Cereal Research Communications 47(1), pp. 145–156 (2019) DOI: 10.1556/0806.46.2018.065 0133-3720/$20.00 © 2019 Akadémiai Kiadó, Budapest Changes in Heterosis of Maize (Zea mays L.) Varietal Cross Hybrids after Four Cycles of Reciprocal Recurrent Selection A.O. Kolawole 1 , A. Menkir 2 *, E. Blay 3 , K. Ofori 3 and J.G. Kling 4 1 Ladoke Akintola University of Technology, Ogbomoso, Nigeria 2 International Institute of Tropical Agriculture (IITA), Oyo Road, PMB 5320, Ibadan, Nigeria 3 West Africa Centre for Crop Improvement (WACCI), University of Ghana, Legon 4 Department of Crop and Soil Science, College of Agricultural Sciences, Oregon State University, Corvallis, OR 97331 (Received 17, July 2018; Accepted 16 October 2018) Two improved tropical maize composites, TZL COMP3 and TZL COMP4; representing complementary heterotic pools have been subjected to four cycles of reciprocal recurrent selection (RRS) for two decades to enhance varietal cross performance. The objectives of this study were to evaluate the effect of selection on genetic gain in heterosis for grain yield and other agronomic traits of these composites. Ten parental populations representing the C 0 to C 4 of each composite and their crosses plus a varietal check were evaluated in a trial at eight environments in Nigeria. Grain yield of the varietal crosses increased with selection by 3.1% cycle –1 . Mean grain yields of the C 4 × C 4 varietal cross exceeded that of a popular improved reference variety by 23%. Selection also reduced anthesis-silking interval, improved ear characteristics, phenotypic appeal and resistance to foliar diseases. Mid-parent heterosis (MPH) increased from 4% at C 0 × C 0 to 24% at C 4 × C 4 . The average rate of genet- ic gain in heterosis for grain yield in population crosses was 3.1% possibly because of pres- ence of non-additive gene effects. The results of our study present the potential usefulness of the advanced selection cycle as sources of diverse inbred lines with improved combining ability as well as improved varietal crosses that can be multiplied and deployed in areas with limited market access. Keywords: genetic gain, heterosis, composites, varietal crosses Introduction Maize (Zea mays L.) is a major staple food and primary source of daily calorie for hu- mans in several countries in sub-Saharan Africa (Olaniyan 2015). Increasing maize pro- ductivity per unit area will lead to more food, reduced food prices for the urban and rural poor and extra produce for sale. The development and commercialization of maize varie- ties and hybrids with high yield potential is therefore important for increased productiv- ity in the diverse production environments (Menkir et al. 2006). This can be achieved through improvements in breeding efficiency to accelerate the rate of genetic gains. *Corresponding author; E-mail: a.menkir@cgiar.org