Relationship between the Nitrogen Content and the Heat of Combustion Value of Sheep Urine

The relationship between the calorific value and nitrogen content of sheep urine was studied. Urine was obtained from 45 sheep during two 10-day intervals of a 196-day feeding period. Each animal received either a low, medium, or high level of one of the following diets: chopped hay, the same hay in pelleted finely ground form, or a pelleted mixture of 55% of the same finely ground hay and 45% of corn meal. The energy-to-nitrogen ratio of the urine produced by sheep ingesting the pelleted mixture of hay and corn meal was lower (P<.05) than that of the urine excreted by sheep ingesting either chopped hay or pelleted finely ground hay. Level of intake had very little effect on the ratio. Equations for predicting the energy value (Y) (kcal./100 ml.) from the nitrogen concentration (X) (gm./100 ml.) of the urine produced by sheep fed the two kinds of diets are as follows: chopped and pelleted hay, Y= 16.103X + 2.00; and pelleted hay and corn meal, Y=18.167X + 0.18. The standard errors of estimate were 1.0 and 1.1 kcal. per 100 ml. of urine, respectively. From all data (representing three levels of intake of the three diets) the following equation was computed: Y=17.257X + 1.09. The standard error of estimate was 1.3 kcal. per 100 ml. of urine. Regression of the total energy output (Y) (kcal./day) on the total nitrogen output (X) (gm./day) resulted in the following equation in which dietary treatment is disregarded∶ Y= 15.257X + 33.14. The standard error of estimate was 20.5 kcal. per day. The separate regressions for the two kinds of diets were as follows∶ chopped and pelleted hay, Y= 14.497X + 44.58; and pelleted corn meal-hay, Y=16.259X + 15.05. The standard errors of estimate were 20.0 and 17.3 kcal. per day, respectively. The regression coefficients for the chopped and pelleted hay diets were not statistically different. Also, the regression coefficient for the pooled data representing the two hay diets was not significantly different from that for the corn meal-hay diet. Thus, all data were pooled to compute a general equation. Because of the small error attached to the prediction of the calorific value from the nitrogen concentration or content of urine, it is indicated that this relationship could circumvent the need to determine directly the heat of combustion value of urine in many experiments in which diets similar to those examined in this study are fed.