Capacidade de combinação e seleção para rebrota em Megathyrsus maximus

Abstract

Megathyrsus maximus (syn. Panicum maximum) is a forage grass of paramount importance in pasture-based beef production systems, with regrowth being a fundamental trait in the breeding of this species. However, regrowth capacity (RC) is traditionally obtained through scores resulting from a combination of visual ratings assigned to its component traits: tiller density and growth rate, which can compromise selective accuracy. Therefore, this study aimed to investigate whether using the measurement of leaf elongation rate (LER) as a proxy for growth rate and a finer scale discretization for evaluating tiller density (DENS), combined through an additive index (AI), would contribute to increasing selective accuracy. Furthermore, the objective was to estimate genetic parameters, combining abilities, and genotype × environment (fertility levels) and genotype × harvest interactions related to regrowth capacity and its component traits. For this purpose, 81 progenies derived from a partial diallel involving 10 apomictic and 10 sexual parents were evaluated in an augmented block design. Data were analyzed using the mixed model approach, with variance components estimated by residual maximum likelihood. Genetic variability was detected among parents for RC, DENS, and LER, and among progenies for RC and DENS, enabling the selection of superior parents and progenies. Individual heritability values were low: 8%, 5%, and 16% for RC, DENS, and LER, respectively, highlighting the significant influence of the environment on the phenotypic expression of regrowth. The genotype × environment and genotype × harvest interactions were significant, but predominantly of the simple type. Traditional evaluation (RC) proved to be more effective than AI, with predicted genetic gains of 3.2%, 1.4%, and 8.2% for apomictic parents, sexual parents, and progenies, respectively.