Effect of subacute dietary nitrate on production traits and plasma analytes in Suffolk ewes.

Elevated dietary nitrate (NO3-) is associated with production losses in ruminant livestock, resulting in substantial economic losses incurred by producers. Severe drought, fertilization practices and poorly maintained pastures increase the risk of elevated NO3- intake among cattle and sheep. Nitrate is metabolized to nitrite (NO2-) in the rumen and further reduced to ammonia. Ruminants consuming high dietary NO3- vary in ability to efficiently reduce excess NO2- to ammonia. This leads to methemoglobin formation and ultimately NO3- toxicity signs. Variation in individual tolerance to elevated dietary NO3- can be partially attributed to rate and duration of exposure, rate of elimination, metabolism, species and dose. Our objectives were to confirm and quantify variation in individual tolerance to subacute levels of dietary NO3-, and determine if individuals could be identified as highly or lowly tolerant to elevated dietary NO3- based on production traits, plasma analytes and(or) signs of subacute NO3- toxicity. Purebred Suffolk ewes were administered supplement mixed with tap water (control; n = 8) or potassium nitrate (NO3- treated; 300 mg NO3-/kg BW daily; n = 47) for 8 days. Coefficients of variation (CV) indicated that supplement intake was more variable in NO3- treated ewes (CV = 59.3%) than in control ewes (CV = 13.6%). Among NO3- treated ewes, six ewes highly tolerant and six ewes lowly tolerant to elevated dietary NO3- were identified based on individual performance, NO3- treated supplement intake, and signs of toxicity. Supplement intake was lower (P < 0.0001) in NO3- treated ewes than in control ewes, indicating elevated dietary NO3- influences feed intake. Supplement intake differed (P < 0.0001) between control, highly tolerant and lowly tolerant ewes. Supplement intake of highly and lowly tolerant ewes was 82% and 23%, respectively, of the control ewes' intake. Weight change and plasma concentrations of NO2-, cortisol, glucose and retinol were not different (P 0.38) among control, highly tolerant and lowly tolerant ewes. Plasma urea nitrogen (PUN) levels were not different (P = 0.25) between control and lowly tolerant ewes, but were lower (P = 0.02) in highly tolerant ewes than in control ewes. Furthermore, PUN and NO3- treated supplement intake were highly correlated (0.71; P < 0.0001) in lowly tolerant ewes. These results confirm and quantify variation in response to subacute levels of dietary NO3- and indicate that individuals can be identified as highly or lowly tolerant to elevated dietary NO3- based on their performance and NO3- toxicity signs.