Chlorophyll meter as a tool for forecasting wheat nitrogen requirements after application of herbicides

Because limited information is available about the validated use of a chlorophyll meter for predicting nitrogen requirements for optimum growth and yield of wheat after application of herbicides, field experiments were carried out in the winter seasons of 2011/2012 and 2012/2013 under different weed and N fertilization treatments. Five weeded treatments, application of herbicides 25 days after sowing (DAS), hand pulling once at 55 DAS and a weedy check were combined with four N application rates. Weeds were completely absent in the non-fertilized plots, either with metribuzin or hand pulling as well as in isoproturon-treated plots fertilized with 190 or 285 kg N ha-1. The grain yield was similar in the treatments of isoproturon × 190 kg N ha-1, isoproturon + diflufenican × conditional N treatment (113.9) or 190 kg N ha-1, hand pulling × conditional N treatment (104.8) or 285 kg N ha-1 and metribuzin × 190 kg N ha-1. Under weeded practices, conditional N treatment recorded the maximum nitrogen use efficiency and almost equaled the grain protein content of the 190 kg N ha-1 application rate. N application based on SPAD readings saved about 40.0% and 44.8% N with isoproturon + diflufenican or hand pulling, respectively, compared to the recommended rate (190 kg N ha-1) without noticeable yield loss. © 2014 © 2014 Taylor & Francis.