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 NO3−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 (R2=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 × 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.
Contribution from the Div. of Soil Sci., Coll. Of Agric., Univ. of Idaho. Approved for publication by the director of the Idaho Agric. Exp. Stn. as Research Paper 87-7-38.