Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 2008
Publication Date: July 31, 2009
Citation: Grabber, J.H., Coblentz, W.K. 2009. Polyphenol, Conditioning, and Conservation Effects on Protein Fractions and Degradability in Forage Legumes. Crop Science. 49:1505-1516. Interpretive Summary: Valued as a high protein forage for livestock, alfalfa is the fourth most widely grown crop in the United States. Unfortunately, alfalfa protein undergoes excessive breakdown prior to gastrointestinal digestion, severely impairing protein use by livestock. To mitigate poor protein use, alfalfa is often substituted with feeds possessing more desirable protein characteristics but this can increase feed costs, impair animal health, or increase reliance on energy intensive and potentially polluting row crops by farms. In the future, the expression of protein-binding polyphenols such as condensed tannins and o-quinones in alfalfa should provide a sustainable approach for improving protein use by dairy cattle. In this study, alfalfa and several polyphenol-containing forage legumes were extracted with buffer and detergents, digested with a commercial protease, or incubated in the rumen of dairy cows to assess how conditioning and conservation methods interact with polyphenols to alter crude protein degradability. The presence of polyphenols, shifting from roll conditioning to maceration, and conservation as hay rather than silage shifted protein fractions from readily degraded to slowly degraded forms. Estimates of protein degradability, however, often differed considerably between chemically-based and biologically-based analytical methods. Therefore improvement of protein degradability assays is needed to properly formulate diets to maximize forage protein utilization by dairy cattle.
Technical Abstract: Forage legume proteins were fractionated by the Cornell Net Carbohydrate and Protein System or ruminally incubated to assess how conditioning and conservation methods interact with polyphenols (condensed tannins or o-quinones) to alter protein degradability. 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. Rumen undegradable protein (RUP) calculated from protein fractions for roll-conditioned hays averaged 281 g kg-1 for alfalfa (Medicago sativa L.), 309 g kg-1 for high tannin trefoil (Lotus corniculatus L.), and 352 g kg-1 for o-quinone containing clover (Trifolium pratense L.). Roll-conditioned silages had lower RUP, averaging 132 g kg-1 for alfalfa, 161 g kg-1 for high tannin trefoil, and 241 g kg-1 for clover. Maceration increased calculated RUP by 67 to 124 g kg-1; responses were greatest in clover and high tannin trefoil. Rumen in situ residual protein (RP) was comparable to calculated RUP for silage, but ~75 g kg-1 higher for hay. Hay RP also indicated a greater impact of tannins and a smaller impact of maceration on protein degradability. Discrepancies between calculated RUP, in situ RP and previous protease RUP estimates indicate that routine methods for estimating RUP must be refined so that polyphenol-containing forages can be properly characterized for feeding to ruminant livestock.