Effects of phlorotannins from Ascophyllum nodosum (brown seaweed) on in vitro ruminal digestion of mixed forage or barley grain

The role of terrestrial tannins in livestock nutrition has been extensively investigated, but information on effects of tannins from marine sources on nutrient utilization is limited. This study investigated effect(s) of phlorotannins (PT) from the seaweed Ascophyllum nodosum on ruminal fermentation of mixed forage or barley grain. In 125-ml serum vials, buffered ruminal fluid inoculum (40 ml) was added to barley silage + alfalfa hay mixtures (500 mg/vial; Exp. 1) or ground barley grain (300 mg/vial; Exp. 2), together with crude seaweed extract in quantities to provide 0, 125, 250 or 500 μg PT/ml of medium, and polyethylene glycol (PEG; MW 3500) at 0 or 1 mg/ml. Gas production was measured at 0, 2, 4, 6, 8, 10, 12 and 24 h. Concentrations of volatile fatty acids (VFA) and ammonia, as well as in vitro digestibilities of neutral detergent fibre (aNDF; Exp. 1) and starch (Exp. 2) were determined at 0, 12, 24 and, Exp. 1 only, 48 h (n = 3). Phlorotannins inhibited fermentation (P<0.05; linear), but PEG eliminated this response. Gas production and aNDF digestibility with mixed forage were greatly reduced by PT, with a minimal inhibitory concentration of 10 μg/ml as estimated by regression. With barley grain, regression analysis suggested that gas production and starch digestibility were inhibited at PT concentrations of 100 and 50 μg/ml, respectively. In Exp. 3, the forage mixture (500 mg) was incubated with PT at 0 or 500 μg/ml and PEG at 0 or 1 mg/ml, with 15N-labeled casein at 0.06 (w/v). Methane and total gas production, and concentrations of VFA and ammonia were measured at 0, 4, 8, 12 and 24 h (n = 3). Net production of ammonia-N from casein (i.e., 15N-NH3) was lower (P<0.05) with PT than without, but was not altered by PT + PEG. Methane and total gas production were reduced (P<0.001) by PT throughout the 24-h incubation. With both diet types, in vitro ruminal fermentation and protein degradation were reduced by PT in a dose-dependent manner.