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FORAGE & GRAZINGLANDS

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

^a U.S. Dairy Forage Research Center, USDA-ARS, Madison, WI 53706
^b Institute for Environmentally Integrated Dairy Management, USDA-ARS, Marshfield, WI 54449. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable

^* Corresponding author (john.grabber{at}ars.usda.gov).

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 birdsfoot trefoil (Lotus corniculatus L.), and 352 g kg^–1 for o-quinone–containing red 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.

Abbreviations: A, nonprotein N soluble in buffer–tungstic acid • ADF, acid detergent fiber • ADIP, acid-detergent insoluble protein • B1, buffer-soluble protein precipitated by tungstic acid • B2, buffer-insoluble protein extracted with neutral detergent • B3, protein insoluble in neutral detergent but extracted with acid detergent • calculated RUP, rumen-undegradable protein calculated from B1, B2, sequential B3, and sequential ADIP fractions • CP, crude protein • CT, condensed tannins • DM, dry matter • NDF, neutral detergent fiber • NDIP, neutral-detergent insoluble protein • NPN, nonprotein N • protease RUP, rumen-undegradable protein estimated with an in vitro protease procedure • RP, residual protein remaining after a 10-h in situ rumen incubation • RUP, rumen-undegradable protein • sequential ADF, acid detergent fiber determined after neutral-detergent extraction • sequential ADIP, acid-detergent insoluble protein determined by sequential detergent analysis • sequential B3, neutral-detergent insoluble protein subsequently extracted with acid detergent • SP, buffer-soluble proteins