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The current E. grandis × E. urophylla hybrid breeding strategy of South Africa’s Forestry Industry is to maintain large breeding populations of both parental species in which parents are selected based on their general combining ability (GCA) estimates or predicted individual tree breeding values and are used for interspecific hybrid crosses. The hybrid material is first screened in seedling progeny trials after which superior individuals are selected and tested as clones. Although this strategy has delivered superior clones for commercial production in South Africa, it is a time consuming strategy to follow and more cost effective strategies are being investigated. In order to review the current hybrid breeding strategy, information on the genetic control of the traits of interest is needed for E. grandis × E. urophylla seedling and clonal populations. The main objectives of this study were therefore to firstly estimate genetic parameters for E. grandis × E. urophylla hybrid seedling and clonal populations; secondly to investigate the correlation between E. grandis and E. urophylla parental (GCA) or individual breeding values and their general hybridising ability (GHA); and lastly to determine the correlation between E. grandis × E. urophylla hybrid seedling ortets and their ramets. Results of our study indicated that non-additive genetic variation explained the majority of the total genetic variation in E. grandis × E. urophylla seedling and clonal populations. Due to the pre-eminence of non-additive variance, the pure-hybrid correlations were weak, especially for clonal populations. It would therefore seem that GCA or predicted individual breeding values are not good predictors of GHA for growth performance in the observed populations. Our study also indicated a weak coefficient of correlation between the growth performance of seedling ortets and their ramets. These results suggest that: firstly a hybrid breeding strategy to capture non-additive genetic variation should be adopted; and secondly that the first phase of screening E. grandis × E. urophylla hybrid material as seedlings should be revisited.

Genetic Parameters of Interspecific Hybrids of Eucalyptus grandis and E. urophylla Seedlings and Cuttings