Protein fractions in forage legumes containing protein-binding polyphenols: Freeze-drying vs. conservation as hay or silage

Herbaceous legumes containing protein-protecting polyphenols are frequently freeze-dried for polyphenol and forage quality analyses. In this study, Medicago sativa, Lotus corniculatus, and Trifolium pratense with ∼200 g/kg crude protein were fractionated with buffer, detergents, and protease to assess the association of protein fractions in freeze-dried herbage to hay or silage. Compared to Medicago, freeze-dried Lotus with condensed tannins (CT) and Trifolium with o-quinones had lower concentrations of buffer-soluble proteins and similar or greater concentrations of neutral-detergent insoluble protein, acid-detergent insoluble protein, and rumen-undegradable protein. Analysis of freeze-dried herbage gave reasonably accurate (R2 ≥ 0.80) and unbiased predictions of acid-detergent insoluble protein in hay and silage and of rumen-undegradable protein in hay, but poorer predictions of other protein fractions. Based on rumen-undegradable protein estimates, conservation as hay or silage rather than freeze-drying enhanced protein protection by o-quinones in Trifolium but not CT in Lotus. Although commonly practiced, the analysis of freeze-dried forages will not necessarily represent most protein fractions in forages fed as hay or especially silage.