Ellagic acid and gallic acid reduced methane and ammonia in an in vitro rumen fermentation model

Livestock production must meet the growing demand for animal-source food and reduce the impact on the environment. Different strategies are studied to lower greenhouse gas (GHG) emissions in ruminants, and one of them is tannin supplementation. The dietary supplementation with tannins can affect the enteric fermentation, reducing methane emissions. Following a previous screening, we investigated the effect of two hydrolysable tannins, ellagic acid (EA) and gallic acid (GA), in a long-term in vitro rumen fermentation. EA and GA were supplemented to a control diet (CTR: ryegrass hay and barley concentrate, 10 g DM/day) in an 8-fermenter rumen simulation technique (Rusitec), for 10 days. Three experimental conditions were investigated: (1) EA 75 mg/g DM; (2) GA 75 mg/g DM; (3) EA 75 mg/g DM + GA 75 mg/g DM. The data were collected in the last 5 days of the incubation time. Total gas production was not altered over the last 5 days, whereas daily methane (CH4) production was significantly decreased by EA (-45%) and EA+GA (-60%), compared to control. CH4 production per unit of dietary organic matter (OM) and short-chain fatty acids (SCFA) was also reduced by EA (-48% and -32%) and EA+GA (-65% and -58%), and less by GA (-19% and -22%). Ammonia formation was significantly reduced by EA (-46%), GA (-19%) and EA+GA (-86%). Total SCFA production was decreased by EA and EA+GA (-26%, -16%), but not by GA. Similarly, EA and EA+GA, but not GA, reduced rumen degradability of OM, crude fibre (CF) and crude protein (CP). All the treatments increased the bacterial count and decreased the protozoal count (except for GA). EA and EA+GA modulated the relative abundance of selected rumen bacterial taxa. These data proved that both EA and GA decreased GHG emissions and ammonia formation, with EA being most effective than GA. Nevertheless, GA showed a lower interfering effect on nutrient rumen degradability. Further details on rumen microbiota dynamics and hydrolysable tannins metabolism will complete the outcome of the study.