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

Abstract

Nitrate (NO−3) supplementation is an effective methane (CH4) mitigation strategy for ruminants but may produce nitrite (NO−2) toxicity. It has been reported that rumen protozoa have greater ability for NO−3 and NO−2 reduction than bacteria. It was hypothesised that the absence of ruminal protozoa in sheep may lead to higher NO−2 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% NO−3 in DM. The effects of rumen protozoa on concentrations of plasma and ruminal NO−3 and NO−2, 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 NO−3 and NO−2 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 NO−3 or with the addition of NO−2. 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 NO−3, NO−2 and plasma NO−2 concentrations tended to increase (p < .10) 1.5 hr after feeding in FAU relative to DEF sheep. In vitro results showed that NO−3 reduction to NH3 was stimulated (p < .01) by incoming NO−3 in both DEF and FAU relative to CON digesta. However, adding NO−3 increased (p < .05) the rate of NO−2 accumulation in the SOL fraction of DEF relative to both fractions of FAU digesta. Results observed in vivo and in vitro suggest that NO−3 and NO−2 are more rapidly metabolised in the presence of rumen protozoa. Defaunated sheep may have an increased risk of NO−2 poisoning due to NO−2 accumulation in the rumen.