Genetic parameter estimates for height and stem straightness in Pinus taeda linnaeus and implications for breeding

Pinus taeda L. , loblolly pine, is an important exotic plantation species in Zimbabwe, and other southern African countries. A major constraint to the efficient breeding of this species in these countries is the lack of genetic parameter estimates to assist informed decisions on the most appropriate breeding strategy and, more generally, to monitor genetic progress. This thesis addresses this deficiency using data from, primarily, the Zimbabwean breeding populations of P. taeda. The study focuses on univariate and bivariate estimation of genetic parameters, prediction of optimum selection age, and evaluation of the magnitude and importance of genotype x environment interaction (GE). Genetic parameters for height and stem straightness were estimated using innovative methods used by animal breeders (individual models), and have recently been used by tree breeders. The data originated from four genetic tests representing 140 fullsib families, assessed at four ages: 1.5, 9.5, 13.5 and 22.5 years. Results suggest height is under moderate to high genetic control, which peaked at 9.5 years, while straightness is under weak genetic control at very young ages, which increases to moderate levels with age. Dominance variance was less than additive variance for both traits, except for straightness at very young ages. Analysing data pooled across sites resulted in biased estimates of heritability at each site for both traits. Age-age genetic correlations for height were high; those for straightness were moderate, apart from correlations involving straightness at very young ages, which were negative. Generally, the genetic correlations between height and straightness were low and positive, which suggests that selecting on height alone will improve straightness. Annual genetic gain and optimum selection age for height were predicted using the estimated genetic parameters for P. taeda. Results suggest that choice of model for predicting trends in age-age genetic correlations is critical for accurate estimation of gain and of optimum selection age. Models based on phenotypic correlations underestimated the annual genetic gain, and needlessly delayed selection. Annual genetic gain was maximised by selection at 10 years; if the species could be induced to flower at 3 years, the annual gain could be maximised by selection at 3 years and increased by 100%, indicating the promise of artificially inducing flowering with P. taeda in Zimbabwe. The implications of GE for breeding strategy was evaluated using genetic correlations, parental rank changes and efficiencies of selection. Results show that GE for both traits was brought about primarily by a change in rankings of genotypes among the locations. Since GE in height was unpredictable, it would be difficult to use GE to advantage in the multiple population breeding strategy for P. taeda in Zimbabwe. Evaluating the effect of early selection on one site for predicting mature age performance at another, as suggested in this study, appears to be an efficient approach for GE evaluation in forest trees, since early selection within sites is a common practice in tree breeding programmes. A Bayesian approach, Gibbs sampling, was used to estimate efficiencies of selection across sites and their variances, and the probability that the efficiency of selection lies between certain values. This is the first use of Gibbs sampling for making decisions about optimal selection environment, and results show that it is an efficient approach. Using this approach progeny tests for P. taeda in Zimbabwe should be located at site C. Since one should expect GE within a region, it is necessary to consider dispersing at least three progeny tests within the site C region, in order to get results appropriate to commercial progress.