Cotton Yield Improvements with Addition of the Urease Inhibitor NBPT to Foliar Applied Urea.

Foliar nitrogen (N) application is a common practice to supplement late-season N requirements in cotton. Urea is the most recommended foliar N source, due to its relatively low toxicity, quick absorption, and low cost.  However, in the literature reports of yield increments with foliar urea application are not consistent. Toxic urea and/or ammonia accumulation are thought to be the main causes of the ineffectiveness of foliar urea application in crop yields.  Understanding foliar urea assimilation in plants is crucial for improving foliar N management. The objectives of this research were to study foliar urea assimilation in cotton and to test the effect of the urease inhibitor N-butyl thiophosphoric triamide (NBPT) in cotton foliar urea application. The study was conducted at the University of Arkansas Cotton Branch Station at Marianna. Treatments consisted of: (T1) full recommended N soil rate with no foliar N application; (T2) 75% of recommended N soil rate with no foliar application; (T3) 75% of recommended N soil rate with two foliar urea applications; (T4) 75% of recommended N soil rate with two foliar urea plus NBPT applications. Each foliar urea application was calculated to supply 11.2 kg of N per hectare. The results showed that treatment T4 had a significantly higher seedcotton yield compared to T3, indicating a positive effect of adding NBPT to foliar urea application. No yield differences were observed between treatments T3 and T2; thus application of foliar urea had no effect on seedcotton yield. Yields of the treatments T4 and T1 were not statistically different, showing that two applications of foliar urea with NBPT can supply 25% of the full recommended N soil rate. Three distinct hypotheses could explain these results: (1) addition of NBTP improved the uptake and assimilation of foliar applied urea; (2) NBPT itself improved the physiology of cotton plants; (3) application of foliar urea with NBPT helped the uptake of the portion of urea that was drift to the soil surface. Currently, these hypotheses are being tested in a growth room study at the Arkansas Agricultural Research and Extension Center.