2.8 Selection strategy

2.8.1 Selection on phenotype

The strategy is thus, by adopting the three-step procedure of parent selection, to develop dura x pisifera planting material with a slow height increment and which maintains a high Harvest Index (HI). The latter is achieved, as shown before, by additional selection for high Leaf Area Ratio (LAR) and Leaf-Mg, and low Vegetative Dry Matter Production (VDM), with Leaf Production (LPR) as the main component. Another desired feature is a quick expansion of the crown, see section 2.7.4, through selection for low t_0.95 and high k, until optimal Leaf Area Index (LAI) is reached as determined by Lm, the asymptotic maximum leaf area, (see Breure, 1985).

The scope of selection progress for these characters depends on the genetic variance in the source population, while the efficiency, in terms of the number of generations of breeding, depends on the heritability, defined as the square of the additive genotypic value and phenotypic value (Falconer, 1981).

Although the genotypic variance of the two main objectives of selection, i.e. oil-and-kernel yield and Harvest Index (HI) is usually fairly high, progress in selection is usually hampered by their low heritability. However, various authors have shown that selection for bunch composition is usually higher than for bunch yield; this was confirmed by Breure & Bos (1992).

Characters for indirect selection (HT, LAR, Leaf-Mg and VDM), on the other hand, usually combine a relatively high genetic variance with a fair level of heritability. Good progress can thus be expected from phenotypic selection for the components of oil extraction rate and these secondary characters, in addition to oil-and-kernel yield and HI.

2.8.2 Selection for GCA values

In contrast to phenotypic characteristics, parental General Combining Ability (GCA) values, estimated by progeny testing the parents, reliably predict the performance of the tenera offspring (Breure & Bos, 1992). GCA values for oil-and-kernel yield and HI, as well as secondary characters, are therefore more suitable to develop planting material which maximizes photosynthetic production per hectare and efficiently transfers photosynthates produced per unit area into economic yield of the oil palm. In other words, the desired ideotype can be efficiently shaped through GCA values of the parents.