Corn Yield Response to the Urease Inhibitor NBPT: Five‐Year Summary

Urease inhibitors are applied to reduce NH 3 volatilization and immobilization losses from surface‐applied urea or urea‐NH 4 NO 3 solutions (UAN). The urease inhibitor N‐(n‐butyl) thiophosphoric triamide (NBPT) was evaluated in the USA in 78 trials conducted with corn ( Zea mays L.) from 1984 to 1989. NBPT was applied with urea at rates of 0.25 to 1.0% (w/w) and with UAN at rates of 0.5 to 2.0 lb/acre. When averaged over N and NBPT rates for all locations and years of the study, NBPT increased grain yields by 4.3 bu/acre when applied with urea and by 1.6 bu/acre when applied with UAN. Average responses to NBPT were higher on sites that were able to respond to N conserved by NBPT (5.0 bu/acre for urea and 2.8 bu/acre for UAN). Average responses to NBPT were also greater on sites where nonvolatile N (NVN) treatments gave higher yields than unamended urea (6.6 bu/acre for urea and 2.7 bu/acre for UAN). Results from 21 trials employing multiple N rates showed that maximum grain yields could be obtained using an average of 74 lb/acre less N when NBPT was included with surface‐applied urea. These results demonstrate that NBPT provides an effective alternative to excessive rates of surface‐applied urea that are currently used to ensure that N will not limit grain yields.