Contributions to Peer-Reviewed Journals only


Roessler, R., Mpouam, S.E., E. Schlecht. 2019. Genetic and nongenetic factors affecting on-farm performance of peri-urban dairy cattle in west Africa. Journal of Dairy Science 102(3), 2353-2364. DOI 10.3168/jds.2018-15348

Wassie, S.E., A.I.M. Ali, D. Korir, K. Butterbach-Bahl, J. Goopy, L. Merbold, E. Schlecht, U. Dickhoefer. 2019. Effects of feed intake level on efficiency of microbial protein synthesis and nitrogen balance in Boran steers consuming tropical poor-quality forage. Archives of Animal Nutrition 73(2), 140-157. DOI 10.1080/1745039X.2019.1572343


Altmann, B., G. Jordan, E. Schlecht. 2018. Participatory mapping as an approach to identify grazing pressure in the Altay Mountains, Mongolia. Sustainability 10(6), 1960. DOI 10.3390/su10061960

Amprako, L., M. Alhassan, A. Buerkert, R. Roessler. 2018. Influence of dietary wood charcoal on growth performance, nutrient efficiency and excreta quality of male broiler chickens. International Journal of Livestock Production 9(10), 286-292. DOI: 10.5897/IJLP2018.0486

Fust, P., E. Schlecht. 2018. Integrating spatio-temporal variation in resource availability and herbivore movements into rangeland management: RaMDry - An agent-based model on livestock feeding ecology in a dynamic, heterogeneous, semi-arid environment. Ecological Modelling 369, 13-41. DOI 10.1016/j.ecolmodel.2017.10.017

Houessou, S.O., L.H. Dossa, R.V.C. Diogo, A. Buerkert, E. Schlecht. 2018. Change and continuity of West African Coast countries: A case study from Benin.Agricultural Systems 168, 112-122. DOI 10.1016/j.agsy.2018.11.003

Ingold, M., S. Schmidt, H. Dietz, R.G. Joergensen, E. Schlecht, A. Buerkert. 2018. Tannins in goat diets modify manure turnover in a subtropical soil. Experimental Agriculture 54(5), 655-669. DOI 10.1017/S0014479717000291

Jordan, G., S. Goenster-Jordan, G.-J. Lamparter, B. Ulziisuren, N. Soninkishig, E. Schlecht, A. Buerkert. 2018. Water use in agro-pastoral livelihood systems within the Bulgan River watershed of the Altay Mountains, Western Mongolia. Agriculture, Ecosystems and Environment 251, 180-193. DOI 10.1016/j.agee.2017.09.008

Roessler, R., F. Chefor, E. Schlecht. 2018. Using a portable laser methane detector in goats to assess diurnal, diet- and position-dependent variations in enteric methane emissions. Computers and Electronics in Agriculture 150, 110-117. DOI 10.1016/j.compag.2018.04.010

Soma, T., E. Schlecht. 2018. The relevance of herders' local ecological knowledge on coping with livestock losses during harsh winters in  western Mongolia. Pastoralism 8,3. DOI 10.1186/s13570-017-0108-y

Tsvegemed, M., A. Shabier, E. Schlecht, G. Jordan, M. Wiehle. 2018. Evolution of rural livelihood strategies in a remote Sino-Mongolian border area: A cross-country analysis. Sustainabiliy 10(4), 1011. DOI 10.3390/su10041011

Zampaligré, N., E. Schlecht. 2018. Livestock foraging behaviour on different land use classes along the semi-arid to sub-humid agro-ecological gradient in West Africa. Environment, Development and Sustainability 20(2), 731-748. DOI 10.1007/s10668-017-9907-y


Al-Kindi, A., A. Schiborra, A. Buerkert, E. Schlecht. 2017. Effects of quebracho tannin extract and activated charcoal on nutrient digestibility, digesta passage and faeces composition in goats. Journal of Animal Physiology and Animal Nutrition 101(3), 576-588. DOI 10.1111/jpn.12461

Al-Rawahi, M.N., M. Melapie, E. Schlecht, A. Buerkert. 2017. Carbon and nutrient balances in three mountain oases in Northern Oman. Journal of Agricultural and Marine Sciences 22(1), 75-86.

Brown, M.M., B. Alenyorege, G.A. Teye, R. Roessler. 2017. Phenotypic diversity, major genes and production potential of local chickens and guinea fowl in Tamale, northern Ghana. Asian-Australasian Journal of Animal Sciences 30(10), 1372-1381. DOI 10.5713/ajas.17.0145

dos Santos Neutzling, A., L.H. Dossa, E. Schlecht. 2017. Production and milk marketing strategies of small-scale dairy farmers in the South of Rio Grande do Sul, Brazil. Journal of Agriculture and Rural Development in the Tropics and Subtropics 118(2), 283-295.

Feldt, T., O. Antsonantenainarivony, E. Schlecht. 2017. Feed selection on dry rangelands in southwestern Madagascar: Implications for ruminant nutrition in view of ecological and social challenge. Journal of Arid Environments 144, 81-90. DOI 10.1016/j.jaridenv.2017.03.009

Roessler, R., L. Amprako, A.R. Sayibu, A. Mohammed, R.C. Menezes, D. Hölscher, B. Alenyorege, H.K. Dei, C. Steiner. 2017. Effects of false yam tuber meals and charcoal on broiler chicken production and blood parameters. South African Journal of Animal Sciences 47(6), 842-853. DOI 10.4314/sajas.v47i6.12


Al-Kindi, A., U. Dickhoefer, E. Schlecht, A. Sundrum, A. Schiborra, A. 2016. Effects of quebracho tannin extract (Schinopsis balansae Engl.) and activated charcoal on nitrogen balance, rumen microbial protein synthesis and faecal composition of growing Boer goats. Archives of Animal Nutrition 70, 307-321. DOI 10.1080/1745039X.2016.1175807

Al-Kindi, A., E. Schlecht, A. Schiborra, R.G. Joergensen. 2016. Effects of quebracho tannin extract (Schinopsis balansae) and activated charcoal on feed intake and digestibility by goats and their faecal microbial biomass. Biological Agriculture and Horticulture 32(3), 159-169.

Feldt, T., R. Neudert, P. Fust, E. Schlecht. 2016. Reproductive and economic performance of local livestock in southwestern Madagascar: Potentials and constraints of a highly extensive system. Agricultural Systems 149, 54-64. DOI 10.1016/j.agsy.2016.08.007

Feldt, T., E. Schlecht. 2016. Analysis of GPS trajectories to assess spatio-temporal differences in grazing patterns and land use preferences of domestic livestock in southwestern Madagascar. Pastoralism 6, 5. DOI 10.1186/s13570-016-0052-2

Jordan, G., S. Goenster, T. Munkhnasan, A. Shabier, A. Buerkert, E. Schlecht. 2016. Spatio-temporal patterns of herbage availability and livestock movements: A cross-border analysis in the Chinese-Mongolian Altay. Pastoralism: Research, Policy and Practice 6, 12. DOI 10.1186/s13570-016-0060-2

Raza, M.A, M. Younas, E. Schlecht 2016. In vitro efficacy of selected medicinal plants from Cholistan desert, Pakistan, against gastrointestinal helminths of sheep and goats. Journal of Agriculture and Rural Development in the Tropics and Subtropics 117(2), 211–224.

Roessler, R., S.E. Mpouam, T. Muchemwa, E. Schlecht. 2016. Emerging development pathways of urban livestock production in rapidly growing West Africa cities. Sustainability 8(11), 1199. DOI 10.3390/su8111199


Amadou, H., L. Konaté, Nantoumé, H., A. Buerkert, E. Schlecht 2015. Nutrient use efficiency in peri-urban dairy cattle and sheep holdings in southern Mali. Nutrient Cycling in Agroecosystems 102, 65-78. DOI 10.1007/s10705-014-9666-1

Dossa, L. H., M. Sangaré, A. Buerkert, E. Schlecht 2015. Intra-urban and peri-urban differences in cattle farming systems of Burkina Faso. Land Use Policy 48, 401-411. DOI 10.1016/j.landusepol.2015.06.031

Dossa, L. H., M. Sangaré, A. Buerkert, E. Schlecht 2015. Production objectives and breeding practices of urban goat and sheep keepers in West Africa: regional analysis and implications for the development of supportive breeding programs. SpringerPlus 4, 281. DOI 10.1186/s40064-015-1075-7

Ingold, M., H. Dietz, A. Sradnick, R.G. Joergensen, E. Schlecht, A. Buerkert 2015. Effects of activated charcoal and quebracho tannin amendments on soil properties in irrigated organic vegetable production under arid subtropical conditions. Biology and Fertility of Soils 51, 367-377. DOI 10.1007/s00374-014-0982-z

Schiborra, A., M. Bulang, A. Berk, A. Susenbeth, E. Schlecht. 2015. Using faecal near-infrared spectroscopy (FNIRS) to estimate nutrient digestibility and chemical composition of diets and faeces of growing pigs. Animal Feed Science and Technology 210, 234-242. DOI 10.1016/j.anifeedsci.2015.10.011


Cumming, G.S., A. Buerkert, E.M. Hoffmann, E. Schlecht, S. von Cramon‐Taubadel, T. Tscharntke 2014. Implications of agricultural transitions and urbanization for ecosystem services. Nature 515, 50‐57.

Al-Qamashoui, B., O. Mahgoub, I. Kadim, E. Schlecht. 2014. Towards conservation of Omani local chicken: Phenotypic characteristics, management practices and performance traits. Asian Australasian Journal of Animal Science 27(6), 767-777.

Al-Qamashoui, B., H. Simianer, I. Kadim, S. Weigend. 2014. Assessment of genetic diversity and conservation priority of Omani local chickens using microsatellite markers. Tropical Animal Health and Production 46(5), 747-752.

Al-Rawahi, M.N., K. Brinkmann, E. Schlecht, A. Buerkert. 2014. Effects of changing water availability on land use in irrigated mountain oases of Al Jabal Al Akhdar, northern Oman. ERDE 145, 197-211.

Amadou, H., C. Hülsebusch, A. Berthe, E. Schlecht. 2014. Safety of horticultural and livestock products in two medium-sized cities of Mali and Burkina Faso. African Journal of Agricultural Research 9(8), 735-745.

Raza, M.A., M. Younas, E. Schlecht. 2014. Prevalence of gastrointestinal helminths in pastoral sheep and goat flocks in the Cholistan desert of Pakistan. The Journal of Animal and Plant Sciences 24(1), 127-134.

Raza, M.A., M. Younas, A. Buerkert, E. Schlecht. 2014. Ethno-botanical remedies used by pastoralists for the treatment of livestock diseases in Cholistan desert, Pakistan. Journal of Ethnopharmacology 151, 333–342.

Riedel, S., A. Schiborra, C. Hülsebusch, E. Schlecht. 2014. The productivity of traditional smallholder pig production and possible improvement strategies in Xishuangbanna, South Western China. Livestock Science 160, 151-162.

Schlecht, E., L.G.H. Zaballos, D. Quiroz, P. Scholte, A. Buerkert. 2014. Traditional land use and reconsideration of environmental zoning in the Hawf Protected Area, south-eastern Yemen. Journal of Arid Environments 109, 92-102.

Tariq, M., J. Hagmann, L.H. Dossa, M. Younas, E. Schlecht. 2014. Structural characterization of dairy production systems in Faisalabad, Pakistan as basis for their efficient resource management. Pakistan Journal of Agricultural Sciences 51, 1069-1080.

Zampaligré, N., L.H. Dossa, E. Schlecht. 2014. Climate change and variability: Perception and adaptation strategies of pastoralists and agro-pastoralists across different zones of Burkina Faso. Regional Environmental Change 14(2), 769-783.


Al-Asfoor, A., A. Schiborra, A. Sundrum, E. Schlecht. 2013. Particulate rate of passsage and faeces quality of water buffalo fet diets varying in concentrations of nitrogen and structural and non-structural carbohydrates. Animal Nutrition and Feed Technology 13, 165-180.

Buerkert, A., E. Schlecht. 2013. Agricultural innovations in small-scale farming systems of Sudano-Sahelian West Africa: Some prerequisites for success. Sécheresse 24, 322-329.

Diogo, R.V.C, E. Schlecht, A. Buerkert, M.C. Rufino, M.T. van Wijk. 2013. Increasing nutrient use efficiency through improved feeding and manure management in urban and peri-urban livestock units of a West African city: a scenario analysis. Agricultural Systems 114, 64–72.

Krug,M. E. Schlecht, A. Buerkert. 2013. Agricultural sustainability and food security of Jhum systems in Nagaland, NE India. Outlook on Agriculture 42(2), 133–139.

Olufemi, S.A., O.S. Fanimo, J.A. Agunbiade, A. Susenbeth, E. Schlecht. 2013. Cassava root peel as a replacement for maize in diets for growing pigs: effects on energy and nutrient digestibility, performance and carcass characteristics. Journal of Agriculture and Rural Development in the Tropics and Subtropics 114(2), 159-166.

Rehman, S.U., M. Predotova, I.A. Khan, E. Schlecht, A. Buerkert. 2013. Socio-economic characterization of integrated cropping systems in urban and peri-urban agriculture of Faisalabad, Pakistan. Journal of Agriculture and Rural Development in the Tropics and Subtropics 114(2), 133-143.

Siegfried, K., H. Dietz, D. Amthauer Gallardo, E. Schlecht, A. Buerkert. 2013. Effects of manure with different C/N ratios on yields, yield components and matter balances of organically grown vegetables on a sandy soil of northern Oman. Organic Agriculture 3(1), 9-22.

Tariq, M., M. Younas, A.B. Khan, E. Schlecht. 2013. Body Measurements and Body Condition Scoring as Basis for Estimation of Live Weight in Nili-Ravi Buffaloes. Pakistan Veterinary Journal 33(3): 325-329.

Zampaligré, N., L.H. Dossa, E. Schlecht. 2013. Contribution of browse to ruminant nutrition across three agro-ecological zones of Burkina Faso.Journal of Arid Environments 95, 55-64.


Abdulkadir, A., L.H. Dossa, D.J.-P. Lompo, N. Abdu, H. van Keulen. 2012. Characterization of urban and peri-urban agroecosystems in three West African cities. International Journal of Agricultural Sustainability 10 (4), 289-314.

Al-Asfoor, H., E. Schlecht, A. Sundrum, A. Schiborra. 2012. Varying the dietary supply of C and N to manipulate the manure composition of water buffalo heifers in Oman. Journal of Agriculture and Rural Development in the Tropics and Subtropics 113(2), 125-136.

Amadou, H., L.H. Dossa, D.J.-P. Lompo, A. Abdulkadir, E. Schlecht. 2012. A comparison between urban livestock production strategies in Burkina Faso, Mali and Nigeria in West Africa. Tropical Animal Health and Production 44, 1631-1642.

Blank, B., E. Schlecht, A. Susenbeth. 2012. Effect of dietary fibre on nitrogen retention and fibre associated threonine losses in growing pigs. Archives of Animal Nutrition 66, 86-101.

Buerkert, A., E. Schlecht. 2012. Organic agriculture in developing countries: Status quo and challenges. Rural 21(3), 6-8.

Dickhoefer, U., M. Nagieb, A. dos Santos Neutzling, A. Buerkert, E. Schlecht. 2012. Modeling herd development and revenues for Jabal Akhdar goats under current and intensified management practices. Agricultural Systems 110, 131–141.

Lambertz, C. C. Chaikong, J. Maxa, E. Schlecht, M. Gauly. 2012. Characteristics, socioeconomic benefits and household livelihoods of beef buffalo and beef cattle farming in Northeast Thailand. Journal of Agriculture and Rural Development in the Tropics and Subtropics 113(2), 155-164.

Lompo, D.J.-P., S.A.K. Sangaré, E. Compaoré, M.P. Sedogo, M. Predotova, E. Schlecht, A. Buerkert. 2012. Gaseous emissions of nitrogen and carbon from urban vegetable gardens in Bobo-Dioulasso, Burkina Faso. Journal of Plant Nutrition and Soil Science 175, 846–853.

Riedel, S., M. Meyer, E. Schlecht, C. Huelsebusch, A. Schiborra. 2012. Swamp buffalo keeping – an out-dated farming activity? A case study in smallholder farming systems in Xishuangbanna, Yunnan Province, PR China. Journal of Agriculture and Rural Development in the Tropics and Subtropics 113(2), 137-145.

Riedel, S., A. Schiborra, C. Huelsebusch, M. Huanming, E. Schlecht. 2012. Opportunities and challenges for smallholder pig production systems in a mountainous region of Xishuangbanna, Yunnan Province, China. Tropical Animal Health and Production 44, 1971-1980.


Brinkmann, K., U. Dickhoefer, E. Schlecht, A. Buerkert. 2011. Quantification of aboveground rangeland productivity and anthropogenic degradation on the Arabian Peninsula using Landsat imagery and field inventory data. Remote Sensing of Environment 115, 465-474.

Brinkmann, K., A. Patzelt, A., E. Schlecht, A. Buerkert. 2011. Use of environmental predictors for vegetation mapping in semi-arid mountain rangelands and the determination of conservation hotspots. Journal of Applied Vegetation Science 14, 17-30.

Dickhoefer U., O. Mahgoub, E. Schlecht. 2011. Adjusting homestead feeding to requirements and nutrient intake of grazing goats on semi-arid, subtropical highland pastures. Animal 5, 471-482.

Diogo, R.V.A., A. Buerkert, E. Schlecht. 2011. Economic benefit of gardeners and retailers from cultivating and marketing of vegetables in Niamey, Niger. Outlook on Agriculture 40, 71-78.

Dossa, L.H., A. Abdulkadir, H. Amadou, S. Sangare, E. Schlecht. 2011. Exploring the diversity of urban and peri-urban agricultural systems in Sudano-Sahelian West Africa: An attempt towards a regional typology. Landscape and Urban Planning 102, 197-206.

Dossa, L.H., A. Buerkert, E. Schlecht. 2011. Cross-location analysis of the impact of household socioeconomic status on participation in urban and peri-urban agriculture in West Africa. Human Ecology 39, 569–581.

Schlecht, E., U. Dickhoefer, M. Predotova, A. Buerkert. 2011. The importance of semi-arid natural mountain pastures for feed intake and recycling of nutrients by traditionally managed goats on the Arabian Peninsula. Journal of Arid Environments 75, 1136–1146.

Siegfried, K., H. Dietz, E. Schlecht, A. Buerkert. 2011. Nutrient and carbon balances in organic vegetable production on an irrigated, sandy soil in northern Oman. Journal of Plant Nutrition and Soil Science 174, 678-689.

Safi, Z., M. Predotova, E. Schlecht, A. Buerkert. 2011. Horizontal matter fluxes and leaching losses in urban and peri-urban agriculture of Kabul, Afghanistan. Journal of Plant Nutrition and Soil Science 174, 942-951.



Buerkert, A., S. Al Khanjari, N.A. Al-Saady, E. De Langhe, D. Hölscher, J. Gebauer, S. Behrendt, S. Happe, K. Brinkmann, E. Schlecht. 2010. Uralter Toleranzmechanismus im Oman entdeckt: Wie sich Oasen-Bananen gegen ihre Feinde wehren. Biologie Unserer Zeit, DOI:10.1002/biuz.201010432.

Buerkert, A., U. Dickhoefer, E. Luedeling, K. Lohrer, B. Mershen, W. Schaeper, M. Nagieb, E. Schlecht. 2010. Prospects of mountain ecotourism in Oman: the example of As Sawjarah on Al Jabal al Akhdar. Journal of Ecotourism 9, 104-116.

Dickhoefer, U., A. Buerkert, K. Brinkmann, E. Schlecht. 2010. The role of pasture management for sustainable livestock production in semi-arid subtropical mountain regions. Journal of Arid Environments 74, 962–972.

Diogo, R.V.C., A. Buerkert, E. Schlecht. 2010. Resource use efficiency in urban and peri-urban sheep, goat and cattle enterprises. Animal 4, 1725-1738.

Diogo, R.V.C., A. Buerkert, E. Schlecht. 2010. Horizontal nutrient fluxes and food safety in urban and peri-urban vegetable and millet cultivation of Niamey, Niger. Nutrient Cycling in Agroecosystems 87, 81-102.

Predotova, M., E. Schlecht, A. Buerkert. 2010. Nitrogen and carbon losses from dung storage in urban gardens of Niamey, Niger. Nutrient Cycling in Agroecosystems 87, 103-114.

Predotova, M., J. Gebauer, R.V.C. Diogo, E. Schlecht, A. Buerkert. 2010. Emissions of ammonia, nitrous oxide and carbon dioxide from urban gardens in Niamey, Niger. Field Crops Research 115, 1-8.



Brinkmann, K., U. Dickhoefer, E. Schlecht, A.Buerkert, 2009. Vegetation patterns and diversity along an altitudinal and a grazing gradient in the Jabal al Akhdar mountain range of northern Oman. Journal of Arid Environments 73, 1035-1045.

Buerkert, A., E. Schlecht, 2009. Performance of three GPS collars to monitor goats’ grazing itineraries on mountain pastures. Computers and Electronics in Agriculture 65, 85-92.

Buerkert, A., E. Schlecht, 2009. The biofuel debate – status quo and research needs to meet multiple goals of food, fuel and ecosystem services in the Tropics and Subtropics. Journal of Agriculture and Rural Development in the Tropics and Subtropics 110, 1-8.

Moreau, M., S. Siebert, A. Buerkert, E. Schlecht, 2009. Use of a tri-axial accelerometer for automated recording and classification of goats' grazing behaviour. Journal of Applied Animal Behaviour Science 119, 158-170.

Schlecht, E., U. Dickhoefer, E. Gumpertsberger, A. Buerkert, 2009. Grazing itineraries and forage selection of goats in the Al Jabal al Akhdar mountain range of northern Oman. Journal of Arid Environments 73, 355-363.



2008 and earlier

Graefe, S., E. Schlecht, A. Buerkert, 2008. Opportunities and challenges of urban and peri-urban agriculture in Niamey, Niger. Outlook on Agriculture 37, 47-56.

Schlecht, E., H. Richter, S. Fernández-Rivera, K. Becker, 2007. Gastrointestinal passage of Sahelian roughages in cattle, sheep and goats, and implications for livestock-mediated nutrient transfers. Animal Feed Science and Technology 137, 93–114.

Richter, H., E. Schlecht, 2006. Accounting for marker disassociation when modeling ruminal escape of particles based on the faecal excretion of Ytterbium. Animal Feed Science and Technology, 128, 135-154.

Schlecht, E., A. Susenbeth, 2006. Estimating the digestibility of Sahelian roughages from faecal crude protein concentration of cattle and small ruminants. Journal of Animal Physiology and Animal Nutrition 90, 369-379.

Schlecht, E., A. Buerkert, E. Tielkes, A. Bationo, 2006. A critical analysis of challenges and opportunities for soil fertility restoration in Sudano-Sahelian West Africa. Nutrient Cycling in Agroecosystems 76, 109-136.

Schlecht, E., P. Hiernaux, I. Kadaouré, C. Hülsebusch, F. Mahler, 2006. A spatio-temporal analysis of forage availability, grazing and excretion behaviour of cattle, sheep and goats in Western Niger. Agriculture, Ecosystems and Environment 113, 226-242.

Turner, M.D., P. Hiernaux, E. Schlecht, 2005. The distribution of grazing pressure in relation to vegetation resources in semi-arid West Africa: The role of herding. Ecosystems 8(6): 668-281.

Schlecht, E., A. Buerkert, 2004. Organic inputs on millet fields in western Niger: the implications of farmers’ practices for sustainable agricultural production. Geoderma 121, 271-289.

Schlecht, E., P. Hiernaux, 2004. Beyond adding up inputs and outputs: Process assessment and upscaling in modelling nutrient flows. Nutrient Cycling in Agroecosystems 70, 303–319.

Schlecht, E., C. Hülsebusch, F. Mahler, K. Becker, 2004. The use of differentially corrected global positioning system to monitor activities of cattle at pasture. Applied Animal Behaviour Science 85, 185-202.

Schlecht, E., P. Hiernaux, F. Achard, M.D. Turner, 2004. Livestock related nutrient budgets within village territories in western Niger. Nutrient Cycling in Agroecosystems, 68, 199-211.

Wezel, A., E. Schlecht, 2004. Inter-annual variation of species composition in fallow plant communities of semi-arid Niger. Journal of Arid Environments 56, 265-282.

Schlecht, E., M. Sangaré, K. Becker, 2003. Seasonal variations in gastrointestinal tract fill of grazing Zebu cattle in the Sahel. Journal of Agricultural Science, Cambridge, 140, 461-468.

Schlecht, E., I. Kadaouré, F. Graef, C. Hülsebusch, F. Mahler, K. Becker, 2001. Land-use and agricultural practices in the agro-pastoral farming systems of Western Niger – a case study. Die Erde 132, 399-418.



"Bergoasen im Oman"
erschienen in englischer und arabischer Sprache