Dietary nitrate and presence of protozoa increase nitrate and nitrite reduction in the rumen of sheep

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