Maximizing Irrigated Maize Productivity: Evaluating the Impact of Deficit Irrigation and Nitrogen Rates on Growth, Yield, and Water-Use Efficiency in Southwest Ethiopia

Maize is vital in Ethiopia, particularly in Jimma. However, the benefits of deficit irrigation (DI) under varying nitrogen rates (NRs) for maize remain unknown. A 3-year field experiment assessed maize response to different NRs (${\mathrm{N}}_0=0\mathrm{kg}{\mathrm{ha}}^{-1}$, ${\mathrm{N}}_{23}=23\mathrm{kg}{\mathrm{ha}}^{-1}$, ${\mathrm{N}}_{46}=46\mathrm{kg}{\mathrm{ha}}^{-1}$, ${\mathrm{N}}_{69}=69\mathrm{kg}{\mathrm{ha}}^{-1}$, and ${\mathrm{N}}_{92}=92\mathrm{N}\mathrm{kg}{\mathrm{ha}}^{-1}$) under three DI levels (${\mathrm{I}}_{50}=50$, ${\mathrm{I}}_{75}=75$, and ${\mathrm{I}}_{100}=100\%$ ETc). Factors such as rain-fed findings, soil testing, and local recommendations were considered in determining these rates. The results showed significant effects of DI, NR, and their interactions on grain yield, biomass, water-use efficiency (WUE), nitrogen-use efficiency (NUE), and harvest index (HI). Without nitrogen, 50% DI led to a greater than 50% reduction in yield and biomass compared with that at ${\mathrm{I}}_{100}\times {\mathrm{N}}_0$. However, WUE and HI increased at ${\mathrm{I}}_{50}\times {\mathrm{N}}_{92}$ compared with ${\mathrm{I}}_{100}\times {\mathrm{N}}_0$. Increasing irrigation and nitrogen levels improved yield and biomass. Optimal WUE was achieved with irrigation close to 75% ETc without compromising yields. Agronomic yield significantly improved with NR above ${92\mathrm{kg}\mathrm{N}\mathrm{ha}}^{-1}$ under mild deficit and full irrigation, but higher rates did not provide additional benefits under severe deficit conditions. The economically optimum nitrogen rate ranged from 78 to $80\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ for irrigation levels ranging from 50% to 100% ETc. The study suggests that applying $80\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ with 75%–100% ETc levels is the best approach to enhance maize yield and achieve economic benefits. Further research is needed to explore DIs below 50% and above 100% ETc, as well as nitrogen fertilization rates above $92\mathrm{kg}{\mathrm{ha}}^{-1}$, in order to optimize maize production in the region.

This study examined the benefits of deficit irrigation and fluctuating nitrogen rates for maize production in Jimma, Ethiopia. By adopting deficit irrigation strategies and adjusting nitrogen rates, producers can reduce water withdrawal by approximately 25%. This approach optimizes both water-use efficiency and nitrogen-use efficiency without compromising yields. Increasing irrigation and nitrogen rates above ${\mathrm{I}}_{100}\times {\mathrm{N}}_{92}$ improves agronomic yield, whereas reducing them below ${\mathrm{I}}_{50}\times {\mathrm{N}}_{46}$ significantly decreases yield and WUE. Economically optimum nitrogen rates range from 78 to $80\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ with 50%–100% full irrigation. Balancing input costs and potential benefits is crucial for optimizing irrigation and nitrogen rates for maize production. To maximize agronomic yield, full irrigation with $92\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ (less than $96\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$) is advisable. To optimize water use, 75% of full irrigation with $92\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$ is recommended. To optimize nitrogen (economic yield) and achieve higher net returns, fully irrigating the maize crop with $80\mathrm{kg}{\mathrm{ha}}^{-1}$ of nitrogen fertilization is advised. Farmers should adapt these findings to local conditions, considering factors such as soil characteristics, nitrogen price, and water price, and should consult local experts. Further research is needed to explore the effects of more or less than 50% full irrigation and of nitrogen doses exceeding $92\mathrm{kg}{\mathrm{ha}}^{-1}$.