Estimating Rumen Degradable Protein in Forage Legume Hays and Silages by in Situ Disappearance Kinetics Verses Alternative Methods.

Currently, the in situ kinetic method is considered the Ògold standardÓ for estimating rumen degradable protein (RDP) in forages. When such estimates are impractical (e.g. numerous samples, limited sample quantities, or lack of ruminally fistulated cattle) alternative RDP methods are used, but their suitability for forages with diverse polyphenol compositions and conserved by various methods requires validation. Our objective was to compare RDP estimates derived from in situ kinetics to alternative laboratory or simpler in situ methods. In this study, alfalfa, red clover with o-quinones, and birdsfoot trefoil with condensed tannins from a summer harvest in 2003 were roll conditioned or macerated and conserved as silage or hay. Forage RDP for a 0.06/h ruminal passage rate was estimated from 0 to 72 h in situ degradation kinetics, a 10-h in situ incubation, chemical fractionation by the Cornell Net Carbohydrate and Protein System (CNCPS), or a 16-h in vitro incubation with Streptomyces griseus protease. All forages contained about 230 g/kg of crude protein. In situ kinetic RDP estimates ranged from 534 to 879 g/kg crude protein and were reasonably (P < 0.01) predicted from CNCPS fractions (y = 0.985x + 1.65, R² = 0.86) or a 10-h in situ incubation (y = 0.814x + 204, R² = 0.89). Using a longer incubation time could likely reduce bias in single time-point in-situ values. The current protease method poorly predicted in situ kinetic RDP (R² = 0.24); predictions of RDP in macerated red clover were particularly poor.