Field trials were planted four times over three seasons from November 2005 to April 2008 at the Cedara Research Centre of the KwaZulu-Natal Department of Agriculture and Rural Development (KZN DARD), South Africa (latitude 29°32'S; longitude 30°16'E; altitude 1051 m).
The mean annual rainfall is 880 mm, of which about 745 mm falls from October to April. The mean annual A-pan evaporation is 1655 mm and 6.8 hours of sunshine per day are received during October to March (Camp 1999). The climatic data for 2005 to 2008 was received from an automatic weather station of the Agricultural Research Council: Institute of Soil, Climate and Water (ARC-ISCW) at Cedara. At all the sites the soil was a Hutton form with an orthic A over a red apedal B. Soil analyses showed means of 52% clay and 2.45% organic carbon. The mean pH (KCl) and acid saturation during the experimental period were 4.64% and 2.5% respectively.
2.3.1 Land preparation
Due to the need for crop rotation, a different land was used each season. The lands had been planted to maize in all the preceding seasons. Soil samples were taken from each land during July or August of the same planting year and analyzed by the Fertilizer Advisory Services‟
Laboratories of the KZN DARD based on the Cedara Research Station.
The lands were cultivated twice with a tractor-drawn offset disc-harrow in spring. Prior to planting a tractor–drawn konskilde was used.
39 2.3.2 Fertilization and planting
Based on the analysis of soil samples, phosphorous (P) was applied as single superphosphate (10.5% P) at a rate of 20 to 45 kg of P ha-1 depending on the requirements for each site. The fertilizer was hand-applied to the rows, which were opened using a hand-held V-shaped hoe to a depth of approximately 0.05 m, and thereafter covered with soil. Soil potassium (K) levels were sufficient (above 80 mg K L-1) and therefore no additional K was applied. Nitrogen (N) fertilizer is not recommended for soybean in KwaZulu-Natal, provided the crop is inoculated with bacteria at planting (Birch et al., 1990) and therefore no N was applied. All the seed was inoculated with Bradyrhizobium japonicum Kirchner at planting using a knapsack sprayer equipped with a Lurmark DT 30 flat spray nozzle applying 250 L ha-1 and immediately thereafter covered with 0.02 to 0.03 m of soil using a hand-held rake.
The trials were hand-planted on 17 November 2005, 3 November 2006, 30 October 2007 and 11 December 2007. The first planting in the 2007/08 season experienced a hailstorm on 29 November, which damaged the young plants in the first planting, hence the reason for implementing the second planting in the 2007/08 season. However, the plants in the first planting recovered and the results have been included. Four cultivars, AGS 352 (long-season), AGS 353 (medium-season), AGS 354 (medium-season) and Lightning (long-season) were hand- planted in 0.75 m wide rows at seeding rates of 200 000, 300 000, 400 000 and 500 000 seeds ha-1.
2.3.3 Weed, insect and disease control
The pre-emergence herbicides, S-metolachlor (Dual S Gold® EC, 915 g a.i. L-1, Syngenta1) and imazethapyr (Hammer® SL, 100 g a.i. L-1, BASF2), were applied at 1189.5 and 50 g a.i. ha-1, respectively, immediately after planting, using a knapsack sprayer equipped with a Lurmark DT 30 flat spray nozzle. The post-emergence herbicides bendioxide (Basagran® SL, 480 g a.i. L-1, BASF) and fluazifop-P-butyl (Fusilade Super® EC, 125 g a.i. L-1, Syngenta) were applied five weeks after planting at 1440 and 600 g a.i. ha-1, respectively, with a knapsack sprayer fitted with a Lurmark DT 30 flat spray nozzle applying 250 L ha-1.
1 Syngenta South Africa (Pty), Ltd., Private Bag X60, Halfway House, 1685. Tel.: 011 541 4000.
2 BASF, P.O. Box 444, Umbogintwini, 4120. Tel.: 031 9047860.
40 The insecticide, cypermethrin (Kemprin® 200 EC, 200 g a.i. L-1, Arysta Life Science1), was applied at 400 a.i ha-1 with the pre-emergence herbicides to control cutworm (Agrotis segetum Denis and Schifferműller) and applied during the growing period to control insects, especially African bollworm (Helicoverpa armigera Hűbner). Carbendazim/flusilazole (Punch C®; 125/250 g a.i. L-1, Du Pont de Nemours2) was applied at 50/100 g a.i. ha-1 at early flowering and again three weeks later to control Asian soybean rust (Phakopsora pachyrhizi Sydow) using a knapsack sprayer fitted with a Lurmark DT 30 flat spray nozzle applying 250 L ha-1.
2.3.4 Data collection
Flowering date (R1) was determined when 50% of the plants in the centre two rows had at least one flower. A harvest maturity date (R8) was recorded when 95% of the pods had turned brown.
At harvest maturity the number of plants was recorded in each plot, which consisted of the centre four meters of the two middle rows. Plant height was measured from the ground to the top of the highest pod. Bottom pod height was measured from the ground to the bottom of the lowest seed-bearing pod. Plant height and bottom pod height were only measured in the 2006/07 and 2007/08 seasons. The number of branches plant-1 and the number of 1-, 2- and 3-seeded pods were measured on ten randomly selected plants in each plot in both plantings in the 2007/08 season only.
All the plants in the plot were harvested with secateurs soon after maturity, because the pods tended to shatter readily when dry (Duppong and Hatterman-Valenti, 2005; Zhang and Kyei- Boahen, 2007). The plants were then threshed and the grain weighed. The moisture content was determined using a Sinar GrainPro 6310 Moisture Analyzer (Supplier: Ronin Grain Management Solutions, South Africa). The yields were adjusted to 12.5% moisture content.
1 Arysta Life Science, 7 Sunbury Office Park, La Lucia Ridge, 4019. Tel.: 031 514 5600.
2 Du Pont de Nemours, 1st Floor Block B, 34 Whiteley Road, Melrose Arch, 2196. Tel.: 011 218 8600.
41 2.3.5 Statistical analysis
A randomized complete block design with three replicates was used. Each plot consisted of four rows, 5 m in length and spaced 0.75 m apart. The data was analyzed using the analysis of variance (ANOVA) procedure in the statistical package Genstat (Payne et al., 2007). Differences between treatment means were measured using Fisher‟s Protected Least Significant Difference procedure with P=0.05.