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

Table 3. SUmmary of performance of some of the best crosses in CIMMYT diallels from across
location data.
Grain yield High parent
Trial type Germplasm involved Cross (tjha) heterosis
Diallel-1 Early Subtropical Pop.48 x Pool 27 4.63 113%
Pop.48 x Pool 28 4.64 112%
Pop.46 x Pool 30 4.47 110%
Pop.46 x Pop. 48 4.43 108%
Diallel-2 Intermediate Pop. 42 x Pop. 47 5.82 111%
Subtropical Pop.42 x Pop. 34 5.64 108%
Diallel-3 Tropical, Pop.32 x Pop. 44 6.65 113%
SUbtropical Pop.43 x Pop. 42 6.85 111%
Pop.43 x Pop. 44 6.71 109%
Diallel-4 Tropical Late Pop.27 x Suwan-1 6.77 115%
Yellow Pool 26 x SU>'Ian-1 6.60 112%
Pop.24 x Suwan-1 6.97 110%
Pop.24 x Pop. 36 6.98 110%
Diallel-5 Tropical Late Pop. 29 x Pop. 32 7.20 113%
White Pop.21 x Pop. 25 7.16 112%
Pop.21 x Pop. 32 6.94 108%
Diallel-6 Tropical, Early, Pop.26 x Pool 21 7.01 110%
intermediate Pop. 23 x Pool 20 7.34 109%
Diallel-7 QPM Subtropical Pool 33 aPM x Pool 7.43 110%
34 aPM
Diallel-8 aPM Tropical Pop.65 x PR 7737 6.77 120%
Pool 25 QPM x PR 7737 6.15 111 %
The level of inbreeding adversely affects ear size, seed yield, vigor, and in some cases
even the reproductive behavior. Thus the problem of inbreeding depression is not only important
but quite crucial in hybrid breeding. The problem is widely known but is highly neglected in
research. More serious efforts are thus needed to correct this problem and to increase efficiency
and probability of extracting superior lines in a much higher frequency than is currently
possible. At the very inception of CIMMYT's hybrid maize breeding program, efforts were devoted
to improving this trait as part of an on-going inbred line development program. Studies conducted
in both tropical and subtropical germplasm using S3 recurrent selection have proved conclusively
that it is possible to improve this trait (Table 5). The S1 bulks developed from Cycles
0, 1, and 2 showed that C2-S1 bulk was superior over C1 which in turn was better than CO. The
performance of S1 bulk as a percentage of F2 full vigor material of the corresponding cycle also
showed improvement indicating reduced inbreeding depression in later cycles. These inbreeding
tolerant germplasm are available on request and would greatly aid hybrid efforts in the developing
countries.
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