Evaluating the efficacy of late nitrogen application: Physiological and environmental perspectives

In-season split-nitrogen (N) applications made after canopy closure in corn ( Zea mays L.) are becoming more common. A better understanding of the physiological and environmental factors that influence their effectiveness is needed. The objectives of this study were to i) quantify yield and N recovery efficiency (NRE) responses to late N strategies that delay increasing proportions of N from planting to V13, and test whether these responses are modified by N source and placement, ii) identify soil and weather factors influencingyield response to late N, and iii) assess how vegetative N status modifies yield response to late N. A three-year field study (2021–2023) was conducted at five locations: Elora, Ridgetown, and Winchester in Ontario (Canada), Windfall in Indiana, and Macomb in Illinois (USA). 179 kg N ha⁻¹ was split-applied in three proportions at planting (20, 50 and 70 %) with the remaining N applied at V13. The V13 in-season N was applied using one of six different N application methods: injection or surface-application of urea or UAN (urea ammonium-nitrate), with surface-applications applied with or without a urease inhibitor (UI). Measurements included dry matter and N uptake at R1 and physiological maturity (R6), grain yield and yield components. Various nitrogen use efficiency (NUE) parameters were calculated, including (NRE) and the nitrogen nutrition index at R1 (NNI R1 ). On average, reducing the proportion of N applied at planting from 70 % to 20 % reduced grain yield from 14.6 to 13.6 Mg ha⁻¹ , NRE from 69 % to 56 % and NNI R1 from 1.06 to 0.96. However, yield response to late N applications were environment-specific. Six location-years have negative yield responses, whilefive were neutral or positive. Negative yield responses became more likely as: i) environmental capacity for post-R1 N uptake (PostN) was reduced, and ii) as the difference in NNI R1 between the 20 % and 70 % up-front treatments increased. Variation in NNI R1 was primarily explained by the amount and evenness of precipitation between V13 and R1. Environmental capacity for PostN was reduced in environments with high clay content. Corn yield became less dependent on PostN as NNI R1 increased past sufficiency (>1.0). N application source and placement did not affect corn yield or NUE parameters. Yield reductions from late (V13) N applications are greater if environmental capacity for PostN is low. Supplying sufficient early N and increasing crop N uptake above sufficiency levels appears to reduce this risk by reducing crop reliance on PostN. • Late N response depended on vegetative N status and its uptake conditions. • Uniform rainfall after late N reduced stress by retaining N until V13. • Coarse-textured soils showed greater capacity for post-R1 N uptake. • Crop dependence on post-R1 uptake declined as early N increased.