Late-Fall, Winter, and Spring Broadcast Applications of Urea to No-Till Winter Wheat II. Fertilizer N recovery, Yield, and Protein as Affected by NBPT

Surface application of urea to winter wheat (Triticum aestivum L.) during the late-fall, winter, or s pring is a common practice in the northern Great Plains (NGP). However, surface applications are susceptible to volatilization, and management strategies are needed to enhance fertilizer-N recovery (FNR). This study was conducted to determine the effect of urea application timing (late-fall, winter, spring) and N-(n-butyl) thiophosphoric triamide (NBPT) on FNR in grain (FNRG) and grain plus straw (FNRT), and grain yield and protein. Replicated small-plot experiments were conducted in no-till winter wheat fields over 3 yr in Montana, and FNR was quantified using N-15-enriched labeled urea (5% atom). Fertilizer-N recovery was greater (P < 0.05) for spring (FNRG 36.5%, FNRT 46.1%) than late-fall (FNRG 24.9%, FNRT 31.7%) and winter (FNRG 26.8, FNRT 34.1%) applications. Addition of NBPT to urea improved FNR of all urea timings, but the response was greater for late-fall and winter compared with spring applications. Fertilizer-N recovery among the management strategies tested was directly related to cumulative NH3 emissions (r(2) = 0.77, P < 0.0001) measured in a companion study, indicating that volatilization represented an important pathway for N loss. Grain yield was not sensitive to application timing or NBPT. However, grain protein increased 6 to 8 g kg(-1) from spring applications or NBPT additions. This study found that management strategies to enhance urea FNR in the NGP should consider delaying applications until the spring or addition of NBPT for late-fall and winter applications that are more susceptible to NH3 volatilization.