Abstract

Nitrogen (N) is essential for the growth and development of crops. It is a major soil nutrient for soybean. Deficiency of nitrogen in soil before soybean root nodule formation constitutes the most limiting factor after soil moisture for its production. This study was undertaken to determine starter N application rate for optimum soybean yield for two soybean varieties; assess Cropgro-soybean model and project soybean yield under different N use.

Two field experiments were set up in a 5 × 2 split-plot arrangement in a randomized complete block design in 2009 and 2010. The N doses of 0 (control), 5, 15, 25 and 35 kg N/ha were applied one week after planting using urea to form the main plot and soybean varieties (TGx1485-1D and TGx1448-2E) constituted the sub-plot with three replications. Designated samples were used to evaluate 100 seed weight, plant dry biomass and harvest index. Data obtained were subjected to analysis of variance and means separated with least significant difference at p ? 0.05. Crop phenological data and yield at harvest were used for Decision Support System for Agrotechnology Transfer (DSSAT) Cropgro-Soybean model calibration and yield projection under different N use. Root mean square error (RMSE) and percentage error (PE) were used to analyze model outputs. The soil water balance output of the model was reviewed for explaining the components of water output in the environment.

Field studies showed significant differences (p ? 0.05) in varietal response to seed weight and biomass production where variety TGx1448-2E produced a significantly higher 100 seed weight and higher plant dry biomass. Genotypic superiority of TGx1448-2E over TGx1485-1D is suggested. There was high accuracy in prediction of soybean phenology (i.e. PE < 10%) for both varieties, although high PE (> 15%) were obtained for yields of TGx1448-2E. The model accurately predicted soybean phenology to within 0-1 day of the field observed values. The soil water balance indicated 39-45 % loss of the total seasonal rainfall to runoff and 3-11 % through deep drainage.

The application of N starter-dose was not effective in enhancing yield of soybean in the study-sites owing to the occurrences of intensive rainfall events that led to runoff and the loss of applied N during the cropping season.

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