Evaluation of iso-α-acid and β-acid extracts from hops (Humulus lupulus L.) on fermentation by rumen microbes in dual-flow continuous culture fermenters

•Hops beta extract linearly increased mean rumen pH and time between pH 5.8 and 6.2 while decreasing time below pH 5.8.•Iso-alpha extract from hops linearly increased mean pH and tended to increase time spent above pH 6.2.•Increasing concentrations of hops beta extract did not affect nutrient digestion, VFA profile or nitrogen metabolism.•Increasing dose of hops beta iso-alpha extract did not affect nutrient digestion, VFA profile or nitrogen metabolism.•Rumen microbes appear resistant to the bacteriostatic properties of hop beta and iso-alpha acids. Hops (Humulus lupulus L.), primarily used in the brewing industry, have been shown to have bacteriostatic properties against Gram-positive bacteria. Active antimicrobial compounds contained within hop flowers include iso-α-acids (isohumulones) and β-acids (lupulones). Previous experiments have demonstrated effects of hop extracts on rumen fermentation in batch culture. The objective of this study was to determine the direct effects of iso-α-acids and β-acids on rumen fermentation in continuous culture fermenters. Two experiments were conducted using dual-flow continuous culture fermenters. Each experiment utilized eight fermenters in two consecutive 10 d periods. Within both experiments, the same basal diet consisting of 44 % corn silage, 14 % alfalfa hay, 13 % ground corn, 11 % protein mix, 10 % corn gluten feed, 5 % cottonseed and 3 % liquid vitamin and mineral supplements on a DM basis was provided to the fermenters at a rate of 75 g of DM/L of fermenter volume/day. In experiment 1, hop beta-extract was added daily to the artificial saliva buffer to supply 0 (CON), 600 (LOW), 1200 (MED), or 1800 (HIGH) mg of β-acids/kg of diet DM/day. In experiment 2, hop iso-α extract was provided to fermenters via the artificial saliva buffer to supply 0 (CON), 600 (LOW), 1200 (MED), or 1800 (HIGH) mg of iso-α-acids/kg of diet DM/day. Data in both experiments were analyzed as a randomized complete block design with experimental period serving as a block and all treatments equally represented within each block. Data were statistically analyzed using GLM procedure of SAS with a model including the fixed effects of experimental period (block), treatment, and the interaction of treatment and period. In experiment 1, mean and maximum fermentation pH increased (P = 0.005) linearly with increasing levels of beta extract inclusion. Additionally, time spent below pH 5.8 increased and time between pH 5.8 and 6.2 decreased (P = 0.00