عنوان مقاله [English]
Introduction The increased level of production of dairy cows increased the losses of nitrogen in feces and urine. The first approach to reduce the nitrogen excretion in the urine or feces, or both is the reduction of nitrogen usage in food rations. This approach is achieved when there is no significant effect on the reduction of animals function. Another way to reduce nitrogen excretion in animals is to improve the nitrogen use efficiency. Mathematical models have been used to predict the potential of nitrogen loss in dairy cows. These models are naturally experimental and not based on a specific process. The animal components in these models are based on CNCPS which these systems contain a large number of experimental elements (Empirical models) and are based on some controversial hypothesis. Jonker et al. (1998) presented a model based on the whole animal with a collection of data and showed that this model could predict the excreted nitrogen of urine from ureic nitrogen of milk precisely. However, they showed that when nitrogen loss is due to the quantity of protein, protein degradation, inadequate energy, etc, the model cannot identify and predict. Therefore, there is a need to develop a model to evaluate the biological processes in animals and nutritional changes to determine their exact role in the assessment of environmental pollution. The purpose of Kebreab et al’s model was to develop a dynamic model based on a certain method that has the ability to predict the quantity and nature of nitrogen excretion in dairy cows under different diets, and also the model could be evaluated by other experiments data. The purpose of this study is the evaluation of the model presented by Kebreab et al (2002). The mathematical models have been used to predict the potential for nitrogen excretion in dairy cows.
Materials and methods To evaluate a dynamic model of nitrogen metabolism in dairy cows by Kebreab and et al. (2002), two master's thesis with titles “Effect of NFC on Performance of Holstein Dairy Cow and Expression of its Mathematical Model on Nitrogen Balance” Kamel (2014), and “Effect of cutting time alfalfa hay harvested at early bud with different ratios of forage to concentrate on chemical composition, fermentation Parameters and performance of early lactation Holstein dairy cows” Firuzi (2014) were used. In this study, to evaluate Kebreab et al’s model, 16 Holstein cows in two 4 × 4 Latin square design were used. Cows were fed with 8 different diets. For each experiment, the total of consumed nitrogen and excreted from feces, urine and milk nitrogen were measured. In addition, the concentration of feed component including dry matter, crude protein, NDF, ADF, starch, non-fibrous carbohydrates, ash, calcium, phosphorus, ammonia nitrogen, pH and metabolisable energy were determined.
Results and Discussion Similar to the results obtained by Kebreab et al. (2002), by increasing nitrogen intake, fecal nitrogen excretion also increased linearly. As the result of Kebreab et al. (2002) and Castillo et al (2000), the nitrogen excreted in the urine predictions match better with the observations.
Conclusion In general, the lowest average of percentage error and the difference of RMSPE with its optimum level are for nitrogen excretion of urine and feces, nitrogen secreted in milk, nitrogen excreted in the urine and the nitrogen excreted in the feces, respectively. Therefore it could be concluded that the model presented by Kebrab et al (2002). has the best estimation and prediction in summation of excreted nitrogen from urine and feces while has the least precision in estimation of excreted nitrogen from feces. Although the results did not show a bi-linear relationship between N intake and output, but our results are very similar with Kebreab’s results.
Key Words: model evaluation-nitrogen-dairy cow-prediction