The influence of addition of gallic acid, tannic acid, or quebracho tannins to alfalfa hay on in vitro rumen fermentation and microbial protein synthesis

Tannins may reduce rumen degradability of protein, increase the proportion of feed protein reaching the lower digestive tract for enzymatic digestion and thereby increase the efficiency of protein utilization. The objective was to assess the effects of different types and levels of tannins on rumen in vitro gas production and its kinetics, in vitro true degradability (IVTD) and rumen degradability of protein (IVRDP), and microbial protein synthesis by incubating alfalfa (Medicago sativa L.) hay in buffered rumen fluid. Alfalfa was incubated in buffered rumen fluid with and without the addition of different levels of gallic acid (GA), quebracho tannin (QT), or tannic acid (TA). Tannins at the lower inclusion levels had minimal effects on fermentation products compared to the higher levels. Addition of QT and TA reduced ammonium-N (NH4+-N) concentration. Addition of QT at 20, 40, and 60 g/kg DM decreased NH4+-N by 2, 7, and 12% compared with control whereas addition of TA reduced NH4+-N by 5, 6, and 12% when added at 20, 40 and 60 g/kg DM, respectively. In experiment 2, addition of QT at 50, 100, and 150 g/kg DM, resulted in reduction of NH4+-N by 12, 30, and 51%, respectively, compared with the control. Addition of TA at 50, 100, and 150 g/kd DM reduced NH4+-N by 14, 26, and 47% compared with control. Inclusion of QT at 50, 100, 150 DM reduced IVRDP by 13, 30, and 36% compared with control whereas at these levels of inclusion, TA resulted in reduction of IVRDP by 14, 25, and 48%. Rate of gas production decreased (P<0.001), while asymptotic gas production increased (P<0.0001) with increasing level of GA and TA. Quebracho tannin decreased (P<0.0001) both the rate and asymptotic of gas production. Gallic acid had a positive effect on fermentation as indicated by increased gas production and total short chain fatty acids (SCFA) production. Quebracho tannin decreased 24 h gas production, IVTD, and total SCFA production. Acetate to propionate ratio increased with the addition of GA and but decreased when QT was added. Addition of tannins did not markedly increase total purines but numerical values tended to be higher in the presence of tannins compared with the control. Efficiency of microbial growth was lower in the presence of GA and unaltered by TA, but higher in the presence of QT compared with the control. The effect of tannins on rumen fermentation and protein degradation varied with type and level of tannins. In vivo studies will be conducted to validate the in vitro results.