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This work was carried out in collaboration between all authors. Author EN designed the study, wrote the protocol and wrote the first draft of the manuscript. Author SMA managed the literature searches, analyses of the study performed the spectroscopy analysis and author CLAA managed the experimental process and author EN identified the species of plant. All authors read and approved the final manuscript.

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Field trials were conducted during 2017 and 2018 rainy season, to investigate the effect of nitrogen fertilizer rates and time of application on the growth and yield of cowpea. The treatment consisted of three levels of urea: 0, 40, 80 kg N ha-1 and the times of fertilizer application: during planting of cowpea, 2 weeks after planting, 4 weeks after planting and 6 weeks after planting; which were factorially combined to form twelve treatments and laid in Randomized Complete Block Design (RCBD). The results showed that, Nitrogen fertilizer rates and time of application had a significant increased on all the growth and yield parameters of cowpea in both years of trials. Application of 80 kg N ha-1 produced the highest number of cowpea leaves (134.45 and 139.23), number of branches (9.45 and 10.54), vine length (78.23cm and 80.12cm) and highest seed weight of 0.52 t ha-1 in both 2017 and 2018 trials. However, the control plots produced plants with the highest number of root nodules (14.78 and 12.49) in both years of cropping. Also, application of nitrogen fertilizer at two weeks after planting of cowpea produced the highest growth and seed weight of 0.68 t ha-1 in 2017 cropping; while in 2018 trial, application of nitrogen fertilizer at two weeks after cowpea planting produced seed yield of 0.69tha-1 which is statistically similar with application of nitrogen fertilizer on the fourth week after planting which produced 0.55 t ha-1 of cowpea seeds. The interaction between nitrogen fertilizer application rates and time of application did not showed any significant difference in cowpea yield and yield components in both years of cropping.