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1613
Inhibition of methanogenesis by nitrate, with or without defaunation, in continuous culture
With regard to the focus of methane (CH4) mitigation in ruminant production systems, nitrate (NO3) serves as an alternative sink for aqueous hydrogen [H2(aq)] accumulating in the rumen, producing ammonium via NO3 reduction pathways and thereby decreasing CH4 production. Defaunation has also been correlated in meta-analyses with decreased methanogenesis. We hypothesized defaunation might increase the CH4 mitigation effect of NO3 by removal of a symbiotic source of substrate to methanogens. In the present study, we applied a 2 × 2 factorial treatment arrangement in a 4 × 4 Latin square design to continuous culture fermenters (n = 4). Treatments were control (–NO3) versus nitrate (+NO3; 1.5% of diet DM), factorialized with control (faunated, FAUN) versus defaunated (DEF). Fermenters were fed once daily (40 g DM; 50:50 forage:concentrate diet); four periods lasted 11 d with 3 d of sample collection. Buffer dilution and solids passage rate were maintained at 7.0%/h and 5.0%/h, respectively. There were no main effects of DEF or interaction of faunation status with addition of NO3 (P > 0.05). The main effect of +NO3 increased (P < 0.05) H2(aq) compared with –NO3 by 11.0 µM. The main effect of +NO3 also decreased (P < 0.05) daily CH4 production compared with –NO3 by 8.17 mmol CH4/d. Because there were no treatment effects on NDF digestibility (P > 0.10), the main effect of +NO3 also decreased (P < 0.05) CH4 production compared with –NO3 by 1.43 mmol CH4/g NDF digested. There were no effects of treatment (P > 0.10) on other nutrient digestibilities, N flow, or microbial N flow per gram of nutrient digested. These data support the existing literature that NO3 incorporation in the diet can decrease the methanogenesis by dairy cattle. More importantly, methanogens are not necessarily inhibited by defaunation in a highly reduced environment. However, practical considerations such as nitrite toxicity and on-farm dietary adaptation to NO3 should be considered before implementing this practice in U.S. dairy production systems.
Keywords: nitrate, methane, defaunation