EFFECT OF ACCLIMATION TO HIGH NITRATE INTAKES ON SOME RUMEN FERMENTATION PARAMETERS IN SHEEP

Abstract

Four sheep, fed a basal diet which included 44% cereal grain and 50% hay and which was supplemented with KNO3, were progressively acclimated to a KNO3 intake of 2.5 g∙kg body wt−1∙day−1. Nitrate and nitrite reducing activity in strained rumen fluid (SRF) collected 2 h after feeding showed maximum values of 45.3 μg N∙mL−1∙h−1 and 39.4 μg N∙mL−1∙h−1, respectively, at an intake of 1.5 g∙kg body wt−1∙day−1. The rate of nitrate reduction was threefold higher (P < 0.01), and that of nitrite reduction fivefold higher (P < 0.01), than in SRF from sheep not receiving KNO3. When the KNO3 supplement was withdrawn the reducing activities fell to their initial levels within 3 wk. In a second experiment, nitrate, nitrite and volatile fatty acids in SRF, and methemoglobin in peripheral blood, were estimated at 30-min intervals after feeding in four sheep, two of which received 1.5 g KNO3∙kg body wt−1∙day−1. In the animals fed KNO3 the peak concentration of nitrate in SRF (13.30 μg NO3−–N∙mL−1) was reached 30 min after feeding, and that of nitrite (2.90 μg NO2−–N∙mL−1) 60 min after feeding; the presence of nitrate in SRF was associated with an increase in the molar proportion of acetate (P < 0.01) and a decrease in the proportion of n-butyrate (P < 0.01). The blood methemoglobin concentration did not exceed 2% (wt/wt) of total hemoglobin at any sampling time. In these animals 30–35 g KNO3 cleared the rumen within 3 h after feeding with no symptoms of nitrate toxicity. Enumeration of rumen bacteria by a direct isolation procedure indicated that the proportion of nitrate reducers in SRF was threefold higher (P < 0.01) in acclimated animals than in control animals. It was concluded that safe acclimation of sheep to high levels of dietary nitrate involved an increase in the rates of nitrate and nitrite reduction in the rumen, a narrowing of the ratio of these activities, and an increase in the proportion of nitrate reducing rumen bacteria. The residual effect of nitrate on the fermentation following clearance from the rumen was short-lived. Key words: Rumen fermentation, sheep, nitrate toxicity, nitrate reduction, nitrite reduction, rumen bacteria