A dynamic model of N metabolism in the lactating dairy cow and an assessment of impact of N excretion on the environment1

Abstract

Improving N utilization in dairy cows and especially reducing N output in excreta is desirable due to global concerns of agricultural contribution of N to environmental pollution, particularly as ammonia. Data from five N balance experiments were used to develop a dynamic model that was evaluated with independent data. Model predictions of feces, urine, and milk outputs were close to observed values. Statistical analysis showed that 96% of mean square prediction error for feces and urine N output predictions was due to random variation. However, the model tends to overpredict milk N output, especially at higher N intake levels. Evaluation of model predictions for independent experimental observations from Agricultural Development Advisory Service at Bridgets (U.K.) showed good agreement between predicted and observed urine N output (95% due to random variation). However, there was a slight underprediction for fecal N output (14% mean square prediction error due to bias) and overprediction of milk N output (22% of mean square prediction error due to bias). The model predictions of N outputs in excreta were sensitive to changes in energy concentration of the diet. Dietary protein degradability had only a small influence on predicted fecal N output. However, the model was sensitive in its predictions of urine N when protein degradability was varied. Application of the model to assess reduction in ammonia emissions from dairy cows showed that increasing the energy concentration could potentially reduce ammonia emissions by up to 25% per cow. Similarly, reducing CP concentration in the diet to about 16% could reduce ammonia production by 20% and lower degradability of CP to match microbial requirement by 19% per cow. The model is a first step toward a mechanistic approach of nutrient modeling, and it is a valuable method for predicting N excretions and estimating N emissions from dairy systems.

Aspects of nitrogen metabolism in sheep-fed mixed diets containing tree and shrub foliages

“…There appears no direct literature available on prediction of N excretion to compare the linear and nonlinear models with other studies. However, a bilinear relationship between NI and FN excretion was observed in other studies in cattle (7,34) . The linear increase of FN in response to NI was expected because dietary indigestible N, which contributes to the FN, increased with the amount of protein consumed (7) .…”

“…However, a bilinear relationship between NI and FN excretion was observed in other studies in cattle (7,34) . The linear increase of FN in response to NI was expected because dietary indigestible N, which contributes to the FN, increased with the amount of protein consumed (7) . The bilinear relationship between NI and FN was explained by Kebreab et al (7) that microbial FN increased with increasing available energy for the growth of microbial cells, but as the amount of microbial synthesis approached optimum according to the available energy, the contribution to MFN remained constant so the increase in FN was mostly as indigestible N from the protein consumed.…”

“…Therefore, the general observation that foliages in the diets due to the presence of tannins may alter N excretion pathway is perhaps not true for diets containing N less than 15 g/kg DM. Nonetheless, the present study suggests that inclusion of foliages in high-protein diets may shift N excretion from urine to faeces, which is beneficial for environmentally reducing ammonia emission (7,29) . Urinary urea is rapidly converted to ammonia and nitrous oxide; whereas FN is comparatively stable to volatisation (30) , which give adequate time before it reaches to storage facilities for the treatment of manure.…”

“…The linear increase of FN in response to NI was expected because dietary indigestible N, which contributes to the FN, increased with the amount of protein consumed (7) . The bilinear relationship between NI and FN was explained by Kebreab et al (7) that microbial FN increased with increasing available energy for the growth of microbial cells, but as the amount of microbial synthesis approached optimum according to the available energy, the contribution to MFN remained constant so the increase in FN was mostly as indigestible N from the protein consumed. In support of the monomolecular model that predicts N excretion better, Kebreab et al (35) noted that the monomolecular equation was best at determining efficiencies of phosphorus utilisation for body weight gain compared with the sigmoidal model.…”

“…The quadratic response of UN (% of NI) as the digestible OM intake by sheep increased was probably due to the increased efficiency of microbial capture of the available N, which decreased N escaping from the rumen as ammonia and the proportion of N excreted in urine (7,32) . The efficiency of N retention was improved with digestible OM intake (33) (hence metabolisable energy intake) probably because of improved energy status of sheep, and therefore the amount of N metabolised for energy purpose diminished.…”