ROW SPACING EFFECTS ON LIGHT EXTINCTION COEFFICIENTS OF CORN, SORGHUM, SOYBEAN, AND SUNFLOWER

In many crop models, light intercepted by a canopy (IPAR) is calculate d from a Beer's Law equation: IPAR = PAR x [1 - exp(-k x LAI)], where k is the extinction coefficient, PAR the photosynthetically active rad iation, and LAI the leaf area index. The first objective of this study was to investigate the effect of row spacing on k for corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.], and sunflower (Helianthus annuus L.) to provide information fo r modeling. Data from literature and from an experiment conducted at T emple, TX, were evaluated. The second objective was to investigate eff ects of time of day and stage of crop development on k for different r ow spacings. Seeds of all four species were sown in rows 0.35, 0.66 or 1.00 m apart. Measurements of canopy light interception were taken ne ar solar noon on two dates before anthesis. At anthesis, extinction co efficients were determined at 0845, 1015, and 1145 h (solar time). The extinction coefficient showed a linear decrease as row spacing increa sed. For each crop, the effect of row spacing on k was described by on e linear regression for most data. Stage of crop development and stage of development x row spacing interaction did not significantly affect k during the period of measurements. The effect of time of day was si gnificant for all four crops, and the time of day x row spacing intera ction was significant for soybean and sunflower. Thus, modeling light interception for different row spacings should account for these effec ts.