Gastrointestinal nematodes remain a major constraint on the health, welfare and production of small ruminants. Over the past decades, the usual mode of control of this parasitism has mainly relied on the repeated use of chemical anthelmintics. However these treatments are nowadays facing some limits among which the most important is the development and widespread diffusion of resistance to these chemical molecules within worm populations. Consequently, the need to find complementary or alternative solutions is becoming urgent. The possible exploitation of forage legumes, rich in condensed tannins, with anthelmintic properties, by incorporation in the diet of sheep or goats, seems a promising option to reduce the reliance on chemical molecules. However, one of the main difficulties to use these plants as nutraceuticals relates to the variations in results. By using sainfoin (Onobrychis viciifoliae) as a model of tannin-containing legume and based on in vitro methods on the infective third stage larvae, the main objectives of this PhD were i) to examine the influence of environmental, genetic (40 different varieties) and technological (mode of preservation) factors on the anthelmintic properties of sainfoin and ii) to analyse whether differences in phenolic compounds might explain the variations. Whatever the factor considered, a high variability in results was observed. The main variations due to the environmental factors depended on the year and the cycle of cutting, as well as on the site of cultivation. Among the 38 varieties tested, only 9 have shown AH activity over 50 %. Last, a higher antiparasitic activity was found in the dried or ensiled forms compared to the fresh samples. A comparison of the biochemical profiles associated with these variations indicated a role of proanthocyanidins plus other phenolic compounds in the anthelmintic properties. The dose-response relationship between the AH activity and the ability to form complex with proteins was defined. The role of condensed tannins was confirmed, particularly those with a low degree of polymerization. The respective importance of prodelphinidins vs procyanidins remains to be further investigated. Moreover, the possible role of other flavonoids, in particular of flavan-3-ols and flavonols was also confirmed. The difference in activity between dried or ensiled forms compared to fresh sainfoin samples was partly explained by the presence of flavonol aglycosides. These results should favour the development of measurements in order to identify sainfoin samples with higher anthelmintic properties.