211 Crop Breeding and Applied Biotechnology 12: 211-214, 2012 LCB Carvalho et al. Crop Breeding and Applied Biotechnology 12: 211-214, 2012 Brazilian Society of Plant Breeding. Printed in Brazil Phenotypic correlations between combining abilities of F 2 cowpea populations Leonardo Castelo Branco Carvalho 1 , Kaesel Jackson Damasceno e Silva 2 *, Maurisrael de Moura Rocha 2 , Massaine Bandeira de Sousa 1 , Carolline de Jesús Pires 1 and José Airton Rodrigues Nunes 3 Received 13 February 2012 Accepted 11 June 2012 Abstract – Cowpea is a crop that has become socio-economically relevant, mainly in developing countries. Correlation studies are important to determine the association between quantitative traits and yield to guide the selection, i.e., choose direct or indirect selec- tion. The objective was to estimate the correlations between six agronomic traits in cowpea as well as the correlations between the estimates of combining abilities of parents. Genotypes with high pod weight and pod length, 100-grain weight, and number of beans per pod should be used to improve grain yield in cowpea. The breeder should preferably insert plants into his group of crosses that have a high combining ability for pod length, number of grains per pod and yield per plot. Key words: Vigna unguiculata, diallel, selection. 1 Universidade Federal do Piauí (UFPI), Campus da Socopo, Centro de Ciências Agrárias, 64.049-550, Teresina, PI, Brazil 2 Embrapa Meio Norte, Avenida Duque de Caxias, 5650, Bairro Buenos Aires, 64.006-220, Teresina, PI, Brazil. *E-mail: kaesel@cpamn.embrapa.br 3 Universidade Federal de Lavras (UFLA), Departamento de Biologia, CP 3037, 37.200-000, Lavras, MG, Brazil INTRODUCTION An annual and extremely robust crop, cowpea (Vigna unguiculata (L.) Walp.) is undemanding in soil fertility and tolerant to high temperatures and drought. At temperatures between 18 °C and 34 °C, considered ideal for its develop- ment, cowpea has higher yields than other crops (Passos et al. 2007, Shimelis and Shiringani 2010). Particulalry in developing countries, cowpea has become a socio-economically relevant crop. According to Vijayku- mar et al. (2010), in recent years, the crop was planted on an area of more than 12 million hectares worldwide and is cultivated in all tropical areas. In Brazil, cowpea has been widely planted in the North, Northeast and Midwest of Brazil (Oliveira et al. 2003, Xavier et al. 2005). Most farmers grow it on small fields, mainly in the north and northeastern regions, achieving low yields due to the limited use or even absence of technological inputs. In the agribusiness and large-scale farming regions of the Midwest, in the states of Mato Grosso and Mato Grosso do Sul, for example, the crop has produced average yields that are 1,159 and 883 kg ha -1 higher, respectively, than the national average of 369 kg ha -1 (Freire Filho et al. 2011). In breeding for yield, different traits and their inter- relationships are considered in the evaluation of cultivars. The low heritability of most quantitative traits of economic interest complicates the selection process, while other traits are hard to measure and identify, making the under- standing of the relationship between the traits extremely important (Cruz et al. 2004). In the selection of lines, the correlations among the target traits for crop improvement are also considered. Correlation studies are important to determine the association between grain yield and other quantitative traits, providing an orientation for the selec- tion, i.e, whether a given trait of interest can be indirectly selected by selection for another (Mohammed et al. 2010). Work carried out by Mohammed et al. (2010) indicates that the number of pods per plant, seeds per pod and seeds per plant can be used in both direct and indirect selection for grain yield in cowpea. High heritability estimates reflect a major contribution of the genotypic variance, which in some cases allows for the direct use of phenotypic selection, while low-heritability traits can be improved by indirect selection for other, high- heritability traits, correlated with the former (Shimelis and Shiringani 2010). NOTE