Effect of nitrogen fertilizer timing on nitrogen use efficiency and grain yield of winter wheat in Ireland

The objectives of this work were to determine the effect of initiating fertilizer nitrogen (N) application to winter wheat at different growth stages on grain yield and N use efficiency (NUE). A factorial experiment was carried out in two growing seasons (2011 and 2012) with five timings of first N application (GS 24/26 (tillering), GS 30, GS 31, GS 32 or GS 37) and an unfertilised control, two sowing densities (100 and 400 seeds/m2) and a cattle slurry treatment (with or without slurry). The latter was included to simulate variation in soil N supply (SNS). Delaying the first application of N from the tillering stage until GS 30 had no significant effect on grain yield in either year. Further delaying the initial N application until GS 31 caused a significant yield reduction in 2011 but not in 2012. Differences in efficiency of recovery and use of fertiliser N by the crop between the first three application timings were small. There was no evidence to support altered timing of the first application of N in response to low plant density. Slurry application did not influence SNS so the interaction between SNS and fertiliser N timing could not be determined. It is concluded that in order to maximise yield and NUE the first N application should be applied to winter wheat between late tillering and GS 30 and that delaying the first N until after GS 31 can lead to yield reductions compared to earlier application.