In situ protein degradation of alfalfa and birdsfoot trefoil hays and silages as influenced by condensed tannin concentration

Authors: Coblentz, Wayne; Grabber, John

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2013
Publication Date: 4/22/2013
Citation: Coblentz, W.K., Grabber, J.H. 2013. In situ protein degradation of alfalfa and birdsfoot trefoil hays and silages as influenced by condensed tannin concentration. Journal of Dairy Science. 96:3120-3137.

Interpretive Summary: Although alfalfa and other legumes are known for their high crude protein concentrations, this valuable nutritional resource often is used inefficiently by ruminants, such as dairy cows, because much of the protein is degraded in the rumen, therefore less of it is used by the cow. This can create an environmental burden because unused protein nitrogen is voided from the cow (primarily in the urine). Condensed tannins, which are found naturally in the legume birdsfoot trefoil but not in alfalfa, may improve the efficiency of protein use by slowing and/or limiting protein degradation in the rumen. In this study, 24 alfalfa and birdsfoot trefoil hays and silages were evaluated. Depending on the computational strategy used, estimates of protein degradation in the rumen decreased at a rate of 3.2 to 4.6 percentage units of crude protein for each percentage unit of condensed tannin within the legume forage. Generally, this rate was consistent across hays and silages; however, overall protein digestion for silages was greater than for hays. Within the limits of this experiment, condensed tannins modestly limited protein degradation in the rumen and may offer some potential for improving efficiency of crude protein use within ruminants.

Technical Abstract: Dairy cattle often make poor use of protein when offered diets comprised of high proportions of alfalfa (Medicago sativa L.) hay or silage because non-protein N (NPN) formed during forage conservation and ruminal fermentation exceeds requirements for rumen microbial protein synthesis; however, condensed tannins (CT) are known to improve the efficiency of CP use within ruminant diets. Two harvests yielding 12 hays and 12 silages made from alfalfa and birdsfoot trefoil (Lotus corniculatus L., BFT) that varied in concentrations of CT were evaluated for in situ disappearance kinetics of CP within 6 ruminally cannulated lactating Holstein dairy cows (627 ± 56.3 kg). Prior to conservation, alfalfa contained no detectable CT, while CT in fresh lyophilized BFT ranged from 0.77 to 1.85% of DM. Concentrations of CT were determined by fully extracting and quantifying CT with a modified acetone-butanol-HCl assay, using a BFT standard of 66% purity as determined by thiolysis. Percentages of CP remaining at each incubation time were fitted to nonlinear regression models with or without a discrete lag time. When calculated with a ruminal passage rate of 6%/h, effective ruminal disappearance of CP (RDP) calculated by all models declined linearly with increasing CT concentrations (r2 = 0.615 to 0.971). Generally, tests of homogeneity showed that conservation type (hay or silage) or harvest (silage only) affected intercepts, but not slopes within regressions of RDP on CT. A significant positive relationship of lag time with CT concentration suggested that a lag adjustment (LADJ) should be included within calculations of RDP. Regression intercepts were mainly affected by conservation method and estimates of RDP calculated with a lag adjustment (RDPLADJ) averaged 77.5 and 88.7% of CP for hay and silage, respectively, when no CT was present. Greater estimates of RDP for silages were related to extensive proteolysis in laboratory silos resulting in conversions of protein into NPN forms, which readily washed out of Dacron bags. When both RDPLADJ and CT were expressed on a CP basis, regression slopes indicated each unit of CT protected 0.92 units of CP from ruminal degradation in both hays and silages. Applying this relationship to a typical mid-maturity forage legume containing 21% CP suggests that a CT concentration of 2.5% of DM would be required to reduce RDP from 81% to a 70% target considered optimal for improving protein utilization and milk yields by dairy cattle.