An important objective in lucerne breeding is the improvement of feeding value. An understanding of the inheritance of digestibility and cell wall related traits would facilitate the breeding of varieties with higher feeding value. The aim of this experiment was first to determine the genetic control of dry matter digestibility and related biochemical and morphological traits, and to assess phenotypic and genetic correlations between these traits. Quantitative genetic parameters were estimated for enzymatic solubility, neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL), protein content, plant height, and leaf-to-stem ratio. A 7 × 7 diallel design with reciprocals and without selfing among parents from different populations and a 7 × 7 factorial design within the ‘Flamande’ population were studied. In the diallel study, effects due to general combining ability (GCA) were higher than those due to specific combining ability (SCA) for all characters tested. In the factorial study, the F1 progeny effect was significant for all characters. The male effect was highly significant for all traits and higher than the female effect except for plant height. The additive variance was higher than the dominance variance for all characters except for plant height. The inheritance was predominantly additive. The highest narrowsense heritabilities were found for NDF and ADF and leaf-to-stem ratio. As a consequence, NDF or ADF would be more efficient selection criteria than enzymatic solubility in a breeding program for improved feeding value. In both mating designs, NDF, ADF, and ADL were positively correlated with one another and negatively with enzymatic solubility and protein content.

-Proteins in forage legumes are rapidly degraded in the rumen, inducing a poor dietary protein efficiency, risk of bloat, and nitrogen loss detrimental to the environment. The aim of this study was to investigate the variation in ruminal protein degradability among legume species and cultivars. Four species, each represented by 1 to 16 cultivars, were studied: lucerne (Medicago sativa), white clover (Trifolium repens), birdsfoot trefoil (Lotus sp.) and crownvetch (Coronilla varia). In a first experiment, forage samples of 16 cultivars of lucerne harvested in the autumn of 1998 were incubated in nylon bags in 3 fistulated cows in order to obtain the kinetics of ruminal degradation of crude protein and dry matter. The phenotypic variation in crude protein degradation was not significant, and was partly related to the dry matter degradation. In a second experiment, lucerne (5), birdsfoot trefoil (5), white clover (4) and crownvetch (1) cultivars were harvested in two cuts in 2000, and dry matter and crude protein degradation were analysed at 3 incubation times (2, 8 and 48 h) in the rumen of the fistulated cows. Crude protein degradation was higher for lucerne than for white clover, and these two tannin-free species exhibited greater crude protein degradation than crownvetch and birdsfoot trefoil. In birdsfoot trefoil, crude protein degradation was negatively correlated to condensed tannin content but positively correlated to dry matter degradation. Except for birdsfoot trefoil, the range of genetic variation within species for in situ crude protein degradation was low. des échantillons de 16 variétés de luzerne récoltés durant l'automne 1998 ont été introduits dans des sachets de Nylon et placés dans le rumen de trois vaches fistulées pour obtenir la cinétique de dégra-dation ruminale des protéines et de la matière sèche. La variabilité phénotypique pour la dégradabilité des protéines n'était pas significative, et elle était en partie reliée à la dégradation de la matière sèche. Dans une seconde expérience, des variétés de luzerne (5), de lotier corniculé (5), de trèfle blanc (4) et de coronille (1) ont été récoltés en deux coupes en 2000, et les dégradations des protéines et de la matière sèche ont été analysées à trois temps d'incubation (2, 8 et 48 h) dans le rumen de vaches fistulées. La dégradation des protéines était plus élevée pour la luzerne que pour le trèfle blanc, et ces deux espèces sans tannin avaient une plus forte dégradation des protéines que le lotier et la coronille. Pour le lotier, la dégradation des protéines était corrélée négativement à la teneur en tannins condensés, mais corrélée positivement à la dégradation de la matière sèche. A l'exception du lotier, la variabilité à l'intérieur d'une espèce pour la dégradabilité des protéines in situ était faible. dégradation des protéines / dégradation de la matière sèche / Medicago sativa / Lotus sp. / Trifolium repens / Coronilla varia

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Abstract.-Alfalfa is a forage legume for which vegetative biomass (stems and leaves) is harvested. Lignification of the oldest internodes of the stems contributes to reduced stem digestion by ruminants, but also glucose recovery in biofuel production. We described and quantified morphological and histological structure of the stem of two genotypes. At the top of the stems, four to six internodes had increasing length and diameter, non-lignified tissues and separated vascular bundles. The following internodes had a constant or decreasing length, but an increasing diameter from the top to the bottom of the stems. In parallel, xylem thickness and pith parenchyma area increased, but the cortex thickness remained almost constant. The cell walls of the xylem were progressively thicker. These results can be used in other studies aiming at describing genetic or environmental variation for stem histological traits.Key words : Medicago sativa -histology -image analysis -morphology -stem.Résumé.-La luzerne est une légumineuse fourragère dont on récolte la biomasse végétative (tiges et feuilles). La lignification des entre-noeuds les plus âgés des tiges contribue à réduire la digestion des tiges par les ruminants, mais aussi l'extraction de glucose pour la production de biocarburants. Nous avons décrit et quantifié la structure morphologique et histologique des tiges de deux génotypes. En haut des tiges, quatre à six entre-noeuds ont des diamètres et des longueurs en augmentation, des tissus non lignifiés et des vaisseaux conducteurs séparés. Les entre-noeuds suivants ont une longueur constante ou décroissante, mais un diamètre qui augmente depuis le haut vers le bas des tiges. En parallèle, l'épaisseur du xylème et la surface de parenchyme médullai-re augmentent, mais l'épaisseur du cortex reste constante. Les parois cellulaires du xylème s'épaississent progressivement. Ces résultats pourront être utilisés dans d'autres études cherchant à décrire la variabilité d'origine génétique ou environnementale des caractères histologiques des tiges de luzerne.