Determination of energy, protein and amino acid requirements for maintenance and growth in ostriches

Animal Feed Science and Technology 72 Ž1998. 283–293

Determination of energy, protein and amino acid requirements for maintenance and growth in ostriches S.C. Cilliers a , J.P. Hayes b, A. Chwalibog c , J. Sales J.J. Du Preez b

b,)

,

a Little Karoo Agricultural Centre, PO Box 313, Oudtshoorn, 6620, South Africa Department of Animal Science, UniÕersity of Stellenbosch, Stellenbosch, South Africa DiÕision of Animal Nutrition, Department of Animal Science and Animal Health, Royal Veterinary and Agricultural UniÕersity, BulowsÕej 13, 1870 Frederiksberg C, Denmark b

c

Received 3 March 1997; accepted 21 November 1997

Abstract Requirements for maintenance and the utilization of dietary true metabolisable energy, corrected for nitrogen ŽN.-retention ŽTME n ., effective energy, corrected for N-retention, protein and amino acids were assessed in 44 young ostriches Ž7 months of age. by means of a comparative slaughter technique. Response in nutrient gain Ženergy, lipid, protein and amino acids. in feathers, legs, hides and carcasses were separately studied by scarifying 8 birds at the beginning and 12 birds at the end of a 21-day feeding period. TME n required for maintenance was 0.425 MJrempty body weight ŽEBW., kg 0.75rday or 7.96 MJrday, while maintenance requirements for effective energy, corrected for N-retention, amounted to 0.311 MJrEBW, kg 0.75rday or 8.90 MJrday. Utilisation efficiencies for TME n were estimated as 0.414 " 0.006 ŽMJrEBW, kg 0.75rday. and 0.443 " 0.016 ŽMJrday., whereas a value of 0.568 " 0.009 was determined for effective energy, corrected for N-retention. Digestible maintenance protein requirements of 0.678 " 0.027 grEBW, kg 0.75rday was found, and by altering this estimate to requirement for total dietary protein, 1.05 " 0.038 grEBW, kg 0.75rday was calculated. Maintenance requirements for lysine, methionineq cystine, threonine and valine compared favourable to literature values for poultry, but substantially higher values than for poultry, however, were determined for leucine, arginine and histidine. Net utilisation estimates for digestible amino acids varied between 0.948 " 0.025 for the slow turnover amino acids Žarginine. and 0.569 " 0.015 for the fast turnover

)

Corresponding author. Queensland Poultry Research and Development Centre, PO Box 327, Cleveland, QLD 4163, Australia. Tel.: q61-07-3824-3081; fax: q61-07-3824-4316. 0377-8401r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 7 - 8 4 0 1 Ž 9 7 . 0 0 1 8 8 - 0

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amino acids Žcystine. with a mean value of 0.747. Results in the present study presented essential information for the establishment of requirement estimates for ostriches. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Ostriches; Nutrient requirements; Maintenance; Growth

1. Introduction The domestication of ostriches and the commercial farming of ostriches on concentrated diets over the past few years offered exceptional possibilities ŽSwart and Kemm, 1985., but various raising difficulties are encountered due to extrapolation of values from poultry nutrition. Literature on ostrich nutrition is limited and mainly build on myths ŽUllrey and Allen, 1996.. In order to increase the profitability of ostriches as meat, hide and feather producers, determination of nutrient requirements of ostriches are essential. Cilliers et al. Ž1994. and Cilliers Ž1995. conducted a number of balance studies with ostriches in which the true metabolisable energy, corrected for nitrogen ŽN.-retention, ŽTME n . values of most of the common ingredients, used in diets of ostriches, were determined. These values were simultaneously compared to values derived for poultry, and significantly higher values were observed for ostriches. The additivity of these TME n values in a complete diet, compiled by various ingredients, was confirmed by Cilliers Ž1995., indicating that reliable metabolisable estimates were now available for ostriches from six months to maturity. The present study was conducted to establish information on the maintenance requirements and utilisation efficiencies of dietary energy, protein and amino acids for ostriches.

2. Materials and methods 2.1. Diets and animal husbandry Prior to experimentation, 44 ostriches Ž Struthio camelus var. domesticus., 7 months of age, were accustomed to weighing procedures and metabolism crates. At the beginning of the trial, a representative sample of 8 ostriches Žmean body mass of 70 " 2.03 kg. were sacrificed to estimate the initial carcass characteristics of the remaining birds, used for the response study Žweight gain. in the metabolism crates. These birds were divided at random in three groups of 12 birds each. The three groups were offered a complete diet, comprising 7 ingredients ŽTable 1., at daily intake levels of 1000 g, 1500 and 2000 g per bird, respectively. Different intake levels were necessary in order to determine TME n , as described by Cilliers et al. Ž1998., and apparent amino acid and protein availability, as described by Cilliers et al. Ž1997. by means of a balance method. TME n and amino acid and protein availability were determined for 5 days after an initial adaptation period of 7 days. After termination of the balance trial, birds were kept individually for an additional 9 days in cages. Daily feed intake was accurately measured and precautions were taken to

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Table 1 Composition, determined analysis and digestibility of the experimental diet fed to growing ostriches Ingredients

Dietary levels Žgrkg.

Barley Yellow maize Wheat bran Lucerne hay Žchopped. Ostrich meat and bone meal Soybean oil cake meal Fish meal Limestone powder Vitaminrmineral premix a

169.48 141.14 150.00 400.00 43.70 30.00 61.54 0.65 3.50

Nutrient Protein Ether extract Ash Threonine Serine Alanine Valine Methionine Phenylalanine Histidine Lysine Isoleucine Tyrosine Arginine Cystine Leucine TME n ŽMJrkg. Effective energy

Content Žgrkg fresh weight. 209.88 32.25 76.11 7.00 6.76 8.20 9.60 3.01 9.02 4.53 11.12 8.23 5.36 11.24 3.05 15.00 11.25"0.072 8.23"0.054

Digestibility coefficient b Žfresh weight basis. 0.646 0.870 0.831 0.849 0.937 0.862 0.816 0.909 0.854 0.832 0.829 0.816 0.780 0.806 0.859

a

Provided per kg diet: 12.5 g calcium, 4.5 g available phosphorus, 150 mg manganese, 120 mg zinc, 12 000 IU vitamin A, 3000 IU vitamin D 3 , 100 IU vitamin E, 3 mg vitamin K, 3 mg vitamin B1 , 8 mg vitamin B 2 , 60 mg niacin, 18 mg panthothenic acid, 2 mg folic acid, 4 mg vitamin B 6 , 0.2 mg biotin, 1500 mg choline, 0.01 vitamin B12 . b Cilliers et al. Ž1997.. TME n : true metabolisable energy, corrected for nitrogen ŽN.-retention derived from Cilliers et al. Ž1998.. Effective energy: TME n y4.67=digested proteiny3.8=faecal organic matterq12=digested N-free extract.

eliminate feed wastage. Water was available at all times. After 21 days, the trial was terminated and a representative sample of 12 birds, four from each feeding level, were selected and killed. 2.2. Method of slaughtering and sampling Birds selected for slaughtering were killed without fasting immediately after weighing by injection of 1 mlr5 kg live weight pentobarbitone sodium. Complete defeathering was conducted by hand and feather weight recorded. Birds were then eviscerated by making a ventral incision posterior to the keel and towards the cloaca. The digestive

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tract was carefully removed, and the gut fill emptied and weighed. The hide and all subcutaneous lipid were removed. Legs were cut at the femur just above the heel. Blood was collected in plastic bags and added together with body lipid, and lipid removed from the hide and the empty viscera, to the carcass. The latter can be defined as the proportion of the empty body after removing feathers, hide and legs. These four body components were individually weighed and stored in plastic bags at y208C in a freezer. Carcasses and legs were cut into small pieces with a band saw after complete freezing took place. The complete legs, hides and carcasses of individual birds were then separately minced in a Wolfmaster mincer Ž6 times. and 5 samples per bird per body component were taken. Samples were freeze-dried and used for analyses. Feathers were dried in a forced draft oven at 808C, and representative samples per bird were taken after feathers were hammermilled. 2.3. Analytical procedures Gross energy ŽGE. determinations were carried out using a solid-state bomb calorimeter ŽModel CP 500, Digital Data Systems, PO Box 35872, Northcliff, Johannesburg.. Samples were defatted with petroleum ether Žboiling point 40 to 608C. for 14 h, and the extracted residue used for protein ŽKjeldahl procedure. and amino acid analysis. Amino acid analyses of dried samples were conducted by means of ion-exchange chromatography of the acid-hydrolysed protein, while methionine and cystine were determined after oxidation by performic acid. Amino acids were separated on a Beckman, Model 6600 amino acid analyser, using lithium- and sodium citrate-based buffers. Procedures used for amino acid determinations were similar to that described by Fisher and Scougall Ž1982.. Ash content was determined in a muffle furnace at 5608C. 2.4. Calculation of energy, protein and amino acid requirements and efficiency of utilisation Retained energy ŽRE. during the trial period, MJrkg 0.75 body mass, was regressed against TME n intake according to the following model: RE Ž kg 0.75 . s a q b = ME intake kg 0.75 where the coefficient b estimates the efficiency of TME n utilisation for RE Žkpf. in growing birds and a = by1 predicts daily requirements for maintenance energy ŽTME n . per kg 0.75 body mass ŽChwalibog, 1991.. Retained carcass energy was also regressed on effective energy, the unit proposed by Oldham and Emmans Ž1990. to correct TME n for differences in heat loss of fats, carbohydrates and metabolism during metabolism. Effective energy of the experimental diet was calculated from TME n y 4.67 = digested proteiny 3.8 = faecal organic matter q12 = digested N-free extract ŽOldham and Emmans, 1990. and is presented in Table 1. The maintenance effective energy requirements was calculated according to the formula, based on the protein content of the carcass, proposed by Emmans and Fisher Ž1986.: EE m Ž MJrday. s 1.63 = Pmy0 .27 = P

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where Pm Žkg. is the potential mature non-feather protein weight and P Žkg. is the non-feather protein weight at the particular growth stage of the bird. The model of Emmans and Fisher Ž1986. was used to calculate maintenance requirements for total protein ŽTPm . and amino acids ŽAA m .: TPm Ž grday. s 0.008 = Pmy0 .27 = P or AA m Ž grday. s 0.008 = Pmy0 .27 = P = AAC where 0.008 represents the ideal requirement for poultry, and AAC the amino acid content in defeathered protein Žgrkg protein.. Protein and amino acids needed for body maintenance was assumed to have identical composition as that of the empty body. Gross efficiency of utilisation of TP and total amino acids ŽTAA. were calculated by the ratio of TP or TAA retention to TP or TAA intake. Digestibility coefficients of protein and amino acids presented by Cilliers et al. Ž1997. and shown in Table 1, were used to calculate amino acid intake from feed consumption figures. Net efficiency of utilisation of TP and TAA Žefficiencies above maintenance. were calculated by the ratio of TP or TAA retention to retainable TP or TAA intake. Energy, protein, lipid, ash, moisture and amino acids were determined in feathers, legs, hides and carcasses, respectively. Nutrient values for the body components were then proportionally combined according to the established ratios to complete empty body. These combined nutrient contents were used in calculations. 3. Results Substantial differences were observed between the gut fill of the initial and the final group of birds selected for slaughtering. While the ratios of feathers and legs to empty body weight ŽEBW. remained relatively constant over the 21-day experimental period, the ratio of hide to EBW substantially increased ŽTable 2.. Noticeable differences were observed between the nutrient contents Ženergy, dry matter, protein, lipid, ash and amino acids. of feathers, legs, hides and carcasses. Similar

Table 2 Ratios between body components for the initial and final group of ostriches used for carcass analysis

LBW Žkg. Wet gut content Žkg. EBW Žkg. Feather weightrEBW Žgrkg. Leg weightrEBW Žgrkg. Hide weightrEBW Žgrkg. Carcass weightrEBW Žgrkg.

Initial group Ž ns8.

Final group Ž ns12.

SEM

70.0 8.579 61.4 22.5 63.8 48.5 865.2

74.7 5.876 68.8 20.8 63.2 55.6 860.4

0.861 0.236) 0.915 0.327 1.036 0.702) 1.512

LBWs Live body weight. EBWs Live body weightywet gut content. Carcass weights EBWyfeather weightyLeg weightyHide weight. ) P - 0.05.

288

Nutrient content

Hide

Legs

Carcass

Feathers

Defeathered EBW

Complete EBW

Gross energy ŽMJrkg. Dry matter Žgrkg. Protein Žgrkg. Ether extract Žgrkg. Ash Žgrkg.

26.40"0.11 395"3.2 247"1.7 131"3.8 9.8"1.24

16.33"0.16 533"2.1 268"2.5 79.7"2.70 145"3.4

23.15"1.26 331"3.0 190"1.4 80.9"2.99 37.9"0.72

21.55"0.09 824"10.2 713"9.8 8.5"0.36 21.1"2.28

22.39"1.10 340"2.9 194"1.5 81.7"2.95 42.4"0.90

22.85"1.10 357"3.1 209"1.7 81.9"2.96 42.9"0.95

Amino acids (g r16 gN) Threonine Serine Alanine Valine Methionine Phenylalanine Histidine Lysine Isoleucine Tyrosine Arginine Cystine Leucine

2.53"0.05 3.14"0.06 3.58"0.12 2.98"0.04 1.16"0.02 3.24"0.07 1.76"0.11 4.85"0.10 2.31"0.03 2.00"0.06 6.96"0.27 0.86"0.04 4.64"0.06

2.17"0.04 2.65"0.05 2.61"0.25 2.57"0.05 1.04"0.02 2.63"0.05 1.60"0.04 3.72"0.06 1.92"0.03 1.55"0.03 5.72"0.19 0.60"0.01 3.94"0.06

3.58"0.05 3.02"0.05 6.41"0.09 4.28"0.05 2.05"0.02 4.48"0.13 2.76"0.09 6.75"0.11 3.88"0.05 3.11"0.07 6.91"0.25 0.90"0.02 6.78"0.08

5.41"0.14 7.84 a 3.95"0.21 8.50"0.10 0.26 a 3.47"0.11 0.70 a 1.27"0.04 5.05"0.06 3.15"0.03 5.54"0.28 5.64"0.11 8.68"0.09

a

Taken from results presented by Du Preez Ž1991..

3.36"0.04 2.94"0.05 5.89"0.10 4.01"0.05 1.90"0.02 4.20"0.12 2.57"0.08 6.31"0.10 3.59"0.05 2.88"0.07 6.69"0.24 0.86"0.02 6.34"0.074

3.47"0.05 3.11"0.05 5.97"0.10 4.19"0.05 1.90"0.02 4.28"0.12 2.59"0.08 6.34"0.10 3.70"0.05 2.95"0.07 6.81"0.25 0.98"0.02 6.53"0.08

S.C. Cilliers et al.r Animal Feed Science and Technology 72 (1998) 283–293

Table 3 Carcass characteristics of various body components viz. hide, legs, carcass and feathers of ostriches slaughtered at 74.7 kg Žmean"standard deviation; fresh weight basis.

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Table 4 Relationship between carcass energy retention and energy intake in ostriches Žmean"standard deviation. Retained energy

Intercept

Slope

R2

In TMEn system kg 0.75 MJrday

y0.176"0.018 y3.528"1.011

0.414"0.022 0.443"0.056

0.97 0.86

In effectiÕe energy system Retained energy

y0.176"0.010

0.568"0.031

0.95

nutrient contents per unit of body component, however, were measured between individual birds, thus individual contents were pooled and are presented in Table 3. The combined nutrient contents of the empty body as a unit Žfeathers, legs, hide and carcass. and defeathered EBW were used in calculations. Energy retention rate was estimated as 0.357 MJrEBW, kg 0.75rday with a mean energy conversion ŽMJ intakerMJ deposited. of 1.559 " 0.028. Significant intercepts and slopes were determined for retained energy on energy consumption ŽTable 4.. TME n required for maintenance, was 0.425 MJrEBW, kg 0.75rday or 7.96 MJrday, while maintenance requirements for effective energy, corrected for N-retention, amounted to 0.311 MJrEBW, kg 0.75rday or 8.90 MJrday. Although substantially different, maintenance values for TME n and effective energy can at least be regarded as being of the same order. In the alteration of the ME content of the experimental diet to effective energy, a markedly reduced value of 8.20 MJrkg was computed, and the ratio between effective energy and ME was 0.729. Retention rates for the various amino acids are shown in Table 5. Gross utilisation of amino acids ŽAA. varied between 0.312 for cystine and 0.556 for arginine with a mean value of 0.435. In calculating maintenance requirements for protein and amino acids, potential mature defeathered protein weight was required. Using mean ratios for gutfill to live body weight ŽLBW., feather weight to EBW and the protein content of defeathered EBW, a mature defeathered protein weight of 20.66 " 0.901 kg was computed. With a digestibility coefficient of 0.646 for protein, total daily dietary protein of 1.05 grEBW, kg 0.75 is required for maintenance. Correction of total AA intakes for AA m revealed substantially higher net utilisation figures varying from 0.569 to 0.948 with a mean value of 0.747 for all amino acids.

4. Discussion Considerable quantities of gut fill Ž8–15% of LBW. were observed in ostriches ŽSwart et al., 1993a. and major variation occurs between individuals at a specific time. It thus is critical to use EBW rather than LBW in calculations.

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Nutrient content Žgrkg. Energy Protein Lipid Ash Threonine Serine Alanine Valine Methionine Phenylalanine Histidine Lysine Isoleucine Tyrosine Arginine Cystein Leucine

Maintenance requirement Žgrday.

Maintenance requirement ŽmgrEBW, kg 0.75 rday.

44"1.2

678"27.1

3.73"0.11 3.42"0.10 5.58"0.17 4.20"0.12 2.45"0.07 4.40"0.13 3.07"0.09 5.90"0.18 3.90"0.12 3.32"0.10 6.23"0.18 1.38"0.04 5.92"0.18

57"2.4 53"2.2 86"3.6 65"2.7 38"1.6 68"2.8 47"2.0 91"3.8 60"2.5 51"2.1 96"4.0 21"0.9 91"3.8

Retention rates Žgrday.

Retention rates ŽmgrEBW, kg 0.75 rday.

8.2"0.37) 75"3.1 33"3.3 14.4"0.88 3.90"0.14 3.66"0.10 6.65"0.21 4.70"0.18 2.12"0.07 4.83"0.14 2.90"0.09 7.102"0.217 4.126"0.136 3.299"0.104 7.700"0.221 1.212"0.059 7.244"0.266

358"6.8 a 3276"58.0 a 1442"52.2 a 629"15.0 a 60"2.8 56"2.3 102"4.4 72"3.5 33"1.4 74"3.0 45"1.9 109"4.6 63"2.8 51"2.2 118"4.9 19"1.1 111"5.2

)Unit for energy retention rate ŽMJrday.. Protein, lipid and ash retention rates mgrEBW Žkg 0.75 rday. and energy retention is kJrEBW Žkg 0.75 rday..

a

Gross efficiency of utilisation

Net efficiency of utilisation

0.523"0.027

0.672"0.015

0.43"0.01 0.40"0.01 0.55"0.01 0.36"0.01 0.45"0.01 0.42"0.01 0.47"0.01 0.486"0.011 0.383"0.009 0.477"0.011 0.556"0.011 0.312"0.019 0.359"0.009

0.71"0.02 0.62"0.03 0.97"0.05 0.70"0.04 0.78"0.02 0.65"0.02 0.88"0.04 0.733"0.016 0.682"0.039 0.904"0.046 0.948"0.025 0.569"0.015 0.569"0.026

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Table 5 Maintenance requirements, retention rates and efficiency of utilization of various nutrients in ostriches Žmean"standard deviation.

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4.1. Energy Higher ME utilisation values were measured in the present study as opposed to a value of 0.32 reported by Swart et al. Ž1993b. for ostriches of a similar age, but not at a similar weight. ME utilisation values for ostriches are low in comparison to values of 0.72 for broilers ŽChwalibog et al., 1985; Chwalibog, 1991. and 0.82 for piglets ŽHuang et al., 1981.. The maintenance energy requirements ŽME m . derived in the present study are in agreement to the findings of Swart et al. Ž1993b., who calculated a value of 0.44 MJrLBW, kg 0.75rday. Swart et al. Ž1993c,d. showed that fermentative digestion of plant fibres could contribute to the energy requirements of ostriches. In the comparative study between ostriches and roosters with respect to TME n values for various ingredients ŽCilliers, 1995., it was evident that the higher metabolisabilities observed in ostriches were probably due to enhanced digestion of the fibrous components. However, the utilisation of volatile fatty acids ŽVFA., produced from fermentative digestion of fibre in ostriches, is unknown, and the reduced kpf estimates could be ascribed to impaired utilisation of energy from VFA. This theory was confirmed by the observation of Swart et al. Ž1993b. that efficiency of ME utilisation deteriorated as dietary levels of fibre increased. Similar observations were reported by Eggum and Chwalibog Ž1982. for rats. Swart et al. Ž1993a. observed that energy conversion ŽMJ inrMJ retained. increased as birds mature, although feed conversion efficiency deteriorated. Ostriches have the behaviour of constantly consuming stones and foreign objects that could contribute to the ash content of excreta. This probably may explain why no significant model could be established between ash intake and ash excreted ŽCilliers, 1995.. In the calculation of effective energy from the dietary characteristics, it was possible that due to variable ash excretion, fecal organic matter ŽFOM. could be altered. As negligible methane losses were observed in ostriches ŽSwart et al., 1993b., effective energy for the experimental diet was estimated according to digestible protein, FOM and digestible ether extract and amounted to 0.729 of the determined ME content. This ratio was substantially lower than 0.94 reported by Emmans and Fisher Ž1986. for poultry. Using effective energy intake instead of ME to relate to the congruent RE deposition as lipid and protein tissues, a higher kpf, which compared more favourable to reported estimates for other species, was derived. 4.2. Protein Carcass characteristics of defeathered body protein are in agreement to the findings of Du Preez Ž1991., although reduced levels of lysine, methionine and cystine were determined in the present study. Emmans and Fisher Ž1986. emphasised the importance of distinguishing between feather and body protein growth due to differences in protein retention rates and amino acid contents. It can be concluded that future research on growth responses in ostriches

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should be aimed at defining growth in the various components, rather than for the body as a complete unit. Daily retention rate varies between species, but expressing protein growth in relation to metabolic weight, yields a rather constant value of 6 g proteinrbody weight, kg 0.75rday for all species ŽFisher, 1980.. This is explained by the consistency in the nutrient composition per unit body weight. In ostriches however, protein retention was 3.28 grEBW, kg 0.75rday which was similar to rates reported for layers ŽFisher, 1980.. Since metabolic body weight relates body weight to surface area, and the fact that there is no merit in relating body surface to protein metabolism, requirements for maintenance of amino acids were expressed as functions of EBW. The determination of maintenance requirements is not well described and various approaches were reviewed by Fisher Ž1983.. Boorman and Burgess Ž1986. summarised some of the problems associated in estimating requirements for maintenance, and concluded that these values served only as guidelines, as requirement estimates derived for older birds are not necessarily applicable to younger birds. RE is generally measured in adult cockerels. Although requirements for maintenance expressed per metabolic body surface are more uniform between birds of different sizes, increased basal metabolic processes result in higher unit requirements for maintenance protein and amino acids in younger birds. 4.3. Amino acids Maintenance requirements ŽmgrEBW, kg 0.75rday. for lysine, methionineq cystine, isoleucine, threonine and valine for ostriches compared favourable to values of 85, 60, 50, 40 and 60 respectively determined for poultry. Substantially higher values were however derived for leucine, arginine and histidine in ostriches. Fisher Ž1980. ascribed the marked difference between net and gross utilisation figures as the result of imbalances of amino acids in dietary protein. Requirements for maintenance and variability between individuals could account for the remaining differences found for net and gross utilisation efficiency values. Boorman and Burgess Ž1986. suggested a mean utilisation efficiency estimate of 0.7 for total dietary amino acids, while Emmans Ž1989. recommended a mean value of 0.8 for utilisation of available amino acids. Baker Ž1991. estimated a mean utilisation efficiency of 0.76 for digestible amino acids with improved utilisation for slow-turnover amino acids such as lysine Ž0.80. and impaired utilisation for fast turnover amino acids such as isoleucine Ž0.61.. Net utilisation values of digestible amino acids for ostriches varied between 0.569 and 0.948 with a mean estimate of 0.747. Similar to the findings of Baker Ž1991., the fast turnover amino acids showed reduced utilisation efficiencies as opposed to the slow turnover amino acids. Adjusting the net utilisation value of digestible amino acids to net utilisation of total dietary amino acids, by using the mean amino acid digestibility value of 0.826, yielded a reduced value of 0.617. Results in the present study were obtained from a response study in which the response in nutrient gains was obtained by means of regulating intakes and not experimental diets. Severe imbalances and unnecessary stress due to diet change were consequently prevented.

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