Journal of Experimental Agriculture International

Herbicide adulteration has emerged as a critical constraint to sustainable weed management and crop productivity among smallholder farmers in Southwestern Nigeria. This study assessed the socio-economic, institutional, and agronomic factors influencing farmers’ exposure to adulterated herbicides and their impact on weed management and crop production in Oyo and Ogun States. A mixed-methods, cross-sectional survey was conducted in the 48 cropping communities, using Cochran’s formula (n = Z²pq/e²; Z = 1.96, p = 0.982, e = 0.05). The minimum sample (385) was expanded by 30% to 500 households (250/state). Data were analyzed with descriptive statistics, χ², logistic regression, t-tests, and ANOVA (p < 0.05). The findings revealed that farming was dominated by middle-aged farmers with an average age of 47.4 ± 12.1 years, indicating an ageing agricultural workforce with limited youth participation. Male farmers constituted 64.6% of respondents, while the average household size was 5.6 persons, reflecting strong dependence on family labour for weed control operations. Farmers had an average of 8.8 years of formal education, and educational attainment significantly reduced exposure to adulterated herbicides (OR = 0.832, p = 0.001), underscoring the importance of literacy in agrochemical verification and decision-making. Similarly, extension contact significantly reduced exposure (OR = 0.820, p = 0.009), whereas larger farm size increased vulnerability (OR = 1.236, p = 0.015). Membership in farmer associations showed a near-significant protective effect (OR = 0.708, p = 0.052). Glyphosate-based herbicides were used by 96.0% of respondents, while 71.6% relied on paraquat despite increasing global restrictions due to toxicity concerns. Nearly half (44.8%) of the sample size sourced herbicides from informal markets, thereby increasing exposure to counterfeit products. Herbicide expenditure accounted for 16.9% of total production costs, indicating substantial economic dependence on chemical weed control. Although 70.0% of farmers were aware of herbicide adulteration, detailed knowledge of counterfeit indicators, environmental risks, and herbicide resistance remained low. Weak regulatory enforcement, inadequate retailer quality assurance, low training participation (24.4%), and poor access to credit (31.6%) further exacerbated farmers’ vulnerability. Herbicide adulteration is both a weed management and institutional governance challenge. Strengthening regulatory surveillance, promoting certified agro-dealer systems, expanding extension services, improving farmer literacy, and introducing digital authentication technologies are essential for safeguarding weed management efficiency, reducing herbicide resistance risks, and ensuring sustainable crop production and food security in Nigeria.