Why Proven Sorghum Technologies Do Not Close the Yield Gap in Smallholder Sub-Saharan Africa: An Interrogative Review of the Constraint and Adoption Evidence

Submitted manuscript, not yet peer reviewed. Sorghum (Sorghum bicolor [L.] Moench) is the main food source for about 300 to 500 million smallholder farmers in the semi-arid regions of sub-Saharan Africa, yet farm yields have remained at 30 to 40% of water-limited yield potential for three decades, one of the most persistent yield gaps among the major cereals. Integrated soil fertility management, water harvesting techniques, Striga-resistant varieties and push-pull pest management are all well evidenced, and none of them has closed it. This review asks why, and interrogates the evidence base rather than summarising it. Three limitations are identified in the literature itself. Yield responses are reported without the partial budgets that determine whether a practice is adoptable, so the largest documented nutrient response coincides with a net financial loss. The largest reported effect sizes are measured against comparators no farmer faces, including infested controls and on-station monocrops, and matched-comparison evidence from the same region shows that unmatched arms can reverse a published gradient. The quantitative backbone of the yield gap narrative rests on a small number of sites, so recommendations carry more confidence than their geographic coverage supports. Beneath these lies a structural problem: agronomic research measures kilograms per hectare while adoption research measures percentages of farmers, and the two are not commensurable, so no single study can attribute the gap between what a technology yields and what it delivers. Working from that diagnosis, the review proposes a definition of the sorghum yield gap anchored in locally attainable yield rather than modelled potential, examines four agronomic constraint domains alongside four classes of adoption barrier, and specifies the constraint-barrier pairings through which documented solutions fail to reach farmer practice. It presents a three-tier framework comprising a Constraint Layer, a Mediation Layer and an Outcome Layer, connected by a feedback pathway through which persistent yield gaps erode household assets and deepen adoption barriers. The framework reframes yield gap closure as a systems problem and gives researchers, extension services and policymakers a diagnostic instrument for smallholder sorghum systems in sub-Saharan Africa. It also yields three propositions that a two-round farm survey could refute, which makes the structure testable rather than only descriptive.