, water spillage, and rain during the manure storage period. Cui et al. [18] report a figure of 10–14 t per horse and year, with used, not specified, bedding material comprising 25% of the total weight, while Steineck et al. [65] estimate 8–10 t per horse and year, consisting of up to 90% straw. Borhan et al. [59] analysed the nutrient content in soiled and fresh horse bedding material (flax shivers and pinewood shavings) and showed that soiled bedding material in most cases had significantly higher nutrient content than fresh material. Total carbon content was reduced in used bedding, through decomposition of organic matter in the soiled bedding. Analyses of bedding materials by Olsson et al. [81] revealed a higher content of phosphorus and potassium in straw than in peat and wood shavings, while peat had a higher content of ammonium. Type of bedding material affects the degradability and thereby the biogas potential. Experiments in studied literature have shown that straw as bedding material gives higher specific methane yield than sawdust and wood pellets [21] and higher methane potential than wood chips [55,56]. Bedding material adds methane potential, probably by adhered manure and absorbed urine [18,19]. However, Mönch-Tegeder et al. [21] found slightly higher specific methane yield for raw straw than for fresh horse manure (a mixture of horse dung and straw). Degradation processes in excreta and in the bedding mixture produce potentially noxious gases such as ammonia, nitrous oxide, carbon dioxide, and methane in deep-litter bedding systems [7,32]. Formation of these gases and water-binding capacity are interesting aspects when choosing bedding material. Garlipp et al. [7] reported less release of methane, nitrous oxide, and carbon dioxide from wood shavings than straw in deep litter bedding systems. Fleming et al. [50] observed lower ammonia concentrations in the stable in experiments where wheat straw was used as bedding material and found that not mucking out but adding bedding material daily resulted in lower emissions of particles, while partly mucking out (only faeces removed and bedding material added daily) was favourable for ammonia emissions.

3.3. Outdoor keeping Outdoor keeping (Fig. 2) allows free exercise, social interaction with other horses, and, if kept on pasture, grazing by domesticated horses. For horses as herd animals, adapted to life on open plains, and herbivores, almost constantly foraging, being outdoors has positive physical and physiological effects on horse welfare [47,61– 64]. Outdoor keeping affects the possibilities for collecting horse manure and urine, and manure from grazing and paddocks are sometimes considered unavailable for biogas production [67]. Faeces are often deposited in excretion areas, increasing the possibility for collecting manure, but during the cold season it is difficult to clean outdoor areas [45]. Paddocks are managed outdoor areas for horses, often situated close to stables. Under Swedish conditions, horses are fed outside and kept in paddocks for about 8–12 h per day. Feed, urine, and manure leave significant quantities of phosphorus and nitrogen, especially in horse feeding and excretion areas. The potential for phosphorus and nitrogen losses is higher from horse paddock soils than from other soils, according to studies by Parvage et al. [6,20]. Thus, there is a risk of a negative impact of horse paddocks on surface water and groundwater [20]. According to estimates reported by Oenema and Tamminga [82], about 50% of the nitrogen is lost when excreta from animal production systems (faeces and urine) are left in grazing areas and not handled. An investigation of the biogas potential of waste in Sweden [67] estimated that 50% of horse manure is left on pasture, while Oenema et al. [10] reported that 30–40% of livestock excreta are left in pastures in Europe. Grazing in the summer period creates equilibrium between uptake of nutrients in growing plants and nutrients in horse manure, although distributed and concentrated to specific areas. If horses are fed extra during the grazing period, e.g. concentrate or hay, this creates an additional input of nutrients. In the winter, there is no uptake of nutrients by growing plants and this situation creates a risk of nutrient leaching to streams or groundwater [65]. Enrichment of nutrients in soil has been identified in feeding and defecation areas in paddocks and pastures, increasing the risk of nutrient leaching [66]. The environmental impact from grazing originates in a combination of vegetative damage, soil damage, erosion, trampling, and defecation. At the same time, grazing confers benefits by reducing the need to purchase forage and grain and avoiding the use of bedding material. If arable land is used as pasture, the fertilisation cost and labour costs for spreading manure are reduced [61]. Manure deposited in grazing areas gives rise to methane emissions, but the presence of oxygen makes the rate relatively low [29]. 3.4. Manure storage Manure is stored to enable composting of the material for better utilisation of the nutrients and to wait for a suitable time for spreading. Estimates of manure generation by animal production systems indicate that 50% of the nitrogen left in barns and sheds is stored and/or treated and finally spread, but only half of it ends up on agricultural land. In Europe, 60–70% of the livestock manure collected in housing systems is in the form of liquid manure/slurry or solid manure [10]. During storage (Fig. 2), emissions to air occur from the bedding and from the pile of manure. Storing manure without a cover permits losses of nutrients to soil and water. Losses to water mainly occur if manure is stored for a long time. The environmental impact from manure storage is mainly due to nitrogen leaching and emissions contributing to global warming, e.g. of CH4 and N2O [10,31]. Horse manure is dry and has a low bulk density and high

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carbon-nitrogen ratio due to a high proportion of bedding material, and its spontaneous decomposition is slow [31]. Nitrogen in both the manure and the soil is used by microbes in the decomposition process [59]. It is positive for the availability of plant nutrients if the horse manure is composted before spreading and if periodic spreading is carried out. Composting reduces the amount of manure, makes it easier to spread, and increases the plant nutrient concentration per unit mass of manure [31]. In a study of three storage methods for equine stall waste [68], a covered turned composting system for horse manure proved more beneficial regarding mass reduction, compost nutrient content, and nutrient runoff concentrations than a static pile or a turned pile. The mass reduction during composting was 43% for the covered and turned pile, compared with 17% for the static pile and 38% for the turned pile [68]. Depending on bedding material, horse manure decomposes quickly or more slowly. One composting study showed that only horse manure with peat as bedding material was ready to spread after the composting period [54]. Storage duration also has an effect on methane production, e.g. horse manure stored for at least four weeks underwent a reduction in easily degradable compounds, followed by an increase in acid detergent lignin content, and gave lower methane yields than fresh manure [21]. About 25% of total nitrogen is lost when horse manure is stored, although some studies indicate lower emission rates [73]. According to Oenema et al. [10], horses and other animals (including sheep and goats) contributed 16% to total nitrogen excretion in the EU-27 countries in the year 2000. The total loss of excreted nitrogen from all livestock was 48% during storage and application to land, indicating that 52% of the excreted nitrogen was recycled as fertiliser [10]. Methane is released from horse manure stored as solid manure, composted, or left on grazing. Methane emissions depend on storage duration and manure composition. Long-term storage results in greater degradation of organic material. When manure is handled as a solid (e.g. in stacks or piles) or when it is deposited on pasture or rangeland, it tends to decompose under more aerobic conditions and less methane is produced. Composting of manure emits methane if there are oxygen-free areas in the material [29]. The default methane conversion factor for manure from paddocks, pastures, and grazing areas which is deposited and not managed is 1%. This is within the same range as when manure is stored in a dry lot, i.e., a paved or unpaved open confined area where manure may be removed periodically [26]. Solid storage involves storing manure in unconfined piles or stacks, typically for a period of several months. Solid storage has a default methane conversion factor of 2%, while composting processes with aeration and mixing have a default methane conversion factor of 0.5% (all factors cited are for cool temperate regions) [26]. Methane emissions factors for manure management presented in IPCC [26] show that those from horse manure management are 1.56 kg per horse and year in developed countries and 1.09 kg per horse and year in developing countries (both in cool temperate regions). In addition to methane, nitrous oxide is produced, directly and indirectly, during the storage and treatment of manure before it is applied to land or otherwise used for feed, fuel, or construction purposes. The nitrous oxide emissions generated by manure in the ‘pasture, range, and paddock’ system occur directly and indirectly from the soil [26]. 3.5. Fertilisation Soil fertilisation is not always a part of horse keeping, e.g. due to horse keepers’ lack of land and machinery [77], but it is included here as it is part of current manure management system (Fig. 2). Application to fields is one of the most common outlets for

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horse manure in Sweden [9]. More than 60% of all horse keepers in Sweden spread horse manure on their own land and more than 30% have agreements with farmers for manure management. After stockpiling, the manure is spread using a tractor and a spreader for solid manure [69]. However, an interview study in the United States found that the majority of the horse manure from horse farms in the study was not used, due to lack of agriculture on the properties [70]. Desalegn et al. [72] reported that riding stables face a challenge in disposing of manure because horse owners do not have land of their own or access to other land for spreading the manure. Other problems mentioned in literature are lack of equipment for manure spreading and high costs [71]. The plant nutrient content in horse manure varies depending on horse size and workload. For an individual horse, the amount of phosphorus ranges between 6.4–10.9 kg/yr, nitrogen 33.4–60.6 kg/ yr, and potassium 41.6–66.6 kg/yr. The lower values represent a 300 kg pony in light to moderate work and the higher values represent a 500 kg horse in intense (heavy) work [31]. The amount of nutrients remaining in horse manure depends on the feed ratio as most of mature horses′ nutrients intake remains in faeces and urine. Growing horses (foals, weanlings, and yearlings) and pregnant or lactating mares are somewhat different as they use nutrients for growing and foetal during pregnancy and milk production during lactation [31]. One study on phosphorous balance in forage fed growing horses in training showed that horses fed with high phosphorous (P) diets excreted more P in faeces, but also retained somewhat more P per day [42]. Sedentary horses′ high level forages diets has a lower digestibility and a lower intake of dry matter than exercised horses’ ratios containing more dry matter and higher digestibility concentrate diets [41]. In addition, horses in training, (demanding, regular, high intensity, and long term), digest feed better [83]. Excretion of nutrients in manure from mature horses evaluated by Lawrence et al. [41] showed that excretion of potassium and phosphorous was not affected by exercise, while excretion of nitrogen was. Muhonen [36] mentions that nitrogen retention increases when horses are subject to increased exercise. Deficient data prohibited the evaluation of excretion from growing horses, pregnant mares, and lactation mares in the Lawrence et al. [41] review. Type of feed affects digestibility and also urinary nitrogen excretion in Woodward et al. [84] as legume hay gave higher nitrogen excretion rates than grass hay from mature moderate trained horses. Matching the need for and quality of protein in feed for exercised horses is important to reduce nitrogen excretion according to Lawrence et al. [85] as less quality protein demands increased amount of crude protein in feed, followed by increased excretion. This is in accordance with Bott et al. [77] concluding that precision feeding, waste management, feed choices, and feed combinations contribute to reduction of excretion of nitrogen and reduced environmental impact from horse operations. The nutrient content of deep litter horse manure is similar to that of deep litter cattle manure, with only slightly less total nitrogen [57]. According to Luostarinen (ed.) [75], around 2952 ktonnes of fresh solid horse manure were produced in Sweden in 2010, about the same amount as for pigs (slurry and solid manure) in the same year. The nutrient content in this annual production of horse manure amounts to 16.6 ktonnes of nitrogen, 2.8 ktonnes of phosphorus, and 16.9 ktonnes of potassium. Compared with pig manure, horse manure has a higher content of nitrogen and potassium, while the phosphorus content is lower. Spreading solid manure results in nitrogen losses in the range of 10–90% depending on application method, time of year and weather, and how incorporation is done [73]. Application of digested or undigested horse manure to soil leads to environmental impacts from spreading equipment (CO2 emissions from vehicles) and direct emissions due to losses of

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nitrogen as ammonia or nitrous oxide, which cause acidification, eutrophication, and also global warming. The magnitude of these losses depends on soil conditions, in combination with the method used for the application [74]. 3.6. Transport Transport related to manure management concerns feed, bedding material, and manure (Fig. 2). Transport does not affect the amount or the characteristics of the manure, but there is an environmental impact related to how much transport is required and also which vehicles and fuels are used. Information about transport distances for horse feed and horse manure is lacking, but there is some information indicating differences in transport distance depending on feed supply practices and horse manure management. Transport affects the amount of horse manure available for biogas production and fertilisation. According to a Swedish study [9], 50% of Swedish horse keepers grow forage for their horses and 40% grow enough to meet their whole forage requirement. Most Swedish horse keepers rely on others to grow the concentrate their horses require. Flysjö et al. [27] assumed that forage was produced and consumed on-farm or by a neighbour but concluded that imported concentrate feed gives a significant environmental impact due to long transport distance. Case descriptions in Berglund and Falkhaven [29] show a variety of transport of feed, varying between transports for feed production for horses on-farm to transport of all forages to the farm. Transport of bedding material is not investigated in this study. Manure is transported when it is used as a fertiliser on the farm or, if land area for spreading manure is lacking, it is moved to storage and handling. One study found that the majority of the horse manure in the study area was not used on horse farms [70]; some horse farms used manure to grow hay, but many just left it in manure piles and some hauled it off to a landscaping company. In Sweden, commercial horse stables (for example A-trainers, riding schools) usually have agreements with farmers or other actors for manure management [9].

4. Factor analysis The literature review showed that different activities in horse keeping affect the amount and characteristics of horse manure and lead to environmental impacts to different extents. Conclusions that can be drawn from previous sections are summarised in Table 1 (explanations below). The activities in Table 3 corresponds to Fig. 2 which

in combination form the conceptualisation of horse keeping, manure production, and the potential for energy and nutrient recovery. Most of the horse management practices listed in Table 3 have potential positive and negative effects on horse manure amount and characteristics. Overfeeding is an inefficient use of resources causing unnecessary production of feed and excess nutrients, which increases the environmental impact. Amount of horse manure collected depends on the type of bedding material used and mucking out regime for faeces indoors and outdoors. A large proportion of bedding material, depending on material used, is more or less negative for biogas production since bedding is generally slow to degrade, whereas the manure and urine it contains add some methane potential to the feedstock. Mucking out frequency and amount of bedding material required affect both energy and nutrient recovery. Collecting horse manure from paddocks and pastures mitigates emissions to air and leaching of nutrients and increases the amount of manure, but also increases the risk of impurities, e.g. sand and gravel, in the collected manure. Manure storage seems to reduce methane potential and nutrient content, in particular if the storage is open. Horse manure is spread on soil using application methods for solid manure, and nutrient losses are strongly affected by application and incorporation techniques used. Transport enables both energy recovery and nutrient recycling, through the delivery of horse manure to biogas facilities and arable land. The actual location of the horse in relation to the place where feed and bedding are purchased, and where manure is treated and spread, affect transport distance and thereby the emissions. To better understand what affects manure amount and/or content, factors corresponding to different choices of practices, often made by the horse keeper, and the effects were identified for each activity, aided by findings in the literature (Table 4). Choices downstream related to the need for pre-treatment, type of anaerobic digestion, etc. were deemed to be outside the system boundaries of this study, but have to be evaluated to fully capture the potential of horse manure as a substrate for biogas production. In Table 4, the amount of horse manure is characterised as produced, collected, and utilised. The amount produced is the sum of faeces and urine (which are affected by feeding and the number, age, and size of horses) and the amount of bedding material (which is affected by indoor mucking regime, which in turn is affected by type of bedding material in terms of absorbency, etc.). All manure produced indoors is mucked out and thereby collected and seen as a potential resource. Manure produced outdoors (paddocks, grazing areas, etc.), however, is not entirely collected. Whether and how manure is collected from outdoor areas, in combination with residence time outdoors, may add to the

Table 3 Conclusions from the literature review. Activity

Manure amount

Manure content

Environmental impact

Feeding

More feed produces more manure.

More feed produces more nutrients in excreta.

Indoor housing

Type of housing and bedding material affects amount of manure collected. Mucking out regime affects amount of bedding material added. Collecting ability from pastures and paddocks.

Type of bedding material affects content of lignocellulosic material, degradability, and biogas potential. Bedding material adds nutrients.

Different types of feed have different environmental impacts and have different concentrations of nutrients that are transferred to the manure. Emissions to air from bedding material and mucking out regime. Air emissions from vehicles. Bedding material affects resource consumption.

Outdoor keeping Manure storage

Manure volume is decreased during composting.

Fertilisation Transport

Amount of manure utilised.

Outdoor collection increases risk of solid impurities. Storage leads to higher share of plant-available nitrogen and lower methane potential. Nutrient leaching differs between storage methods. Type of bedding material affects spreading method.

Air emissions and leakage from trampling and defecation. Emissions to air and risk of leachate of nutrients.

Emissions from spreading. Emissions to air.

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Table 4 Critical factors for different activities in horse keeping, related to manure production and management and the effects of those choices, based on information in the listed references and the analysis in the present study. Critical factors Feeding Feed amount Feed type

What is affected

References

Amount produced, content and concentration of nutrients in excretion, biodegradability

[38,41,42,55]

Indoor housing Amount of bedding materiala Type of bedding material

[18,19,21,53–56] Amount produced, biodegradability, methane potential, nutrient content

Outdoor keeping Outdoor residence time Outdoor collection regime

Amount produced and collected, biodegradability (no bedding, impurities, etc.)

[20,45,67]

Amount utilised, nutrient content, nutrient availability, manure volume, leachate of nutrients, methane potential

[21,31,68]

Manure storage Storage type Storage time

Fertilisation Spreading methodb Soil conditions Transport Transport distance Type of fuel

Amount utilised, application tech- [69,73,74] nique, season, drainage conditions, etc., affect losses Amount utilised

a Amount of bedding is in turn affected by mucking out regime and type of bedding. b Horse manure is spread as solid manure. Bedding material affects spreading.

amount collected. The amount of the collected manure that is utilised is dependent on nutrient and carbon losses (and thus also emissions) from storage and fertilisation. Manure transport may affect the amount available for biogas production and fertilisation. Further analysis of manure characteristics is not feasible, as it has to be made in combination with knowledge about the treatment processes (anaerobic digestion, composting, and combustion).

5. Discussion and conclusions 5.1. Horse manure as feedstock in anaerobic digestion The critical factors identified related to the different activities in horse keeping are depicted in Fig. 3 and further discussed below in terms of how these choices support the use of horse manure as feedstock for anaerobic digestion. Feeding is one of the horse keeping practices which poses a risk of inefficient use of resources. A notable finding is that horses are

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often overfed, causing resource inefficiency and also negative impacts on horse health. Spillage of feed not composted and recycled causes lost nutrients in the natural cycle. Indoor housing, where manure is easily collected, is preferable from a pure manure management and biogas utilisation perspective. Changing the mucking out regime from complete daily mucking out to daily removal of faeces only, would result in lower use of bedding material. Changing the bedding material used, e.g. from wood products to straw, could result in a more suitable feedstock for biogas. Horses kept outdoors are better able to express their natural behaviour, but extensive outdoor horse keeping increases the challenges of collecting horse dung and loss of urine, if not soaked into bedding material in open shelters. Outdoor keeping in paddocks or dry lots with ground prepared for intensive wear near stables gives the opportunity to collect horse dung without added bedding material, which affects biogas production. Disadvantages for the collection ability and nutrient recycling are urine losses to soil and water, impurities like gravel and stones in the manure and challenges that occur in the winter period. Faeces left in pasture can be seen as a naturally spread fertiliser, but is challenging to collect for energy purposes and is therefore considered unavailable for biogas production in some studies. Results in the literature indicate that storage is positive for nutrient availability in horse manure. Storage should preferably impervious to rain and leachate and the material should be turned during storage. The product is a material with lower volume, higher concentration of nitrogen and lower losses of nutrients than static unmanaged piles. Poorly decomposed material after composting is a problem for nutrient recovery [16,31,54,68]. Getting horse manure with a high content of solids to actively decompose may be a problem in manure storage, but is a positive aspect if the horse manure is used as a biogas feedstock. This provides potential for longer storage periods for horse manure before composting. Stored, probably decomposed, material gave lower methane potential than fresh horse manure in one study, indicating a negative impact of composting preceding biogas production [21]. Nitrogen losses from stored horse manure are estimated to range from 25% to 50% [58,73]. While most horse keepers in Sweden store manure on concrete plates or in containers, about 25% store manure on land other than arable land [9]. This increases the risk of leaching to soil and water of e.g. phosphorus, especially if stored manure is not covered. How many horses or how much manure this 25% of horse keepers represents is not known, but the results could be interpreted as meaning that not storing manure covered or on a concrete plates results in a feedstock with lower methane potential and a storage system with higher environmental impacts. This represents an area for potential improvement. When horse manure is not used for fertilising arable land, its nutrient potential is lost. Different references mention the troublesome fact that horses today are kept in close proximity to urban areas, where there is limited areas to spread horse manure and also a lack of

Feeding

Indoor housing

Horse keeping Outdoor Manure keeping storage

Fertilisation

-amount feed -type of feed

-amount bedding -type of bedding

-time outside -collection regime

-spreading method -soil conditions

-type -time

Transport -distance -fuel

Amount and characteristics of horse manure

Biogas and biofertiliser Fig. 3. Summary of critical factors for using horse manure as a biogas feedstock.

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equipment for spreading [68,71]. Long storage time and a treatment such as composting are favourable for utilising the nutrients in horse manure. How to solve the combination of biogas utilisation, collection and storage needs to be further investigated to identify optimal conditions for energy recovery and nutrient recycling. Figures on transport distances are not presented in the literature, but estimates indicate that transport of forage is mostly associated with the production phase, while concentrates may involve longer transport distances. Collecting horse manure sometimes requires the use of motor vehicles and transport to storage, from storage to the field and for spreading in arable land, which contribute to varying degrees to the transport requirements in horse manure management. Horse manure left unmanaged in piles requires no transport but leads to environmental impacts from leaching and emissions to air.

so as to make the manure interesting as a substrate for biogas production. The results compiled here can be used as a basis for obtaining more knowledge on the motives governing horse keepers when deciding on practices. In addition, more knowledge is needed on what can motivate horse keepers to change their current practices in order to decrease the environmental impact and increase the handling of manure to promote better use of resources. Based on this new knowledge, horse keepers should be informed about their role and what they can change to facilitate environmental benefits. Action could also be taken by society in terms of changes in policy etc. to promote a transition towards using all horse manure as a resource. Due to lack of proper registration and supervision of horses, facts and figures on the suspected increasing waste management problem are lacking, which may lead to underestimation of the importance of this problem.

5.2. Method used

5.4. Conclusions

This literature review aimed to qualitatively describe general characteristics in horse manure management and their environmental impact, which varies depending on conditions and different choices and assumptions. Results for a Swedish context are presented for leachate from paddocks, horse manure management, horse keeping and greenhouse gas emissions, while values for biogas potential are mostly from international studies. Information on the environmental impact of horse keeping originated from both Swedish and international studies. As a review of existing information in the horse manure management area, the results are not transferable to certain specific circumstances, but they show examples of existing horse manure management practices and the environmental impacts from horse keeping. The review compiles information from different studies performed as laboratory experiments, full-scale tests and questionnaire- and interview-based surveys. The quantitative studies reviewed, involving surveys or interviews in specific areas, can be seen as samples of practices in reality. Specific conditions affect environmental impacts in horse keeping, e.g. resource use and environmental emissions. Information about the environmental impact of horse keeping and nutrients in horse manure was obtained from all types of sources mentioned above. Biogas research to date mostly comprises laboratory experiments, with some pilot tests and some full-scale tests. Specific information about horse manure as a biogas substrate and methane potential from horse manure has mostly been obtained in laboratory experiments, although such experiments represent optimal circumstances for biogas production and probably differ from those in full-scale tests. Published studies on the environmental aspects of horse keeping or horse health issues (respiratory diseases, problems from air emissions) are often based on laboratory tests, e.g. on nutrient runoff from paddocks, biogas potential of horse manure and/or bedding material or air emissions from horse manure. Laboratory tests indicate the general risks of environmental impacts from horse keeping. Compiling and analysing results from a wide range of references poses a challenge to interpretation. The use of ‘reality checks’ in the present study, e.g. interviews and study visits, supported the literature review in this respect.

Factors identified as affecting the amount and content of horse manure were: amount and type of feed, amount and type of bedding, mucking out regime, time spent outside, time and type of manure storage, spreading of manure and soil conditions, transport distances and type of fuel (cf. Fig. 2). These factors arose in all parts of the current management system in horse keeping and their environmental impacts comprised emissions to air, soil and water and unnecessary use of resources. Areas with potential for improvements to reduce this environmental impact and to increase the opportunities for energy recovery and nutrient recycling were also identified. These potential improvements are to a large extent within the control of horse keepers and the choices they make, e.g. regarding the ration fed, the mucking out regime, how horse manure is stored and whether nutrients are recycled to arable land. These results are relevant when performing environmental systems analysis with numerical calculations on different set-ups for biogas systems and other treatments. In such assessments, the relevance and importance of the critical factors identified here and corresponding conditions to achieve potential improvements can be examined and the most promising system set-up can be devised.

5.3. Implementation of results Based on the present review of literature providing scientific and statistical information on horse keeping, horse manure and related environmental impacts, it can be concluded that vital and relevant information is lacking, e.g. with respect to the number and location of horses, statistics on manure management and the environmental impacts related to current manure management practices. This knowledge gap has implications for how horse keepers and society should work together. However, the results also indicate the importance of designing horse keeping systems

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