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Nitrate (NO3–) supplementation is an effective methane (CH4) mitigation strategy for ruminants but may produce nitrite (NO2–) toxicity. It has been reported that rumen protozoa have greater ability for NO3– and NO2– reduction than bacteria. It was hypothesised that the absence of ruminal protozoa in sheep may lead to higher NO2– accumulation in the rumen and a higher blood methaemoglobin (MetHb) concentration. An in vivo experiment was conducted with defaunated (DEF) and faunated (FAU) sheep supplemented with 1.8% NO3– in DM. The effects of rumen protozoa on concentrations of plasma and ruminal NO3– and NO2–, blood MetHb, ruminal volatile fatty acid (VFA) and ruminal ammonia (NH3) were investigated. Subsequently, two in vitro experiments were conducted to determine the contribution of protozoa to NO3– and NO2– reduction rates in DEF and FAU whole rumen digesta (WRD) and its liquid (LIQ) and solid (SOL) fractions, incubated alone (CON), with the addition of NO3– or with the addition of NO2–. The results from the in vivo experiment showed no differences in total VFA concentrations, although ruminal NH3 was greater (p < .01) in FAU sheep. Ruminal NO3–, NO2– and plasma NO2– concentrations tended to increase (p < .10) 1.5 hr after feeding in FAU relative to DEF sheep. In vitro results showed that NO3– reduction to NH3 was stimulated (p < .01) by incoming NO3– in both DEF and FAU relative to CON digesta. However, adding NO3– increased (p < .05) the rate of NO2– accumulation in the SOL fraction of DEF relative to both fractions of FAU digesta. Results observed in vivo and in vitro suggest that NO3– and NO2– are more rapidly metabolised in the presence of rumen protozoa. Defaunated sheep may have an increased risk of NO2– poisoning due to NO2– accumulation in the rumen. |
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