INTRODUCTION
Sunflower is the most important oilseed crop in South Africa. The sunflower oil market has shown a steady increase of approximately three percent per year in the past few years, with a current demand of 600 000 tons of seed for oil extraction (Pakendorf, 1998).
In the past, sunflower in South Africa was considered to be an alternative crop to maize, i.e. if a maize crop could not be successfully produced due to drought or any other constraint. This led to a situation where sunflower cultivation was not done under optimal conditions, leading to low and erratic yields and consequently gaining a reputation of being uneconomical compared to maize.
The areas planted during the 2002/2003 season were, Free State 275 000 ha, Mpumalanga 40 000 ha, Limpopo 37 000 ha, Gauteng 10 000 ha and North West 220 000 ha with a total of approximately 582 000 ha (Beukes, 2003). It is evident that the largest concentration of sunflower is in the Free State and North West province. This is generally the drier or western part of South Africa with more sandy soils. However in the Limpopo province most of the sunflower is planted very late in Arcadia type soils with very high clay content. Another factor typical to these areas is that the evaporation is up to three times the value of the annual rainfall. Economics is an important factor that influences the expansion of sunflower. In areas where maize has a low average yield, sunflower is a good alternative crop (Parkendorf, 1998).
The above-mentioned areas of cultivated sunflower vary considerably in soil, climate and elevation. Although it is widely accepted that sunflowers have a good general adaptability, the planting date and rainfall have an influence on the performance of hybrids. The instability of hybrids creates difficulty in selection in breeding programs. Most decisions are based on limited information from one or two years with a normal ANOVA and cross site analysis. No effects of environment x genotype interaction are taken into consideration.
According to Becker and Leon (1988) successful new varieties must show good performance for yield and other essential agronomic traits. Their superiority should be reliable over a wide range of environmental conditions. Plant breeders generally agree on the importance of high yield stability, but there is fewer consensuses on the most appropriate definition of �stability� and on methods to measure and to improve yield stability.
The basic cause of differences between genotypes in their yield stability is the wide occurrence of genotype x environment interactions, i.e. the ranking of the genotype depends on the particular environmental conditions where it is grown. Very few researchers use statistical measures of yield stability in their breeding programs. A deeper insight into the relation among the numerous stability parameters and their similarity may be obtained by comparing the resulting stability rank orders of different genotypes which are derived by applying different concepts of phenotypic stability (Huehn, 1990).
The aim of this study was to compare various statistical procedures
� For assessing genotype x environmental interaction and yield stability of South African sunflowers.
� To determine the most suitable parametric procedure to evaluate and describe sunflower genotype performance under dryland conditions in South Africa.
� To recommend to breeders the most appropriate procedure to estimate genotype performance and stability most accurately.
Individuals and seed companies plant the trials co-coordinated by the Agricultural Research Council (ARC) as a trade for participation to the research. This trial system ensures good quality hybrids in the market since intercompany competition is very active and the advantage of having hybrids with good yields and good ranking in this trial setup would ensure good sales. Part of the system requirements is to have all entries registered on the cultivar list after a Difference in Uniqueness System (DUS) test run by the Registration Department in Roodeplaat. This, in turn, ensures the quality of the seed reaching the millers and the oil press. The independent evaluation of data run by the ARC, gives the farmer an advantage of a choice of improved hybrids, proven to have good yields without extra cost.
In the map of general agricultural regions (Fig.1) it is evident that the Free State is mostly utilized for cereal production and to the west for mixed farming. In the areas of cereal cultivation, sunflowers are used in rotation with wheat and maize. The western areas are traditionally maize areas. During the last five years the percentage of sunflower hectares has greatly increased in the North West and decreased in the Mpumalanga province.
Fig. 1 Map of agricultural activities. The main sunflower production areas are indicated (â) and the test sites by (â) (Dept. of Foreign Affairs and Information, 1982)
The large circle depicts the area actually planted in the Free State and part of the adjoining North West and contains two of the ARC trial sites namely Potchefstroom and Koster. The bottom smaller circle would be the very early plantings in the southern Free State. The circle above Johannesburg represent the area with the dark Arcadia type soils known as the �Springbok flats� with the Warmbaths site and the circle west of Johannesburg would represent the North West province and contains the Lichtenburg site.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-08262004-161357 |
| Date | 27 August 2004 |
| Creators | Schoeman, Lourens Jurgens |
| Contributors | PROF M T LABUSCHAGNE |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://lourie.uovs.ac.za//theses/available/etd-08262004-161357/restricted/ |
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