Managing the rumen to limit the incidence and severity of nitrite poisoning in nitrate-supplemented ruminants

Abstract

Inclusion of nitrate (NO3−) in ruminant diets is a means of increasing non-protein nitrogen intake while at the same time reducing emissions of enteric methane (CH4) and, in Australia, gaining carbon credits. Rumen microorganisms contain intracellular enzymes that use hydrogen (H2) released during fermentation to reduce NO3− to nitrite (NO2−), and then reduce the resulting NO2− to ammonia or gaseous intermediates such as nitrous oxide (N2O) and nitric oxide (NO). This diversion of H2 reduces CH4 formation in the rumen. If NO2− accumulates in the rumen, it may inhibit growth of methanogens and other microorganisms and this may further reduce CH4 production, but also lower feed digestibility. If NO2− is absorbed and enters red blood cells, methaemoglobin is formed and this lowers the oxygen-carrying capacity of the blood. Nitric oxide produced from absorbed NO2− reduces blood pressure, which, together with the effects of methaemoglobin, can, at times, lead to extreme hypoxia and death. Nitric oxide, which can be formed in the gut as well as in tissues, has a variety of physiological effects, e.g. it reduces primary rumen contractions and slows passage of digesta, potentially limiting feed intake. It is important to find management strategies that minimise the accumulation of NO2−; these include slowing the rate of presentation of NO3– to rumen microbes or increasing the rate of removal of NO2−, or both. The rate of reduction of NO3− to NO2− depends on the level of NO3− in feed and its ingestion rate, which is related to the animal’s feeding behaviour. After NO3− is ingested, its peak concentration in the rumen depends on its rate of solubilisation. Once in solution, NO3− is imported by bacteria and protozoa and quickly reduced to NO2−. One management option is to encapsulate the NO3− supplement to lower its solubility. Acclimating animals to NO3− is an established management strategy that appears to limit NO2− accumulation in the rumen by increasing microbial nitrite reductase activity more than nitrate reductase activity; however, it does not guarantee complete protection from NO2− poisoning. Adding concentrates into nitrate-containing diets also helps reduce the risk of poisoning and inclusion of microbial cultures with enhanced NO2−-reducing properties is another potential management option. A further possibility is to inhibit NO2− absorption. Animals differ in their tolerance to NO3− supplementation, so there may be opportunities for breeding animals more tolerant of dietary NO3−. Our review aims to integrate current knowledge of microbial processes responsible for accumulation of NO2− in rumen fluid and to identify management options that could minimise the risks of NO2− poisoning while reducing methane emissions and maintaining or enhancing livestock production.

Low Emission Feed : using feed additives to decrease methane production in dairy cows

“…Aschenbach et al (2009) reported that nitrate impaired acetate uptake through the rumen wall in vitro. Nolan et al (2016) suggested this to be a possible explanation for a shift in VFA profile towards acetate that has been reported upon nitrate supplementation, both in vitro and in vivo (Zhou et al, 2012;Guyader et al, , 2016de Raphélis-Soissan et al, 2016a). However, in the study of Aschenbach et al (2009) only acetate uptake was measured, and not uptake of propionate and butyrate.…”

“…As the VFA uptake mechanisms generally are not VFA specific, it is likely that nitrate will also have inhibited the uptake of other VFA, instead of being specific for acetate. Nolan et al (2016) showed that methemoglobin (MetHb) levels in sheep receiving a diet containing 2% nitrate remained constant and low (>12% of Hb) until 10 h after the first meal, when the diet was fed in hourly portions of 42 g. The MetHb levels in sheep receiving the same amount of feed (1 kg/d) not in hourly portions, but in either one or two meals per day, peaked during the hours after feeding. The authors discuss that the rate of nitrate reduction is increased when animals eat rapidly and/or when a feed restriction is applied.…”

“…Frequent feeding will also reduce the peak levels of MetHb in blood, with much lower peak levels in sheep fed once a day compared with sheep fed meals at hourly intervals (de Raphélis-Soissan et al, 2016b). The likelihood of nitrate poisoning is reduced by the inclusion of fermentable energy sources (concentrates) in nitrate-containing diets (Nolan et al, 2016). However, substituting fibre rich feeds for starch or sugar rich concentrates may not be attractive from a human-edible feed efficiency viewpoint.…”

“…The formation of MetHb occurs as a result of nitrite absorption into the bloodstream. It is, therefore, likely that a more gradual intake pattern will alleviate nitrite accumulation in the rumen, and subsequent negative effects on digestion and animal health Nolan et al, 2016).…”

“…In view of the potentially toxic effects of nitrite, Nolan et al (2016) discussed several control points in nitrate metabolism in the rumen with the goal to alleviate toxic effects of nitrite. Slowing the rate of presentation of nitrate to rumen microbes reduces the risk of nitrite accumulation.…”