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

Authors: Grabber, John

Submitted to: Animal Feed Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2009
Publication Date: 2/26/2009
Citation: Grabber, J.H. 2009. Protein Fractions in Forage Legumes Containing Protein-binding Polyphenols: Freeze-drying vs. Conservation as Hay or Silage. Animal Feed Science And Technology. 151:324–329.

Interpretive Summary: Alfalfa, the fourth most widely grown crop in the United States, is highly valued as forage, but unfortunately livestock such as cattle and sheep poorly utilize its protein. In the future, feeding alfalfa engineered to express protein-binding polyphenols (e.g. condensed tannins or o-quinones) or feeding forages naturally producing these compounds should greatly improve protein use by livestock. In most studies, polyphenols are measured in herbage that has been quickly frozen after cutting and then slowly dried at low temperature and high vacuum (freeze drying). Forages processed in this manner are then frequently analyzed for various protein fractions to predict how polyphenols would impact protein use by livestock. In this study we evaluated whether the analysis of freeze-dried herbage accurately reflects protein fractions in alfalfa and several polyphenol-containing forages conserved as hay or silage. As in previous work, we found that polyphenols in freeze dried herbage shifted protein fractions to less degradable forms that should be used more efficiently by livestock. The analysis of freeze-dried herbage gave reasonably accurate and unbiased predictions of only the most stable protein fractions in hay and silage but poor predictions of more labile protein fractions. Our results, demonstrate that the analysis of freeze-dried herbage will not accurately reflect protein characteristics of forages fed as hay or especially silage. These findings will help scientists to develop improved forages and feeding systems to enhance nutrient use by livestock farms.

Technical Abstract: We compared protein fractions in freeze-dried herbage to hay or silage of forage legumes containing about 200 g/kg of crude protein. Protein was partitioned with buffer and detergents into rapidly (A and B1), moderately (B2), and slowly (B3) degraded and undegradable acid-detergent insoluble protein (ADIP) fractions. Rumen undegradable protein (RUP) was calculated from protein fractions and estimated in vitro with Streptomyces griseus protease. On average, crude protein in freeze-dried Medicago sativa without polyphenols contained 380 g/kg of A plus B1, 523 g/kg of B2, 68 g/kg of B3, 28 g/kg of ADIP and 256 g/kg of RUP. Condensed tannins in Lotus corniculatus and o-diphenols in Trifolium pratense shifted protein fractions to less degradable forms, increasing RUP by up to 30% compared to Medicago. Analysis of freeze-dried herbage gave reasonably accurate (R2  0.80) and unbiased predictions of ADIP in hay and silage and RUP in hay but poor predictions of other protein fractions. Based on RUP estimates, conservation as hay or silage rather than freeze-drying enhanced protein protection by o-diphenols in Trifolium but not tannins in Lotus. Although commonly practiced, the analysis of freeze-dried polyphenol-containing forages will not accurately reflect most protein fractions in forages fed as hay or especially silage.