Winter wheat (Triticum aestivum L.) is grown on the Southern Great Plains under dryland conditions and under varying irrigation regimes. Relationships between water and fertilizer needs are not well defined. Field studies were conducted to determine: (i) the interacting effects of N fertilization and irrigation on N and P needs, wheat yields, and yield components; and (ii) the effects of timing of water deficit periods on N and P needs, wheat yields, and yield components. Studies were conducted on a Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll). Respective N and P rates ranged from 0 to 210 kg ha-1 and 0 to 40 kg ha-1. Irrigations were applied or withheld to allow the crop to be nonstressed (I-1), stressed during heading and grain filling (I-2), stressed during tillering and jointing (I-3), and stressed throughout spring (I-4). Two-year average data showed that 140 kg N ha-1 was sufficient for maximum grain yields on treatment I-1, while 70 kg N ha-1 was sufficient on treatments on I-2 and I-3, and no N response occurred on treatment I-4. Compared to treatment I-1, grain yields were 27, 32, and 52% less on treatments I-3, I-2, and I-4, respectively. Water use efficiency (WUE) increased with increments of N through 140 kg ha-1 on treatment I-1, and through 70 kg ha-1 on treatments I-2, and I-3 but applied N did not affect WUE on treatment I-4. In 1981, WUE was highest on treatment I-1 and lowest on treatment I-4; in 1982, however, WUE was highest on treatment I-4 and lowest on treatments I-1 and I-3. For limited irrigation, irrigating during tillering and jointing would be preferable to allowing stress then, and irrigating during heading and grain filling, because there is still potential for high yields if precipitation occurs during heading and grain filing.
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