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Journal of Animal Science - Animal Nutrition

Energy requirements for growth in male and female Saanen goats1

 

This article in JAS

  1. Vol. 93 No. 8, p. 3932-3940
     
    Received: Oct 20, 2014
    Accepted: May 08, 2015
    Published: July 10, 2015


    2 Corresponding author(s): izabelle@fcav.unesp.br
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doi:10.2527/jas.2014-8632
  1. A. K. Almeida*,
  2. K. T. Resende*,
  3. N. St-Pierre,
  4. S. P. Silva*,
  5. D. C. Soares*,
  6. M. H. M. R. Fernandes*,
  7. A. P. Souza*,
  8. N. C. D. Silva*,
  9. A. R. C. Lima* and
  10. I. A. M. A. Teixeira 2*
  1. * UNESP, Univ. Estadual Paulista, Department of Animal Science, Jaboticabal, SP. 14884-900, Brazil
     Department of Animal Sciences, The Ohio State University, Columbus 43201

Abstract

The aim of this study was to investigate the energy requirements of female and intact and castrated male Saanen goats. Animals were randomly assigned to 1 of 2 experiments designed to investigate the energy requirements for maintenance and gain. To determine the maintenance requirements, 85 goats were used (26 intact males, 30 castrated males, and 29 females) with an initial BW of 30.3 ± 0.87 kg. Thirty goats (8 intact males, 9 castrated males, and 13 females) were slaughtered to be used as the baseline group. The remaining goats were assigned in a split-plot design using a 3 × 3 factorial arrangement (3 sexes—intact males, castrated males, and females—and 3 DMI levels—ad libitum and restricted fed to 75 or 50% of the ad libitum intake). The NEg was obtained using 65 goats (20 intact males, 22 castrated males, and 23 females) fed ad libitum in a completely randomized design. Eight intact males, 9 castrated males, and 13 females were slaughtered at 30.5 ± 1.53 kg BW. Seventeen goats (6 intact males, 6 castrated males, and 5 females) were slaughtered at 38.1 ± 0.49 kg BW. The remaining goats were slaughtered at 44.0 ± 0.50 kg BW. The NEm did not differ between the sexes (P = 0.59; 258.5 kJ/kg0.75 BW), resulting in a ME for maintenance of 412.4 kJ/kg0.75 BW. The estimated energy use efficiency for maintenance was 0.627. During the growth phase, NEg differed between the sexes (P < 0.001); intact males, castrated males, and females showed an average NEg equal to 15.2, 18.6, and 22.7 MJ/kg of empty weight gain, respectively. The energy requirements for growth differed between the sexes. The difference was found to be due to distinct NEg and partial efficiency of ME utilization for growth in intact and castrated males and females during the late growth phase. This study may contribute to adjustments in feeding system energy recommendations regarding the NEm and NEg found for goats during the late growth phase.



INTRODUCTION

The Saanen goat breed is extensively used in dairy herds around the world, and mature females and males are more numerous in herds aimed at breeding. Nutrition plays an important role at the beginning of the reproductive life in these animals, especially with respect to energy supplementation. Animals that are either underfed or overfed may exhibit reproductive problems throughout their lives.

Although knowledge of energy requirements is crucial for proper feeding management, information about sex differences in energy demands in goats is controversial. The NRC (2007) stated that intact males require 15% more energy for maintenance than females and castrated males, based on the greater quantities of protein found in the bodies of intact males (Webster, 1981; Geay, 1984) and differences in the stage of maturity exhibited at equivalent BW. On the other hand, Ash and Norton (1987) did not find sex differences in the energy requirements of goats, and Chizzotti et al. (2007) reported similar findings in beef cattle.

Regarding the effect of sex on the energy requirements for growth in goats, the NRC (2007) recommended equal energy requirements for growth for males and females at equivalent BW. Because growth requirements depend on body composition, which is ultimately dictated by sex, an important hypothesis to test is whether females and intact and castrated males have different requirements. In fact, a previous study on beef cattle supports this hypothesis (Chizzotti et al., 2007). However, no studies have taken into account the effect of sex on the energy requirements of late growth stage goats.

Because of this gap in knowledge, the objective of this study was to determine the energy required for maintenance and gain in female and intact and castrated male Saanen goats weighing from 30 to 45 kg. The results presented here can directly assist in the formulation of optimal diets for goats in the final phase of growth that maximize energy use and profitability.


MATERIALS AND METHODS

The study was conducted at the Goat Laboratory of the Universidade Estadual Paulista (Unesp, Jaboticabal campus, São Paulo State, Brazil; 21°14′05″ S, 48°17′09″ W, and 595 m altitude). Humane animal care and handling procedures were followed in accordance with the university’s Animal Care Committee (Comissão de Ética e Bem Estar Animal), under protocol number 004972-09.

Two experiments were designed to determine the energy requirements for maintenance and gain. Until the beginning of the experiments, all goats were fed ad libitum and had free access to fresh water. In both experiments, the experimental diet (Table 1) was formulated to meet the energy and protein requirements of a 45-kg growing goat according to the NRC (2007). The diet (DM based) consisted of dehydrated corn plant (46.2%; prepared as described by Bompadre et al., 2014), ground corn grain (30.9%), soybean meal (15.2%), soybean oil (1.9%), limestone (1.0%), and mineral supplement (4.8%).


View Full Table | Close Full ViewTable 1.

Ingredient and chemical composition of the diet

 
Ingredients composition,2 g/kg DM
Ingredient DM1 OM CP Fat NDF NFC GE
Dehydrated corn plant 850 960 85.7 16 568 290 19.2
Soybean meal 861 931 541.9 22 158 216.1 20.3
Ground corn grain 858 989 84.8 47 127 728.3 19.2
Soybean oil 995 997 991 38.6
Limestone 999 3
Mineral supplement3 969 56
Total diet 866 907 147.2 48 312 391 18.5
1DM expressed in grams per kilogram, as-fed basis.
2NFC = nonfibrous carbohydrates. GE is expressed in megajoules per kilogram DM.
3Composition, per kilogram, as-fed basis: 190 g Ca, 92 g Cl, 73 g P, 62 g Na, 44 g Mg, 1.35 g Zn, 1.06 g Fe, 0.94 mg Mn, 0.73 g F (maximum), 0.34 g Cu, 18 mg Se, 16 mg I, and 3 mg Co.

Feed ingredients were dried at 55°C for 72 h and ground through a 1-mm screen using a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA). They were then analyzed for ash (AOAC, 1990; method number 924.05); fat (AOAC, 1990; method number 930.15); protein (N analysis performed via Dumas combustion using a LECO FP-528LC Nitrogen/Protein Determinator; LECO Corp., St. Joseph, MI; Etheridge et al., 1998); NDF, with amylase and without sodium sulfite (Van Soest et al., 1991); ADF (Goering and Van Soest, 1970); and GE, using a bomb calorimeter (IKA Calorimeter system C 2000 basic/control; IKA Works, Cincinnati, OH).

Experiment 1: Energy Requirements for Maintenance

To determine the maintenance requirements, 85 goats were randomly assigned as baseline (BL) animals (n = 30) or pair-fed animals (n = 55).

The BL animals were used to develop equations to predict the initial empty body energy content, which was used to estimate energy retention. In the BL group, 8 intact males slaughtered at 31.1 ± 1.81 kg BW and an age of 281 ± 33 d, 9 castrated males slaughtered at 30.6 ± 1.46 kg BW and an age of 274 ± 31 d, and 13 females slaughtered at 29.8 ± 1.34 kg BW and an age of 283 ± 33 d were used. A simple linear regression equation was developed to estimate initial empty BW (EBW; kg) from BW (kg) using the BL animals’ data. Likewise, nonlinear regression equations were developed to determine initial energy content (MJ) from initial EBW (kg).

The pair-fed animals comprised 18 intact males (30.9 ± 0.88 kg initial BW), 21 castrated males (30.2 ± 0.73 kg initial BW), and 16 females (29.8 ± 0.34 kg initial BW). These animals were pair fed in slaughter groups: 6 and 7 complete groups of intact males and castrated males, respectively, and 6 incomplete groups of females. One female from group 4 and another from group 5 from different treatments were removed due to sickness during the experiment. Each group included 3 goats randomly assigned to 1 of 3 DMI levels: ad libitum and 75 or 50% of ad libitum intake. Therefore, pair feeding was established within each group based on the intake of the animal fed ad libitum. The daily intake of the restricted-fed animals within a group was determined by the DMI of the animal fed ad libitum within the same group on the previous day. The animals were fed twice a day (0700 and 1600 h), and the intake of animals fed ad libitum was adjusted to allow for 20% daily leftovers. Each group was slaughtered when the animal fed ad libitum in the group reached approximately 45 kg BW; as a result, all animals in a slaughter group were slaughtered after the same number of experimental days.

When the animals fed ad libitum reached 38.3 ± 0.71 kg BW, representing the midpoint of the maintenance trial, a metabolism assay was performed to determine DE, ME, and energy metabolizability (ME:GE ratio [q]). All animals were housed in individual metabolic cages. Feed intake was recorded, and orts, feces, and urine were collected for 5 d after a 3-d period of adaptation to the cage. Urine was acidified daily with 20 mL of 20% sulfuric acid (vol/vol). Feed and orts 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 feed, orts, and feces were dried at 55°C for 72 h and ground through a 1-mm screen using a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA). Urine composites were passed through a sieve and subsampled to determine energy content. Gross energy was determined for feed, orts, feces, and urine using a bomb calorimeter (IKA Calorimeter system C 2000 basic/control; IKA Works). Energy lost due to the gaseous products of digestion was estimated according to Blaxter and Clapperton (1965). Metabolizable energy was computed from the DE and the energy lost due to the gaseous products of digestion and urine.

The animals were not subjected to a fasting period. Immediately before slaughter, the animals’ BWs were measured. At slaughter, the animals were first stunned with a captive bolt pistol followed by severing the jugular vein and carotid artery. Blood and organs were collected and weighed. The digestive tract was weighed before and after it was emptied and flushed with water. The EBW was computed as BW at slaughter minus the weight of the contents of the digestive tract, bladder, and biliary vesicle. The empty whole body was initially frozen at –6°C and then cut into small pieces, ground with a large screw grinder (Grinder CAF 114DS inox NR 12; C.A.F. Máquinas, Rio Claro, São Paulo, Brazil) through a plate with 0.32-cm holes, and mixed by 2 additional passes through the grinder. After grinding and homogenization, samples were collected, frozen again, and freeze-dried for DM determination. These samples, which weighed approximately 30 g, were analyzed for fat, protein, and GE as previously described.

Daily heat production (HP; kJ/kg0.75 EBW or BW) was calculated as the difference between daily ME intake (MEI; kJ/kg0.75 EBW or BW) and daily retained energy (RE; kJ/kg0.75 EBW or BW). The antilog of the intercept of the linear regression between the log10 of HP and MEI was used to estimate the NEm (kJ/kg0.75 EBW or BW; Lofgreen and Garrett, 1968). A nonlinear model was also used to describe the relationship between HP and MEI and was performed according to Eq. [1]; the ME required for maintenance (MEm; kJ/kg0.75 EBW or BW) was calculated by iteration, assuming that the maintenance requirement equals the value at which HP is equal to MEI:in which HP is the daily HP (kJ/kg0.75 EBW or BW), β0 and β1 are the parameter estimates, MEI is daily MEI (kJ/kg0.75 EBW or BW), and ε is the residual.

The efficiency of energy utilization for maintenance (km) was calculated as NEm/MEm. The partial efficiency of ME utilization for growth (kg) was assumed to be the slope of the linear regression of RE on MEI above the maintenance level, assuming that RE is null when MEI above maintenance = 0 (model intercept equal to 0), according to Galvani et al. (2014).

Alternatively, the approach proposed by Luo et al. (2004) was used to estimate MEm, in which daily MEI (kJ/kg0.75 BW) was regressed against ADG (g/kg0.75 BW); the intercept was defined as the MEm, and the slope was considered the ME required for growth (MEg).

Experiment 2: Energy Requirements for Growth

Net requirements for growth were obtained using 65 goats (20 intact males, 22 castrated males, and 23 females) in a completely randomized design. The animals were fed ad libitum twice a day (0700 and 1600 h) and slaughtered at 30.5 ± 1.53 kg BW (8 intact males, 9 castrated males, and 13 females) and 38.1 ± 0.49 kg BW (6 intact males, 6 castrated males, and 5 females), representing intermediate body composition. The remaining goats were slaughtered when they reached 44.0 ± 0.5 kg BW (6 intact males, 7 castrate males, and 5 females). Restricted-fed animals were not included in this data set because their growth pattern differs from that of goats fed ad libitum. Slaughter procedures and laboratory analyses were similar to those previously described. Allometric equations (Eq. [2]) were used to calculate the energy concentration (kJ) from the EBW (kg; ARC, 1980) as follows:in which “energy” is the amount of energy in the EBW (MJ), β0 and β1 are the parameter estimates, EBW is expressed in kilograms, and ε is the residual.

The first derivative of Eq. [2] with respect to EBW yields an estimate of the NEg at various EBW (Eq. [3]):in which NEg is the concentration (KJ/kg) per unit of EBW (empty BW gain [EWG]; kg), β0 and β1 are the parameter estimates of the allometric equation (Eq. [2]), and ε is the residual.

Statistical Analysis

The allometric equations (baseline and NEg) were fitted according to the nonlinear mixed model methodology using the SAS macro %NLINMIX (SAS Inst. Inc., Cary, NC; version 9.4). The REML was used as the method of estimation, and the zero-order expansion method was used to fit the model (Littell et al., 2006). Nonlinear procedures, whether for fixed effects (PROC NLIN) or mixed effects models (PROC NLMIXED and %NLINMIX), do not directly accommodate independent discrete (class) variables in the model (such as sex). Ways to circumvent this computational problem include imposing a restriction on the parameter estimates (Noftsger et al., 2005) or the use of so-called dummy variables (Draper and Smith, 1998). Both methods would yield identical results. We chose the dummy variables approach to assess the effect of sex on the regression parameters. That is, 3 dummy variables (z1, z2, and z3) were created. For intact males, z1 = 1, z2 = 0, and z3 = 0; for females, z1 = 0, z2 = 1, and z3 = 0; and for castrated males, z1 = 0, z2 = 0, and z3 = 1. Using these dummy variables, one can estimate separate regression parameters for each of the 3 sexes. CONTRAST statements were used for testing whether a regression parameter differed across the 3 sexes.

Data from Exp. 1 were analyzed as an unbalanced split-plot design with a 3 × 3 factorial treatment structure (3 sexes—intact males, castrated males, and females—and 3 DMI levels—ad libitum and 75 and 50% of ad libitum intake). A mixed model with the fixed effects of sex (df = 2), intake level (df = 2), and their interactions (df = 4) and the random effect of group nested within sex (df = 15) were used. The residual was the random effect, assumed independent and identically distributed (iid) N(0, σ2ij). When significantly different (P < 0.05), treatment means were compared using Fisher’s protected LSD. The equation for the nonlinear regression of HP on MEI was also fitted using the SAS macro %NLINMIX (SAS Inst. Inc.; version 9.4) as the allometric equation. The between-group variability was modeled by introducing the parameters u1 and u2 to the β0 and β1 parameters, respectively. The effect of sex was tested as described before using dummy variables.

Experiment 2 used a completely randomized design with fixed effects of sex (df = 2), weight (df = 2), and their interactions (df = 4). The residual was the random effect, assumed iid N(0, σ2ij). Linear regression analyses in both experiments were performed with the MIXED procedure (SAS Institute Inc.; version 9.4). When sex was found to be significant (P < 0.05), indicating a different intercept for at least 1 sex, 3 CONTRAST statements were used to conduct all 3 pairwise comparisons of sex. Likewise, 3 CONTRAST statements were used to conduct all 3 pairwise comparisons when the interaction between sex and regressor effects was found to be significant (P < 0.05), indicating that at least 1 sex had a different slope. Residuals were plotted against predicted values to check model assumptions. Studentized residuals were plotted against predicted values to verify the model assumptions, and outliers were defined as those values with a Studentized residual outside the ±2.5 range values.


RESULTS

Experiment 1: Energy Requirements for Maintenance

Body Composition.

A regression equation was developed from BL animals to estimate initial EBW from BW. A significant relationship between EBW and BW was found. This regression equation (Eq. [4]; n = 30; P < 0.0001, σ2e = 0.33) was the same irrespective of sex (P = 0.87):in which iEBW is the initial EBW (kg) and iBW is the initial BW (kg).

Body energy content of the initial EBW was given by Eq. [5] and Eq. [6] (n = 30; P < 0.0001, σ2e = 1,004) because the intercept and slope differed between the sexes (P = 0.01):in which iEBW is expressed in kilograms and “energy” is the initial body energy (MJ).

Equations [4], [5], and [6] were used to estimate the initial body composition of goats in Exp. 1 to calculate energy retention.

Intake, Digestibility, and Performance.

Sex affected body composition, excluding protein concentration in the EBW (mean of 16.8 ± 0.47%; P > 0.05). Fat and energy concentrations were greatest in females (P < 0.05) followed by castrated males and then intact males (means of 28.1, 24.3, and 19.3% fat in the EBW and 15.1, 13.2, and 10.4 MJ/kg EBW, respectively). On the other hand, water concentration showed the opposite pattern (Table 2).


View Full Table | Close Full ViewTable 2.

Performance and body composition of intact male, castrated male, and female Saanen goats under different levels of intake at the maintenance experiment

 
Level of intake2
Intact males
Castrated males
Females
P-value4
Variable1 AL 75% 50% AL 75% 50% AL 75% 50% SEM3 Sex Lvl Lvl × Sex
BW, kg 44.1 37.8 33.1 44.4 37.2 30.9 43.6 36.3 30.5 0.69 0.30 <0.001 0.25
EBW, kg 37.6 31.6 26.7 37.7 31.1 25.3 37.8 30.8 24.8 4.85 0.38 <0.001 0.21
DMI, g/d 1,113 818 582 1,106 793 561 1,039 769 534 26.6 0.07 <0.001 0.94
MEI, kJ/kg0.75BW 791 617 489 736 549 487 684 622 494 20.8 0.22 <0.001 0.22
ADG, g/d 139.5 90.4 3.4 137.4 90.9 2.4 120.7 75.3 2.4 10.5 0.89 <0.001 0.77
EWG, g/d 133.8 78.8 2.8 131.9 71.7 2.0 102.6 66.7 1.6 7.4 0.68 <0.001 0.97
Water, % EBW 55.9 60.3 60.9 52.2 53.6 57.1 47.9 51.9 53.8 0.74 <0.001 <0.001 0.60
Fat, % EBW 22.6 18.1 17.1 26.7 25.4 20.8 31.5 26.9 25.8 0.99 <0.001 <0.001 0.45
Energy, MJ/kg EBW 11.9 10.3 9.1 14.4 13.4 11.9 17.1 14.9 13.3 0.61 <0.001 <0.001 0.77
1EBW = empty BW; EWG = empty BW gain; MEI = ME intake.
2Ad libitum (AL) or 75 or 50% of the AL intake.
3The SEM are slightly different for the different sexes due to the removal of 2 females during the experiment because of sickness; the largest SEM (i.e., that of females) are reported. The SEM for intact or castrated males are equal to 0.887 times the reported SEM.
4Significance of the main effects of sex (Sex), level of intake (Lvl), and their interaction.

The fat and energy concentrations in the EBW of goats fed ad libitum were greater than those restricted fed (means of 26.9 and 22.4% fat in the EBW and 14.5 and 12.4 MJ/kg EBW, respectively). The opposite relationship was observed for water concentration; goats fed ad libitum showed lower water concentrations than those restricted fed. Not surprisingly, due to greater nutrient intake, animals fed ad libitum showed better performance than those with lower intake levels (Table 2).

Females and intact males presented greater energy digestibility compared with castrated males (72.8 vs. 70.4; Table 3). However, ME, ME:DE ratio, and q did not differ among the sexes, averaging 12.04 MJ/kg DM, 0.86, and 0.65, respectively.


View Full Table | Close Full ViewTable 3.

Energy digestibility and metabolizability in castrated male, intact male, and female Saanen goats under different levels of intake

 
Level of intake2
Intact males
Castrated males
Female
P-value4
Variable1 AL 75% 50% AL 75% 50% AL 75% 50% SEM3 Sex Lvl Lvl × Sex
DE/GE 0.609 0.705 0.770 0.694 0.675 0.742 0.706 0.779 0.802 0.035 0.39 <0.001 0.31
ME/DE 0.885 0.875 0.893 0.870 0.873 0.874 0.876 0.882 0.895 0.007 0.09 <0.001 0.26
q 0.63 0.64 0.64 0.60 0.59 0.65 0.62 0.64 0.65 0.027 0.19 <0.001 0.86
1Proportion of GE that is digestible (DE/GE) and proportion of DE that is ME (ME/DE); q = ME:GE ratio.
2Ad libitum (AL) or 75 or 50% levels of the AL intake.
3The SEM are slightly different for the different sexes due to the removal of 2 females during the experiment because of sickness; the largest SEM (i.e., that of females) are reported. The SEM for intact or castrated males are equal to 0.887 times the reported SEM.
4Significance of the main effects of sex (Sex), level of intake (Lvl), and their interaction.

Energy Requirements for Maintenance and Efficiency of Energy Utilization.

All of the parameter estimates of the equations presented in Table 4 (Eq. [7], [8], [9] and [10]) did not differ between the sexes (P = 0.59). The linear-shaped equations (Eq. [7] and [8]) were used to demonstrate the significant relationship between MEI and HP. The general equation chosen to describe the relationship between MEI and HP for male and female Saanen goats was the one that had the lowest residual variance (Eq. [10]; Table 4).


View Full Table | Close Full ViewTable 4.

Parameters for the different models to estimate heat production (HP) at the maintenance experiment

 
Parameter estimates
Equation Model1 β0 SE β1 SE (× 1,000) n P-value σ2e BIC2
[7] Log10 HP, kJ/kg0.75 EBW 2.47 0.0252 0.000434 0.036 53 <0.001 0.00119 –129
[8] Log10 HP, kJ/kg0.75 BW 2.41 0.0240 0.000498 0.039 53 <0.001 0.00113 –133
[9] HP nonlinear, kJ/kg0.75 EBW 299.2 16.7 0.000979 0.074 53 2,300 563
[10] HP nonlinear, kJ/kg0.75 BW 258.5 13.9 0.001133 0.082 53 1,643 546
1EBW = empty BW.
2BIC = Bayesian information criteria.

The best-fit equation is displayed in Fig. 1, by which the daily NEm was estimated to be 258.5 kJ/kg0.75 BW. The MEm was calculated assuming that HP was equal to MEI at maintenance, resulting in 412.4 kJ/kg0.75 BW daily. Therefore, the predicted km was 0.627.

Figure 1.
Figure 1.

Relationship between daily ME intake (MEI) and heat production (HP; Eq. [10]; Table 4, solid line) of Saanen growing goats of different sexes ( represents observations of intact males, represents observations of females, and represents observations of castrated males).

 

Likewise, using the approach suggested by Luo et al. (2004), the estimated MEm was 498 kJ/kg0.75 BW (Eq. [11]; n = 55; P < 0.001, σ2e = 5,009):in which MEI is daily MEI (kJ/kg0.75 BW) and ADG is expressed in grams per kilogram0.75 BW.

Additionally, no significant effect of sex on the percentage of organs in the EBW (heart, liver, kidneys, and gastrointestinal tract) was found (P > 0.05). The means in the EBW were 0.531 (±0.016), 1.852 (±0.061), 0.309 (±0.010), and 5.92% (±0.19) for heart, liver, kidneys, and gastrointestinal tract, respectively.

This data set was also used to estimate the kg using the regression equation of MEI above the maintenance level on RE, assuming zero RE when MEI was equal to MEm (Eq. [12], [13] and [14]; n = 55; P < 0.001, σ2e = 948). The kg for intact males, castrated males, and females was 0.32, 0.40, and 0.55, respectively.

in which RE is daily RE (kJ/kg0.75 EBW) and MEI is daily MEI (kJ/kg0.75 EBW).

Experiment 2: Energy Requirements for Growth

Energy Requirements for Gain.

Sex did not affect the relationship between BW and EBW (P = 0.78); therefore, a general equation is presented (Table 5). Nonetheless, parameter estimates of the allometric equations used to predict energy body composition differed between the sexes (P < 0.03). When the BW ranged from 30 to 45 kg, the body energy concentration increased from 9.0 to 11.3 MJ/kg EBW for intact males, from 10.5 to 13.3 MJ/kg EBW for castrated males, and from 12.3 to 15.9 MJ/kg EBW for females. This may be due to different daily fat gain observed in the EBW for each sex. Females showed greater EBW fat gain than castrated males followed by intact males (P < 0.05 and means of 54.4, 46.5, and 31.2 g/d, respectively). No differences were found for protein gain in the EBW during this phase (mean of 19.2 g/d; P > 0.05).


View Full Table | Close Full ViewTable 5.

Allometric equations to estimate retained energy of female, intact male, and castrated male Saanen goats

 
Parameter estimates2
BW,4 kg
Variable1 β0 SE β1 SE σ2e 30 37.5 45
EBW, kg –2.74 0.78 0.916 0.049 0.984 24.7 31.6 38.4
Energy, MJ/kg EBW3
    Intact males 1.76 0.285 1.509 0.061 9.01 10.2 11.3
    Castrated males 1.92 0.239 1.537 0.074 975 10.5 12.0 13.3
    Females 1.92 0.239 1.588 0.096 12.3 14.2 15.9
1EBW = empty BW; regressed linearly against BW.
2Parameter estimates (β0 and β1) of the simple regression analysis or allometric equation.
3Energy was obtained by nonlinear allometric equation of the total body energy content (MJ) by EBW.
4Energy concentration in the EBW calculated from allometric equations.

The first derivatives of the equations yielded estimates of NE requirements for gain for intact males, castrated males, and females. Our results revealed that, during the growth phase investigated here, NEg differed between the sexes (P < 0.001). Therefore, a different equation was proposed for each sex (Eq. [15], [16], and [17]):in which NEg is the daily NEg (MJ/kg EWG) and EBW is expressed in kilograms.

As BW increased from 30 to 45 kg, the NEg rose from 13.6 to 17.0 MJ/kg EWG, from 16.5 to 20.9 MJ/kg EWG, and from 20.1 to 26.0 MJ/kg EWG in intact males, castrated males, and females, respectively.

Application of the kg estimated for each sex (Eq. [12], [13], and [14]) yielded MEg estimates of 52.4, 46.9, and 53.8 MJ/kg EWG for intact males, castrated males, and females, respectively, each at a weight of 45 kg.


DISCUSSION

Given the gap in knowledge and controversy regarding the role of sex in the energy requirements of goats, the effect of sex on the energy requirements for maintenance and weight gain was tested in Saanen goats weighing from 30 to 45 kg. Sex did not affect the energy requirements for maintenance. However, it did influence the energy requirements for gain.

The energy maintenance requirement comprises basal metabolism (e.g., cellular activity, respiration, blood flow, etc.) plus energy expenditure due to feeding, digestion, and absorption (NRC, 1981). The NRC (2007) states that the energy maintenance requirements of intact males are 15% greater than those of females and castrated males. These recommendations are based on the concept that males have more protein in the body (Webster, 1986; Emmans, 1994) and because of differences in the stage of maturity observed in animals with the same BW but of different sexes. The Commonwealth Scientific and Industrial Research Organization (CSIRO, 2007) also recommends the same correction factor for sex. The findings of this study do not support this recommendation; although females exhibited a greater proportion of mature weight than intact and castrated males, no difference in body protein content was found among the sexes.

Variation in maintenance energy requirements is strongly associated with mass and the metabolic activity of visceral organs such as the gut and liver (Baldwin et al., 1985; Ferrell, 1988; Ferrell and Jenkins, 1998). As shown here, the mass of the main organs (heart, liver, kidneys, and gastrointestinal tract) involved in maintenance did not differ between the sexes, which may explain why no difference in maintenance energy requirements was observed between the sexes at similar BW in this study. In addition, the absence of differences in intake between the sexes raises the question of whether the energetic costs associated with HP due to the digestive process were similar, supporting the premise that maintenance energy requirements should not be considered sex dependent. Consistent with these results, other researchers have not found differences in maintenance energy requirements between sexes at the same weight in beef cattle (Chizzotti et al., 2008) or cashmere goats (Ash and Norton, 1987).

The MEm estimated here was 412.4 kJ/kg0.75 BW, 16% lower than that reported by the Agricultural and Food Research Council (AFRC, 1998; 490 kJ/kg0.75 BW). The MEm recommended by the NRC (2007) is, on average, 19% greater than that estimated here for all sexes. Females and castrated males had MEm values that were 4% lower than those reported by the CSIRO (2007), and the MEm of intact males was 21% lower than reported previously by the CSIRO. Both the NEm and MEm estimates were similar to those reported by Ferreira et al. (2015) for castrated male Saanen goats weighing from 20 to 35 kg BW.

From the standpoint that body composition is dictated by sex, energy requirements for gain should also depend on sex. The data revealed that NEg differed between the sexes in accordance with differences in body composition; greater fat deposition in females resulted in greater NEg values in females followed by castrated males and then intact males. This result was most likely due to differences in fat gain observed among the sexes (i.e., females showed 17 and 57% greater EBW fat gain than castrated and intact males, respectively). Accordingly, previous studies have reported that the relative proportion of fat in the female body is greater than in intact males, whereas the relative proportion of fat in castrated males is intermediate in value (Geay, 1984; Mahgoub et al., 2005; Chizzotti et al., 2007).

The data set that generated the results presented here was exclusively composed of Saanen goats evaluated using the comparative slaughter technique. The AFRC (1998) adopted the same technique and took into account the impact of BW changes in the composition of the gain. Because of that, the AFRC (1998) also reported values of NEg that may be suitable for estimating the NEg of castrated males yet underestimate the NEg of females (33%, on average) and overestimate the NEg of intact males (12%, on average). The AFRC values are more suitable for castrated males because the data set that generated the AFRC equations for gain primarily comprised castrated males. Recommendations for females based on this feeding system might not be appropriate for females because the average body composition of a typical female dairy goat in the AFRC (1998) system was based on males and calculated from a small and heterogeneous data set (i.e., combining females of different breeds, ages, and weights and even different physiological stages).

The NRC (2007) assumed a unique MEg regardless of sex based on a response-to-feeding-level method in which MEI was regressed against ADG (Luo et al., 2004). Actually, the results presented here show that ADG did not differ between the sexes (Table 2); as a consequence, when using the same approach as that adopted by the NRC (2007; Eq. [11]), MEg was not found to differ between the sexes either. Although the data are easy to obtain, dose–response experiments depend on the specific rearing conditions and feed digestibility (CSIRO, 2007). Indeed, the current work demonstrated that nutritional requirements for gain should take into account not only the rate of BW gain but also the composition of the gain, which results in different NEg values for each sex. This result has direct implications for herd management, primarily for the phase studied here, when fat deposition is becoming more important than protein gain, and suggests that different feeding strategies should be used depending on sex.

An important factor that may contribute to the paradoxical information presented here are the values adopted for the efficiencies (km and kg). In fact, the km may change according to many factors related to environmental conditions, animal traits, and diet quality (AFRC, 1998; Birkett and de Lange, 2001; Corbett and Freer, 2003). Despite the great importance of energy use efficiencies in the making of recommendations, it remains unclear how those factors influence km and kg. Due to the methodology chosen, it was possible to present NE requirements, which is a strength of the present study because the choice of efficiencies that suitably express the requirements in metabolizable terms may vary with the conditions to which the animals are subjected. The Agricultural Research Council (ARC, 1980) proposed a way to estimate efficiencies based on diet characteristics (qm), which was adopted by the AFRC (1998) and CSIRO (2007). This proposal yielded km estimates similar to those shown here. Tovar-Luna et al. (2007) reported a km of 0.68, with small variations due to differences in diet. However, these researchers also reported different kg values for roughage and concentrate-rich diets (0.32 vs. 0.61). In accordance with this research, it is proposed that requirements be treated in net terms, in which there is less variation, and the researchers should focus on describing efficiencies more accurately for a broad range of external conditions. Furthermore, from a bioenergetics standpoint, differences in the protein and fat deposition efficiencies reported in the literature (e.g., varying from 0.1 to 0.40 for protein and from 0.60 to 0.84 for fat, according to the ARC, 1980) may explain the differences in kg found here. Because females retained more energy as fat than did castrated males, which in turn retained more than intact males, the kg likely reflected the fat gain pattern.

To our knowledge, this is the first study aiming to estimate the maintenance and gain requirements of goats of different sexes during the late growth phase. The hypothesis that energy requirements for growth differ among the sexes was accepted. It was shown that the existing difference is due to distinct NEg and kg values in intact and castrated males and females during the late growth phase. These findings may be useful to help improve the accuracy of energy recommendations and to promote correct nutritional management, preventing economic losses and environmental issues due to diet inadequacy.

 

References

Footnotes


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