Relationship between Plasma and Saliva Urea Nitrogen Concentrations in New Zealand Red Deer Calves (Cervus elaphus).

Simple Summary: The relationship between the saliva urea N (SUN) and plasma urea N (PUN) concentrations was investigated in red deer calves (n = 23) over a five-month period with the objective of creating an easy tool to quickly evaluate circulating urea nitrogen concentrations. The experimental design comprised a 2 × 2 factorial arrangement with different seasonality phenotypes (high and low) and two different forage-based diets, namely a medium-quality diverse treatment (Diverse) or a low-quality perennial ryegrass–white clover treatment (PRG), which were offered ad libitum. Blood and saliva samples for the determination of the PUN and SUN were evaluated monthly at civil dawn between April and September of 2022. A moderately strong relationship between the PUN and SUN was identified by simple linear regression (R2 = 0.65; p < 0.001). The interaction between diet and the date of sampling had a significant effect on the relationship between the SUN and PUN (p < 0.001). This interaction is likely explained by the seasonal variation in the crude protein (CP) content in the diet. The use of a mixed-model procedure improved the model fit, increasing the R2 by 0.12 and reducing the root mean square error by 0.341. There was no effect of seasonality on the prediction estimates of the PUN from the SUN, although a tendency for a sex–seasonality interaction was observed (p = 0.09). Therefore, an evaluation of the diet and date of sampling can be used to improve the reliability of the prediction estimates of the PUN from the SUN in red deer calves. Further investigation into the effects of significant factors on this relationship is required to improve the reliability of the model before the SUN can be used to predict the PUN. Red deer (Cervus elaphus), like other ruminants, excrete approximately 70% of the nitrogen they ingest. Developing ways in which to reduce the rate of loss, such as manipulating the diet or selecting for efficiency of growth, requires close monitoring of the plasma urea N (PUN) concentration which, in turn, requires a simple, safe, and reliable method for collecting samples. Saliva is easier to collect than blood, but the relationship between the salivary urea N (SUN) and the PUN is not known for red deer. This was therefore evaluated in two strains of mixed-sex red deer calves (Cervus elaphus): a phenotype with a high seasonality of growth (H, n = 10) and a phenotype with a low seasonality of growth (L, n = 13). Both phenotypes were divided into two groups, which were each offered one of two forage-based diets ad libitum: a medium-quality diverse treatment and a low-quality perennial ryegrass–white clover treatment. Blood and saliva samples for the determination of the PUN and SUN were collected at dawn every four weeks for five months (April to September 2022). There was a strong linear relationship between the PUN and SUN in the pooled sample (R2 = 0.65, p < 0.001). The estimations of the PUN were significantly improved by adding diet and the date of sampling into the model (p < 0.001), but not phenotype (p > 0.75). SUN represents a reliable index of the PUN, and collecting saliva therefore represents a simple and inexpensive alternative to collecting blood samples in studies of nitrogen metabolism in red deer. [ABSTRACT FROM AUTHOR]