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Biomass, corn, silage maize, in vitro digestibility, hybrid, inbred line, genetic improvement. Travail réalisé par le groupe maïs de l'I.N.R.A., avec la collaboration plus particulière de Y. MoNTn!nrvT, A.

en-Chaussée, F 80200 Péronne ( * ) I. N. R. A., Station expérimentale du Mai's, F 40590 Saint-Martin-de-Hinx ( ** ) LN.R.A., Station de Pathologie végétale, Route de Saint-Cyr, F 78000 Versailles.

The utilization of exotic germ plasm is difficult due to its non-adaptability. This study investigates the possibility of exotic germ plasm loss during adaptation, and the effect of an additional cross with elite material on the breeding value of exotic x adapted material. The study was conducted on a temperate x highland tropical composite (or pool) developed in order to broaden the genetic variability of maize in north western Europe. The frequency of unique exotic alleles and the isoenzymatic polymorphism at four loci were analysed in the pool itself, in the pool after mild selection, and in the selected pool crossed with elite material. Based on these data, no significant deviation seemed to occur during the mild selection and the cross. The pool and the pool x elite germ plasm cross were evaluated in testcrosses with two complementary testers for both grain and forage production. The pool was later in maturity, more susceptible to lodging, and yielded less than the pool x elite germ plasm crosses for all evaluations. The highest estimates of genetic variance were obtained in the pool for earliness and height traits, and for yield. However, based on the predicted genotypic mean of the selected population, the pool had a lower breeding value than the pool x elite germ plasm cross. The pool x elite germ plasm cross is thus preferred to initiate selection.