An investigation into the heritability of commercially important traits in a sugarcane population under dryland conditions.
Inheritance studies have previously been undertaken at the South African Sugar Association Experiment Station (SASEX) under irrigated conditions. Since most sugarcane is grown in South Africa under dryland (raingrown) conditions, heritability estimates were calculated under these conditions in this study and compared to those previously obtained under irrigated conditions. A sugarcane population consisting of 12 crosses, 32 offspring in each cross, and their parents were planted in the first two selection stages of the SASEX selection programme to ascertain which stage provided the most useful information when selecting parent cultivars. Data collected from Stage 2 was more reliable than data collected from Stage 1. Variance components, narrow and broad sense heritabilities, correlations among traits, and clonal repeatabilities between seasons were determined for 11 sugarcane traits at Stages 1 and 2. These traits studied included: stalk population; stalk diameter; stalk height; cane mass; dry matter % cane; fibre % cane; brix % cane; brix % dry matter; purity; pol % cane; and ers % cane. Narrow sense heritabilities of the sugarcane traits were estimated by mid-parent offspring regression . Alternative heritability estimates were obtained through restricted maximum likelihood (REML) analysis of the unbalanced North Carolina design II at Stage 2. Although narrow sense heritabilities determined by mid-parent-offspring regression were comparable with those previously determined at SASEX and by other workers, REML was more efficient than regression. Use of REML enabled additive and non-additive genetic variance components to be estimated by allocating degrees of freedom to treatments and the interactions between the different treatments. Heritability estimates varied for different traits and compared favourably with those obtained under irrigated conditions and by other workers. Additive genetic variance was more important than non-additive genetic variance for some characters, but not for stalk population, cane mass, and dry matter % cane, for which both variances were important. Selection of parent cultivars for all sucrose-related traits, fibre % cane, and stalk diameter should be as successful under raingrown as under irrigated conditions, provided that the environmental variation is determined efficiently under raingrown conditions. Environmental correlations were observed between some traits, particularly between the yield related traits, and may have influenced heritability estimates for those traits determined by mid-parent offspring regression. Stalk diameter, fibre % cane, and brix % dry matter were the most repeatable traits between seasons. Cane mass was the least repeatable trait between Stages 1 and 2 but was highly repeatable between plant (-P) and ratoon (-R) crops of Stage 2. Stalk diameter was positively correlated with brix % dry matter (0.457-P and 0.623-R) and strongly negatively correlated with stalk population (-0.790-P and -0.711-R) and fibre % cane (-0.628-P and -0.651-R). Cane mass was strongly positively correlated with brix % dry matter (0.638-P and 0.679-R). By selecting for brix % dry matter and stalk diameter, indirect selection for cane mass would be possible. Brix % dry matter was determined as the most reliable trait on which to base parental and commercial cultivar selection because it was highly heritable, highly repeatable and highly positively correlated with stalk diameter and cane mass.