Effects of five additives with antimicrobial properties on in vitro rumen fermentation and methane production of a commercial diet for dairy cows

This study explored effects of four plant extracts (allyl-sulfyde, AS; cinnamaldehyde, CI; eugenol, EU; limonene, LI) and one synthetic compound (monensin, MO), all with antimicrobial properties, on in vitro rumen fermentation and methane (CH4) production of a diet for dairy cows (NDF=37.0%; CP=15.2%; lipids=3.3%, on DM). Four incubations at 24 h were carried out using a gas production (GP) system made up of bottles wireless-connected to a PC. Bottles (317 ml) were filled with 1.0\ub10.010 g of diet, additive, and 150 mL of buffered rumen fluid, and incubated at 39\ub10.4\ub0C. Two dosages of each additive were tested: 3 or 30 mg/g diet for AS, CI, EU, and LI; 0.015 or 0.030 mg/g diet for MO. For each bottle GP was recorded every minute. Gas was vented when the pressure into the bottles reached 6.8 kPa. The experimental design was: 4 incubations\ua55 additives\ua52 dosages\ua53 replications, plus 24 bottles as control (without additive; 6/run) and 12 as blanks (without diet and additive; 3/run), for a total of 156 bottles. At the end of each incubation, gas (10 ml) was sampled from headspace of each bottle and analyzed for CH4 by GC.Fermentation fluids were treated with neutral detergent solution to compute degradability of NDF (NDFd, %) and of true DM (TDMd, %). Data were submitted to ANOVA considering the treatment (n=11; 5 additives\ua52 dosages, plus control) and incubation (n=4) as variation sources. High dosage of LI strongly depressed (P<0.001) both NDFd (-68%) and TDMd (-14%) compared to the control; less marked (P<0.05) reductions were noted for high dosage of MO (-16% and -3%, for NDFd and TDMd, resp.). No effects emerged for other additives, irrespective by dosage. Compared to the control, high dosage of AS, CI, LI, and MO reduced (P