Effects of Nitrapyrin, N Rates and N Timing On NUE in Maize Fertilized with UAN.

The frequent occurrence of low nitrogen use efficiency (NUE) in maize suggests that a relatively high proportion of the fertilizer N applied is not taken up by the plants. Certain individual fertilizer management practices are reported to increase NUE, but few studies have investigated combined management approaches to advancing maize NUE. The objectives of this study are to assess the impacts of integrated N management practices (timing, rate, and nitrification inhibitor) on harvested grain yield, total plant N uptake, and on maize NUE and its components using data from 3 site-years in Indiana, USA. The main treatments were N rate (0, 90, 180 kg N ha^-1 applied as urea-ammonium nitrate), nitrapyrin (presence or absence), and timing of N application (pre-emergence or side-dress at V6 stage). Whole-plant biomass samples were collected at silking (R1) and physiological maturity (R6), and R6 samples were further fractioned into grain and stover to determine N uptake (NU) and other NUE components.  As expected, higher N rates resulted in significantly higher grain yield (GY), as well as N uptake (NU) at R1 and R6. The mean grain yields at the N rates applied were 6.32, 9.97, and 12.12 Mg grain ha^-1 for 0, 90, 180 kg N ha^-1, respectively. Delaying the timing of application to V6 significantly increased NU at R1 (132.23 and 146.39 kg N ha^-1, for pre-emergence and side-dress respectively) and R6 (147.85 and 163.37 kg N ha^-1 for pre-emergence and side-dress respectively). The gain in NU for side-dress timing was particularly evident at R6 for the 90 kg N ha^-1 rate. Increasing the N rate from 90 to 180 kg N ha^-1 significantly reduced NUE (40.56 to 32.73 kg grain kg N applied ^-1), but nitrapyrin use significantly enhanced NUE when averaged across the two N rates (from 33.80 to 39.48 kg grain kg N applied ^-1). The timing of application, N rate by timing of application interaction, and nitrapyrin treatments significantly affected N recovery efficiency (NRE). Nitrapyrin inclusion increased NRE at R6 (0.44 kg N uptake kg N^-1 applied without nitrapyring to 0.55 kg N uptake kg N^-1 applied with nitrapyrin). Delaying the timing of application also  enhanced NRE from 0.41 at pre-emergence to 0.59 kg N uptake kg N applied ^-1 for side-dress). The timing by N rate interaction was evident in that 90 kg N ha^-1 side-dress applied had significantly higher NRE than any other combination of N rate and timing. However, N internal efficiency (NIE), averaged across N rates and nitrapyrin levels, was significantly reduced by delayed N application (117.54 to 67.57 kg grain kg N uptake^-1, for pre-emergence and side-dress, respectively).