Repetibilidade e seleção em Panicum maximum Jacq.

• Autor(a) principal: Martuscello, Janaina Azevedo
• Data de Publicação: 2007
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/1911
• topic: Forrageiras tropicais; Melhoramento genético; Tropical forrage crops; Breeding; CNPQ::CIENCIAS AGRARIAS::ZOOTECNIA::PASTAGEM E FORRAGICULTURA

The experiment was done with the objective to evaluate the repeatability coefficient of some agronomic characteristics in hybrids of Panicum maximum, and to select superior genotypes in half-sib populations obtained from crosses between sexual plants and apomictic accessions. It was also proposed to compare selection strategies between and within families, combined selection, mass selection and stratified mass selection. For this, ten plots of sexual plants were randomly distributed among 230 plots of apomictic accessions. After natural pollination, seeds of each sexual plant constituted a half-sib family. Thirty plants of each female progenitor were evaluated in a random blocks design experiment, with five plants per plot and six replications (for the repeatability estimate) and five replications (to evaluate the genetic parameters). The coefficient of repeatability, based on five evaluation harvests, was estimated by the methods of Analysis of Variance, Principal Components (correlation and covariance matrices) and Structural Analysis. For the evaluation of genetic parameters, five evaluation harvests were done in the rainy seasons and two in the dry seasons in all plants. Immediately after the harvests, the forage was weighed to obtain green matter production. Samples were separated into leaf blades, stems + sheath and dead matter, which were air-forced dried and later weighed for the determination of dry matter and percentages of leaves, stems and dead matter of each genotype. The estimates of the repeatability coefficient by the different methods, for all evaluated characteristics varied from 0,51 to 0,86 and may be considered high, as were the determination coefficients. It was observed that the repeatability coefficients obtained for the four characteristics evaluated by the method of analysis of variance, were almost always smaller than the estimates from the other methods. By the principal components method (based on the covariance matrix) the estimates were always greater than from the other methods. Considering as satisfactory a level of 80 or 90% confidence (R2) for the decision on the relative superiority of the hybrids, for all the evaluated characteristics, the five harvests were sufficient for the choice of the best plant. The exclusion of harvests 1 and 2 promoted an increase in the coefficients of repeatability and determination. There was significant genetic variability for all the production characteristics, with the exception of dead matter both in the rainy and the dry periods, which shows the possibility of considerable gains with selection. The selection criteria used (selection between and within families, combined, mass and stratified mass selections) were efficient for application in P. maximum breeding, with medium to high gains for most characteristics evaluated. The highest genetic gains were obtained from combined selection. However, mass and stratified mass selections promoted high genetic gains and may be used in P. maximum breeding. The sexual progenitors identified as numbers 7, 1, 3 and 5 are promising and may be used in future crosses, as well as for increase of the genetic variability in the P.maximum genebank.