Tannins in tropical browses: Effects on in vitro microbial fermentation and microbial protein synthesis in media containing different amounts of nitrogen

Four species of browses (Acacia angustissima, Acacia salicina, Calliandra calothyrsus, and Dichrostachys cinerea) were used to study the effect of tannins on microbial fermentation and microbial protein synthesis in incubation media containing high nitrogen (HN) and low nitrogen (LN) in the presence and absence of polyethylene glycol (PEG, MW 6000). The additional nitrogen in HN medium was supplied through ammonium bicarbonate. The use of HN medium significantly (P < 0.05) increased the in vitro gas and short-chain fatty acid (SCFA) production and microbial protein synthesis compared to the LN medium. Incubation of tannin-containing browses alone produced significantly (P < 0.05) lower gas and SCFA compared to in the presence of PEG in both HN and LN media. Inclusion of PEG in tannin-containing browses significantly (P < 0.05) reduced the molar proportion of propionate compared to in its absence. Higher N in the media resulted in 10.4 and 9.9% increases in in vitro gas and SCFA production, respectively, whereas inclusion of PEG to tannin-containing feed to remove the effect of tannins increased the in vitro gas and SCFA production by 186 and 195%, respectively, indicating that the low fermentation of tannin-containing browses could be due to the depressive effects of tannins on microbial activity and only partially accounted for by unavailability of N for rumen microbes. Incubation of browses with straw significantly (P < 0.05) decreased ammonia nitrogen concentration but increased the in vitro gas and SCFA production and microbial protein synthesis compared to straw alone.