Energy and protein requirements for maintenance and growth of Boer crossbred kids 1

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1 Published December 8, 2014 Energy and protein requirements for maintenance and growth of Boer crossbred kids 1 M. H. M. R. Fernandes,* 2 K. T. Resende,* L. O. Tedeschi,# J. S. Fernandes Jr.,* H. M. Silva,* G. E. Carstens,# T. T. Berchielli,* I. A. M. A. Teixeira,* and L. Akinaga* *UNESP - São Paulo State University, Jaboticabal, SP , Brazil; #Texas A&M University, College Station, TX 77843; and Postgraduation Program of Animal Science, UNESP, Jaboticabal, Brazil ABSTRACT: Meat production by goats has become an important livestock enterprise in several parts of the world. Nonetheless, energy and protein requirements of meat goats have not been defined thoroughly. The objective of this study was to determine the energy and protein requirements for maintenance and growth of 34 ³ ₄ Boer ¹ ₄ Saanen crossbred, intact male kids (20.5 ± 0.24 kg of initial BW). The baseline group was 7 randomly selected kids, averaging 21.2 ± 0.36 kg of BW. An intermediate group consisted of 6 randomly selected kids, fed for ad libitum intake, that were slaughtered when they reached an average BW of 28.2 ± 0.39 kg. The remaining kids (n = 21) were allocated randomly on d 0 to 3 levels of DMI (treatments were ad libitum or restricted to 70 or 40% of the ad libitum intake) within 7 slaughter groups. A slaughter group contained 1 kid from each treatment, and kids were slaughtered when the ad libitum treatment kid reached 35 kg of BW. Individual body components (head plus feet, hide, internal organs plus blood, and carcass) were weighed, ground, mixed, and subsampled for chemical analyses. Initial body composition was determined using equations developed from the composition of the baseline kids. The calculated daily maintenance requirement for NE was 77.3 ± 1.05 kcal/kg 0.75 of empty BW (EBW) or 67.4 ± 1.04 kcal/kg 0.75 of shrunk BW. The daily ME requirement for maintenance (118.1 kcal/kg 0.75 of EBW or kcal/kg 0.75 of shrunk BW) was calculated by iteration, assuming that the heat produced was equal to the ME intake at maintenance. The partial efficiency of use of ME for NE below maintenance was A value of 2.44 ± 0.4 g of net protein/kg 0.75 of EBW for daily maintenance was determined. Net energy requirements for growth ranged from 2.55 to 3.0 Mcal/kg of EBW gain at 20 and 35 kg of BW, and net protein requirements for growth ranged from to g/ kg of EBW gain. These results suggest that NE and net protein requirements for growing meat goats exceed the requirements previously published for dairy goats. Moreover, results from this study suggest that the N requirement for maintenance for growing goats is greater than the established recommendations. Key words: comparative slaughter, goat, net requirement, nutrition, production 2007 American Society of Animal Science. All rights reserved. J. Anim. Sci : doi: /jas INTRODUCTION Production of meat from goats, especially the Boer breed, has increased considerably during the last decade, becoming an important livestock enterprise in several parts of the world (Goetsch and Sahlu, 2004). Boer goats are highly fertile and yield high-quality lean 1 We thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, São Paulo-SP, Brazil, Proc. 03/ ) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brasília-DF, Brazil) for providing the financial support for Márcia H. M. R. Fernandes. 2 Corresponding author: mhmrocha@hotmail.com Received February 27, Accepted November 28, meat, particularly when harvested at a young stage of maturity (Malan, 2000). Meat goats can also be used in crossbreeding programs to improve the quality and rate of growth of dairy goat male kids (Goetsch and Sahlu, 2004). The ease of adaptability to intensive or harsh extensive conditions help to ensure a high adoption of the Boer breed to produce animal protein for human consumption (Bradford, 1999; Malan, 2000). Energy and protein requirements of Boer kids and Boer crossbreds are not well established, which limits the development of more efficient feeding systems. The NRC (1981a) and the AFRC (1998) recommendations assumed similar energy and protein requirements between meat and dairy goats and no differences among breeds. Recently, Luo et al. (2004a,b,c) evaluated a database of 349 treatment means and reported that the 1014

2 Energy and protein requirements of goats 1015 Table 1. Ingredient and chemical composition of the diet Item Dietary ingredient, % of DM Dehydrated corn plant 46.8 Cracked corn grain 25.6 Soybean meal 19.2 Molasses 4.5 Soybean oil 0.9 Mineral supplement Limestone 0.9 Diet composition DM, % as-fed 92.9 Ash, % of DM 7.9 CP, % of DM 14.5 Crude fat, % of DM 3.7 GE, kcal/kg of DM 3,964 DE, kcal/kg of DM (0.075 DMI 2 ) NDF, % of DM 39.6 ADF, % of DM Composition, per kg, as-fed basis: 190 g of Ca; 92 g of Cl; 73 g of P; 62 g of Na; 44 g of Mg; 30 g of Sn; 1.35 g of Zn; 1.06 g of Fe; 0.94 mg of Mn; 0.73 g of F (maximum); 0.34 g of Cu; 18 mg of Se; 16 mg ofi;3mgofco. 2 DMI as a percentage of shrunk BW daily. MP requirement for maintenance was equal for all biotypes of growing goats; however, the MP requirement for gain was greater for meat (>50% Boer) than for dairy or indigenous goats. Those authors suggested that the factors responsible for the latter finding remain unclear and may be due to a higher protein concentration in BW gain in meat goats. The main objective of this study was to use body composition data from a comparative slaughter trial of ³ ₄ Boer ¹ ₄ Saanen crossbred goats to determine the energy and protein requirements for maintenance and gain. A second objective was to derive equations to estimate fat and protein content of BW gain in growing goats. MATERIALS AND METHODS Animal and Management Description This study was conducted at the Goat Center at Faculdade de Ciências Agrárias e Veterinárias - FCAV/ UNESP (São Paulo, Brazil) during 2003 and Humane animal care and handling procedures were followed according to the university s animal care committee. Thirty-four ³ ₄ Boer ¹ ₄ Saanen crossbred male kids were used in this trial. The kids were dehorned and intact. After weaning at approximately 60 d of age, all kids were fed the experimental diet (Table 1) for ad libitum consumption until the beginning of the experiment (20.5 ± 0.24 kg of initial BW). The baseline (BL) group was 7 randomly selected kids slaughtered at 21.2 ± 0.36 kg of BW. An intermediate slaughter group consisted of 6 randomly selected kids, fed for ad libitum intake, that were slaughtered when they reached 28.2 ± 0.39 kg of BW. On d 0, the remaining kids (n = 21) were allocated randomly to 3 treatments that consisted of 3 levels of DMI: ad libitum or restriction to 70 or 40% of ad libitum intake. These kids were pair-fed in 7 slaughter groups. A slaughter group consisted of 1 kid from each treatment, and they were slaughtered when the ad libitum treatment kid reached 35 kg of BW. The 70 and 40% of ad libitum intake levels were determined daily, based on the DMI of the kids in the ad libitum treatment on the previous day. The experimental diet (Table 1) consisted of dehydrated corn plant, cracked corn grain, soybean meal, molasses, soybean oil, limestone, and mineral supplement and was fed as a total mixed diet. Dehydrated corn plant was whole corn plants (60 to 70% moisture) chopped when the kernel milk line was approximately two-thirds of the distance down the kernel. The chopped material was air dried for approximately 72 h or until it reached 8 to 10% moisture, and then the dried chopped material was ground to pass a 4-mm screen (Wiley-type mill). During the trial, the kids were fed once daily (0800) in individual pens that were protected from rain and wind and with free access to water. Feed ingredients were dried at 60 to 65 C for 72 h and ground through a 1-mm screen using a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA). Ingredients were analyzed for fat (by loss in weight of the dry sample upon extraction with petroleum ether in a Soxhlet extraction apparatus for 6 h; AOAC, 1990), protein [N analysis via Dumas combustion using Leco FP-528LC (St. Joseph, MI); Etheridge et al., 1998], ash (complete combustion in a muffle furnace at 600 C for 6 h; AOAC, 1990), NDF with amylase and without sulfite (Van Soest et al., 1991), ADF (Goering and Van Soest, 1970), and GE using a bomb calorimeter (Parr Instrument Co., Moline, IL). Determination of Diet Digestibility A digestion trial with a completely randomized design was conducted in parallel with the comparative slaughter trial and used 15 male kids (24 ± 3.8 kg of BW) at 3 levels of intake (ad libitum or restricted to 70 or 40% of the ad libitum intake) to determine dietary DE and ME, energy metabolizability (q m ), digestible protein, and the biological value of the protein. These animals were housed in individual metabolic cages. Feed intake, feed refusals, feces, and urine were collected for 5 d after a 5-d adaptation period to each new level of intake. Urine was acidified daily with 20 ml of 6 M HCl. Feeds and feed refusals were sampled daily, and the samples were stored at 20 C. Feces and urine were weighed daily, and a 10% sample was collected and stored at 20 C. Composites of feeds, feed refusals, and feces were dried at 60 to 65 C for 72 h and ground through a 1- mm screen using a Wiley mill. Composites of urine were passed through a sieve to remove the large particles, and a subsample was taken for N determination. Gross

3 1016 energy was determined for feeds, feed refusals, feces, and urine using a bomb calorimeter (Parr Instrument Co.). Digestible energy was computed from the GE of the feeds, feed refusals, and feces. The DE values were regressed on daily DMI (% of shrunk BW; SBW) of the goats in the digestibility trial, and the regression was subsequently used in the comparative slaughter trial to estimate the DE content and intake. The SBW was measured after feed and water were withdrawn for 16 h. Dietary ME was computed from the DE, GE, and level of intake using Eq. [1] (Tedeschi et al., 2002), which included adjustments for DE at maintenance (ARC, 1980), methane production (Blaxter and Clapperton, 1965), urinary losses (Blaxter et al., 1966), and fecal energy losses (ARC, 1980), as follows: ME, Mcal/kg = [ GE (1 L)] + {DE m [ (0.163 L)]}, [1] where GE is the dietary GE (3.96 Mcal/kg); L is the ratio of DE intake to DE required for maintenance; and DE m is the DE when fed at maintenance intake, in Mcal/kg. The dietary DE m varied from 3.08 to 3.09 Mcal/ kg, and L varied from 1.2 to 2.5. The dietary ME ranged from 2.40 to 2.46 Mcal/kg, with an average of 2.43 Mcal/kg. Slaughter and Body Composition Techniques Body weights were measured immediately before feed and water were withdrawn for 16 h. Shrunk BW was measured immediately before slaughter and after feed and water were withdrawn for 16 h. At slaughter, the kids were stunned with an electric shock and killed by exsanguination using conventional humane procedures. Blood was weighed and sampled. The body was separated into individual components, which were then weighed separately. The components included internal organs (liver, heart, lungs and trachea, tongue, kidneys, and spleen), emptied and cleaned digestive tract (rumen, reticulum, omasum, abomasum, and small and large intestines), hide, head, feet, and carcass. The digestive tract was weighed before and after emptying and flushing with water. All body components were initially frozen at 6 C, then cut into small pieces, ground with a large screw grinder through a plate with 0.32-cm holes, and mixed by 2 additional passes through the grinder. After grinding and homogenization, the samples were collected, frozen again, and freeze-dried for DM determination. These samples, approximately 30 g, were analyzed for fat, protein, and ash as described previously. Empty BW (EBW) was computed as SBW at slaughter minus digestive tract contents. Because the sum of fat, protein, water, and ash content of the empty body did not add up to the observed EBW (105.6 ± 4.4%), the components were proportionally scaled to match the observed EBW. Fernandes et al. Data Calculation and Analyses Calculation of Initial Body Composition. The procedures used to compute retained energy (RE, Mcal) and energy requirements for maintenance were similar to those of Lofgreen and Garrett (1968), except that a regression equation was developed from the BL animals to determine the initial composition of the EBW and SBW rather than using their mean BW and body composition. The initial EBW was computed from SBW (Eq. [2], r 2 = 0.82, root mean square error [RMSE] = 0.34): EBW, kg = 0.68 (± 3.2) + [0.80 (± 0.17) SBW, kg]. [2] Initial empty body protein was estimated from EBW for each animal (Eq. [3], r 2 = 0.70, RMSE = 0.07): Empty body protein, kg = 0.50 (± 0.67) + [0.14 (± 0.04) EBW, kg]. [3] Initial empty body fat, water, ash, and energy were estimated from the mean BW and body composition of the BL animals. Energy Partitioning. Rates of EBW gain (EWG, kg/ d) and of body fat and protein gain were calculated as the difference between the initial and final weights of the respective body components, divided by number of days on trial. The caloric value of retained fat was assumed to be Mcal/kg of fat (Blaxter and Rook, 1953), and that of retained protein was assumed to be Mcal/kg of protein (Garrett, 1958). Heat production (HP, kcal/kg 0.75 of EBW) was calculated as the difference between ME intake (MEI, kcal/ kg 0.75 of EBW) and RE (kcal/kg 0.75 of EBW). The antilog of the intercept of the linear regression between the log of HP and MEI was used to estimate the maintenance requirement for NE (MRNE; kcal/kg 0.75 of EBW; Lofgreen and Garrett, 1968). The maintenance requirement for ME (MRME, kcal/kg 0.75 of EBW) was computed by iteratively solving the semilog linear regression equation until HP was equal to MEI. Linear regressions (Lofgreen and Garrett, 1968) of the log of HP on MEI were used to calculate the energy utilization for maintenance (k m ), which was computed as MRNE/ MRME. The slope of the regression of RE on MEI above maintenance was assumed to be the partial efficiency of energy utilization for growth (k g ). Protein for Maintenance. A linear regression of the retained N in the daily gain (g of N/kg 0.75 of EBW) on N intake (g of N/kg 0.75 of EBW) was used to calculate net N requirement for maintenance. The intercept of the regression was assumed to be the endogenous and metabolic losses of N, which is assumed to be the maintenance requirement for net protein (MRNP). Requirements for Growth. Estimates of the composition of the gain were obtained in 2 phases. In the first phase, the logs of allometric equations (Eq. 4) were

4 Energy and protein requirements of goats 1017 calculated to predict the protein, fat, water, ash, or energy concentration from EBW (ARC, 1980), as follows: Log 10 (component amount) = a+[b log 10 (EBW, kg)], [4] where component amount is the nutrient amount in the EBW. In the second phase, Eq. [4] was differentiated to compute the estimates of the composition of the gain at various EBW (Eq. [5]): [Component] = b 10 a EBW (b 1), [5] where [Component] is the nutrient or energy concentration per unit of EWG (in g/kg of gain or kcal/kg of gain); EBW is in kilograms; and a and b are coefficients determined from a linear regression (Eq. [4]). Statistical Analysis The data were analyzed as a completely randomized design using SAS (SAS Inst. Inc., Cary, NC). The linear regressions analyses were conducted with PROC REG. The analysis of DMI, DE, and MEI, body composition, ADG, and EBW were performed using PROC GLM. Residuals plotted against the predicted values were used to check the assumptions of the model for homoscedasticity, independency, and normality of the errors. A data point was deemed to be an outlier and removed from the database if and only if the Studentized residual was outside the ±2.5 range values. The comparison of the means was performed using the least squares means option of SAS. RESULTS AND DISCUSSION Performance and Body and Gain Compositions As expected, DMI and MEI were greater (P = ) for kids in the ad libitum treatment than for those in the restricted treatments (Table 2). Therefore, kids in the ad libitum treatment had greater (P = ) ADG, EWG, RE, and HP, indicating that HP increased as MEI increased. The ratio of EBW to SBW was greater (P = 0.002) for the kids fed to ensure ad libitum intake than those fed 40% of ad libitum intake (0.87 and 0.81, respectively). The equations to predict rate of fat gain (FG; Eq. [6], r 2 = 0.91, RMSE = 0.005) and rate of protein gain (PG; Eq. [7], r 2 = 0.97, RMSE = 0.002) are FG, kg/d = (± 0.006) + [0.012 (± 0.002) REc] + [0.211 (± 0.014) EWG, kg/d], and [6] PG, kg/d = [ (± 0.067) FG] + [0.192 (± 0.013) EWG, kg/d], [7] where REc is concentration of RE (Mcal/kg of EWG). There were 2 outliers in the FG regression. Equations to predict percentage of fat in the gain (FIG, Eq. [8], r 2 = 0.92, RMSE = 1.6) and percentage of protein in the gain (PIG, Eq. [9], r 2 = 0.99, RMSE = 0.004) are: FIG, % of EWG = (± 1.71) + [9.71 (± 0.59) REc], and [8] PIG, % of EWG = [17.58 (± 0.006) REc] [1.654 (± 0.001) FIG], [9] where REc is concentration of RE (Mcal/kg of EWG). There were 2 outliers in the FIG regression. Equation [9] is very similar to that derived by Tedeschi et al. (2002) using Nellore bulls (PIG, % = [17.6 REc] [1.65 FIG] ). However, our equation to predict FIG had a lesser slope and an intercept closer to zero than did the equation of Tedeschi et al. (2002; FIG, % = [11.5 REc] 10.24), suggesting, at the physiological states of the animals studied, that male kids have leaner carcasses than bulls and that the proportion of RE as protein was greater for goats than for cattle. The proportion of RE as protein was estimated from the REc, as shown in Eq. [10]. There were 2 outliers in the REp regression. REp = (± 0.09) + [ (± 0.58) Exp ( (± 0.22) REc)], [10] where REp is protein energy as a proportion of RE (Mcal/Mcal); and REc is the concentration of RE (Mcal/ kg of EWG). The pattern of Eq. [10] is very similar to that reported by Tedeschi et al. (2004) for intact and castrated cattle. Figure 1 shows the relationship of REp and REc, and the REp predicted using Eq. [10] and that reported by Tedeschi et al. (2004). The equation developed by Tedeschi et al. (2004) for cattle tended to over- and underpredict REp below 2.8 and above 4.2 Mcal/kg; respectively. This was likely due to the high leverage of some points in our database. Nonetheless, the equation developed by Tedeschi et al. (2004) had a satisfactory fit for goats between the interval 2.8 and 4.2 Mcal/kg and was nearly identical to our Eq. [10]. This finding suggests we are able to accurately determine REp, and then, as shown by several studies (Williams and Jenkins, 2003; Tedeschi et al., 2004), REp can be used to compute the partial efficiency of ME use for NE for growth. Energy Requirement for Maintenance Energy required for maintenance is the amount of energy used in basal metabolism and lost as heat when an animal is fasting (also know as fasting HP) plus the heat of activity and the additional energy lost when an

5 1018 Fernandes et al. Table 2. Final body composition and energy balance of ³ ₄ Boer ¹ ₄ Saanen crossbred, intact male kids at baseline (BL), at an intermediate (IM) slaughter, and at 3 levels of intake Level of feed intake 2 Item 1 BL IM AL 70% 40% SEM P-value No. of kids Days on feed BW, kg < SBW, kg < EBW, kg < Fat, % of EBW Protein, % of EBW Water, % of EBW Ash, % of EBW ADG, g/d a 152 b 41 c 14.9 < EWG, g/d a 134 b 27 c 15.9 < FIG, % of EWG PIG, % of EWG b 17.9 b 24.8 a 1.4 <0.01 DMI, g/d a 763 b 442 c 50.2 <0.05 DMI, g/kg 0.75 of EBW a 82.8 b 54.8 c 5.1 <0.02 ME intake a 202 b 135 c 11.8 <0.03 Retained energy a 38 b 9 c 5.4 <0.02 Heat production a 164 b 126 c 8.0 <0.002 EBW/SBW a 0.85 a 0.81 b 0.01 <0.002 a c For level of intake within a row, means without a common superscript letter differ (P < 0.05). 1 SBW = shrunk BW, EBW = empty BW, EWG = EBW gain, FIG = fat in gain, PIG = protein in gain. 2 Ad libitum (AL) or restricted to 70 or 40% of the ad libitum intake. Animals of each group were slaughtered when the ad libitum kid reached 35 kg. Values for IM are for reference only; they were not used in the statistical regressions. 3 kcal/kg 0.75 of EBW daily. animal consumes enough feed to maintain a static body energy content (heat increment at zero energy balance; NRC, 1981b). In our study, changes in body energy content were measured at 3 levels of MEI, and through extrapolation, maintenance energy requirements and partial efficiencies of ME utilization for maintenance and growth were calculated. The linear regression of the log of HP on MEI (r 2 = 0.92, RMSE = 0.026) is shown in Eq. [11]: Log (HP, kcal/kg 0.75 of EBW) = (± 0.02) + [ (± ) MEI, kcal/kg 0.75 of EBW]. [11] Figure 1. Relationship between protein energy as a proportion of retained energy (REp, Mcal/Mcal) and concentration of retained energy in the daily empty BW gain (REc, Mcal/kg of empty BW gain). The lines represent the REp as predicted by Tedeschi et al. (2004; dotted line) and by equation [10] (solid line); REp = (± 0.09) + { (± 0.58) Exp [ (± 0.22) REc]}. The mean of the antilog of the confidence interval values of the intercept of Eq. [11] is the MRNE. The MRNE was 77.3 kcal/kg 0.75 of EBW, which is nearly identical to that reported for beef cattle (Lofgreen and Garrett, 1968; Tedeschi et al., 2002) and was adopted by the beef NRC (1984, 2000). The AFRC (1998) for goats recommended a value of 75.3 kcal/kg 0.75 based on 9 studies from 1906 to Our estimate is equivalent to 67.4 ± 1.04 kcal/kg 0.75 of SBW. The MRNE has been determined in other experiments in conditions very similar to our trial. Teixeira (2004) reported a MRNE (85.1 kcal/kg 0.75 of BW) that was 26% greater than our value for Boer Saanen crossbred goats. However, their kids were lighter (5 to 25 kg of BW) and younger than our kids. In contrast, smaller MRNE were obtained for Saanen kids by Ferreira (2003; 62.5 kcal/kg 0.75 of EBW with BW ranging from 20 to 35 kg), and Medeiros (2001; 69.2 kcal/kg 0.75 of

6 Energy and protein requirements of goats 1019 Table 3. Values of BW, apparent digestibility, N retention, DE, and ME in ³ ₄ Boer ¹ ₄ Saanen crossbred, intact male kids in the digestibility trial, at 3 levels of intake Level of feed intake 1 Item AL 70% 40% SEM P-value No. of kids BW, kg 28.1 a 24.4 b 19.6 c 0.30 <0.001 BW, kg a 10.9 b 9.3 c 0.10 <0.001 DM digestibility, % Energy digestibility, % 72.6 b 76.3 a 77.0 a DE, Mcal/kg 2.92 c 2.98 b 3.06 a 0.01 <0.001 ME, Mcal/kg 2.42 c 2.44 b 2.46 a 0.01 <0.001 CP digestibility, % 76.2 b 73.9 b 81.5 a N, g/kg 0.75 of BW Intake 1.88 a 1.59 a 1.13 b Fecal 0.50 a 0.42 a 0.21 b Urinary Retention a c Within a row, means without a common superscript letter differ (P < 0.05). 1 Ad libitum (AL) or restricted to 70 or 40% of the ad libitum intake. EBW with BW ranging from 5 to 20 kg). These findings suggest that Saanen kids might have a smaller MRNE than Boer kids and that the crossbreds are intermediate. Using a multiple regression approach with 8 breeds, Luo et al. (2004a) reported a MRNE of 71.2 kcal/kg 0.75 of BW for goats consuming diets at, near, or above maintenance. Those authors indicated that variability in estimating MRNE might be attributed partly to differences in mathematical models and accuracy of measurements. Their measurements were based on respiration calorimetry, which yields consistently greater estimates of RE and k g than those obtained using comparative slaughter (Johnson et al., 1997), decreasing HP and consequently MRNE estimates. The MRNE may decrease with advancing age or stage of maturity (Ferrell, 1988; CSIRO, 1990), and differences among cattle breeds suggest that MRNE of various goat breeds might also be variable (Gonçalves et al., 1991). Nonetheless, Luo et al. (2004a) stated that inadequate age information was available to address potential differences among goat genotypes in MRNE. The MRME, which was calculated by iteration assuming HP is equal to MEI at maintenance, was kcal/kg 0.75 of EBW. The partial efficiency of use of ME for NE below maintenance (k m ) was 65%. The k m suggested by AFRC (1998) is calculated from the equation k m = [0.35 q m ] (ARC, 1980), where q m is the diet metabolizability (i.e., dietary ME divided by GE). We calculated a q m for our diet of 0.66 based on the digestibility trial. The k m calculated from the ARC (1980) equation was 0.73, which was approximately 11% greater than our k m, resulting in a lesser MRME (105.3 kcal/kg 0.75 of EBW). Luo et al. (2004a) reported a greater variability in estimates of MRME than those of MRNE, which is expected given higher levels of feeding for direct determination of MRME and greater differences among diets in efficiency of ME utilization for gain than maintenance. Therefore, differences in MRME among genotypes might depend on variations in MRNE or k m, although dietary characteristics may affect MRNE and k m. Protein Requirements for Maintenance Values for apparent digestibility of CP and for N retention are presented in Table 3. Nitrogen retained in the gain (g of N/kg 0.75 of EBW) was regressed on N intake (g of N/kg 0.75 of EBW) to determine the MRNP (Figure 2A). The MRNP is assumed to be the sum of endogenous urinary N, metabolic fecal N, and dermal N losses, multiplied by the factor When N retention in the gain is regressed against a measure of N supply, the negative intercept at zero N intake provides an estimate of minimum N losses, which should be similar to the sum of endogenous urinary N and metabolic fecal N (AFRC, 1998). Our study indicated a value of 391 ± 59 mg of N/kg 0.75 of EBW of minimal N losses, which corresponds to MRNP of 2.44 ± 0.4 g/kg 0.75 of EBW or of 2.04 ± 0.3 g/kg 0.75 of BW. The N intake required for maintenance, at which retained n = 0, was 1.05 g of N/kg 0.75 of EBW, which corresponds to an intake of 6.57 g of CP/kg 0.75 of EBW. The efficiency of MP utilization for maintenance (k pm ) varies widely among feeding systems (i.e., 0.85 for ARC, 1980; 1.00 for AFRC, 1998; and 0.67 for NRC, 2001). The AFRC (1993) concluded that k pm is high relative to efficiencies for other functions in part because maintenance functions are obligatory, with AA available from both absorption and turnover of other tissues. Using k pm of 0.67 and 1.0, the corresponding MP requirements for maintenance are 3.6 and 2.44 g/kg 0.75 of EBW, respectively, which are slightly less than the recommendation of 3.8 g/kg 0.75 of SBW for beef cattle (NRC, 2000). The regression of N retention (g of N/kg 0.75 of BW) on digestible N intake (g of N/kg 0.75 of BW) resulted in a value

7 1020 Fernandes et al. Based on a summary of 7 studies, the NRC (1981a) proposed a digestible CP requirement for maintenance of goats of 2.82 g/kg 0.75 of BW, which is 27.7% less than our value of 3.9 g of digestible CP/kg 0.75 of BW. The MRNP for goats recommended by AFRC (1998, 2.19 g/ kg 0.75 of BW), based on data with sheep and cattle, and by INRA (1989; 2.30 g/kg 0.75 of BW), based on N balance of male goats, are 7.3 and 12.8% greater, respectively, than our value for MRNP of 2.04 g/kg 0.75 of BW. Similarly, Teixeira (2004) reported a MRNP for F 1 Boer Saanen goats of 2.56 g/kg 0.75 of BW. Lesser values were reported by Medeiros (2001) and Ferreira (2003) for Saanen kids (1.31 and 2.16 g/kg 0.75 of BW). In contrast, the greatest values were estimated by Luo et al. (2004b,c) for Angora (3.35 g/kg 0.75 of BW) and for meat, dairy, and indigenous goats (3.07 g/kg 0.75 of BW). The variation in MRNP by goats may be attributed to factors such as breed, experimental conditions, and methods of determination. Collectively, these results suggest that a single recommendation for all breeds and diverse practical conditions may not be feasible; therefore, mechanistic methods that account for these factors are necessary to accurately determine protein requirements for animal- and diet-specific conditions. Published results and our current findings do not support the NRC (1981a) digestible CP recommendation of 2.82 g of digestible CP/kg 0.75 of BW. We conclude that 3.9 g of digestible CP/kg 0.75 of BW is more appropriate for meat goats. Figure 2. Relationship between N retention and N intake of ³ ₄ Boer ¹ ₄ Saanen crossbred, intact male kids. (A) N retention, g/kg 0.75 of empty BW (EBW) = (± 0.06) + [0.41 (± 0.03) N intake, g/kg 0.75 of EBW], r 2 = 0.89, and root mean square error = 0.08; (B) N retention, g/kg 0.75 of BW = (± 0.06) + [0.57 (± 0.04) digestible N intake, g/kg 0.75 of BW], r 2 = 0.88, and root mean square error = of 3.9 g/kg 0.75 of BW for digestible CP required for maintenance (Figure 2B), assuming the N maintenance requirement is the intercept when N retention is zero. Net Requirements for Gain The relative proportion of protein, fat, and energy in the BW at 20, 27.5, and 35 kg are presented in Table 4. Table 4 shows that the proportion of fat and energy increased as BW increased. The proportion of protein was nearly constant, suggesting that fat was the main factor increasing body energy. Some studies have suggested that protein proportion in the body diminished when Saanen (Yáñez, 2002) and F 1 Boer Saanen (Teixeira, 2004) kids reached 21 and 15 kg, respectively. Geay (1984) showed that protein deposition increases with daily gain regardless of BW; however, the increase Table 4. Allometric equations to estimate body composition (protein, fat, ash, and water) and retained energy of ³ ₄ Boer ¹ ₄ Saanen crossbred, intact male kids BW, 3 kg Item 1 Allometric equations R 2 RMSE EBW, kg EBW, kg = 3.86 (± 0.96) + [0.93 (± 0.03) BW] Water, g/kg of EBW Log water, g = 2.94 (± 0.02) + [0.91 (± 0.01) log EBW, kg] Ash, g/kg of EBW Log ash, g = 1.62 (± 0.06) + [0.92 (± 0.05) log EBW, kg] Protein, g/kg of EBW Log protein, g = 2.16 (± 0.03) + [1.05 ± 0.03 log EBW, kg] Fat, g/kg of EBW Log fat, g = 1.61 (± 0.12) + [1.39 (± 0.09) log EBW, kg] Energy, kcal/kg of EBW Log energy, kcal = 3.03 (± 0.07) + [1.24 (± 0.05) log EBW, kg] ,049 2,241 2,411 1 EBW = empty BW. 2 RMSE = root mean square error. 3 Values were calculated from the equations.

8 Energy and protein requirements of goats 1021 Table 5. Estimates of protein and energy concentrations in the equation to predict net requirements for gain of the empty BW gain (EWG) at different BW BW, kg Equation of net requirements Item for gain 1 Empty BW, kg Energy, Mcal/kg of EWG ,322.5 EBW Protein, g/kg of EWG EBW EBW = empty BW; component concentration = b 10 a EBW (b 1), where the component concentration is the nutrient or energy concentration per unit of EWG, EBW is in kilograms, and a and b are coefficients determined from equations of Table 4. is slower as BW increases, suggesting an ever-decreasing pattern before animals reach a plateau. Similarly, the energy content of BW gain increases at decreasing rates as ADG increases. The RE ranged from 2.5 to 3.0 Mcal/kg of EWG, and net protein in gain ranged from to g/kg of EWG as BW ranged from 20 to 35 kg (Table 5). Table 6 summarizes NE and net protein requirements for growth from various experiments using conditions and methods similar to those of our experiment. Except for the results of Ferreira (2003), our values were greater than the findings of Resende (1989), Ribeiro (1995), Medeiros (2001), Teixeira (2004), and AFRC (1998) for dairy breeds. The NRC (1981a) recommended greater requirements for NE for growth (Mcal/kg of gain). A direct comparison of these findings (Table 6) is complex due to differences in experimental design, dietary variation, BW range, and methods and techniques used. Nonetheless, it suggests that the Boer breed, a meat breed, and its crossbreeds might have greater energy and protein deposition (%) than reported in most studies, therefore requiring greater requirements for growth. Partial Efficiency of Conversion of ME to NE for Growth. The k g value was calculated as the slope of the regression of RE (kcal/kg 0.75 of EBW) on MEI (kcal/ kg 0.75 of EBW) above maintenance as shown in Eq. [12] (r 2 = 0.86, RMSE = 9.56). The slope of Eq. [12] demonstrated the k g value of 0.42: RE, kcal/kg 0.75 of EBW = 46.4 (± 7.16) + [0.42 (± 0.03) MEI, kcal/kg 0.75 of EBW]. [12] Changes in body fat and protein with changes in BW and diet are needed to determine NE requirements with consumption of diets differing in diet metabolizability. However, body composition is not frequently measured, in part due to high costs and labor associated with harvest and the determination of chemical composition (Sahlu et al., 2004). Thus, some authors have calculated the energy allowances, not by a factorial method, but by regressing MEI on BW change or ADG (Luo et al., 2004c,d). Figure 3 shows graphically the linear regression between MEI and EWG of animals from this experiment. This regression indicated that MRME is kcal/kg 0.75 of EBW and ME requirements for gain are 6.3 Mcal/kg of EWG. Using the mean RE of 2.78 kcal/ kg of EWG (Table 5), k g would be 0.44, which is similar to k g (0.42) obtained by regressing RE on MEI (Eq. [12]). The AFRC (1998) calculates k g using the equation k g = [0.78 q m ] Using the mean value of q m of 0.66 for our diet, the value of k g would be 0.52, which is 20% greater than those estimated using regressions of RE on MEI or MEI on EWG. Luo et al. (2004c) reported a greater ME requirement for gain for mature Angora goats (8.82 kcal/g). Luo et al. (2004d) reported similar ME requirements for gain for mature goats (6.80 kcal/g), and lower values for meat and dairy growing kids (5.52 kcal/g) and indigenous growing kids (4.73 kcal/g). The estimate of the ME requirement for gain from our study is in accordance with that estimated using regression by Luo et al. (2004c). Increases in dietary energy intake resulted in progressively smaller increases in RE, likely because diet metabolizability de- Table 6. Literature reports of NE and net protein requirements for growth of goats, as determined in various individual experiments and estimated by the NRC (1981a) and the AFRC (1998) NE, kcal/g Net protein, Breed BW, kg of gain g/g of gain References Native Toggenburg or Alpine 5 to to to 172 Resende (1989) Native Toggenburg or Alpine 5 to to to 151 Ribeiro (1995) Saanen 5 to to to 184 Medeiros (2001) Saanen 20 to to to 221 Ferreira (2003) F 1 Saanen Boer 5 to to to 165 Teixeira (2004) Indigenous/West African dwarf 4.0 NRC (1981a) Saanen/Toggenburg 20 to to to 132 AFRC (1998)

9 1022 Fernandes et al. LITERATURE CITED Figure 3. Relationship between ME intake (MEI), kcal/ kg 0.75 of empty BW (EBW) and (A) empty weight gain (EWG, g/kg 0.75 of EBW) and (B) ADG, g/kg 0.75 of BW, of ³ ₄ Boer ¹ ₄ Saanen crossbred, intact male kids, using the regressions suggested by Luo et al. (2004c). creases when intake increases. Thus, the relationship between RE and GE intake might be curvilinear (Geay, 1984), suggesting the efficiency of utilization of ME for growth and fattening may decline as ADG increases. In conclusion, our study indicated that ³ ₄ Boer ¹ ₄ Saanen crossbred goats have a requirement of NE for gain between 2.5 and 3.0 Mcal/kg of EWG and net protein requirements for gain between and g/ kg of EWG, which are greater than previously published values for other goat breeds but less than values recommended by the NRC (1981a). Additional research is needed for other classes of goats, different ratios of concentrate to forage, and diverse production scenarios. AFRC Energy and Protein Requirements of Ruminants. Agricultural and Food Research Council. CAB International, Wallingford, UK. AFRC The Nutrition of Goat. Report N. 10. Agricultural and Food Research Council. CAB International, Wallingford, UK. AOAC Official Methods of Analysis. 15th ed. Assoc. Off. Anal. Chem. Arlington, VA. ARC The Nutrient Requirements of Ruminant Livestock. Commonw. Agric. Bureaux, Slough, UK. Blaxter, K. L., and J. L. Clapperton Prediction of the amount of methane produced by ruminants. Br. J. Nutr. 19: Blaxter, K. L., J. L. Clapperton, and A. K. Martin The heat of combustion of the urine of sheep and cattle in relation to its chemical composition and to diet. Br. J. Nutr. 20: Blaxter, K. L., and J. A. F. Rook The heat of combustion of the tissues of cattle in relation to their chemical composition. Br. J. Nutr. 7: Bradford, E Animal Agriculture and Global Food Supply. Task Force Report No Counc. Agric. Sci. Technol., Ames, IA. CSIRO Feeding Standards for Australian Livestock, Ruminants. Commonw. Sci. Ind. Res. Organ., Melbourne, Australia. Etheridge, R. D., G. M. Pesti, and E. H. Foster A comparison of nitrogen values obtained utilizing the Kjeldahl nitrogen and Dumas combustion methodologies (Leco CNS 2000) on samples typical of an animal nutrition analytical laboratory. Anim. Feed Sci. Tech. 73: Ferreira, A. C. D Body composition and nutritional requirements of protein, energy and macrominerals in Saanen kid goats. PhD Diss. Universidade Estadual Paulista/FCAVJ, Jaboticabal, SP, Brazil. Ferrell, C. L Contribution of visceral organs to animal energy expenditure. J. Anim. Sci. 66(Suppl. 3): Garrett, W. N The comparative energy requirements of sheep and cattle for maintenance and gain. PhD Diss. Univ. California, Davis. Geay, Y Energy and protein utilization in growing cattle. J. Anim. Sci. 58: Goering, H. K., and P. J. Van Soest Forage Fiber Analyses (Apparatus, Reagents, Procedures, and Some Applications). Agric. Handbook No ARS-USDA, Washington, DC. Goetsch, A. L., and T. Sahlu Preface. Small Rumin. Res. 53: Gonçalves, L. C., J. F. Coelho da Silva, S. C. Valadares Filho, and A. C. G. Castro Energy requirements of five genetics groups of steers. Rev. Soc. Bras. Zootec. 20: INRA Ruminant Nutrition: Recommended Allowances and Feed Tables. Institut Nacional de la Recherche Agronomique, Montrouge, France. Johnson, D. E., E. M. Larson, and M. J. Jarosz Extrapolating from ME to NE: Unintended consequences. Pages in Proc. 14th Energy Metab. Farm Animals, Newcastle, UK. Lofgreen, G. P., and W. N. Garrett A system for expressing net energy requirements and feed values for growing and finishing beef cattle. J. Anim. Sci. 27: Luo, J., A. L. Goetsch, I. V. Nsahlai, T. Sahlu, C. L. Ferrel, F. N. Owens, M. L. Galyean, J. E. Moore, and Z. B. Johnson. 2004a. Maintenance energy requirements of goats: Predictions based on observations of heat and recovery energy. Small Rumin. Res. 53: Luo, J., A. L. Goetsch, I. V. Nsahlai, T. Sahlu, C. L. Ferrel, F. N. Owens, M. L. Galyean, J. E. Moore, and Z. B. Johnson. 2004b. Metabolizable protein requirements for maintenance and gain of growing goats. Small Rumin. Res. 53: Luo, J., A. L. Goetsch, I. V. Nsahlai, T. Sahlu, C. L. Ferrel, F. N. Owens, M. L. Galyean, J. E. Moore, and Z. B. Johnson. 2004c. Prediction of metabolizable energy and protein requirements for maintenance, gain and fiber growth of Angora goats. Small Rumin. Res. 53:

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