Maximizing cotton and sorghum yields and water use efficiency by optimizing plant density

Plant productivity on a land area basis (both total biomass and yield) is a function of the available environmental resources (water, radiation, and nutrients). In semi-arid regions like the Southern High Plains of Texas, an adequate water supply throughout the growing season is the greatest yield limiting factor. Establishment of plant population to optimize the expected growing season water supply would reduce interplant competition and increase yield per plant and subsequent yield per unit area. Field studies were conducted near Lubbock, TX, to determine the optimum plant densities which would reduce water stress on individual plants during critical growth stages and maximize yields and water use efficiency. Three row spacing (0.33, 0.67, and 1.00 m) and four plant density (5, 10, 15, and 20 plants m^-2) treatments were utilized to achieve a wide range of plant populations and subsequent interplant competition levels. Irrigation water was applied across all treatments at 0, 50, and 100% replacement of actual evapotranspiration (Eta) as determined by the modified Penman equation and crop coefficients based on percent of ground cover. Yield and yield components were determined at crop maturity. Midseason dry matter harvests were utilized to assess the plant density-water supply interaction (expressed as water supply plant^-1) on growth and development. Plant mapping was performed at first flower and peak bloom in cotton to determine treatment effects on fruiting patterns. Results indicated that fruit m^-2 (seeds or bolls) accounted for over 85% of the variability in yields over a wide range of environmental conditions. Fruit plant^-1 decreased in an exponential manner as plants m^-2 increased. Water supply m^-2 from square initiation to peak bloom was highly correlated with final lint yield m^-2. Water supply m^-2 had the biggest impact on cotton LAI, dry matter, and fruit m^-2 at first flower and peak bloom. In sorghum, water supply m"^ from planting to anthesis was most influential on final seed m^-2, yield m^-2, and LAI and total dry matter m^-2 at anthesis. Individual plant yield and growth parameters exhibited identical response to water supply plant^-1 over the same developmental periods as parameters on a m^-2 basis. In both cotton and sorghum, altering the plant number m^-2 had little impact on productivity on a ground area basis. However, increasing plant density reduced in final yields on a per plant basis due to increased interplant competition.