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This article in AJ

  1. Vol. 86 No. 6, p. 942-948
    Received: June 24, 1993

    * Corresponding author(s): pan@wsuvml.csc.wsu.edu
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Nitrogen Use Efficiency of Split Nitrogen Applications in Soft White Winter Wheat

  1. Karen E. Sowers,
  2. William L. Pan ,
  3. Baird C. Miller and
  4. Jeffrey L. Smith
  1. Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA, 99164-6420



Nitrogen use efficiency (grain weight per unit of N supplied from soil or fertilizer) can be reduced by overfertilization, suboptimal yields, and N losses. Nitrogen is typically fall-applied in eastern Washington for soft white winter wheat (Triticum aestivum L.) production, and is therefore subject to overwinter losses or accumulation in deep soil layers. Increasing grain protein levels of soft white winter wheat have been attributed in part to excessive N application rates and high residual N levels. Spring N applications were evaluated over four site-years as an option to all-fall applied N for reducing N inputs and improving N use efficiency, thereby allowing producers to maintain productivity while controlling grain protein. Five to six N rates ranging from 0 to 140 kg ha−1 for the all-fall N applications were compared with fall-spring split applications of 84 to 140 kg total N ha−1. Nitrogen was applied in the spring by topdressing (TD) or with a spoke-wheel point-injection (PI) system. A 15N experiment was conducted at two locations during the second year to quantify N uptake from fertilizer and soil N. High preplant residual N conditions resulted in limited responses to added N. Nitrogen use efficiency was 26 to 44% lower in the 140 kg ha−1 fall-applied N treatments than in the zero-N control. Reduced N use was related to N losses from the system and to decreased N utilization efficiency (grain weight produced per unit plant N). Spring-applied N, with point injection or topdressing, maintained or increased N use efficiency compared with equivalent all-fall N rates of 84 and 112 kg N ha−l. More 15N-labeled fertilizer (7–16% more) was recovered with a split N application than with fail-applied N at the same total rate of 112 kg N ha−1. At maturity, 68% of plant N was from soil sources with a split application, compared with 80% with an all-fall application. These results suggest spring-N applications with point injection or topdressing can improve fertilizer N recovery and N use efficiency over preplant applications in dryland winter wheat.

WSU Crop and Soil Sciences Dep. Paper no. 9201-66.

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