SIMULATED GROWING-SEASON PRECIPITATION AND NITROGEN EFFECTS ON WINTER-WHEAT YIELD

Available water (precipitation plus stored soil water) is the principal factor that determines wheat (Triticum aestivum L.) yield potential and response to N in the Northern Great Plains. Field studies were conducted in 1987 and 1988 to determine the interactive effect of water and available N on 'Redwin' and 'Centurk' winter wheat yield. Seven fertilizer N levels and a line-source sprinkler system created a wide range of N and simulated growing-season precipitation (SGSP) regimes on a Yegen sandy loam (fine-loamy, mixed, Typic Argiboroll). Wheat yield-N-SGSP functions revealed that yield and response to water were much greater in 1987 than 1988, when return from N was maximized (maximum economic yield or MEY). At 100, 200, and 300 mm SGSP, predicted MEY in 1987 averaged 2350, 4019, and 5434 kg ha-1, respectively. In 1988, MEY was 9, 17, and 24% lower, respectively. High available N reduced yield from the maximum across all SGSP regimes in 1988, but not in 1987. High temperatures during grain fill in 1988 (23.1-degrees-C, 4.7 degrees-C warmer than 1987), were likely the reason for differing yield-N-SGSP relations. Though Centurk yielded more than Redwin for most SGSP conditions, the N required for MEY or optimum N level (ONL) was not greatly affected by cultivar. Due to less favorable growing conditions, the ONL was approximately 27% lower in 1988 than 1987. An acceptable fertilizer N strategy might be to raise the ONL level to 100 kg ha at 100 mm growing-season precipitation, then adjust N upward 1 kg ha-1 for every 3 mm rise in available water.