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Sensitivity Analysis of SIMED¹

SIMED, a basic crop growth model for alfalfa (Medicago sativa L.), uses weather data to predict the physiological rates which over time control the accumulation of dry matter in the leaves, stem, and roots. The objective of the work presented was to demonstrate the response of SIMED to two of the most important environmental variables, temperature and solar radiation, and to compare its response to current information on alfalfa growth.

Computer simulations can be conducted to determine how a crop responds to wide ranges and many combinations of environmental factors. Such sensitivity analyses were made using combinations of five temperature and five solar radiation regimes for a period of 6 weeks, representing early spring growth or regrowth following a harvest. Data on herbage yield and crop growth rate indicate the interaction of these two important environmental parameters on alfalfa growth. Plants grown at lower temperatures maintain vegetative growth for longer periods than those grown at high temperatures. Also, radiation levels exert less influence on alfalfa crop growth rates as temperatures increase. These data further illustrate the importance of maturity factors and temperature factors utilized in the physiological rates computed in SIMED.

Key Words: Temperature • Radiation • Growing degree days • Yield • Crop growth rate • Alfalfa

² Research agronomist, ARS, USDA and professor of plant physiology, Dep. of Botany and Plant Pathology, Purdue Univ., West Lafayette, IN 47907; formally, graduate instructor, Dep. of Agricultural Engineering, presently assistant professor, Dep. of Agricultural Engineering, Clemson Univ., SC 29631; associate professor of agronomy, Dep. of Agronomy, Purdue Univ.; and research agronomist, ARS, USDA and professor of agronomy, Dep. of Agronomy, Purdue Univ.

Received for publication February 28, 1977.