Market Integration and Price Discovery in the Yam Market Across Nigerian Geopolitical Zones

Yam is Nigeria's largest root crop by production volume and a major food security staple traded across all geopolitical zones in domestic and regional markets. Understanding the degree of market integration and the speed of price discovery across spatially separated yam markets has direct implications for food policy, producer pricing strategies, and inter-regional trade facilitation. This study analyses market integration and price discovery in the Nigerian yam market using monthly wholesale price data from twelve wholesale markets across all six geopolitical zones spanning 2019 to 2024. Spatial price integration was examined using the Johansen cointegration test and the Engle-Granger two-step approach. Price leadership and discovery were assessed using Granger causality tests and variance decomposition from vector error correction models. Results indicate cointegrating relationships across most market pairs, confirming long-run price integration in the national yam market. Lagos and Onitsha markets exhibited the strongest price leadership roles, consistent with their status as high-volume consumer and transshipment markets. Adjustment speeds to price disequilibria were slowest for markets in the North East zone, reflecting higher transaction costs associated with distance and poor road infrastructure. The study identifies seasonal price patterns that suggest harvest period market glut in producing states is inadequately transmitted to consumer price reductions in distant urban markets. Recommendations target improved yam price information dissemination through a national commodity market information system and investment in yam storage infrastructure to dampen seasonal price swings. Keywords: yam market, market integration, price discovery, cointegration, Nigeria