Combining ability for grain yield and agronomic traits among early drought tolerant quality protein maize (QPM) inbred lines
Quality protein maize (QPM), a bio-fortified form of maize has twice the content of limiting amino acids, lysine and tryptophan, compared with conventional maize. It has been developed to, among other uses, help alleviate human malnutritition in areas where protein deficiency is a major problem in diets as in the dry areas of Eastern Kenya. Information about how F 1 hybrids produced from early drought tolerant QPM inbred lines compare with one of the popular QPM single-cross (CMLI44/CMLI59) is likely to speed up release of early drought tolerant QPM cultivars in this area. Prior knowledge on the combining ability of these inbred lines would be beneficial to breeders in deciding how to best develop single-crosses, three-way, double-cross hybrids and synthetic varieties. The objectives were to (i) compare grain yield and agronomic performance of F 1 hybrids produced from early drought tolerant QPM single-crosses with a reference QPM check (CMLI44/CMLI59); (ii) estimate general combining ability (GCA) and specific combining ability (SCA) among early drought tolerant QPM inbred lines for grain yield and other agronomic traits; (iii) assess the relative importance of general combining ability and specific combining ability in determining the progeny performance of the early drought tolerant QPM inbred lines; (iv) to predict grain yield of three-way and double-cross hybrids from data obtained from the early drought tolerant singlecrosses; and (v) to establish the relationship between grain yield and other agronomic traits. Fourteen inbred lines were crossed in half diallel mating design. The 91 diallel hybrids together with 18 experimental checks and one reference check were planted in one trial and evaluated in five environments in Kenya including under four optimum and one drought condition. Data on grain yield, time to anthesis, time to silk, anthesis-silking interval, ears per plant, root lodging, stalk lodging, ear rots and ear aspect were recorded. The experimental checks yielded more grain than the diallel hybrids. One diallel hybrid entry 3 and one experimental check entry 72 had higher grain yields and were earlier in maturity than the reference check. The combined analysis of variance showed that the mean squares due to genotypes and general combining ability (GCA) were significant (p<O.OOl) for all the eight traits studied. However, the mean squares due to specific combining ability (SCA) was significant (p<O.01) for anthesis date only. Inbred Line 4 had superior GCA effects for grain yield. It was indeed a parent to twelve single-crosses among the selected top performing, dominant male parent to all the best predicted threeway hybrids and common parent to all the double-cross hybrids confirming its superiority in GCA effects. The same inbred had good GCA for shorter anthesis- silking interval, good alleles for increased number of ears per plant, resistance to ear rots and good ear aspect. Inbred Lines 5, 12, 7 and 9 had good alleles for reduced anthesis date. Line 6 and 10 had good alleles for root lodging and stalk lodging. Additive gene action appeared to have been more important than non additive gene action for grain yield and the other agronomic traits. Average grain yield for the single-crosses, predicted three-way and double-crosses was found to be the same, suggesting that the grain yield of the singlecrosses may accurately reflect grain yield of the three-way and four-way crosses and should therefore be confirmed in field evaluations. An increase in grain yield was associated with a reduction in anthesis-silking interval and barrenness, and better ear aspect in the set of inbred lines used in this study.