Dec. 2009, Volume 3, No.12 (Serial No.25) Journal of Agricultural Science and Technology, ISSN 1939-1250, USA 36 Cardinal temperatures and required biological days from sowing to emergence of three millet species (common, foxtail, pearl millet) Morteza Eshraghi Nejad, Behnam Kamkar, Afshin Soltani (Department of Agronomy, Gorgan University of Agricultural Science and Natural Resources, Gorgan 49189-43464, Iran) Abstract: The modeling of germination and seedling emergence is required for the construction of a simulation model of three species of millet (panicum miliaceum, pennisetum galucum and setaria italica). This study provides the necessary temperature parameters to model these processes. For this purpose, different non-linear regression models including flat, logistic, quadratic, sigmoidal, dent-like, segmented, beta and curvilinear were used. Root Mean Square of Errors, coefficient of determination and regression coefficients of predicted values versus observed were used to find the appropriate model. Investigating regression coefficients indicated that dent-like model has the least RMSE and a coefficient (RMSE=0.000009, a=0.0006) and the biggest R 2 and b coefficient (R 2 =0.96, b=0.98) in common millet. These coefficients were (RMSE=0.01, a=0.005) and (R 2 =0.94, b=0.97), and (RMSE=0.004, a=0.05) and (R 2 =0.99, b=0.99), for beta in foxtail and pearl millet, respectively. According to these coefficients, dent-like, was chosen as the best model to describe the response of common millet germination to temperature (T b =7℃ and T c =49.50℃). Also beta, was chosen for foxtail millet (T b =7 ℃, T c =49.50 ℃). Beta, was chosen as the best model for pearl millet (T b =6.5 ℃ and T c =4 ℃ ). These parameters can be used in millet simulation models to predict sowing to emergence duration based on a thermal time concept. Also, required biological days from sowing to emergence using these models varied from 3.57, 4.29 and 5.54, for common millet, foxtail millet and pearl millet, respectively. Key words: cardinal temperature; germination rate; nonlinear fitting; millet 1. Introduction Emergence is probably the single most important event that affects the success of an annual crop [1] and also success or failure of crop production [2] . Rapid, Morteza Eshraghi Nejad, M.SC student; research field: agronomy. E-mail: eshraghi_398@yahoo.com. Corresponding author: Behnam Kamkar, Ph.D.; research field: agroecology. E-mail: behnamkamkar@yahoo.com. uniform and complete emergence of vigorous seedlings leads to high grain yield potential by shortening the time from sowing to complete ground cover, allows the establishment of optimum canopy structure to minimize interplant competition, maximizing crop yield and providing plants with time and spatial advantages to compete with weeds [1] and also reduces water and wind erosion [3] . Seed germination is a complex biological process that is influenced by various environmental and genetic factors [4] . Seedling emergence is controlled by species-specific requirements and the availability of favorable seedbed conditions [5] . Environmental conditions directly surrounding a seed determine germination success and subsequent seedling emergence and establishment [5] . Temperature and water mainly drive the rate of seed germination when aeration is not restrictive [6] . Temperature is the most important driving force influencing crop development rate [7] . The effects of temperature on plant development are the basis for models used to predict the time of germination. Three cardinal temperatures (maximum, minimum and optimum) describe the range of temperature over which seeds of a particular species can germinate [8] . Estimation of the cardinal temperatures, including base, optimum, and maximum, is essential because rate of development increases between base and optimum, decreases between optimum and maximum, and ceases above the maximum and below the base temperatures [4-9] . A portion of a crop model is to predict the time of crop development processes (phenology) [10] . Many mathematical models have been developed to