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

Div. of Soil Sci., Univ. of Idaho, Moscow, ID 83843

^* Corresponding author.

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 (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 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.

Key Words: Nitrogen-use efficiency • Ammonium nitrate • Urea-ammonium nitrate • Leaf burn • Weed control • Fertilizerherbicide tank mix • Triticum aestivum L.

Received for publication July 24, 1987.

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