Calibration and validation of the crop growth model LINTUL for grain amaranth (<em>Amaranthus</em> sp.)

Abstract

Grain amaranth, a C₄ plant, is a promising pseudocereal due to its valuable grain components. Knowledge of crop growth parameters is crucial for the introduction of a new crop, and the use of a crop model can help to understand yield formation and yield limiting processes. The aim of the study was to parameterise and validate the model LINTUL for grain amaranth. Basically, LINTUL estimates dry matter production from daily intercepted radiation and light use efficiency under potential growth conditions, i. e. without occurrence of any other limiting factors. A field experiment with the A. hypochondriacus genotypes „Neuer Typ“ and „Anderer Typ“ was carried out under semiarid conditions in 2004 and 2005. Field data of individual years were used for parameterisation while independent observations of the other year allowed for cross-validation, respectively. The estimated light use efficiency ranged between 2.5 and 2.8 g MJ^-1 (total biomass per accumulated PAR). Specific leaf area estimates were lower in observations of 2004 (0.014 m² g^-1) than in observations of 2005 (0.018 m² g^-1). The light extinction coefficient of both genotypes measured before heading was 1.1. The effective sum of temperature (above a given threshold of 8°C) to anthesis was 554°C d for the genotype „Neuer Typ“ and 640°C d for „Anderer Typ“ in both years. The effective sum of temperature to maturity was 1127°C d in 2004, and 1249°C d in 2005 independent of the genotype. Model predictions of total biomass agreed well (RMSE: 92 to 136 g m^-2) with the observed biomass throughout the growing cycle including final harvest (between 749 and 1172 g m^-2). The estimated grain yield over time (RMSE: 47 to 112 g m^-2) matched the field observations including final grain yield (between 220 and 367 g m^-2) with less accuracy. The leaf area index was overestimated throughout the growing cycle from heading onwards to seed filling. The sharp initial increase in grain yield was underestimated suggesting that currently produced assimilates could not meet the growth capacity of the young seeds but might be complemented by internal re-translocation of biomass.