Nitrate Metabolism of Microorganisms in the Rumen of Sheep Fed High Nitrate Italian Ryegrass Silage

Abstract

This study was carried out to clarify whether ensiled Italian ryegrass of high nitrate content would cause so-called nitrate poisoning in ruminants. The chopped high nitrate silage containing 0.35g of nitrate per 1kg of body weight was introduced into the rumen of sheep through fistula twice a day for four days. On the first day of feeding trial with the high nitrate silage, ruminal nitrate disappearance, nitrite appearance and followed disappearance rates seemed to be generally slow. However, these rates increased gradually from the second up to fourth day of trial period, and the maximum nitrite concentration decreased simultaneously as rumen microorganisms adapted to nitrate. The decrease of lactic acid, pH restoration, and the increase of total volatile fatty acids and ammonia were faster in the rumen of animal showed rapid disappearance of nitrate and nitrite. More vigorous reduction of nitrate and nitrite in vitro was observed without the marked fall of media pH, when rumen microorganisms adapted to nitrate. These reduction rates were accelerated by the addition of protozoal fraction. From above results, it was deduced that feeding of ensiled high nitrate grass might reduce nitrate toxity for ruminants due to rapid reduction of nitrate and nitrite by rumen microorganisms, presumably as the result of providing hydrogen donor such as lactate produced in the silage fermentation.