Irrigated corn (Zea maysL.) is a relatively new crop on the Southern High Plains where the groundwater supply for irrigation is declining. Field studies were conducted to determine the plant nutrient needs for corn on a predominant soil of the area, the effects of timing and duration of drought stress periods, and the interacting effects of N levels and drought stress on N nutrition and production of corn. Fertilizer N rates ranged from 0 to 350 kg/ha. Irrigations were applied or deleted to allow the crop to be unstressed (I‐1), stressed 2 weeks during late vegetative growth (I‐2), stressed 2 weeks during early vegetative growth (1‐2a), stressed 4 weeks during vegetative growth (I‐3), stressed about 4 weeks during grain filling (I‐4), and stressed about 2 weeks during grain filling (1‐5). The soil was Pullman clay loam (fine, mixed, thermic Torrertic Paleustolls). Fouryear average data (three with graded furrow irrigation, one in level borders) showed that 140 kg N/ha were sufficient for maximum yields. However, simultaneous experiments conducted on graded furrows and level borders showed that while 140 kg N/ha gave maximum yields on graded furrows, 210 kg N/ha were required in level borders. This comparison illustrates the possible fallacy in conducting fertilizer trials under one method of irrigation and extrapolating results to another. Two and 4 weeks of plant water stress during vegetative growth reduced yields of adequately fertilized (210 kg N/ha) corn 23 and 46%, respectively. Two‐week stress periods during late (1‐2) and early (I‐2a) vegetative growth had similar effects on grain yields. Relationships between lengths of stress periods during grain filling and yield showed that yields were reduced 1.2% for each day stress was imposed during grain filling; however, as indicated by the r2(0.31), there was considerable variation in the data. A N × water stress interaction occurred on grain yields. Adequate N slightly increased corn grain yield under stress and greatly increased yield with full irrigation. Excessive N did not reduce yield even with severe water stress, thus, there would be no reason to reduce N rates to reduce water stress. As grain yields increased, the ratio of grain yield to N yield decreased until maximum yields were attained and then remained constant, indicating little luxury consumption of N. At maximum yield, typical grain yield/N yield ratios were about 52:l. At harvest soil NO−3‐N levels showed that plants had removed most of the N applied at rates up to 140 kg/ha, but increasing amounts remained when N application rates were above 140 kg/ha.

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 riming 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 filling.

Irrigated corn (Zea mays L.) is an important feed grain crop on the Southern High Plains where the supply of water for irrigation is declining. Limited irrigation-applying less water than is required to meet potential evapotranspiration-is extensively practiced on the drought tolerant crops of grain sorghum (Sorghum bicolor (L.) Moench), wheat (Triticuf!i aestivum L.), and cotton (Gossypium hirsutum L.). The purpose of this paper is (i) to report the effects of timing and duration of water deficit periods on growth and yield components of corn, (ii) to evaluate the seasonal evapotranspiration requirements of corn, and (iii) to give further information regarding the adaptation of corn for limited irrigation in a region of normally high evaporative demand climate. In a 4-yr study, corn was grown under five irrigation treatments: adequate water, 2-and 4-week water deficit periods during vegetative growth, and 2-and 4-week water deficit periods during grain filling. Water deficits imposed 41 days after planting reduced leaf, stalk, and ear yields, while those imposed 55 days after planting reduced only stalk and ear yields. Deficits during vegetative growth reduced kernel numbers but had little effect on weight per kernel. Deficits during grain filling did not affect leaf and stalk yields but reduced ear yields. Kernel numbers were not affected by water deficits during grain filling unless severe deficits were imposed early in the period; thus, grain yield reductions were proportional to reductions in weight per kernel. With adequate water, seasonal water use averaged 964 mm on graded furrows and 834 mm in level borders. Although water use efficiency (WUE) was sometimes increased slightly when plants were subjected to water deficits, the data indicate that limited irrigation of corn would not be feasible on the Southern High Plains. ~---------------Additional index words: Water use, Limited irrigation, Weight per kernel, Kernel number, Zea mays (L.).

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