Le maïs-fourrage. I-Origine, évolution, ressources génétiques et méthodes de sélection

Forage maize. I- Origin, evolution, genetic resources, and breeding methods

The use of the vegetative parts of maize for feeding ruminants really began only a few decades ago, while the use of its grain for the feeding of humans and animals is very old. The monophyletic origin of the Monocotyledons is now well accepted. The divergence between Rice and Maize dates back 60 to 80 M years, that between Sorghum and Maize only 15 to 20 M years. Maize was progressively domesticated in Central America from a plant close to Teosinte, and the oldest known vestiges of Maize are 7000 years old. Because of the very long history of Maize evolution, a very large genetic diversity is available to farmers and plant breeders, but most of this variation is related to grain traits. Available resources are of interest to breeders only when concerning a specific useful trait such as digestibility or stress tolerance. Moreover, genetic variation should be organized so as to take profit of heterotic patterns. Recurrent breeding allows the accumulation of favourable traits and epistatic relationships in elite genotypes, but these should not be unbalanced by too large introgressions of non-adapted germplasm. The large development of molecular biology will widen the possibilities of using genetic resources in silage maize breeding. From the theoretical and physiological knowledge of the maize genome, and from the knowledge of synteny between genomes, the traits of interest would be easily recognized in resources. It would thus be possible to introduce only the useful part of genomes into the elite lines. All the genomes, through genetically modified organisms, could in the future constitute the genetic resources of silage maize breeding.