Nitrogen Use Efficiency of Hybrid Maize Under Split Application of Urea and Neem-Coated Urea

Abstract

Hybrid maize responds strongly to nitrogen, yet a large share of the urea applied to it never reaches the crop, and the question of how to apply that nitrogen more efficiently is argued on agronomic, economic and environmental grounds at once. This narrative mini review asks what published work can and cannot say about the nitrogen use efficiency of hybrid maize when urea is split across the season and when it is coated with neem. It draws on literature covering the definitions of efficiency, the timing of crop demand, the fate of urea nitrogen in soil, enhanced-efficiency fertilisers and maize hybrid physiology, and it reports no new data. Split application is well supported as a way of matching supply to demand, particularly on soils and in seasons where losses are large. Neem coating has a plausible mechanism, a slowing of nitrification, but its benefit appears modest and inconsistent across soils and seasons. Efficiency metrics differ in meaning and are often compared as if they were interchangeable, and gains in yield per unit of nitrogen may sit beside unchanged or even larger losses to the environment. The review closes with the reporting that would make future trials comparable.