Effect of Tannins in Acacia nilotica, Albizia procera and Sesbania acculeata Foliage Determined In vitro, In sacco, and In vivo

The nutritive value and the effect of tannins on the utilization of foliage from three commonly used legumes, Acacia nilotica, Albizia procera, and Sesbania acculeata, were determined. Three mature rumen-fistulated bullocks were used to study in sacco degradability and twelve adult sheep were randomly allocated on the basis of live weight to 4 groups of 3 in each to study the in vivo digestibility of the foliages. In all foliages, the contents of crude protein (17 to 24% of DM) were high. Fibre was especially high in Albizia (NDF 58.8% of DM vs. 21% in Sesbania and 15.4% in Acacia). Contents of both hydrolysable (4.4 to 0.05%) and condensed tannins (1.2 to 0.04%) varied from medium to low in the foliages. Acacia contained the highest level of total phenolics (20.1%), protein precipitable phenolics (13.2%) and had the highest capacity to precipitate protein (14.7%). Drying in shade reduced the tannin content in Acacia and Albizia by 48.6 and 69.3% respectively. The foliages ranked similarly for each of the different methods used to estimate tannin content and activity. Acacia and Sesbania foliage was highly degradable (85-87% potential degradability of DM in sacco), compared to Albizia (52%), indicating a minimal effect of tannins in Acacia and Sesbania. Yet, in vitro, the tannins in the Acacia inhibited microbial activity more than those in Albizia and Sesbania. Following the addition of polyethylene glycol to neutralise the tannins, gas production and microbial growth increased by 59% and 0.09 mg RNA equiv./dg microbial yield respectively in the Acacia, compared to 16-17% and 0.06 mg RNA equiv./dg microbial yield in the other foliages. There was a trend for low in vivo apparent digestibility of N in the Acacia (43.2%) and Albizia (44.2%) compared to the Sesbania (54.5%) supplemented groups. This was likely to be due to presence of tannins. Consistent with this was the low N retention (0.22 and 0.19 g N/g NI) in sheep supplemented with Acacia and Albizia compared to that for the Sesbania (0.32). Similarly, a trend for poor microbial N yield was observed in sheep fed these foliages. Across the foliages tested, an increase in tannin content was associated with a reduction in ruminal fermentation, N digestibility and N retention. For overall nutritive value, Sesbania proved to be the superior forage of the three tested.