RESPONSE OF RAIN-FED BREAD AND DURUM-WHEAT TO SOURCE, LEVEL, AND TIMING OF NITROGEN-FERTILIZER ON 2 ETHIOPIAN VERTISOLS .1. YIELD AND YIELD COMPONENTS

In trials conducted at two highland Vertisol sites in Ethiopia in 1990 and 1991, two locally popular spring bread wheat (Triticum aestivum L.) and durum (T. durum Desf.) cultivars were supplied with nitrogen (N) fertilizer at 0, 60, and 120 kg N/ha in the form of large granular urea (LGU), standard urea prills, or ammonium sulfate. Nitrogen was applied all at sowing, all at: mid-tillering, or split-applied between these two stages (1/3:2/3). Bread wheat exhibited the highest productivity and was the most responsive to applied N. Grain and biomass yields and most grain yield components were positively affected by increasing N rate and yield responses in each site-season were economic up to the highest rate of N used in this study. Split application of N had a favorable effect on grain yield as did the use of LGU as the N source, particularly under conditions of severe water-logging and at the high N rate. Split applications of N tended to nullify the positive effects of LGU, presumably by approximating the delayed release of N achieved with LGU. Grain yield was closely related to the number of grains/m(2) and grains/spike.