212. Ruminal and Gastrointestinal Degradability of Protein in Alfalfa and Tannin-containing Birdsfoot Trefoil Estimated by an Improved Three-step Procedure.

Abstract Birdsfoot trefoil (BFT, Lotus corniculatus L.) fed as pasture often contains a desirable type and quantity of condensed tannins (CT) for improving the utilization of crude protein (CP) by ruminant livestock. However, dairy cattle are usually less responsive to BFT fed as hay or silage, and this might be related to the relative partitioning of rumen-undegradable protein (RUP) into gastrointestinal-degraded protein (GIDP) and truly undegradable protein (UP). To investigate this, we prepared hay and silage from two cuttings of one alfalfa (Medicago sativa L.) cultivar lacking CT and five BFT cultivars containing up to 3.1% CT on a dry matter (DM) basis and subjected them to a three-step in-situ ruminal incubation followed by an in-vitro gastric and intestinal digestion with enzymes. Linear relationships between various CP fractions and CT, expressed as a percent of CP, were examined at P = 0.05, using a means model for intercepts and an effects model for slopes. A standard 0-h washing machine extraction of samples within in-situ bags indicated insoluble CP increased from 45.3 to 51.4% in hay and from 18.7 to 32.0% in silage as CT increased from 0 to 16% of CP. Buffer extraction and filtration revealed that an average of 8.3 percentage points of particulate-CP was washed out from in situ bags. The three-step procedure indicated UP increased from 4.5% in alfalfa to 10.5% in BFT containing 16% CT on a CP basis. A 2-point kinetic model, based on the disappearance rate of insoluble degradable CP between 0- and 12-h of ruminal incubation, was developed to more accurately estimate RUP in the three-step procedure. Two-point RUP increased from 15.3 to 25.3% in hay and from 9.9 to 17.9% in silage while GIDP increased from 10.9 to 13.9% in hay and from 5.5 to 8.1% in silage as CT increased from 0 to 16% of CP. Correction for washout of particulate-CP increased 2-point RUP and GIDP by an average of 4.2 percentage points. Hay and silage had similar slopes and intercepts for UP, and similar slopes but differing intercepts for RUP and GIDP. Our results indicate 40% of the RUP formed by CT was GIDP while 60% was UP having no nutritional value. Overall, our study suggests CT are an inefficient means for increasing GIDP from conserved forage. This suggests other approaches, such as formulating diets with a proper balance of CT-free forage, RUP supplements, and other feedstuffs may be more effective for improving the protein use efficiency and productivity of ruminant livestock.