RESPONSE OF NO-TILL WINTER-WHEAT TO NITROGEN-FERTILIZATION AND DROUGHT STRESS

The yield of recrop winter wheat (Triticum aestivum L.) is a function of the interaction between agronomic management and the prevailing environment. Eight field trials were conducted over 2 yr on Dark Brown and Black Chernozemic soils in Saskatchewan to determine the influence of fertilizer-N rate and time of application on the early-season crop development and water use of no-till seeded winter wheat. Ammonium nitrate was surface broadcast on one of three schedules: as early as possible (early): 67 % early and 33 % at the beginning of stem elongation (split): or 3 wk after early (late), at rates of 0, 67. 134 and 202 kg N ha-1. In 1987, N fertilization resulted in the development and maintenance of a larger leaf-area index (LAI) and increased leaf conductance, leading to higher dry matter (DM) yield at anthesis and harvest. High air temperatures increased evaporative demand in 1988 and hastened crop development. Early-season response of both LAI and tiller number to fertilizer-N were abruptly terminated. followed by rapid pre-anthesis senescence in 1988. On average, 43% of harvest DM had accumulated by anthesis in 1987, compared with 78% in 1988. Although early N application increased and maintained LAI over late N in three of the eight trials, tiller responses to early N application were lost before anthesis under the environmental stress encountered. Increases in water-use efficiency of DM production with added N were a reflection of DM responses and not water use. Most of the soil water was extracted pre-anthesis, with on average 98% of post-anthesis evapotranspiration (ET) coming from rainfall. Maximum ET was associated with periods of high rainfall. Pre-anthesis DM yield increases associated with fertilizer-N, and dependence of post-anthesis ET on rainfall, resulted in increased plant stress arid reduced leaf conductance during grain filling with fertilizer-N additions. Early correction of N deficiencies were required to efficiently utilize rainfall and stored soil water for biomass production under the recrop conditions used to produce no-till winter wheat in Saskatchewan's semi-arid environment.