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

Due to a wide range of irrigated and dryland production systems and hi ghly variable weather, planting date of winter wheat (Triticum aestivu m L.) grown in the Southern High Plains varies from August to November . A 2-yr field study was conducted at Bushland, TX, to evaluate planti ng date effects on soil water extraction and grain yield. 'TAM 107' wh eat 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 ther mic Torrertic Paleustoll) and grown to maturity without further irriga tion. Gravimetric soil water content to 3.6 m depth was determined at the beginning and end of winter dormancy and at anthesis. Water extrac tion prior to winter dormancy occurred to a depth of 1.8, 0.9, and 0.1 5 m for the early, normal, and late planting dates, respectively. Dept h of water extraction at anthesis was 2.4, 2.4, and 1.2 m for the thre e dates. The fraction of total preanthesis soil water extraction that occurred during a critical 60-d period immediately prior to anthesis w as 0.22, 0.59, and 0.91 for the early to late planting dates, respecti vely. Planting in August greatly increased depth and amount of soil wa ter extraction prior to winter dormancy; however, water deficits in th e spring contributed to low head density and grain yield. Wheat plante d in early October extracted as much soil water as the August planting by anthesis and produced high grain yield. Wheat planted in early Nov ember rooted only one-half as deep, extracted two-thirds as much water , and yielded two-thirds as much grain as the October planting date. I n conclusion, planting in August as compared with October did not incr ease soil water extraction at anthesis and grain yield was reduced bec ause the period of major soil water extraction was shifted from spring to fall. Planting in November reduced grain yield because of insuffic ient rooting to fully extract soil water from below 1.2 m.