Field Performance of a Modified Serrated-Disc Stem-Gripping Mechanism for a Tractor-Mounted Cassava Harvester on Sandy-Loam Tropical Soil

Manual cassava harvesting in sub-Saharan Africa demands 44–62 labour-days per hectare and causes substantial tuber damage through breakage. Most existing mechanical harvesters address soil lifting but fail at stem engagement due to disc clogging and insufficient torque. This study evaluated a tractor-mounted cassava harvester modified by replacing the plain disc with a serrated disc (six 10-cm peripheral gaps) and upgrading the drive from a single-chain belt to a double-chain spring-loaded sprocket system. Field trials were conducted on sandy-loam soil (bulk density 1.49 g/cm³; moisture content 5.13% dry basis; penetration resistance 1.02 MPa) at the Step-B Demonstration Farm, Federal University of Technology Akure, Nigeria, using cassava variety TME 419 at nine months after planting. Four disc angles (30°, 45°, 50°, 60°) and three operating depths (30, 40, 50 cm) were evaluated in a factorial design with three replicates. At 30° disc angle and 30 cm depth, the modified mechanism achieved 90.2% harvesting efficiency, 0.084 ha/h field capacity, and 12% root damage without requiring stem pre-cutting. Mean tuber extraction force fell from approximately 1,250 N to under 160 N after machine passage — an 87–100% reduction. One-way ANOVA confirmed that disc angle significantly affected harvesting efficiency (F = 15.52, p = 0.001) but not root damage, unharvested roots, or field capacity (p > 0.05). Linear regression showed disc angle explained more than 94% of the variance in harvesting efficiency at all tested depths (R² = 0.946–0.970). The serrated disc fully eliminated soil clogging observed on the baseline plain disc. Total modification cost was ₦60,000 (approximately US$146), placing this upgrade within reach of smallholder cooperatives in south-west Nigeria.