Ensiling willow (Salix acmophylla) fodder modifies the contents of plant specialized metabolites, but not nutritional attributes

Abstract

Willow (Salix spp.) is grown for energy biomass in temperate regions and is also a promising source of feed for ruminants. Willow fodder contains high concentrations of beneficial plant specialized metabolites (PSMs). We investigated the efficacy of ensiling willow fodder in 1.5 L jars without (control-C) or with silage additives (lactobacilli-LAB, molasses-M, or the combination MLAB), and the effects on nutritional attributes of ensiling willow in 60 L drums with LAB. We also assessed the effect of ensiling willow fodder on PSMs by liquid chromatography / time-of-flight / mass spectrometry (LC-TOF-MS). Exact or tentative compound identification was established, depending on availability of appropriate standards. In jars, LAB-supplemented silages reached lowest pH (4.2). M-supplemented silages had lower aNDF and aADF contents. IVDMD was higher in M than in C, and the contents of crude protein (CP) were not affected by silage treatments. Ensiling in drums did not affect the major nutritional components of willow fodder. Even though phenylalanine, arginine, tyrosine, histidine, and methionine, but not tryptophan, were increased 3.8–7.9-fold by ensiling, the finding is of low importance, as free amino acids are totally degraded in the rumen. In contrast, the main PSM, i.e., the salicinoid salicin was 74 % de-glycosylated, concomitantly with a ten-fold increase in salicylic acid concentration and with the appearance of acetyl-salicylic acid; likewise, the flavonoids hyperin and rutin were 66 % de-glycosylated, consequent with a 48-fold increase in quercetin concentration. As the bio-availability of the quercetin aglycone is inferior to that of its glycosylated counterparts, ensiling might affect the nutraceutical value of willow fodder. Likewise, catechin, gallocatechin and epicatechin were totally metabolized by ensiling. Overall, ensiling willow fodder (with LAB) does not affect the nutritional attributes of willow fodder but it greatly alters its PSM concentrations and, possibly impairs its nutraceutical value. However, as anti-oxidative activity is supposed to increase, following de-glycosylation, further research is needed to evaluate a possible trade-off on nutraceutical value between the glycosylated and aglycone forms of phenolics contained in willow fodder.