WHEAT PLANTING DATE EFFECTS ON SOIL-WATER EXTRACTION AND GRAIN-YIELD

Due to a wide range of irrigated and dryland production systems and highly variable weather, planting date of winter wheat (Triticum aestivum L.) grown in the Southern High Plains varies from August to November. A 2-yr field study was conducted at Bushland, TX, to evaluate planting date effects on soil water extraction and grain yield. 'TAM 107' wheat was planted in late August to simulate a grazed production system, early October to represent an optimum date for grain production, and early November on preirrigated Pullman clay loam soil (fine mixed thermic Torrertic Paleustoll) and grown to maturity without further irrigation. Gravimetric soil water content to 3.6 m depth was determined at the beginning and end of winter dormancy and at anthesis. Water extraction prior to winter dormancy occurred to a depth of 1.8, 0.9, and 0.15 m for the early, normal, and late planting dates, respectively. Depth of water extraction at anthesis was 2.4, 2.4, and 1.2 m for the three dates. The fraction of total preanthesis soil water extraction that occurred during a critical 60-d period immediately prior to anthesis was 0.22, 0.59, and 0.91 for the early to late planting dates, respectively. Planting in August greatly increased depth and amount of soil water extraction prior to winter dormancy; however, water deficits in the spring contributed to low head density and grain yield. Wheat planted in early October extracted as much soil water as the August planting by anthesis and produced high grain yield. Wheat planted in early November rooted only one-half as deep, extracted two-thirds as much water, and yielded two-thirds as much grain as the October planting date. In conclusion, planting in August as compared with October did not increase soil water extraction at anthesis and grain yield was reduced because the period of major soil water extraction was shifted from spring to fall. Planting in November reduced grain yield because of insufficient rooting to fully extract soil water from below 1.2 m.