Effects of nordihydroguaiaretic acid on in vitro fermentation profiles of rumen bacteria.

Nordihydroguaiaretic acid (NDGA) is a secondary plant metabolite with antimicrobial properties, and therefore may have potential as a rumen modifier. Two in vitro experiments were conducted to determine the usefulness of NDGA as a rumen modifier. Exp. 1 evaluated the effect of adding 0, 5, 10, 50, and 100 mg/mL NDGA on growth of pure and mixed cultures of rumen bacteria. Growth of all cultures except Butyrivibrio fibrisolvens H17c was inhibited at 50 mg/mL NDGA (P < 0.05). Cultures from whole rumen fluid and B. fibrisolvens H17c were inhibited with 100 mg/mL NDGA (P < 0.05). Exp. 2 evaluated additions of NDGA on IVDMD (48 h) and VFA production. Three dietary substrates simulating different resources available for livestock production and 5 concentrations of NDGA were compared with monensin (47.5 μg/mL, MON, Elanco Animal Health, Indianapolis, IN). Substrates included (DM basis) 100% meadow hay (H100), 50% alfalfa with 50% ground corn (H50), and 90% ground corn with 10% alfalfa (H10). Treatments were 0 (Control; CON), 20, 40, 60, 80 μg/mL NDGA and MON. Treatment means were compared using 2 single degree of freedom contrasts (0 μg/mL NDGA vs. MON and NDGA vs. MON) and orthogonal polynomial contrasts within NDGA concentrations. Monensin fermented with H100 had the least (P < 0.01) IVDMD. A linear increase in IVDMD was observed for H50 (P < 0.01) but not H10 or 100 (P > 0.40). Acetate was quadratic for all substrates tested with NDGA (P < 0.01) and adding NDGA vs. MON resulted in 9% greater values (P < 0.01). Propionate increased by addition of MON compared with CON, which was opposite for acetate. Propionate showed the greatest increase with addition of MON and was dependent on diet vs. CON (H100 vs.H50 vs. H10; 22.5%, 44.4%, and 30.2%, respectively). When H100 was used, total VFA declined linearly by 61% with increasing NDGA (P < 0.01), whereas H50 and H10 were quadratic (P < 0.01) with the greatest total VFA resulting from 40 and 80 mg/mL NDGA for H50 and H10, respectively. Addition of NDGA tended to decrease total VFA (P =0.06) for H100 and H10 by 18.5% and 9.0%, respectively; however, H50 did not differ (P = 0.82) compared with MON. Butyrate increased linearly with increasing NDGA for H10 (P < 0.03) and quadratic for H50 and H100 (P < 0.01). The lowest overall acetate:propionate ratio was obtained with addition of MON to H10 (1.35) and the greatest ratio resulted from adding 60 μg/mL NDGA to H100 (3.63). Rumen fermentation was responsive to NDGA, and the response is dependent on diet.