Despite the high level of production of sorghum in Nigeria its threshing and cleaning operation re-mains among the major challenges to farmers especially in the northern part of the country. Efficient sorghum threshers are not available to Nigerian farmers. The prototype thresher developed at Institute of Agricultural Research (IAR) was associated with many problems such as: low capacity, and higher mechanical grain damage. To address these problems a modified model was designed, constructed and evaluated. The major components modified include feed hopper, cylinder and concave. The machine performance was evaluated at various levels of moisture contents, speed and feed rates. The results showed that threshing efficiency and throughput capacity of the modified thresher were 99.980 % and 253.960 kg/hr respectively while that for mechanical grain damage was 3.700 %. In comparison with the existing IAR thresher the results indicated an increased by 0.980, and 56.960 % for threshing effi-ciency and throughput capacity respectively While mechanical grain damage decreased by 3.840 % The best combinations of independent variable for optimum operations were 13.000 % moisture con-tent, 12.670 m/s cylinder speed and 5.000 kg/min feed rate. At these levels the improved prototype sorghum thresher gave performance values of 297.000 kg/hr throughput capacity, while threshing effi-ciency and mechanical grain damage were 99.800 and 3.400 % respectively.
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