Abstract
The perennial grass crop model ANDRO, simulating N limits to growth of Andropogon gayanus during the rainy season in sub-Saharan Africa was developed to quantify biomass and forage production potentials, and to evaluate the main crop constraints under different management condition in West African agro-environments. The model was calibrated by adjusting two model parameters, initial radiation use coefficient (1.8 g MJ- 1) and initial shoot N use coefficient (7.25 g-1 day-1). Subsequently, crop biomass and forage production were correctly simulated by the model in four major zones of West Africa covering different lengths of rainy season, N supply and exploitation regimes, and production levels. Production potentials and constraints of A. gayanus were assessed with the model for four different sites in the Sahel and Sudan zones. With ample N supply, simulated biomass and forage production, N uptake, and crop transpiration increased from minimum values in the Sahel proper zone (Nara) to maximum values in the Sudan zone (Koutiala), due to increasing length of the season (69-136 days). Without forage removal, maximum above ground biomass was 8000 kg ha-1 at Nara and 27 000 kg ha-1 at Koutiala for year with normal dates for the start of the rainy season. Nitrogen productivity was approximately 160 kg above ground biomass per kilogram N uptake for cases of N-limited growth, where the length of the growth period, relative to total uptake, permitted maximum N dilution. Cutting once every 15 or 30 days decreased biomass production and increased shoot N content, irrespective of N supply regime. Estimates of seasonal evapotranspiration were below the annual rainfall for years with normal dates for the start of the rainy season. It is unlikely, even under ample N supply, that A. gayanus in the study area would suffer severe water deficits during the rainy season, if all rainfall could be stored in the root zone.
