The structural diversity of condensed tannins in willows (salix spp.): a first screening to improve the nutritional quality of ruminant products

Introduction Condensed tannins (CT) are polyphenols widely investigated in recent plant and animal research. Forage
plant CT can in fact interact with animal physiological processes and with the metabolism of the microbial population in the rumen. Both these processes can generate useful benefits for ruminant nutrition and health, productivity, and product quality (Vasta et al., 2010). However, some tannins can also have dose-dependant negative nutritional effects. Thus, all tannins were often mistakenly described in the past as anti-nutritional factors. Actually, plants produce a wide range of tannin structures, differing enormously between plant species, varieties and plant parts. This explains why some tannin-containing forages are much more effective than others. Therefore, investigations into the beneficial or detrimental effects of CT need to consider all aspects of their structural variation. Within the EU project ‘Tannin StrACTure QTLs’, willow (Salix spp.) has been selected as model plant to investigate the biological effects of plant CT structural traits. The UK National Willow Collection, maintained at the Rothamsted Research Institute, is a worldwide and unique willow germplasm collection that currently includes about 1300 accessions representing over 100 different willow species. Willows contain CT (2-3% of leaf dry matter), and their leaves and twigs have been used in the past as ruminant feeds and feed supplements in extensive systems in Northern Europe (Waller et al., 2001) and Bhutan (Roder, 1992), while researchers from New Zealand have investigated their potential as substitutive feed to deal with summer drought (Moore et al., 2003). As a first step in order to evaluate the potential use of willows as novel sustainable feed and the biological effect of the different CT structural traits, this work reports the CT composition of 18 diverse accessions of the UK National Willow Collection, determined by direct thiolysis. This method is suited to quantify the variable proportions as terminal or extension units of the CT monomers, named flavan-3-ols (catechin, epicatechin, gallocatechin, and epigallocatechin), that define CT structural traits.
Materials and methods Leaves from 18 accessions of willow were collected on July 2010 from the National Willow
Collection (BBSRC Rothamsted Research, Harpenden, Hertfordshire, UK), then freeze-dried for 48 hours, ground to pass a 0.5 mm sieve, and stored in the darkness. The thiolysis extraction was performed on the dried-leaves samples (200 mg
each) after prewashing in acetonitrile to remove free catechin. Then, after HPLC separation, CT structural information (Total extracted condensed tannins, mean Degree of Polymerization – mDP, Procyanidin/Prodelphinidin ratio - PC/PD, and cis/trans ratio) were obtained according to Gea et al., 2011.
Results Results showed a great and partially unexpected range of variability for each tannin trait among the accessions.
Total CT content varied from 0.30 (accession n:1000, Salix myrsinifolia) to 2.50 g\100g DM (n:449, S. appendiculata).
mDP, related to CT molecular weight, varied from 4.1 (n:1000, S. myrsinifolia) to 26.1 (n:987, S. hookeriana). PC/PD ratio
varied from 14.8/85.2 (n:99, S. triandra) to 97.7/2.3(n:956, S. eriocephala). Also cis/trans ratio showed large variations
among accessions, ranging from 1.7/98.3 (n:99, S.triandra) to 81.3/18.7 (n:945, S. myricoides).
Conclusions This screening has demonstrated that a wide range of different combinations of CT traits exists in willow
germplasm. This structural richness is coupled to a CT concentration that is close to the optimum considered for feed to potentially obtain benefits for animal health and nutrition (Dixon et al., 2005).
Further investigations are therefore planned in the project “Tannin StrACTure QTLs” to link willow CT structural traits
and their biological activity. Future results will be useful for the researchers investigating how to optimize in a sustainable way novel feeds for ruminant nutrition, in order to improve animal health, productivity, and the quality of derived products.
Acknowledgements The research project ‘Tannin StrACTure QTLs’ is funded by the European Union (FP7/2007-2013)
under grant agreement n PIEF-GA-2009-253905.
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