Model Predicted DMI, Nitrogen (N) Excretion and N Use Efficiency Utilizing Plasma Urea Nitrogen (PUN) Versus Values Estimated in Conjunction with Viable Dry Matter Intake Estimates in Lambs Grazing Pasture

A unique Plasma Urea Nitrogen (PUN) model was presented that utilizes animal PUN to predict nitrogen excretion and the efficiency of its utilization in sheep. However, this model has not been evaluated due to limited availability of measured data. Our goal was to evaluate the PUN model's efficacy in a pasture situation using lamb PUN, herbage N, pasture dry matter intake (DMI) estimates, and lamb productivity data from a previous experiment carried out on pasture in the Appalachian mountain region of the eastern USA. Based on the model alone, estimated lamb N intake and DMI values were calculated utilizing known herbage N contents and individual lamb PUN values. The model's DMI estimates were much lower (P<0.01) than those utilized in our experiment, and modeled DMI estimates were unrealistic when related to actual animal performance. The model predicted lower (P<0.0001) intake N and fecal N, and greater (P<0.0001) N use efficiency than that predicted with experimentally utilized DMI estimates. These results indicate the model needs to be combined with viable DMI estimates to generate realistic outputs, and as we illustrate, viable estimates are easily attainable. We present a method of adjusting the PUN model that would make it a viable tool to help evaluate N excretion and N use efficiency within a pastoral system.