Proceedings of the Fifth Asian Regional Maize Workshop - Search ...

hybrids (Hallauer and Miranda, 1988).
Visual selection, however, does receive consideration by breeders during development of
lines (Bauman, 1981). Among the traits considered during inbreeding, visual selection was
considered to be effective for some traits (Table 9). The breeders also rated the traits for
their relative importance in lines and hybrids. There was not a close agreement (r = 0.58)
between the importance of traits and effectiveness of visual selection for these traits (Table
9). Maize breeders, however, continue to practice rigid selection during the development of
lines because the eventual value of the lines is their relative performance in hybrids. Although
the traits of the lines have low predictability for their hybrids and visual selection is
generally not effective for many important traits, selection certainly remains effective for
monitoring maturity, conforming to standar~s for plant and grain type, screening for disease and
insect resistance to enhance resistance of their hybrids, and developing lines that can be
maintained and used in seed production at reasonable costs. Visual selection will continue to
have a role in maize breeding, but evaluation of lines in hybrids is an imperative.
Table 9. Ratings for the relative importance and effectiveness of visual selection for 17 plant
and ear traits in US Corn Belt breeding programs (Adapted from Bauman, 1981).
Traits
Ratings (1 to 4)
Importance of
Effectiveness of
traitt visual selection ~
Grain yield 1.2 3.2
Stalk strength 1.2 2.5
Root strength 1.4 2.6
Stalk rot 1.5 2.3
Silk emergence 1.6 1.4
Leaf blights 1.7 1.6
Ear rots 1.8 2.1
Flowering date 1.9 1.3
Pollen shed 1.9 1.7
Seed] ing vigor 1.9 1.9
Insect damage 2.0 2.3
Kerne 1 Qual iti es 2.1 1.9
Plant and ear height 2.2 1.5
Smut (Ustilago maydis) 2.3 2.2
Plant appearance 2.3 1.8
Erect leaf habit 3.0 1.6
Plant color 3.2 1.5
t Rating of 1 trait considered more important and rating of 4 trait less important.
t Rating of t visual selection is effective and rating of 4 where visual selection
is not effective.
Testing for combining ability:
The importance of testing inbred lines for their performance in hybrids was recognized and
emphasized by Shull (1909); only then was the true merit of the new lines determined. Shull also
indicated that a group of inbred lines that random crosses among them will not all be equally
productive. Hence, one has to make the crosses between pairs of lines and evaluate the crosses
in replicated trials before one can determine which pair of lines produces the superior hybrid.
Shull (1909), therefore, either had a premonition that appearance of lines would not predict
their performance in hybrids or had not visualized how important many traits would be considered
during inbreeding to develop lines and evaluate in hybrids.
Methods to increase the efficiency and effectiveness for evaluation of lines in crosses
have always been a major focus in the study of hybrid breeding methods. Studies and suggestions
were made for appropriate generation of inbreeding to test (early testing vs. late testing),
relative importance of additive (GCA) and nonadditive (SCA) effects in crosses, types of testers
(good or poor performance), extent of testing, and relative value of among and within progeny
testcrossing (Hallauer and Miranda, 1988). Jenkins and Brunson (1932) presented evidence that
the topcross method was useful for making an initial screening of 1ines for combining abil ity.
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