The Influence of Nitrogen Application Time and a Urease Inhibitor On Winter Wheat Yield.

Nitrogen (N) is generally the most-limiting nutrient for soft red winter wheat (Triticum aestivum L.) production in Arkansas. Late winter environmental conditions vary annually and may influence N loss from surface-applied urea. Our objectives were to evaluate how N application time influences wheat yield and whether wheat yields benefit from urea treated with the urease inhibitor N-(n-Butyl)-thiophosphoric triamide (NBPT). A preliminary study was conducted in 2007 with four N application times (10 February - 25 March) comparing 84 kg N ha^-1 from urea and urea+NBPT. In 2008, tests were established at two sites and three N-fertilizers sources (urea, urea+NBPT, and urea+NBPT+dicyandiamide) were applied at 84 kg N ha^-1 and compared to 140 kg N ha^-1 as urea+NBPT and a no N control. In 2007, wheat yields, averaged across N application times, were 243 kg ha^-1 greater for urea+NBPT than urea (P = 0.0417). However, the non-significant interaction suggested that wheat yields were greater for urea+NBPT only when N was applied 25 March. In 2008, wheat yield was affected (P <0.05) by the N application time × N source interaction. Wheat receiving 140 kg N ha^-1 as urea+NBPT produced greater yields than all N sources applied at 84 kg N ha^-1 when N was applied 15 February or 1 March. Wheat yields were uniform among all N sources, regardless of N rate, when N was applied 15 March and 1 April. Comparing N application dates within each N source applied at 84 kg N ha^-1 showed that wheat yields were generally equal when N was applied from 15 February through 15 March, but occasionally declined when N was delayed until 1 April. When 140 kg N ha^-1 was applied yields were greatest for the 15 February application date, intermediate for the 1 and 15 March application dates, and lowest for N applied on 1 April. Results suggest that loss of fertilizer N via NH₃ volatilization, leaching, and denitrification were low and urease and/or nitrification inhibitors provide little benefit for wheat grown in Arkansas. A sufficient rate of N should be applied beginning in mid-February to maximize wheat yield potential.