Yield prediction and economic evaluation of integrated NPK fertilizer and poultry manure use in eggplant (Solanum melongena L.) production in Guinea-forest savannah zone of Gambia

Abstract

This study investigated the relationships between growth parameters and fruit yield of eggplant as well as the economic returns associated with different fertilizer regimes under field conditions. A field experiment was conducted at the College Horticultural Garden, University of Education, Brikama, Guinea-forest savannah, The Gambia, during the 2022 and 2023 cropping seasons. Treatments consisted of Control, T1 (NPK at 125 kg ha⁻¹), T2 (poultry manure at 10 t ha⁻¹), T3 (NPK at 125 kg ha⁻¹ + poultry manure at 10 t ha⁻¹), and T4 (NPK at 250 kg ha⁻¹ + poultry manure at 20 t ha⁻¹) and laid out in a randomized complete block design (RCBD) with three replications.  Growth parameters evaluated included plant height, stem girth, leaf area index, number of fruits per plant, and average fruit weight per plant, while economic variables comprised total production cost, gross revenue, net return, and benefit–cost ratio (BCR). Data were subjected to Pearson correlation, multiple linear regression, and partial budget analyses to determine yield predictors and profitability of fertilizer applications. Result of correlation analysis revealed significant positive associations between fruit yield and most growth traits, particularly number of fruits per plant (r = 0.87), leaf area index (r = 0.94) and plant height (r = 0.79). Multiple linear regression analysis indicated that plant height leaf area index and number of fruits per plant were the strongest predictors of fruit yield, contributing significantly to the model fit (). T3 produced the highest fruit yield and the most favorable economic returns. This treatment generated a net return of $ 2,060 ha-1 and a benefit–cost ratio of 3.42, indicating high profitability and efficient resource use compared with other treatments. The study concludes that integrated fertilizer management enhances both yield predictability and economic returns in eggplant production. T3 therefore represents a sustainable and economically viable nutrient management strategy capable of improving productivity and farm income among eggplant growers in the Gambia.