Performance assessment of cattle manure management technologies : a case of Sio-Malaba-Malakisi River Basin, Uganda
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Most Ugandan households are engaged in agriculture with 58% in livestock farming. With up to 16 million Tropical Livestock Units (TLU), the manure generated is yet to be well managed to reduce associated environmental hazards such as greenhouse gas (GHG) emissions. Under shade manure storage (USMS), open manure storage (OMS) and bio-digester with under shade slurry storage (USSS) are some of the technologies used by farmers to manage manure however, their uptake is still low in Uganda. This study assessed these technologies from a technical, environmental and economic viewpoint to create awareness and develop recommendations for adoption. To determine the commonly practiced manure management technologies particularly in Sio-Malaba-Malakisi River Basin (SMMRB), a questionnaire survey was conducted. Experiments were set up to monitor losses of nutrients and gaseous emissions from the different technologies during three months period of manure storage. In addition, an economic assessment of the manure management technologies was performed using net present value, benefit-cost ratio and internal rate of return techniques. The study established that about 34% of cattle farmers in SMMRB practiced open dumping, 1.2% used it as feedstock for bio-digester, 1.2% burnt it while 63.6% stored manure either in open OMS or in USMS. For the farmers who stored manure, the majority practiced OMS (89.4%) while 9.6% and 1% practiced USMS and storage in sacks respectively. Results revealed that, bio-digester and USSS retained 52.9% (w.b) of the original mass of fresh manure while OMS and USMS respectively retained 46.2% (w.b) and 34.4% (w.b) of the initial mass. For nutrient retention, bio-digester and USSS retained the highest TN and TC of 67.5% and 61.8% respectively followed by USMS (59.8% and 56.8%) and lastly OMS (47.8% and 45.4%). Conversely, bio-digester and USSS retained the least TP and TK of 65.1% and 40.8% respectively followed by OMS (98.0% and 46.5%) and finally USMS (99.8% and 99.2%). Environmental performance showed that OMS had the least emissions of CH4 at 0.09% while USSS and USMS had 0.11% and 0.13% respectively. The respective contribution to global warming in carbon dioxide equivalent were 40.04 g CO2-eq kg-1 waste, 54.88 g CO2-eq kg-1 waste and 60.20 g CO2-eq kg-1 waste for OMS, USSS and USMS, excluding carbon dioxide gas. Similarly, OMS and USSS each had 0.13 ppm of NH3 emissions compared to 0.83 ppm for USMS. Translating the NH3 emissions to phosphate equivalents revealed that the eutrophication potentials for OMS and USSS were each 7 x 10-5 g PO4-3-eq kg-1 waste and 5.3 x 10-4 g PO4-3-eq kg-1 waste for USMS. Economic analysis suggested that at Bank of Uganda interest rate (16.1%), OMS had the highest NPV (UGX. 2,170,963) while USMS and bio-digester with USSS had UGX. 99,477 and UGX. 530,602 respectively. At the same interest rate, the respective B/C of the three technologies were 25.12, 1.24 and 1.08. However, at interest rate charged by commercial banks (22.0%), OMS was the only technology with positive NPV (UGX. 1,619,970) and B/C above one. The IRRs were respectively 426%, 21% and 18% for OMS, USMS and bio-digester with USSS. In general, bio-digester and USSS retained more mass, TN and TC. However, OMS was the most environmentally friendly as well as economically viable manure management technology compared to USMS and bio-digester with USSS.