Benefits of condensed tannins in forages fed to ruminants: importance of structure, concentration and diet

Condensed tannins (CTs, syn. proanthocyanidins) account for up to 13% of the dry matter in forage legumes used as ruminant feeds, such as sainfoin (Onobrychis viciifolia. Scop), sulla (Hedysarum coronarium), sericea lespedeza (Lespedeza cuneata), Lotus spp (e.g birdsfoot trefoil and big trefoil), the flowers of white clover, as well as seed coats of Lucerne. They also occur in weeds (e.g. Rumex), many herbaceous plants and tree/shrub species. Concentrations are usually higher in leaves than stems, but this does not deter their consumption by ruminants.

Animal responses to CTs have included improved growth, milk production, fertility, wool growth and reduced methane emissions and ammonia volatilization from dung or urine. Perhaps most important are the ability of forages with CTs to combat the effects of some gastro-intestinal (GI) parasites, and even kill the nematodes as well as reduce egg hatching, larval development, exsheathment and fertility. However, benefits may be variable and are not universal, especially when dietary protein concentrations are low. Inconsistent animal responses to CTs were initially attributed to concentration in the diet, but recent research has highlighted the importance of their molecular structures, as well as concentration, and also the composition of the diet containing the CTs.

The importance of tannin structural traits cannot be underestimated; one structure will kill GI nematodes, another will have no effect. Interdisciplinary research is the key to unraveling the relationships between tannin traits and bioactivities, whether positive or negative, and will enable future on-farm exploitation of these natural plant compounds. Research is also needed to provide plant breeders with guidelines and screening tools to optimize tannin characteristics and concentrations, in both the forage and in the whole diet. In addition, the competitiveness and agronomic traits of tannin-containing legumes needs improvement and a better understanding of options for their inclusion in ruminant diets. Farmers need varieties that are competitive in mixed swards and have predictable bioactivities.

This review focuses on recent results from multidisciplinary research involving sainfoin, but also provides an overview of current developments with other tanniniferous forages. Tannin chemistry is now being linked with agronomy, plant breeding, animal nutrition and parasitology. The past decade has yielded significant progress, but also generated more questions; an enviable consequence of new knowledge!