Sources and Timing of Spring Topdress Nitrogen on Winter Wheal in Idaho

Fall‐applied N is poorly utilized by winter wheat ( Triticum aestivum L.) in northern Idaho, because heavy winter precipitation leaches much of the N from the root zone. Nitrogen‐use efficiency can be improved by spring application. This study evaluates two N fertilizer sources, urea‐ammonium nitrate solution (UAN; 32% N) and prilled ammonium nitrate (AN; 34% N), as spring topdress materials and determines the growth stage at which N application would produce maximum yield. Four field studies were established on cropland initially containing less than 50 kg NO 3− N ha −1 . Winter wheat was seeded at 90 kg ha −1 at each site. Nitrogen was topdressed at 100 kg ha −1 in the spring as either UAN, AN, or a UAN‐bromoxynil/MCPA [3,5‐dibromo‐4‐hydroxybenzonitrile/(4‐chloro‐2‐methylphenoxy) acetic acid] tank mix at eight different growth stages. Parameters evaluated included: (i) leaf burn, (ii) stand vigor, (iii) tillering, (iv) weed control, and (v) yield. Temporary yellowing of plant leaves was noted following UAN applications at air temperatures > 14°C. The degree of yellowing or leaf burn exhibited a significant curvilinear correlation with air temperature ( R 2 =0.89). Nitrogen application date affected stand vigor, tillering, weed control, and yield, with the best ratings occurring with N applications before Zadoks' growth stage 31. When study sites and N sources were pooled, yields were 5.4,4.9, 4.8, 4.6, 4.3, 4.1, 3.6, and 3.1 Mg ha −1 for wheat fertilized at growth stages 22,24,26,28,31,32,37, and 43, respectively. Significant N source X time of application interactions for winter wheat yield were observed at three study sites. At early N application dates (growth stages) yield differences attributable to AN or UAN were not observed; however, when N was applied after growth stage 31, UAN plots produced yields between 1 and 24% (average 6%) greater than AN‐fertilized plots. This was attributable to dry soil conditions that reduced AN movement into the soil plant‐root zone, coupled with foliar absorbtion of UAN by wheat plants.