Plant material
Fifteen soybean genotypes, introduced from diverse agricultural institutions, were subjected to a preliminary screening for agronomic performance in a greenhouse initially. In order to select the top five genotypes, the 15 genotype were subjected to a base index that uses economic weight for each trait developed and used by maize breeders at the International Institute of Tropical Agriculture (IITA)
(Meseka et al., 2011). The base index used for selecting promising soybean for this study combined seed yield, number of days to 95% maturity, number of productive branches per plant, number of pods per plant, number of seeds per pod, pod length and 100-seed weight for each of the genotypes. Because each parameter was standardized with mean “0” and standard deviation of “1” to minimise the effects of different scales, a positive value was considered an indicator of good agronomic performance, whereas a negative value indicates poor performance. Five genotypes combining high seed yield with other desirable agronomic traits (100-seed weight, number seeds per pod and number of pods per plant), were selected for further evaluation.
The sources of five selected genotypes originated from the International Institute of Tropical Agriculture (IITA), Nigeria and ORNAS Company in Sudan (Table 1). The experiment was conducted during 2009 and 2010 minor cropping seasons at El Gantra in the Range and Pasture Farm, Sennar State in Sudan. The yield performances of these soybean genotypes were evaluated under ten different sowing dates.
Experimental procedures
The experimental site was El Gantra farm, located in Sennar State, Sudan (latitude 14°C 24'N and longitude 33°C 29'E with an altitude of 127.41 m above sea level). Sennar State, one of the semi-arid agro-ecological zones of the Sudan, is characterized by erratic rainfall and temperature variability (Table 2). The soil was 60% clay, with pH 8.2; and low organic (0.5%) and nitrogen (0.05%) content and available phosphorus (2.8 mg kg
-1).
Field preparation included ploughing, harrowing and ridging were done before sowing. The experiment was arranged in a split-plot design with three replications. Each plot size was 2.4 m × 5 m, consisting of 4 rows 5 m-long. The plots were pre-irrigated three days before sowing, to ensure sufficient moisture in the soil during planting. Two seeds were planted per hill on ridges, spaced 60 cm apart and 10 cm between hills. The seeds were inoculated with
Rhizobium japonicum strain to ensure the process of nodulation. The experiment was implemented using five sowing dates during each of the cropping seasons of 2009 and 2010. Each sowing date was considered an environment as they differed in weather patterns of temperatures and precipitations (Table 2).
Inoculation of seeds with the nitrogen-fixing bacteria strain was carried out once in the 2009 season not as a treatment and the 2010 trial was also planted in the same field with the residual inoculum effect in the soil. Standard cultural practices recommended for soybean production, such as plant population, row planting, planting date and insect scouting were applied uniformly to all the plots to monitor pests and disease build up. In cases of poor germination, re-sowing was done seven days after planting, followed by the second irrigation. Seedlings were thinned to one seedling per hill at three weeks after planting. Plots were manually kept weed-free throughout the season. The chemical-based insecticide Malathion was used for control of pest such as aphid, bean leaf beetle and green clover worm whenever necessary. To avoid drought stress, the field was irrigated twice a week using gravity flow irrigation system.
Data collection
Field data were collected in accordance with the International Plant Genetic Resources soybean descriptors
(IBPGR, 1983). The agronomic traits in this study were recorded on plot basis. Plant height was measured three weeks after 50% flowering, from the ground surface to the base of primary stem of the mother plant, for 10 randomly selected plants. Number of branches was recorded as mean count of branches of randomly selected plants with 3-week-old pods. Leaf area was computed following the empirical relations determined by
Iamauti (1991). First pod height (cm) was measured 7 days after pod formation. Lodging, number of pods per plant and number of seeds per pod were recorded at physiological maturity when the seed or pod was completely yellow. A hundred seed weight was determined by randomly counting 100 seeds from a bulked seed for each plot and weighed using a digital weighing-scale. Seed yield (kg ha
-1) was measured on plot bases after harvest.
Data analysis
Correlation coefficients were computed to determine the relationships between seed yield and yield components using SAS ver. 8 [
SAS Institute Inc, (2000) Cary, North Carolina, USA]. Combined analysis was performed for seed yield to determine variability among the soybean genotypes and the effect of GEI across the 10 environments. For stability analyses, Additive Main Effect and Multiplicative Interaction (AMMI) and Genotype and Genotype × Environment (GGE) biplot analyses using GEA-R ver. 4.1
(Pacheco et al., 2015), were used to determine the effects of genotype × environment interaction (GEI) on seed yield across the environments. To generate a visual AMMI biplot, the following statistical model equation was used: