TIMING OF THE FIRST IRRIGATION IN CORN AND WATER-STRESS CONDITIONING

Delaying the first irrigation is thought to encourage root growth and to condition crop plants for stress later in the growing season. We tested this common practice for corn (Zen mays L.) grown under arid conditions in Tucson, AZ, in 1989 and 1990. In field studies conducted on an Agua sandy clay loam (coarse-loamy over sandy, mixed, thermic Typic Torrifluvent), the first irrigation was applied at the 2-leaf, 4-leaf, or 6- to 8-leaf stages, followed by irrigation or water stress at anthesis. Delaying the first irrigation did not encourage root growth but actually restricted it in 1989. Seasonal water use was decreased from an average of 796 mm to 705 or 652 mm if the first irrigation was delayed from the 2-leaf to the 4-leaf or 6- to 8- leaf stages, respectively. Timing of the first irrigation did not affect yield in 1989, or in 1990 if irrigations were withheld at anthesis. However, grain yield was decreased from 830 to 693 g m(-2) and total plant yield was decreased from 2490 to 2185 g m(-2) when the first irrigation was delayed past the 2-leaf stage in 1990 and the crop was well-watered at anthesis. Delaying the first irrigation of corn, with 148 mm (1989) and 170 mm (1990) plant available water in the top 1.5 m of soil at the beginning of the season, did not appear to condition the crop for water stress later in the season in an arid environment, but may substantially decrease yield if adequate water is available for the remainder of the growing season.