Urease Inhibitor (Agrotain) Effects on N-Use Efficiency in No-Till CottonJac J. Varco1, Robert E. Earnest1, and Jimmy D. Johnson2 Plant and Soil Sciences Dept. (1) Mississippi State University, Mississippi State, MS, (2) Agrotain International, Collierville, TNINTRODUCTIONFindings from a 10-y study in Miss. (Parvin et al., 2003): AN highly profitable due to high yield achieved at a lower rate of N (80 lb/acre) Subsurface banded UAN efficient, but lower profit due to lower yield Surface dribbled UAN profitable at a higher rate of N (120 lb/acre) Urea most profitable at a high rate of N (120 to 180 lb/acre) Current trends: Decreasing availability of ammonium nitrate to growers Increasing fertilizer N prices Increased acreage of no-till cotton Research results: decreased NH3 loss with urease inhibitor (Beyrouty et al., 1988) N losses up to 30% of surface applied urea (Beyrouty et al., 1988) N-(n-butyl) thiophosphoric triamide (NBPT) is an effective urease inhibitor (Bremner et al., 1991) NBPT treated urea increased N-use- efficiency for rice (Aly et al., 2001)N Fertilizer Use in MS (Nutrient short tons, 2004) Ammonium nitrate 20,089 Nitrogen solutions (UAN)57,119 Urea 31,867Price/lb N AN$0.448 UAN (32%) $0.328 Urea$0.364Average N rate, MS 2005 =120 lb/acUrea HydrolysisIncreased potential for NH3 volatilization w/no-till due to greater urease activity on crop residues(NH2)2CO + H2O 2NH3 + CO2 Urease enzyme is a catalyst for hydrolysis reactionOBJECTIVES Determine effects of fertilizer N source and placement on N-use-efficiency by no-till cotton Determine effects of a urease inhibitor (UI) on N-use-efficiency of urea N-sources by no- till cottonMETHODS General No-till N rate 120 lb/acre, split 50% planting and early square15N enriched fertilizer 1.4% Varieties: ST 4892 BR (2005), DP 445 BR (2006) Urease inhibitor in Agrotain N-(n-butyl) thiophosphoric triamide or NBPT Automated rainfall data collection Treatments No N check Ammonium nitrate (AN) broadcast Urea broadcast Urea w/UI broadcast UAN subsurface banded UAN dribbled UAN w/UI dribbled Randomized complete block, four replicationsUrea, AN, and urea treated with UI were broadcast within microplots using a pump type sprayer UAN subsurface banded, UAN dribbled, and UAN w/UI dribbled were applied 8” from drill using a 2 oz syringe Screens placed under rows to collect residuesElemental and isotope analysis were determined w/an automated C/N analyzer coupled to a mass spectrometerRainfall/Fertilization 2005Rainfall/Irrigation/Fertilization 2006Cotton N Content ResponseTreatmentTotal N content, lb/acre200520062-y ave.No N check629679Ammonium nitrate88151120Urea6912195Urea w/UI100134117UAN banded120135128UAN surface dribbled111117114UAN dribbled w/UI 136144140LSD0.0522.427.5N Derived From Fertilizer ResponseTreatmentN derived from fertilizer, %200520062-y ave.Ammonium nitrate39.629.935Urea19.324.822Urea w/UI34.328.231UAN banded45.835.841UAN surface dribbled46.427.937UAN dribbled w/UI45.430.638LSD0.054.55.1TreatmentFertilizer 15N recovery, %200520062-y ave.Ammonium nitrate28.143.035.6Urea11.129.620.4Urea w/UI28.736.132.4UAN banded46.248.247.2UAN surface dribbled43.534.839.2UAN dribbled w/UI 49.943.846.9LSD0.0522.427.5Fertilizer N Recovery ResponsePrice of Agrotain $45/gal 1 gallon treats 1 ton dry urea 2.3 quarts treats 1 ton UAN 32% Price of urea w/Agrotain /lb N$0.413 Price of UAN 32% w/Agrotain /lb N$0.368 Price of AN$0.448Cost Benefit at 120 lb N/acreUrea $43.68/acre w/o Agrotain $49.56/acre w/Agrotain 22 lb N/acre increase in efficiency $0.36/lb urea N = $8.01/acre= $2.13/acre cost benefitUAN 32% $39.36/acre w/o Agrotain $44.16/acre w/Agrotain 26 lb N/acre increase in efficiency $0.33/lb UAN N = $8.53/acre = $3.73/acre cost benefitCONCLUSIONS UI treated urea improved no-till cotton N use efficiency comparable to AN UAN dribbled with UI was as effective as soil incorporation in no-till cotton productionWhen to use a urease inhibitor? Conditions for NH3 volatilization no-till temperatures above 65F high organic matter/surface residues high humidity unincorporated urea