Modeling the emergence of seedling johnsongrass (Sorghum halepense (L.) Pers.)

Research was conducted to investigate the influence of burial depth and temperature on seedling johnsongrass emergence and to use the generated data in the formulation of predictive models. Constant temperature growth chamber experiments indicated that there was a significant interaction between burial depth and temperature. Percent emergence and time to first emergence were highly dependent upon this interaction and needed to be evaluated on an individual basis. Generally, emergence ranged between 0 and 54% while time to first emergence ranged between 3 and 19 days. The constant temperature-burial depth data and previously published emergence data were used to formulate an emergence model using the Fermi-Dirac distribution function. The Fermi-Dirac distribution function was found to be an adequate model for describing seedling emergence, as influenced by burial depth, of various weed species including johnsongrass, downy brome, and round-leaf mallow. Model adequacy was determined by a good visual fit of the model to the data, narrow confidence intervals for the model parameters, and an appropriate regression analysis of the modeled vs. an independent observed data set. The poikilotherm rate equation was fit to the seedling emergence data that was generated from 0 to 2.5 cm burial depths. The equations developed from this data were combined to form a four-parameter poikilotherm rate equation that could be used to model the emergence of seedling johnsongrass from these depths at any constant temperature between 20 and 44 C. When this single poikilotherm rate equation model was validated against independent emergence data, a good visual fit of the model to the data and an appropriate regression analysis of the observed and modeled data was observed.