Polyphenol, Conditioning, and Conservation Effects on Protein Fractions and Degradability in Forage Legumes

Authors: Grabber, John; Coblentz, Wayne

Submitted to: American Forage and Grassland Council Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 5/15/2009
Publication Date: 6/21/2009
Citation: Grabber, J.H., Coblentz, W.K. 2009. Polyphenol, Conditioning, and Conservation Effects on Protein Fractions and Degradability in Forage Legumes. In: Proceedings of the 2009 American Forage and Grassland Council Annual Conference, June 21-24, 2009, Grand Rapids, Michigan. 2009 CDROM.

Interpretive Summary:

Technical Abstract: Alfalfa herbage contains excessive levels of proteins that are highly susceptible to proteolysis during ensiling and rumen fermentation. As a result, only 10 to 30% of the protein in alfalfa, principally membrane proteins of inferior nutritional value, undergoes direct gastrointestinal digestion and absorption by ruminants. Therefore, approaches for increasing rumen-undegradable protein (RUP) in alfalfa should improve the performance and sustainability of ruminant-based farming systems. Although alfalfa currently lacks protein-protecting polyphenols, the expression of condensed tannins or polyphenol oxidase with o-diphenols (to form o-quinones) could provide a sustainable approach for increasing RUP in alfalfa-based diets. Previous work also indicates RUP in alfalfa can be modestly enhanced by increased severity of conditioning at harvest and by conserving forage as hay rather than silage. In this study, our first objective was to determine how protein-protecting polyphenols, conditioning methods, and conservation methods interact to influence RUP in forage legumes. Our findings should provide insight into how natural or engineered expression of tannins or o-quinones may influence RUP in conserved forage legumes. A second objective was to assess the consistency of RUP values as estimated by different methods. In 2002 and 2003, first and second cuttings from replicated plots of alfalfa, red clover, and birdsfoot trefoil were conventionally roll conditioned or severely macerated and then conserved as silage or hay. Forage proteins were fractionated with buffer and detergents according to the Cornell Net Carbohydrate and Protein System. Forages were also incubated in situ ruminally for 10 h or in vitro with Streptomyces griseus protease for 16 h. The presence of polyphenols, conditioning by maceration rather than rolls, and conservation as hay rather than silage shifted protein fractions from buffer soluble to detergent extractable forms. As a proportion of crude protein, RUP calculated from protein fractions for roll-conditioned hays averaged 28% for alfalfa, 31% for high tannin trefoil, and 35% for o-quinone containing clover. Roll-conditioned silages had lower calculated RUP, averaging 13% for alfalfa, 16% for high tannin trefoil, and 24% for clover. Maceration increased calculated RUP by 7 to 12 percentage points; responses were greatest in clover and high tannin trefoil. Compared to calculated RUP, the in-situ method gave comparable RUP estimates for silage, but higher RUP estimates for hay. The in situ method also indicated a greater impact of tannins and a smaller impact of maceration on RUP in hays. To further confound matters, the protease assay suggested smaller RUP differences between hay and silage and greater impacts of tannins and o-quinones on in roll-conditioned hays and silages than calculated RUP estimates. The protease assay also indicated more pronounced species-specific and conservation effects of maceration than calculated RUP, most notably a negative effect of maceration on clover RUP. Clearly, these results underscore the need to refine and validate methods for estimating RUP, so that forages with natural or bioengineered protein-protecting polyphenols can be properly characterized for feeding to ruminant livestock.