The prediction of total body nitrogen in the sheep from the composition of individual muscles.

Abstract

THE PREDICTION OF TOTAL BODY NITROGEN IN THE SHEEP FROM THE COMPOSITION OF INDIVIDUAL MUSCLES J. G. MORRIS* AND PATRICIA M. PEPPER* The relationships between both nitrogen and dry matter in the M. biceps femoris, M. semimembranosus plus M. adductor femoris, and M. semitendinosus muscles and total body nitrogen were examined in 36 Border Leicester X Merino wethers ranging from 12.8 to 28.1 kg empty body weight. Linear regression analysis indicated that empty body weight was as efficient in estimating total body nitrogen as was muscle nitrogen or dry matter. Multiple regression equations incorporating empty body weight and muscle nitrogen, particularly that of the M. semitendiosus, provided the most efficient predictors of total body nitrogen and accounted for about 96% of the variance. Summary I. INTRODUCTION Butterfield (1965) presented regression equations for the prediction of the total weight of dissectible muscle, fat and bone in the carcass from the weight of the carcass, the shin group of muscles, the radius and ulna, and the thickness of the subcutaneous fat. While the partition of the carcass into dissectible muscle, fat and bone may be satisfactory for gross anatomical studies, th.ese components are of variable chemical composition and cannot be defined precisely. It appeared possible that the chemical composition of individual muscles was related to total body composition and, in particular, total body nitrogen. This hypothesis was tested in the course of dissection studies with sheep. II. MATERIALS AND METHODS (a) Animals Thirty six Border Leicester x Merino two-tooth wethers which came from a single lambing and which had been selected for uniformity of body weight were used as experimental animals. These animals were used in a survival feeding experiment. Nine were slaughtered at the start of the experiment, and the other 27 were slaughtered as follows : - ten after losing about 20% of their body weight, nine after losing about 40% of their body weight, and eight after losing about 40% of their body weight and then fed ad libitum until initial body weight was restored. *Animal Research Institute, Yeerongpilly, Department qf Primary Industries, Queensland. 303 The muscles M. biceps femoris, M. semimembranosus plus M. adductor femoris, and M. semitendinosus were removed from both hind limbs of the animals according to the procedure described by May ( 1964). All subcutaneous and intermuscular fat was trimmed off, and the tendinous attachments were removed at their junctions with the true muscular tissue. The whole bodies of the sheep including the shorn pelt, but excluding the contents of the gastro-intestinal tract were ground and analysed for water, nitrogen, fat and ash according to the procedure described by Morris and Moir ( 1964). The - weight of material processed plus the dissected muscles was designated as empty body weigh,t. Muscles were dried and analysed by the same methods as used for the carcass samples. Linear regression equations of the form y =I a + bx and multiple regression equations of the form y = a + blxl + bzx2 were computed by the least squares method to predict the total body nitrogen. The independent variables which were used singly in the linear regressions were either empty body weight, total body fat, or the weigh(t of nitrogen or dry matter (DM) in the muscles. For the multiple regressions, the independent variable x1 represents nitrogen or DM in the muscles, while x2 represents empty body weight. The effect of the addition of the extra variable was tested by the methods of Williams ( 1959a). The efficiency of the x's in the linear equation as a predictor of y was estimated by the method described by Williams ( 1959b). The fitting of curvilinear regression equations was also investigated. III. RESULTS The mean weights of total body nitrogen and nitrogen and DM in the three muscles are presented in Table 1. The mean empty body weight of the 36 sheep was 20.7 kg with a range of 12.8 to 28.1 kg, and the mean total fat and mean total ash of the bodies were 2.7 1 kg (range 0.11 to 6.79) and 1.15 kg (range 0.78 to 1.52) respectively. There was significant (P < 0.01) linear relationships between total body nitrogen and the following variables : empty body weight, total body fat, and TABLE 1 (b) Dissection Procedure (c) Analytical Methods (d) Statistical Methods Mean values for total body nitrogen, muscle nitrogen and dry matter used for the development of the regression equations The values for a and b in linear regression equations of the form y = a + bx relating either empty body weight (kg), total fat (kg) or the nitrogen or dry matter (g) of the muscles (x), to total body nitrogen (g) (y ) and the standard error of estimate (SEE) and variation accounted for by the regression TABLE 2 both nitrogen and DM of the three muscles. The a and b values for the linear regression analyses are presented in Table 2 together with the standard errors of estimate (SEE) and the percentage of the total variation accounted for by the regression. For the prediction of total body nitrogen from eithler nitrogen or DM of the muscles, there were no significant differences between the regressions for the left and right sides. Regressions incorporating the values for both sides have been computed. None of the correlation coefficients obtained by combining values for the muscles of the left and right sides were significantly different from each oth!er or empty body weight. For the prediction equation using empty body weight only, the value of a was not significantly different from zero. The regression coefficient b of the equation passing through the origin (y = bx) is given in Table 2 in parenthesis. For all other variables (total body fat, muscle nitrogen and DM) the CJ term was significant (P < 0.05). Quadratic regression equations were also fitted, but the addition of the quadratic term was not a significant improvement; also, thle intercept a was still significantly different from zero. Thus if the restriction that the line must pass through zero is imposed, some worsening of fit would be expected. The a, bI and b2 values of multiple regression equations using the combination of muscle nitrogen or DM (xl), and empty body weight (x2), for the prediction of total body nitrogen are presented in Table 3 together with the SEE and percentage of thle total variation accounted for by the regression. As in the case of the linear regressions, there were no significant differences between the multiple regressions for the prediction of total body nitrogen from empty body weight and the nitrogen and DM of the three muscles of the left and right sides. A combined regression was then computed. The values of ct in all equations were significantly different from zero. With one exception, the multiple regressions 305 of muscle nitrogen or DM and empty body weight fitted the results significantly better (P < O.Ol) than the corresponding simple regressions based on either muscle `nitrogen or DM or empty body weight alone. The exception was the multiple regression of DM of the M. biceps fermoris and empty body weight which was significantly better at P < 0.05 level than the prediction of total body nitrogen from empty body weight alone. The `addition of a quadratic term (x12) to the multiple regression equations muscle nitrogen or DM and x2 = body weight) resulted, in general, in no significant improvement in fit. There was no significant relationship between total body ash and total body nitrogen. IV. DISCUSSION Tulloh ( 1963) from regression analysis of published sheep carcass dissection data covering a wide, but discontinuous range of empty live weight, suggested that 'there is a constant differential growth ratio between each carcass tissue' (muscle, fat and bone) 'and empty live weight'. In his analysis, log. empty live weigh.t accounted for 98.0% of the variance in log. weight of dissected carcass muscle. The relationships presented in this paper cover a much narrower range in empty body weights than those used by Tulloh ( 1963 ) , but also indicate that empty body weight alone can account for a large percentage (93.0% ) of the variance in total body nitrogen. The variation in empty body weight in our regressions arises primarily from variation in the plane of nutrition. In Tullohk log - log. regressions, although the values are derived from sheep of variable nutritional' history, age is the major factor determining the variation in body weight. 306 As the regression of total body nitrogen on empty body weight was linear, over the range of empty body weight studied, the change in total body nitrogen was represented by 0.03 times th,e change in empty body weight. Furthermore, as the intercept of the regression was not significantly different from zero, it would appear that this regression may apply also to empty body weights below 13 kg. This constant proportion of the change in empty body weight represented by nitrogen lends support to the statements of Meyer and Clawson ( 1964) based on experiments with lambs that there is 'a striking loss of protein and not a differential loss emphasising fat when growing animals suffer undernutrition' and that 'the composition of the body weight loss during starvation (is) similar to that of the body weight gain'. Although the tests used to compare the correlations of empty body weight with total body nitrogen, and the correlation of muscle nitrogen and DM with total body nitrogen are conditional tests, they provide a useful approximation of the efficiency of th.e predictors. Even though it cannot positively be stated that empty body weight is the more efficient predictor, the ease of measurement would make it more desirable than either muscle nitrogen or DM. The relative growth pattern of the three muscles which were used may have differed from th.e mean growth pattern of the whole body. These three muscles were classified by Butterfield and Berg ( 1966) as having 'high average' growth patterns in cattle, the 'high' phase being not apparent after 84 days of age. However, Butterfield ( 1966) found no significant difference between the relative loss of weight of the proximal muscles of the pelvic limb and the total musculature of cattle over 19 months of age when subjected to semistarvation. Of the variables examined, multiple regression equations using empty body weight with muscle nitrogen, in particular that of M. semitendinosus, provided the best prediction of total body nitrogen. Only about 4% of the variance remained as deviations from the regression. V. ACKNOWLEDGMENTS The authors thank officers of the Biochemical Branch, Animal Research Institute, for chemical analyses and Mr. B. J. McDonald for technical assistance with the carcass dissections. VI. REFERENCES B B , R. M. ( 1965). Res. vet. Sci. 6: 24. UTTERFIELD , R. M. ( 1966). Res. vet. Sci. 7: 168. B UTTERFIELD, R. M., and B ERG. R. T. (1966). Res. vet. Sci. 7: 326. M AY, N. D. S. ( 1964). 'The Anatomy of the Sheep'. 2nd Ed. (University of Queensland UTTERFIELD Press : Brisbane.) , J. H., and CLAWSON, W. J. (1964). J. Anim. Sci. 23: 214. ORRIS , J. G., and M OIR, K. W. (1964). Tech. Conf. on Carcass Composition and Appraisal 1. s of Meat Animals. Melbourne. August 1963. C.S.I.R.O., Melbourne. TULLOH , N. W. ( 1963). Nature, Lond. 197: 809. W ILLIAMS , E. J. ( 1959a). 'Regression Analysis'. p. 34. (Wiley, New York.) W ILLIAMS , E. J. (1959b). Biometrics 15: 135. M M EYER 307