Genetic Gains in Grain Yield of Extra-Early Maturing Maize Cultivars of Three Breeding Eras Under Multiple Stress Environments.

The availability of extra-early cultivars has resulted in the expansion of maize production into new frontiers replacing the traditional crops such as sorghum and millet in the savannas of West Africa (WA). Fifty-six extra-early maturing cultivars were evaluated for 2 yr for grain yield and other traits under 23 multiple stress (drought, Striga and low soil nitrogen) and 29 nonstress environments in Nigeria, Republic of Benin, and Ghana to determine the rate of genetic improvement in grain yield of the cultivars developed during three breeding eras (1995-2000, 2001-2006 and 2007-2012) . Under multiple stresses, grain yield ranged from 1752 kg ha^–1 for cultivars bred during 1995–2000 to 2066 kg ha^–1 for those developed during 2007–2012 with a corresponding genetic gain of 1.42 % per year. Under nonstress conditions, grain yield ranged from 3217 kg ha^–1 for cultivars bred during 1995–2000 to 3673 kg ha^–1 for those developed during 2007–2012 with annual genetic gain of 1.07 %. The average rate of increase in grain yield was 24 kg ha^–1 per year under multiple stress and 34 kg ha^–1 per year under nonstress environments. Genetic gains in yield from first- to third-generation cultivars under multiple stresses was associated with improved ear aspect, increased days to silking, reduced anthesis-silking interval, and decreased root and stalk lodging whereas under nonstress environments  significant gain was not achieved for yield but good progress was made for improved husk cover, increased ears per plant and reduced anthesis-silking interval. It may be concluded that genetic improvement of extra-early maturing maize for stress tolerance in WA has been very effective. The cultivars TZEE-W STR 105 BC1 and 2004 TZEE-W Pop STR C₄ of Era 2 and TZEE-W Pop STR C₅, TZEE-W STR 105, TZEE-W STR 108 and TZEE-W STR 104 developed from the latest era of improvement (Era 3) were identified as outstanding in grain yield and stability across test environments and should be commercialized to contribute to food security in WA.