Shattercane [Sorghum bicolor (L.) Moench] is a serious weed problem. Few methods are available to assess the impact of shattercane competition on crop growth and yield. This study evaluated model sensitivity to variations in light, water, and plant density and assessed the applicability of the SORKAM model for simulating forage sorghum (used to represent shattercane) growth and development. Sensitivity analysis of SORKAM to photosynthetically active radiation (50, 75, and 100% of actual); water (50, 75, and 100% of actual precipitation, and no water stress); and plant densities of 3, 6, 12, 24, 48, and 96 plants m−2 was performed. ‘Rox Orange’ forage sorghum (used to represent shattercane) was grown at three densities, under irrigated and dryland environments over a 3-yr period. Experiments were conducted on Eudora (coarse-silty, mixed, mesic Fluventic Hapludoll) and Kahola silt loam (fine-silty, mixed, mesic Cumulic Hapludoll) soils, in Manhattan, KS. The sensitivity analysis showed that SORKAM responded to a broad range of light, water, and plant density environments, with significant light by water interaction for biomass, and light by density interaction for biomass, leaf area index (LAI), and tillering. The SORKAM model simulated growth of Rox Orange from inputs of weather, soil, and plant data were compared with measured values. There was good agreement between simulated and measured LAI through 60 d after planting (r2 = 0.82, intercept not significantly different from 0, and slope = 0.83). Accurate estimation of LAI during this period of vegetative growth is important, because LAI determines the competition between a crop and shattercane for light and water. Agreement of measured and simulated biomass was good for low plant densities (slope and intercept not significantly different from 1 and with high r2 = 0.78), with progressively greater underestimation at higher densities. Tiller number was generally underestimated, with no apparent correlation tO model under- or overestimation of LAI or biomass. The sensitivity analysis and comparison of measured and simulated LAI indicates that the SORKAM model can be used to assess Rox Orange (shattercane) competition.
Contribution no. 91-1204 of the Kansas Agric. Exp. Stn. Research supported by 87-CRSR-2-3012, 84CRSR-2-2515, F581