Estimating Body Weight and Body Composition of Chickens by Using Noninvasive Measurements 1

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1 Estimating Body Weight and Body Composition of Chickens by Using Noninvasive Measurements 1 J. D. Latshaw*,2 and B. L. Bishop *Department of Animal Sciences, The Ohio State University, Columbus, Ohio 43210; and Computing and Statistical Services, Ohio Agricultural Research and Development Center, Wooster, Ohio ABSTRACT The major objective of this research was to Multiple linear regression was used to estimate BW develop equations to estimate BW and body composition using measurements taken with inexpensive instruments. We used five groups of chickens that were created with different genetic stocks and feeding programs. Four of from body measurements. The best single measurement was pelvis width, with an R 2 = Inclusion of three body measurements in an equation resulted in R 2 = 0.78 and the following equation: BW (g) = (breast, the five groups were from broiler genetic stock, and one cm) (circumference, cm) (pelvis, cm). was from sex-linked heavy layers. The goal was to sample The best single measurement to estimate body fat was six males from each group when the group weight was abdominal skinfold thickness, expressed as a natural logarithm. Inclusion of weight and skinfold thickness resulted 1.20, 1.75, and 2.30 kg. Each male was weighed and measured for back length, pelvis width, circumference, breast = 0.63 for body fat according to the following equa- in R tion: fat (%) = (skinfold, ln cm) 3.87 (wt, width, keel length, and abdominal skinfold thickness. A kg). Inclusion of the result of TOBEC and the effect of cloth tape measure, calipers, and skinfold calipers were sex improved the R 2 to 0.78 for body fat. Regression analysis was used to develop additional equations, based on used for measurement. Chickens were scanned for total body electrical conductivity (TOBEC) before being euthanized and frozen. Six females were selected at weights The body water content (%) = (body fat, %), fat, to estimate water and energy contents of the body. similar to those for males and were measured in the same and body energy (kcal/g) = (body fat, %). way. Each whole chicken was ground, and a portion of ground material of each was used to measure water, fat, ash, and energy content. The results of the present study indicated that the composition of a chicken s body could be estimated from the models that were developed. (Key words: body composition, chicken, pelvis width, skinfold thickness, total body electrical conductivity) 2001 Poultry Science 80: INTRODUCTION A range of techniques is available to gain information about an animal s body mass and body composition. Some of these techniques use simple, inexpensive equipment, and others require sophisticated, expensive equipment. The most direct way to determine an animal s mass is to weigh it. However, under some circumstances, a scale may not be available. An alternative is to measure a body part and relate the measurement to BW. Shank length is the body part that is commonly measured in poultry to 2001 Poultry Science Association, Inc. Received for publication October 16, Accepted for publication March 13, Salaries and research support were provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. 2 To whom correspondence should be addressed: Latshaw.1@osu.edu. 3 Hog Weigh Tape, the Coburn Company, Whitewater, WI relate to BW. A linear relationship between BW and shank length has been reported (Lerner, 1937). However, a general formula that is now used (Tierce and Nordskog, 1985) is as follows: shank length (mm) = W(kg) B. Because body conformation (especially breast width) is different among breeds of chickens, different coefficients and exponents are needed for different breeds of chickens. For livestock, it is more common to estimate weight by measuring a part of the body trunk rather than an extremity. A good estimate of weight can be obtained by measuring the heart girth, the circumference of the animal posterior to the front legs. This estimate is routine for cattle (Heinricks and Hargrove, 1987), horses (Willoughby, 1975), and swine. 3 Information about body composition can be important in several ways. Those producing and selling animal products need information about body composition to track their efforts and produce leaner meat animals. Nu- Abbreviation Key: TOBEC = total body electrical conductance. 868

2 ESTIMATING CHICKEN BODY WEIGHT AND COMPOSITION 869 tritionists also need information about body composition to determine how much of the energy that an animal eats is captured as growth or other forms of production. Body composition and energy content can be determined using a number of methods. The most direct and accurate way is to euthanize the animal, grind the whole animal, sample it appropriately, and then collect appropriate data. Fat content is estimated by using organic solvents (Polleymounter et al., 1995), and energy content is measured by bomb calorimetry. Indirect methods also can be used to estimate body composition (Blaxter, 1989). One method is to estimate body water content by dilution of an injected compound and then estimate body fat content based on the inverse relationship of body fat and body water. A second method is to measure 40 K, which is proportional to lean body mass. A third method uses weighing in water because the specific gravity of fat is less than the specific gravity of the lean body (Pesti and Bakalli, 1997). The latter method was applied in energy studies with beef cattle (Lofgreen, 1964). The empty body weight measured in water was used to estimate the fat content and, subsequently, energy content of cattle carcasses. Skinfold thickness and body density are used to estimate body composition of humans (Lohman, 1981). Abdominal skin thickness was related to abdominal fat in broilers (Pym and Thompson, 1980; Mirosh and Becker, 1984). Indirect methods have been developed to measure energy gain by animals (Blaxter, 1989). One method is to measure carbon and nitrogen balance, and a second is to measure the gaseous exchange of CO 2 and O 2. Recently, electronic devices have been evaluated for their ability to detect body composition. Ultrasonic systems are used to estimate the fat content of specific parts of cattle and hogs (Liu and Stoffer, 1995). Total body electrical conductivity (TOBEC; Roby, 1991), near infrared interactance (Roby, 1991), and x-ray absorptiometry (Mitchell et al., 1997) have been tested with birds. An indirect procedure offers the potential to harmlessly estimate body composition and energy content of live animals (Walsberg, 1988; Staudinger et al., 1995). The principle is based on TOBEC. Equipment has been designed to generate and detect electromagnetic waves. Several factors, including body composition of a bird, alter conductivity in the electromagnetic fields. Body fat is a poor conductor, whereas components of the lean body, especially electrolytes, are good conductors. As a result, the potential exists to estimate whole body fat and energy. There were two objectives of this research. The primary objective was to develop general equations to estimate BW, body fat content, and body energy content of live chickens with different body conformations. Measurements to develop the equations were taken with inexpensive instruments that included a cloth tape measure, calipers, and skinfold calipers. The second objective was to 4 Eagle Nest Hatchery, Oceola, OH determine if additional information about body composition could be provided by the TOBEC procedure for body composition estimation. Chickens MATERIALS AND METHODS Five groups of chickens were used to provide different conformations and body compositions. The Ross strain 4 of meat-type chicken was used for three groups. One group was fed diets appropriate for broiler chickens (National Research Council, 1994). A second group was fed appropriate diets, except the protein content was 3% lower at each age than was fed to Group 1. This low protein diet was used to increase body fat. A third group followed a feeding program similar to that used for broiler breeders. They were fed unlimited amounts of the same diet as Group 1 for 3 wk. At that age they were fed limited amounts of a 17% protein diet each day so that the females would attain an average BW of 2.3 kg at 20 wk of age. A fourth group, Avian strain 4 of meat-type chickens, was fed the same diet as Group 1. The fifth group was Sex- Links 4 and was fed diets, ad libitum, that were appropriate for heavy egg-type chickens. The energy content of diets was 3,200 kcal/kg for Groups 1, 2, and 4 and 2,900 kcal/kg for Groups 3 and 5. Each group was reared in a floor pen, beginning with 25 males and 25 females. All groups were exposed to continuous lighting for 3 d. Groups 1, 2, and 4 were given 23 h of light each day, and Groups 3 and 5 were provided 9 h of light to simulate normal lighting schedules. Body Measurements The size of the chicken that could be used was limited by the TOBEC scanning cylinder, which was approximately 15 cm in diameter. A chicken weighing 1.2 kg was the smallest that could be accurately scanned, and 2.30 kg was the largest to fit into the cylinder for accurate scanning. As a result, target weights for body measurements were 1.20, 1.75, and 2.30 kg. Ross males in Group 1 reached those weights in the fewest days. At an average weight of approximately 1.20 kg, six Ross males were randomly selected for measurement. Six males from Group 1 were again randomly selected at average weights of approximately 1.75 and 2.30 kg. Selection weights of females in Group 1 were the same as for the males. Eighteen males and 18 females from Group 1 were measured. The same procedure was followed for all groups. Six chickens were selected and then weighed and measured individually. A cloth tape measure was used for back length. When the chicken is standing, the neck curves so that the neck is almost perpendicular to the back. The back was measured from the nadir of the curve to the base of the tail. Circumference was measured with the tape at the anterior end of the keel bone. The tape was passed under the wings and anterior to the legs. Keel length was measured with the tape as the chicken was held on its back.

3 870 LATSHAW AND BISHOP Several measurements were taken with calipers 5 that had a span of 20 cm. Width of the pelvis was measured when the chicken was standing. The calipers rested on the back and measured the distance between the outer edges of the thighs. Breast width was measured at the anterior end of the keel while the chicken was held on its back. Abdominal skinfold thickness was measured when the chicken was standing. Abdominal skin below the vent and pelvic bones was drawn out far enough to apply the skinfold meter 6 and measure thickness. Electrical Conductance Total body electrical conductance was measured using an EM-SCAN. 7 The phantom or calibration standard was placed in the measurement cylinder (model-sa-152) 7 for calibration. A chicken s legs were grasped in one hand, and the chicken was placed in the cylinder. Care was taken to position the chicken so that it was in an upright position and that the feather line on the hocks was at the interface of the internal cylinder and the metal encasement. If the chicken struggled when placed in the scanner, it did so for only a few seconds. When voluntary movement stopped, the measurement began. For the first few seconds, the two sets of numbers on the display increased fairly rapidly, at least for the larger chickens. Then they increased or decreased slowly. When this stage was reached, scanning was continued for approximately 15 s before termination and recording results. If a chicken had voluntary activity after scanning began, it was removed from the cylinder, and the procedure was begun again. After being scanned, the chickens were placed in a floor pen with no access to feed until the next morning. They had access to water until 0400 h when the lights were turned off. At approximately 0800 h, the chickens were weighed and TOBEC was measured again. After being scanned, the chickens were killed by cervical dislocation, placed in polyethylene bags intact, and frozen for further analysis. Whole Body Sampling Frozen chickens were thawed and cut into appropriately sized pieces. Pieces from each chicken were ground in a meat grinder (Model 4822) 8 through a die with cm holes. The resulting material was reground in the meat grinder through a die with cm holes, and this material was minced by a meat chopper (Model 48181). 8 An aluminum pan was weighed, a sample of approximately 100 g of ground chicken was added to the pan, and the pan and contents were reweighed and stored at 20 C. 5 Edmunds Scientific, Barrington, NJ Fat-O-Meter, Edmunds Scientific, Barrington, NJ EM-SCAN, Inc., Springfield, IL Hobart Corp., Columbus, OH Parr Instrument Co., Moline, IL TABLE 1. Means and ranges of measurements from chickens that were used to model BW, body fat, and body energy Parameter Mean Range Body measurements Back length, cm to 26.0 Breast width, cm to 8.5 Circumference, cm to 43.8 Keel length, cm to 13.3 Pelvis width, cm to 16.3 Skinfold thickness, cm to 2.00 TOBEC, 1 feed deprived, u to 1,807 TOBEC, 1 fed, u to 1,894 Weight, feed deprived, g 1,650 1,096 to 2,342 Weight, fed, g 1,718 1,154 to 2,456 Body composition Water, % to 71.8 Fat, % to 23.5 Ash, % to 3.9 Protein and carbohydrate, % to 23.3 Energy, kcal/g to Total body electrical conductivity. To dry the samples, pans and contents were placed in a forced-air oven at 65 C for 3 h. Drying was accomplished at a temperature of 45 C for 60 h. Final drying was completed by placing in desiccators for 48 h. The pan and contents were weighed, the contents were stored in a polyethylene bag and frozen, and the sample was ground in a kitchen blender. The ground sample was again placed in a polyethylene bag and stored at 20 C. Standard procedures were used to measure fat and ash content of the dry material (AOAC International, 1995). Protein and carbohydrate contents were found by subtracting water, fat, and ash percentages from 100. The energy content was determined by using a 0.5-g sample in an adiabatic bomb calorimeter. 9 Statistical Analysis Data from 179 chickens were analyzed using multiple linear regression techniques (Draper and Smith, 1981). Five independent variables were used to estimate body weight. They were back length, breast width, circumference, keel length, and pelvis width. Four independent variables, TOBEC value from fed weight, sex, skinfold thickness (transformed to natural logarithm), and fed weight, were used to estimate body fat percentage. An additional model included variables that provided the best fit of data when the TOBEC value was omitted. Two other models were also developed that used body fat content as the independent variable. In one model, body water content was the dependent variable; in the other model, body energy content was the dependent variable. Body fat, ash, and energy were measured on dry material; however, they were expressed as they related to the live chicken. RESULTS The means and ranges of data collected in this experiment are summarized in Table 1. During the sampling

4 ESTIMATING CHICKEN BODY WEIGHT AND COMPOSITION 871 period, average BW approximately doubled. Broilers fed unlimited amounts of feed were sampled beginning at 27 d of age and were completed by 56 d of age. Broilers fed limited amounts of feed that simulated a broiler breeder feeding program were sampled beginning at 58 d of age and were completed at 119 d of age. The Sex-Link chickens were sampled from 81 to 139 d of age. The females in this group had an average BW of 1.76 kg at 139 d; however, the experiment was terminated because combs indicated the onset of sexual maturity. Body measurements had a range of values (Table 1). Except for circumference, the largest measurement for each variable was approximately twice that of the smallest measurement. Ranges of some measurements were more than double the smallest measurement. Components of body composition also had a range of values (Table 1). Fat had the largest range, from 2.6 to 23.5% of the body. As a result, the range of body energy was large. Body fat also affected other body components. A model was developed to estimate BW from body measurements (Table 2). Multiple linear regression indicated no difference in quality of fit (based on R 2 )ifthe weight of fed chickens was estimated or if the weight of feed-deprived chickens was estimated. As a result, the weight of fed chickens was used in the models. The best single body measurement to estimate BW was pelvis width, resulting in R 2 = The model was as follows: weight (g) = (pelvis, cm). If the five measurements of keel length, breast width, pelvis width, length of back, and circumference were included, R 2 = Inclusion of only three body measurements resulted in R 2 = The model was BW (g) = (breast, cm) (circumference, cm) (pelvis, cm). A model was developed to estimate the percentage of body fat by using several measurements. The measurements that were useful in the model were abdominal skinfold thickness (as a natural logarithm), sex of the chicken, BW, and TOBEC measurements. Inclusion of those four parameters in the model resulted in an R 2 = 0.78 (Table 3). When the TOBEC value was removed from the model, the parameters of abdominal skinfold thickness and BW were the most useful to estimate body fat (R 2 = 0.63). Two other models were calculated from the data. One related body water and body fat as follows: body water (%) = (body fat, %) (R 2 = 0.75). The second model related body energy and body fat as follows: body energy (kcal/g) = (body fat, %) (R 2 = 0.93). DISCUSSION Body measurements provided an accurate base of data to estimate BW of chickens that weighed between 1.2 and 2.3 kg. The first research that related BW and shank length in one breed of chickens had an R 2 = 0.66 (Lerner, 1937). Pelvis width and body weight had an R 2 = 0.67 in the present research in which several body conformations were included. Shank length was not measured, because preliminary research indicated almost identical quality of fit if shank length or keel length were included in models. In the present research, the single independent variable of keel length resulted in an R 2 = 0.24 for estimation of BW. Inclusion of more body measurements improved the quality of fit in estimating BW. The variables of breast width and circumference, when included with pelvis width, improved the quality of fit. Addition of the variables keel length and length of back to the previous three variables slightly improved quality of fit. Simple measurements also provided reliable data to estimate the body fat content of chickens in the weight range that was examined. Skinfold thickness was the single variable that provided the best estimate of body fat. The variables of weight and skinfold thickness gave a quality of fit of R 2 = This value is comparable to those found with humans. When skinfold data collected from male college athletes were fit to various models, correlations of skinfold thickness and body fat ranged between 0.70 and 0.85 (Sinning et al., 1985). In that study, linear models provided as good fit of the data as did curvilinear models that were more sophisticated. The R 2 in the present study is also similar to that from research with broiler chickens relating abdominal fat to abdominal skin thickness (Mirosh and Becker, 1984). Based on previous research with chickens (Staudinger et al., 1995), it was expected that the TOBEC procedure would provide the best estimate of body fat. That research estimated the fat-free masses of broiler chicks weighing between 46 and 334 g. There was probably little difference in fat content or body conformation among chicks, because they were from commercial broiler stock. Under those conditions, the TOBEC procedure by itself was able to accurately estimate fat-free mass (R 2 = 0.95). Percentage TABLE 2. Estimating the weight (g) of chickens by using three body measurements Parameter Standard Variable estimate error t-value P > t Intercept < Breast width < Circumference < Pelvis width < Width of the breast (cm) measured perpendicular to the keel at the anterior end. 2 Circumference of the body (cm) measured anterior to the legs of a standing chicken. 3 Distance between the exterior of the thighs (cm) measured across the back when the chicken was standing.

5 872 LATSHAW AND BISHOP TABLE 3. Coefficients to estimate the percentage of body fat in chickens Parameter Standard Scan and variable estimate error t-value P > t Included in model Intercept Skinfold thickness < Sex BW < TOBEC value < Not included Intercept < Skinfold thickness < BW < Natural logarithm of abdominal skinfold thickness (cm). 2 Male = 0, and female = 1. 3 BW (kg). 4 Total body electrical conductivity scan. of body fat could then be calculated from the difference between total mass and fat-free mass. The present research, in contrast, used genetic difference and feeding programs to maximize differences in body conformation and body fat. Body weights ranged from 1,154 g to 2,456 g. Percentage body fat was estimated directly from the TOBEC procedure or from the TOBEC procedure and other parameters. With those conditions the TOBEC procedure, by itself, was not useful for estimating percentage body fat. The results of the procedure, when combined with other parameters, improved the ability to estimate percentage body fat. An explanation for the discrepancy between the expected ability of the TOBEC procedure (Staudinger et al., 1995) to predict body fat and the actual inability to do so in the present study may come from other research. Staudinger et al. (1995) developed a standard curve that related total body mass to total fat-free mass in one trial. In two other trials, the predicted fat-free mass was correlated with the determined fat-free mass, with excellent R 2. Mitchell et al. (1997) also found an excellent fit of the data when the predicted total amount of component in the body was regressed against the chemically determined total amount of body component; however, a poor fit of the data resulted when the percentage of predicted fat and the percentage of chemically determined fat were regressed for chickens weighing less than 2.0 kg. The fit of the data was better at weights greater than 2.0 kg but was not as good as was obtained in the present research. If electronic devices are used to assist in measuring body composition, the TOBEC procedure has some advantages. It does not harm the chicken and requires only manual restraint. It is also a procedure that can be completed in minutes, as opposed to x-ray absorptiometry, which requires anesthesia. Models developed in this research can aid in estimating the complete body composition. Percentage body fat can be estimated using coefficients (Table 3) that include or exclude values from the TOBEC procedure. When fat content is estimated, water content can be estimated by the following equation: body water (%) = (body fat, %). The average body ash content was 2.7%. The combined percentage of protein and carbohydrate can be estimated by subtracting percentages of water, fat, and ash from 100. Finally, this research related body energy to body fat using the following equation: body energy (kcal/g) = (body fat, %). REFERENCES AOAC International, Official Methods of Analysis of AOAC International. 16th ed. AOAC International, Gaithersburg, MD. Blaxter, K., Pages in: Energy Metabolism in Animals and Man. Cambridge University Press, Cambridge, UK. Draper, N., and H. Smith, Applied Regression Analysis. 2nd edition. John Wiley and Sons, New York, NY. Heinricks, A. J., and G. L. Hargrove, Standards of weight and height for Holstein heifers. J. Dairy Sci. 70: Lerner, I. M., Shank length as a criterion of inherent size. Poultry Sci. 16: Liu, Y., and J. R. Stouffer, Pork carcass evaluation with an automated and computerized ultrasonic system. J. Anim. Sci. 73: Lofgreen G. P., A comparative slaughter technique for determining net energy values with beef cattle. Proceedings of the 3rd Symposium of Energy Metabolism of Farm Animals. European Assoc. Anim. Prod. 11: Lohman T. G., Skinfolds and body density and their relation to body fatness: A review. Human Biol. 53: Mirosh, L. W., and W. A. Becker, Comparison of abdominal region components with abdominal fat in broilers. Poultry Sci. 63: Mitchell, A. D., R. W. Rosebrough, and J. M. Conway, Body composition analysis of chickens by dual energy X-ray absorptiometry. Poultry Sci. 76: National Research Council, Nutrient Requirements of Poultry. 9th ed. National Academy Press, Washington, DC. Pesti, G. M., and R. I. Bakalli, Estimation of the composition of broiler carcasses from their specific gravity. Poultry Sci. 76: Polleymounter, M. A., M. J. Cullen, M. B. Baker, R. Hecht, D. Winters, T. Boone, and F. Collins, Effects of the obese gene product on body weight regulation in ob/ob mice. Science 269: Pym, R.A.E., and J. M. Thompson, A simple caliper technique for the estimation of abdominal fat in live broilers. Br. Poult. Sci. 21:

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