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 (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 = 0.007) linearly with greater beta extract inclusion (P = 0.08). However, beta extract did not affect DM, OM, NDF or ADF digestion. Furthermore, beta extract did not modify VFA production or N metabolism. In experiment 2, iso-alpha extract also increased average pH, and tended to increase time above pH 6.2. Iso-α extract did not affect DM, OM, NDF and ADF digestion, VFA production, or N metabolism. Results indicate that increasing the concentration of beta extract or iso-α extract increase rumen pH within continuous culture, but neither extract affects rumen nutrient digestion.