Tannin variation in tree fodder from temperate climates and implications for methane emissions from enteric fermentation

A field trial evaluated the effects of tree species, plant fractions, and season on the dry matter degradability ( DMd ) and tannin profiles of three tree species ( Salix caprea, goat willow ( GW ); Quercus robur, common oak; Acer campestre. , maple). Leaf and twig samples were collected monthly from five trees per species between June and September in Berkshire, UK. Total condensed tannins ( CTs ) concentrations were determined using a Butanol−HCl assay and tannin profiling (mean degree of polymerization ( mDP ), and concentrations of procyanidins ( PC ), prodelphinidins ( PD ), and cis - and trans - flavan-3-ols) were measured via in situ thiolysis assay. In vitro total DMd was determined using the ANKOM Daisy II system. Data were analysed using linear mixed models with tree species, plant fractions, month, and their interactions as fixed factors, and tree ID (nested within species) as a random factor. Goat willow had the highest (P<0.05) DMd, total CTs concentrations, mDP, and trans flavan-3-ols. Oak had the highest (P<0.001) PD concentrations. Maple had the highest final DM, soluble losses, and concentrations of PC, and cis flavan-3-ols. Across all tree species, leaves had a higher (P<0.001) DMd, soluble losses, and total CTs concentrations, and lower DM (P<0.001) and mDP (P=0.043) concentrations than twigs. Soluble losses and DMd increased (P<0.001) with season, from June to September, while the highest (P=0.001) DM content was observed in August. Tannin profiles did not vary with seasons, except for the highest (P<0.001) mDP concentrations in June compared with other months. The impacts of high-PC (96% in CTs; HPC) and high-PD (73% in CTs; HPD) GW on total gas and methane ( CH 4 ) production were assessed in vitro . Experimental diets were 100% grass silage (GS, control), HPC (80:20 grass silage:HPC GW), HPD (80:20 grass silage: HPD GW), plus two diets with the addition of polyethylene glycol ( PEG ) in the HPC and HPD to neutralise tannins. Diets were incubated for 72 h in rumen fluid: medium at 1:9 v/v in triplicate. Gas pressure (psi transducer) and CH 4 concentrations (gas chromatography) were measured throughout. Volatile fatty acids ( VFA ) concentrations and DMd were assessed at 72 h. Data were analysed by linear mixed models using dietary treatment as a fixed factor and batch run (1-3) as a random factor. HPC, HPD, HPC+PEG, and HPD+PEG diets had lower (P<0.001) DMd and 72 h production of cumulative gas and CH 4 than Grass silage ( GS ). Grass silage produced more (P<0.001) total VFA and acetic acid concentrations than the four GW diets and less (P<0.001) butyric acid than HPD. This study concludes that the inclusion of GW reduced in vitro CH 4 production, partly due to its tannin content, with PC showing an improved potential. Species and fraction-specific differences in tannin profiles influenced the nutritive value and CH 4 mitigation potential of tree fodder, and these need to be accounted for when tree fodder is introduced into animal diets. • Tannins composition varies between tree species and plant fractions • Maple had more procyanidins (PC) and less prodelphinidins (PD) than goat willow and oak • Diets containing tree fodder were less degradable in vitro • Both High-PC and High-PD fodder reduced in vitro methane emissions, with High-PC showing stronger effect