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The composition of egg yolk absorbed by fasted ostrich (Struthio camelus L.) chicks from 1 to 7 days posthatching and for ostrich (Struthio camelus L.) chicks from 1 to 16 days posthatching on a prestarter broiler diet
METABOLiSiVI AND NUTRITiON The composition of egg yoik absorbed by fasted ostrich (Struthio camelus L.) chicks from 1 to 7 days posthatching and for ostrich {Struthio camelus L.) chicks from 1 to 16 days posthatching on a prestarter broiier diet M. Viljoen,*!^ T. S. Brand,tí# J. T. Soley,t and E. A. Boomkerf *Cape Institute for Agricultural Training, Elsenburg, Department of Agriculture, Western Cape, Private Bag XI, Elsenburg, 7607, South Africa; fDepartment of Anatomy and Physiology, Faculty of Veterinary Science, University of Pretoria, Private Bag XO4, Onderstepoort, 0110, South Africa; }Elsenburg Animal Production Institute, Department of Agriculture, Western Cape, Private Bag XI, Elsenburg, 7607, South Africa; and #Department of Animal Sciences, University of Stellenbosch, Private Bag XI, Matieland, 7602, South Africa ABSTRACT This study was performed to obtain information on yolk utilization in fasted and fed ostrich chicks posthatching. The fasted trial lasted for 7 d, whereas the fed trial continued for 16 d. Fasted ostrich chicks showed a decrease of 31.3 g of BW, with yolk weight decreasing by 28.9 g daily after hatching. Yolk weight comprised 28% of 1-d-old ostrich chick BW and decreased to 12% at 7 d of age. Only 44.4% of the fasted ostrich chick yolk was assimilated over the trial period. Crude protein content of the yolk decreased by 13.2 g daily. Fat content increased by 1.77% daily, whereas total yolk fat weight decreased with 8.91 g daily. Slaughter weight of fed ostrich chicks increased, with yolk weight decreasing by 16.3 g daily. Yolk content for fed ostrich chicks was 26% of BW at 2 d of age. Ostrich chicks absorb 30% of yolk over the first 4 d, 67% after 8 d, and only deplete the yolk after 14 d posthatch. Key words:
Fasted ostrich chicks absorbed the yolk content at a rate of 28.9 g/d, compared with 22.3 g/d over the first 8 d and 16.3 g/d over the 16 d for fed ostrich chicks. The CP content of the yolk decreased by 6.84 g daily in fed ostrich chicks, whereas fat content of the yolk increased by 1.39% daily, although total yolk fat weight decreased by 6.61 g daily. Yolk weight and total CP decreased faster over the first 7 d in the fasted ostrich chicks compared with the fed ostrich chicks, which indicated that the decrease in yolk weight could be attributed to absorption of protein from the yolk. Fat content decreased faster over the first 8 d from the yolk of the fed ostrich chicks compared with that from the yolk of the fasted ostrich chicks, which could indicate that external feed has a positive influence on the absorption of fat from the yolk content.
yolk, fasted, fed, ostrich chick 2012 Poultry Science 91:1342-1349 http://dx.doi.org/10.3382/ps.2011-01833
INTRODUCTION During the early part of the 20th century, studies were conducted to determine the effect of feeding on yolk assimilation in chickens (Roberts, 1928; Heywang and Jull, 1930). Chicks were fasted for approximately 48 to 72 h posthatching to allow the yolk to be completely absorbed and prevent possible bowel problems that were thought to manifest when yolk assimilation occurred simultaneously with exogenous feed intake (Heywang and Jull, 1930). Since then, various studies have investigated the effect of feeding and starvation on yolk utilization and the development of the digestive tract and enzyme activity in poultry (Bierer and ©2012 Poultry Science Association Inc. Received August 31, 2011. Accepted January 22, 2012. ^Corresponding author: [email protected]
Eleazer, 1965; Chamblee et al., 1992; Noy et al., 1996; Noy and Sklan, 1998; Dibner, 2000; Smirnov et al, 2003; Uni, 2003). It was concluded that yolk utilization was more efficient in chicks that received feed and water after hatching, and that starvation has a negative effect on the development of the digestive tract and digestive enzymes. Much less work has been done on yolk utilization in ostrich chicks (Bertram and Burger, 1981; Deeming, 1995; Mushi et al., 2004), especially with regard to the influence of feeding and starvation on yolk utilization. To establish whether yolk utihzation in ostrich chicks was similar to that reported in chickens, 2 separate trials were conducted. In the first trial, the chemical composition of the yolk of fasted ostrich chicks from 1 to 7 d posthatching was determined. The aim of the second trial was to establish the chemical composition of the yolk of ostrich chicks fed a broiler prestarter diet
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EGG YOLK ABSORBED BY FASTED AND FED OSTRICH CHICKS
1343
Table 1. Mean hatching and slaughter weights (± SE) of fasted ostrich chicks as well as the yolk weight and pH of the yolk on 7 consecutive days after hatching-' Slaughter age (d) 1 2 3 4 5 6 7 LSD
Hatching weight (g) 833 ± 884 ± 836 ± 835 ± 836 ± 768 ± 824 ± 116
102* 81.6" 99.6" 101" 84.4* 86.6* 57.3*
Slaughter weight (g) 833 ± 838 ± 739 ± 782 ± 749 ± 657 ± 661 ± 119
102* 91.9" 148*'' 79.4" 83.2"'' 67.1'' 46.0''
Yolk weight (g) 230 ± 249 ± 206 ± 164 ± 109 ± 78.0 ± 102 ± 78.8
104*'' 74.9" 39.9"'' 76.5'"^ 51.8«' 12.0'' 7.3«'
DM (%)
Yolk pH
48.4 ± 6.23* 45.3 ± 5.60*'' 45.9 ± 2.48*'' 44.0 ± 2.95"'' 36.6 ± 4.98' 48.0 ± 2.41" 41.1 ± 8.22'"^ 2.05
7.34 ± 0.22* 7.18 ± 0.35*'' 7.00 ± 0.28'"^ 6.98 ± 0.18'"= 6.81 ± 0.80<= 6.46 ± 0.30^ 6.69 ± 0.30<=<' 2.05
" ''Column means with different superscripts differ significantly at P < 0.05, n = 5. 'Student's i-test least significant difference (LSD) was used for separation of treatment means.
that could conceivably affect the uptake of the yolk and stimulate growth and development of the digestive tract and digestive enzymes. The 2 trials are not compared, as there were too many variables in the methodology of the trials. The project had ethical approval from the Animal Use and Care Committee (Protocol 36-5-623) of the Faculty of Veterinary Science, University of Pretoria, South Africa.
MATERIALS AND METHODS
Birds The first trial involved 35 ostrich chicks. Fertilized eggs were obtained from the Oudtshoorn Experimental farm of the Department of Agriculture, Western Cape Province, South Africa and transported to the Faculty of Veterinary Science of the University of Pretoria, Onderstepoort, South Africa. Eggs were moved on d 36 of incubation. The eggs were packed by a professional ostrich egg transport company in suitable containers that eliminated excessive shaking and cooling of the eggs during transport, but allowed sufficient ventilation. Eggs were hatched in a poultry incubator (Octagon 100, Brinsea Products Inc., Titusville, FL) at 36°C with the humidity set at 24%. Eggs that did not hatch within the first 2 d of arrival at Onderstepoort were manually cracked to assist with hatching. Assisted chicks were slaughtered first, as they tended to be weaker than the others (Zanell Brand, Little Karoo Agricultural Development Centre, Oudtshoorn, South Africa, personal communication). To make up for losses sustained, 1-dold chicks (n = 8) were obtained from a nearby ostrich farm to maintain the required number of birds for the trial. Chicks were housed in a clean, disinfected room that was kept cool and dark. Noise and human contact were restricted to the minimum to limit stress. Chicks were provided with clean drinking water but no food. Five chicks were slaughtered each day for 7 consecutive days, starting from the day of hatching (day 1). The mean hatching and slaughter weights of the fasted chicks, as well as the yolk weight and pH, are presented in Table 1.
During the second trial, 6 chicks were slaughtered every second day (except on d 8 when 7 chicks were slaughtered) over a period of 16 d. The first group of chicks was thus 2 d old when killed, and a total of 49 chicks was slaughtered. The rationale for starting the trial on d 2 was based on the observation that feed and water does not appear to affect weight or yolk sac utilization of broilers during the first day posthatch (Chamblee et al., 1992). The chicks were obtained from the same source as those used for the first trial and were similarly handled and transported to the Faculty of Veterinary Science at Onderstepoort. The average weight of the 1-d-old chicks was 839 g. The trial continued for 16 d, as it had been reported that yolk was still present in 12-d-old chicks (Mushi et al., 2004). Chicks were kept under similar conditions to those used in the first trial, except for the provision of a poultry prestarter diet (AFCRI Animal Feeds, South Africa). The minimum specifications for the commercial prestarter diet were protein 24.5%, moisture 11.5%, energy 2,866 kcal/kg of feed, fat 6.7%, crude fiber 3.6%, and ash 6.2%. The mean hatching and slaughter weights for the fed chicks are presented in Table 2.
Measurements and Calculations In both trials, chicks were euthanized with CO2 in a closed container. The yolk sacs were immediately removed from the carcasses, weighed, and pH values determined using a Thermo Orion pH meter (Orion Research Inc., Beverly, MA). As much yolk as possible was taken from the yolk sac for proximate analysis and immediately frozen at —20°C. Yolk samples from both trials were freeze-dried and analyzed for DM (method 930.15), CP content (method 976.05), and fat content (method 920.39) according to AOAC (1995). Yolk samples from the first trial (the fasted chicks) were further analyzed for amino acids, fatty acids, and glucose composition. As the same tendencies were observed for CP and fat values in both trials, the analyses for amino acids, fatty acids, and glucose composition were not repeated for the second trial due to financial constraints and were assumed to be similar.
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VILJOEN ET AL.
Table 2. Mean hatching and slaughter weights (± SE) of fed ostrich chicks, as well as the yolk weight and pH on 16 d posthatching'^ Slaughter age (d) 2 4 6 *8 10 12 14 16 LSD
Hatching weight (g)
870 ± 132* 858 ± 65.0* 925 ± 129* 827 ± 129* 767 ± 136* 881 ± 44.2* 829 ± 89.0* 831 ± 116* 127
Slaughter weight (g)
772 ± 787 ± 853 ± 801 ± 902 ± 1,040 ± 1,192 ± 1.342 ± 181
118'' 38.3'' 123'' 167'' 107'='' 170'"= 273*'' 145*
Yolk weight (g)
DM weight (g)
229 ± 61.1* 160 ± 29.6'' 135 ± 42.4'' 75.2 ± 36.7<= 68.6 ± 36.0"= 51.6 ± 20.1'='' 18.4 ± 28.5''" 4.56 ± 4.0"= 41.4
51.7 db 0.16* 51.5 ± 0.33* 51.4 ± 0.29* 51.3 ± 0.54* 51.6 ± 0.28* 51.7 ± 0.28* — — 1.02
Yolk pH
7.28 7.03 6.81 7.01 7.49 7.04 7.40
± 0.16*'' ± 0.17'"= ± 0.16"= ± 0.19'"= ± 0.40* ± 0.21'"= ± 0.25* — 0.30
* "Column means with different superscripts differ significantly at P < 0.05, n = 6, *n = 7. '^Student's i-test least significant difference (LSD) was used for separation of treatment means.
Amino acids were determined on freeze-dried samples by ion-exchange chromatography of the acid-hydrolyzed protein. Samples were hydrolyzed (AOAC, 1995) with 6 M HCl in a sealed tube under N2 for 22 h in an oil bath at 110°C and then stored at -20°C. On the day the analyses were done, the samples were thawed to room temperature. Each sample was then mixed by vortex for 5 to 10 s and centrifuged at 15,000 x 5 for 5 min at room temperature in a Hermle bench centrifuge (HERMLE Labortechnik GmbH, Wehingen). The supernatant (25 fxL) was placed in a glass hydrolysis tube and dried under vacuum for 1 h. The pH was adjusted to pH = 7 by adding 20 |J,L of methanol:water:triethylamine, 2:2:1, and the samples were redried for 1 h. Each sample was derivatised by adding 20 |a,L of derivatising solution [methanol:wate r:triethylamine:phenyhsothiocyanate (PITC), 7:1:1:1]. The mixture was incubated at room temperature for 10 min and then dried under vacuum for a minimum of 1 h and a maximum of 3 h until completely dry. The derivatised dried sample was dissolved in 400 \xL of Picotag sample diluent (Waters, Milford, MA), filtered through a 0.45-|j,m filter and 16 |J-L of sample was subjected to HPLC using a standard method for PTC-amino acid chromatography. Data was collected and analyzed using Breeze software (Waters, Millford, MA). The percentage recovery of standards was determined by analyzing standards of a known quantity. For each batch of samples, at least 2 standards were dried and treated under the exact conditions as the samples. Eatty acid methyl esters (FAME) were prepared according to the method of Morrison and Smith (1964). The EAME were analyzed with a gas-liquid Chromatograph: Varian model 3300, equipped with flame ionization detection and two 30-m fused silica mega bore DB-225 columns of 0.53-mm internal diameter (J & W Scientific, Eolsom, CA). Gas flow rates were hydrogen, 25 mL/min; air, 250 mL/min; and nitrogen (carrier gas), 5 to 8 mL/min. Temperature programming was linear at 4°C/min; initial temperature, 160°C; final temperature, 220°C held for 10 min; injector temperature, 240°C; and detector temperature, 250°C. The EAME were identified by comparison of the retention times to those of a standard EAME mixture (Nu-Chek-
Prep Inc., Elysian, MN) and the milhgrams of fatty acid per gram of tissue sample was calculated. Glucose content was determined enzymatically using glucose oxidase within the ACE clinical chemistry system (Alfa Wassermann Inc., West Caldwell, NJ) according to manufacturer instructions.
Statistics The experimental design for the first trial entailed the selection of 5 ostrich chicks each day from hatching up to 7 d, with daily measurement of the yolk content parameters, with the exception of amino acids and fatty acids that were only analyzed on d 1, 3, 5, and 7. Amino acid and fatty acid composition was subjected to one-way ANOVA using SAS version 9.2 (SAS Institute, 2000). Levene's test was performed to test for homogeneity of daily ANOVA (Levene, 1960). The Shapiro-Wilk test was performed to test for nonnormality (Shapiro and Wilk, 1965). Student's t- test least significant difference was calculated at the 5% confidence level to compare means for age in days (Ott, 1998). The experimental design for the second trial entailed random selection of chicks, with BW and yolk content analysis being made on 6 chicks every second day from hatching up to 16 d. Linear and quadratic regression functions were fitted to weight and yolk variables over both trial periods to quantify changes over time using Proc Reg SAS version 9.2 (SAS Institute, 2000). Regressions are presented in Tables 3 and 4, respectively. In the case of certain variables, the pattern of change showed a linear tendency to decrease or increase, whereas other variables showed a tendency that was more quadratic. The decision of best fit was based on the R^ criteria.
RESULTS AND DISCUSSION
Fasted-Chlcks Triai (Triai 1) There was no difference in the hatching weight between individual chicks {P = 0.56; Table 1) although the average hatching weight was much lower than that reported in previous studies that ranged from 1.4 to 1.6 kg (Keffen and Jarvis, 1984; Mushi et al, 2004). This
EGG YOLK ABSORBED BY FASTED AND FED OSTRICH CHICKS
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Table 3. Regression statistics for the fasted ostrich chick trial Item
Equation
R2
Slaughter weight (g) Yolk weight (g) CP (%) CP content (g) Fat content (%) Fat content (g)
y y y y y y
0.36 0.55 0.48 0.57 0.48 0.48
= = = = = =
878 - 31.3x 3.1x2 _ ;)5.2x + 317 0.07x2 _ 2.21x + 46.8 1.58x2 - 27.2x + 142 0.12x2 _ 0.89x + 40.1 1.35x2 - 21.2x + 129
discrepancy may be due to the inherent characteristics of the particular eggs that were sourced for the trial. There was a decrease {P < 0.05) in slaughter weight from 1 to 7 d posthatching (Table 1). The chicks lost an average of 31.3 g of BW over the first 7 d posthatching (Table 3). This was expected as the chicks did not receive any feed and utilized 44% of their yolk over the trial period. Yolk weight decreased linearly (P < 0.05) over the trial period. On d 1, the average yolk weight was 230 g and decreased daily by 28.9 g to reach an average weight of 102 g on d 7 posthatching (Table 1). Yolk weight, as a percentage of BW, was 28% for 1-d-old and 2-d-old posthatch ostrich chicks and decreased to 12% of total BW on d 7. Romanoff (1960) and Sklan and Noy (2000) reported yolk weight as 20% of BW for chicks immediate posthatch, and Noy and Sklan (1998) reported 13 to 15% for poults at hatch. Only 44.4% of the ostrich chick yolk however, was assimilated over the 7 d posthatching period. Bierer and Eleazer (1965) reported that chickens deprived of feed for the first 7 d posthatching utilized 88.8% of their yolk. This difference could be due to the production cycle of ostriches being much longer than that of poultry, resulting in the utilization of the yolk over a longer period of time (Iji et al., 2003). The total DM percentage of the yolk declined {P < 0.05) from 1 to 7 d posthatching, with a significant drop in DM percentage on d 5, followed by a significant spike on d 6 (Table 1). The average pH of the yolk of chicks slaughtered on d 1 posthatching was 7.34. Yolk pH showed a linear dechne (P < 0.05) of 0.10 daily over the trial period, with an average pH of 6.69 on d 7 (Table 1). No literature could be found on the pH of yolk sac contents posthatch in any poultry. However, the decline in pH could be attributed to the changing composition of the yolk, as it is utilized during the first week posthatch. It has also been reported that a wide range of enzymes has been detected in the yolk membrane of the avian embryo during yolk digestion (Escribano et al., 1988).
P-value <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
Crude protein and fat content as a percentage of yolk composition, as well as total crude protein and fat content in grams is presented in Table 5. Crude protein content of the yolk decreased {P < 0.05) by 1.5% daily from d 1 (44.3%) to d 7 (32.5%) posthatching. This accounts for a daily decrease of 13.2 g in protein content. Fat content, on the other hand, increased (P < 0.05) by 1.77% daily from d 1 (41.2%) to 7 (48.9%) posthatching. Total yolk fat weight, however, decreased daily by 8.91 g from 125 g to 48.2 g. The increase in yolk fat percentage, therefore, simply refiected an increase in concentration of the component as a percentage of total yolk weight. The crude protein content of 43.8% for 2-d-old ostrich chicks was shghtly less than that of fasted poults that reflected a yolk crude protein content of 49.8% two days posthatch (Moran and Reinhart, 1980). Yolk fat content for ostrich chicks 2 d posthatching (42.4%) was also less than that reported for poults (50.6%) 2 d posthatching (Moran and Reinhart, 1980). Chicks reportedly showed a decrease in yolk protein and fat content after 2 d of fasting (Noy and Sklan, 1999). Although no difference (P < 0.05) in the glucose content over the entire trial period (P = 0.36) was observed, the glucose level in the yolk increased (P < 0.05) from d 3 (8.48 mmol/L) to d 7 posthatching (11.8 mmol/L). The same conclusion can be made as for yolk fat percentage; that is, that the apparent increase in glucose content within the yolk was only due to an increase in concentration of glucose as a percentage of total yolk volume. This was probably due to the high protein removal from the yolk. Noy and Sklan (1998) reported that the absorption of glucose did not change with age in poults, but although the absorption of glucose is low in chicks close to hatch, it increased slightly over the first 7 d posthatch (Noy and Sklan, 2001; Sklan, 2003). No literature could be found that referred to the glucose content of the yolk in either fasted or fed birds. Prom this study, it seemed that little glucose, or lipids, was utilized from the yolk as an energy source for the fasted ostrich chick. The fasted ostrich chick
Table 4. Regression statistics for the fed ostrich chick trial Item
Equation
R2
Slaughter weight (g) Yolk weight (g) CP (%) CP content (g) Fat content (%) Fat content (g)
y y y y y y
0.64 0.81 0.47 0.74 0.48 0.73
= = = = = =
3.88x2 - 30.2x + 835 0.82x2 - 29.8x + 278 46.2 - 1.20x 97.4 - 6.84x 38.7 + 1.39x 99.3 - 6.61x
P-value <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
VILJOEN ET AL.
1346 Table 5. Crude protein and fat content 1 to 7 d posthatching-' Slaughter age (d) 1 2 3 4 5 6 7 LSD
I of total yolk content; ± SE) of fasted ostrich chicks from
n
CP (%)
CP(g)
n
Fat (%)
Fat (g)
5 5 5 5 5 4 4
44.3 ± 1.98" 43.8 ± 1.76"'' 41.0 ± 3.81"'' 40.1 ± 2.82'' 36.0 ± 1.88'= 35.4 ± 1.66'= 32.5 ± 4.62'= 3.77
134 ± 23.4'^ 93.0 ± 23.1'' 73.7 ± 26.1'' 69.8 ± 37.9'"= 39.0 ± 17.7'='' 34.3 ± 3.10'' 30.8 ± 11.8'' 31.3
5 5 5 5 5 5 3
41.2 ± 2.49'= 42.4 ± 2.01'= 43.6 ± 4.67'"= 43.5 ± 6.04'= 50.1 ± 1.90" 52.8 ± 2.38" 48.9 ± 6.58"'' 5.32
125 ± 29.1" 90.1 ± 23.0'' 76.5 ± 17.2'"= 70.1 ± 22.7'"= 55.1 ± 26.5'= 49.7 ± 6.02'= 48.2 ± 4.31'= 28.9
" ''Column means with different superscripts differ significantly at P < 0.05. •'Student's t-test least significant difference (LSD) was used for sepai-ation of treatment means.
would therefore have had to use yolk protein as an energy source, which could also contribute to the weight loss observed over the 7-d trial period, as less protein was then available for growth. Amino acid composition of yolk from fasted ostrich chicks is displayed in Table 7. Of the 18 amino acids analyzed, only 7 amino acids showed any differences {P < 0.05) in percentage contribution to the total amount of protein in the yolk over the 1 to 7 d posthatching trial period. Glycine and threonine showed a similar pattern over the 7-d period, with an initial decline in content from d 1 to 3 (although not significant; P > 0.05), followed by an increase {P < 0.05) from d 3 to 5 and declining again on d 7, but not significantly so. Methionine and serine showed the same tendencies, with first a slight increase from d 1 to 3, a decline {P < 0.05) from d 3 to 5, and a shght decrease from d 5 to 7. Histidine declined {P < 0.05) from d 1 to 5, whereas valine decreased {P < 0.05) from d 1 to 5 and then increased {P < 0.05) again from d 5 to 7. No information is available in the literature on the uptake of amino acids from the posthatch yolk contents in either poultry or ratites. It has been reported that amino acids from the yolk are used both as an energy source and as building blocks for the development of the small intestine (Wijtten et al., 2010). Noy and Sklan (2002) reported that amino acids are not easily absorbed from the rich yolk medium, but that absorption increases with age and with development of the hydrophilic conditions in the small
of total yolk content; ± SE) of fed ostrich chicks 16 d
Table 6. Crude protein and fat content posthatching'^ Slaughter age (d) 2 4 6 8 10 12 14 16 LSD
n 6 6 6 6 5 6 1
CP (%) 44.8 39.3 39.7 37.1 32.0 31.9
± 3.77" ± 6.78"'' ± 3.14"'' ± 6.43"'' ± 4.52'"= ± 4.85'"= — — 7.96
intestine (Noy and Sklan, 1999). The results of this trial may indicate that the absorption of histidine and valine is favored by ostrich chicks and that the inclusion of these amino acids in prestarted ostrich chick diets should be considered. Fatty acid composition of the yolk is given in Table 8 and is expressed as percentage contribution to the total amount of fat in the yolk. Saturated fatty acid (SFA) content remained constant from d 1 to 5 posthatching and then increased {P < 0.05) from d 5 to 7. The main contributors to this increase were palmitic acid (16:0), stearic acid (18:0), arachidic acid (20:0), and lignoceric acid (24:0). Results obtained in a study conducted in poults (Reidy et al., 1998) indicated that stearic acid (18:0) also increased posthatch from 10.4 to 11.3% 24 h posthatch. Monounsaturated fatty acids (MUFA) also remained constant over the first 5 d posthatching, but decreased {P < 0.05) from d 5 to 7. This decrease was due to similar decreases {P < 0.05) in oleic acid (18:ln9c) and nervonic acid (24:1). Reidy et al. (1998), however, reported an increase in oleic acid (18:ln9c) from 45.0 to 46.9%. There were no differences in the polyunsaturated fatty acid (PUFA) contents from 1 to 7 d posthatching, regardless of differences {P < 0.05) in fatty acid content within the group, mostly from 5 to 7 d posthatching. The SFA:UFA ratio also remained constant over the first 5 d, but then increased {P < 0.05) from d 5 to 7. This is the result of the large increase in SFA content
CP(g) 103 63.9 53.7 28.3 22.1 16.8 1.25
± 31.5" ± 18.0'' ± 17.4'' ± 17.8'= ± 13.8'= ± 8.04'='' ± 3.05'' — 41.4
n 6 6 6 5 4 6 1
Fat (%) 40.9 46.3 46.5 48.5 51.7 55.9
± ± ± ± ± ±
5.27'' 6.67'='' 3.74'='' 7.70'"='' 1.23'"= 6.56"'' — — 9.22
Fat (g) 93.3 73.7 63.1 30.4 30.2 28.4 2.65
" ''Column means with different superscripts differ significantly aX P < 0.05. ^Student's i-test least significant difference (LSD) was used for separation of treatment means.
± 25.5» ± 14.2"'' ± 21.8'' ± 23.3'= ± 24.3'= ± 9.85'= ± 6.49'' — 21.0
EGG YOLK ABSORBED BY EASTED AND EED OSTRICH CHICKS
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Table 7. Amino acid composition (% of total yolk protein content) of yolk from fasted ostrich chicks from 1 to 7 d posthatching^ Item n Alanine Arginine Aspartate Gy steine Glutamate Glycine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tyrosine Valine
Id
3d
5d
7d
LSD
5 7.18 3.24 8.54 1.80 11.9 5.64»'' 1.84» 4.58 9.50 5.86 2.90*'' 3.78 4.94 12.0"'' 6.49'^'^ 3.51 6.09"
4 7.26 3.15 8.49 1.81 11.7 5.63'' 1.78"'' 4.50 9.53 6.15 2.97" 3.87 4.91 12.1« 6.46= 3.53 5.81''
4 7.23 3.21 8.64 1.84 11.7 5.78" 1.68<= 4.47 9.52 5.98 2.89'' 3.88 4.96 11.8'' 6.69" 3.61 5.85''
3 7.27 3.09 8.53 1.84 12.0 5.75"'' 1.7l'"= 4.58 9.69 5.97 2.95"'' 3.89 4.99 12.0"'' 6.62"'' 3.58 6.00"
0.21 0.22 0.16 0.19 0.34 0.15 0.08 0.22 0.24 0.53 0.08 0.15 0.13 0.29 0.16 0.12 0.11
means with different superscripts differ significantly a t f < 0.05. ^Student's i-test least significant difference (LSD) was used for separation of t r e a t m e n t means.
from d 5 (46.2) to d 7 (77.9). Reidy et al. (1998) reported that poults and chicks also differ in fatty acid utilization from the yolk, and that poults prefer palmitoleic acid (16:1) over oleic acid (18:ln9c), whereas Noble et al. (1988) reported a decrease in oleic acid and an increase in palmitoleic acid with age in chicks. Surai et al.
(1999) reported a marked difference between the fatty acid profiles of the yolk lipids between 4 avian species (chicken, turkey, duck, and goose), even though all the birds in the trial received very similar dietary fatty acids. The authors suggested that the fatty acid profile of layer diets should be designed to accommodate the par-
Table 8. Fatty acid composition {% of total yolk fat content) of yolk from fasted ostrich chicks from 1 to 7 d posthatching^ Item n Palmitic acid Steai'ic acid Arachidic acid Heneicosanoic acid Behenic acid Lignoceric acid Palmitoleic acid Elaidic acid Gleic acid Gondoic acid Erucic acid Nervonic acid Linolelaidic acid Linoleic acid Eicosadienoic acid Brassic acid 7-Linolenic acid a-Linolenic acid Dihomo-"/-linolenic acid Eicosatrienoic acid Arachidonic acid Eicosapentaenoic acid Docosapentaenoic acid Docosahexaenoic acid
G:D2 16:0 18:0 20:0 21:0 22:0 24:0 SFA 16:1 18:ln9t 18:ln9c 20:1 22:ln9 24:1 MUFA 18:2n6t 18:2n6c 20:2 22:2 18:3n6 18:3n3 20:3n6 20:3n3 20:4n6 20:5n3 22:5n3 22:6n3 PUFA UFA SFA:UFA
Id 5 32.8'' 9.82'' 0.07 0.02 0.02 0.18 42.9'' 7.43"'' 0.22 34.1" 0.26 0.03 0.13 42.2" 0.02 6.02" 0.22 0.05 0.22 3.05" 0.12 3.24" 0.02 0.15 0.13 1.15"'' 14.9" 57.1" 0.76
3d 4 33.3'' 9.87'' 0.08 0.03 0.02 0.27 43.6'' 8.47"'' 0.30 33.3" 0.26 0.04 0.09 42.4" 0.03 5..36" 0.18 0.04 0.2lt 2.25"'' 0.15t 3.79" 0.05 0.11 0.12 1.02'' 14.0" 56.4" 0.78
5d
7d
LSD
4 35.2'' 10.4'' 0.08 0.02 0.22 0.27 46.2'' 6.39'' 0.35 36.8" 0.26 0.03 0.10 43.9" 0.03 1.99" 0.15 0.02 0.19 2.78" 0.15 3.18" 0.02 0.18 0.16 1.06'' 9.92" 53.8" 0.87
2 58.6" 18.5" 0.15 0.03 0.04 0.40 78.0" 8.98"* 0.34* 0.07'' 0.59 0.05* 0.10* 10.8'' 0.05* 0.09"* 0.10* 0.03* 0.30* 1.14'' 0.20* 0.22'' 5.19 0.30 0.35 1.63"* 10.3"* 22.1'' 3.65
3.53 3.56 0.02 0.02 0.04 0.11 6.09 2.46 0.07 5.99 0.08 0.03 0.04 6.73 0.03 9.67 0.09 0.02 0.09 1.59 0.07 1.15 0.22 0.07 0.14 0.50 8.54 6.09 0.54
"•''Row means with different superscripts differ significantly at P < 0.05. t' Golumn means with different superscripts have n = 3. ^Student's i-test least significant difference (LSD) was used for separation of treatment means. 2G:D = lipid number (G = number of atoms in a fatty acid: D = number of double bonds in the fatty acid); SFA = saturated fatty acids, i\'IUFA ; monounsaturated fatty acids, PUFA = polyunsaturated fatty acids, UFA = unsaturated fatty acids.
1348
VILJOEN ET AL.
ticular metabolic features of each species with regard to the specific fatty acids of the yolk that are essential for the embryonic development of each individual species. No literature could be found on the fatty acid content of ostrich yolk posthatching, especially with regard to possible preference for specific fatty acid withdrawal from hatching until the yolk has been totally absorbed. The results of this trial may indicate that the ostrich chick has a greater ability to absorb MUFA, especially oleic acid (18:ln9c). The inclusion of oleic acid in prestarter ostrich chick diets should be considered in future studies, as energy from fatty acids may be a better source of energy for sustainable early growth than glucose derived from protein, as is discussed later in this paper.
Fed-Chicks Trial (Trial 2) There was an increase (P < 0.05) in slaughter weight from 1 to 16 d posthatching (Table 2). Yolk weight decreased (P < 0.05) by 16.3 g daily from d 2 (229 g) to d 16 (4.56 g) posthatching (Table 2). Yolk content of 2-d-old fed ostrich chicks in this trial was 26% of BW, which was more than reported for poults (10.1%; Moran and Reinhart, 1980) and broilers (16.7%; Chamblee et al., 1992) at hatch. Several studies on newly hatched chicks reported that the yolk is absorbed during the first 3 d posthatching (Heywang and Jull, 1930; Jull and Heywang, 1930; Heywang, 1940; Chamblee et al., 1992). In contrast, this trial indicated that ostrich chicks only absorb 30% of the yolk over the first 4 d posthatch, 67% after 8 d posthatch, and only deplete the yolk reserve after 14 d posthatch. This observation is supported by the study of Mushi et al. (2004) on ostrich chicks that suggested yolk reserves were considered retained beyond 13 d posthatch. Crude protein as a percentage of yolk composition, total crude protein content in grams, fat content as a percentage of yolk composition, and total fat content in grams are presented in Table 6. Both constituents showed similar tendencies to those reported for crude protein and fat content in the yolks of fasted ostrich chicks. Crude protein content decreased (P < 0.05) by 1.2% daily (Table 4) from d 2 (44.8%) to d 14 (28.7%) posthatching, which amounts to a daily decrease of 6.84 g (Table 4). This result corresponds with work reported by other authors. Noy and Sklan (1999) reported that yolk protein content decreased posthatch for chicks and was depleted 4 d posthatch. Similar results were obtained for poults by Moran and Reinhart (1980), who reported that the yolk protein content decreased from 51.3% 1 d posthatch to 45.3% 2 d posthatch. Fat content of the yolk increased (P < 0.05) by 1.39% daily (Table 4) from d 2 (40.8%) to d 14 (61%) posthatch, although total yolk fat weight decreased by 6.61 g daily (Table 4) from 93.75 g at 2 d of age to 11.25 g at 14 d of age. There was too little yolk left to measure crude protein and fat content at 16 d of age. The
increase in yolk fat percentage could therefore be attributed to fat being slowly assimilated from the yolk content, hence increasing as a percentage of total yolk concentration. These results were in direct contrast to results obtained for chicks and poults. Moran and Reinhart (1980) reported that lipids were utilized faster than protein from the yolk of poults, whereas Romanoff and Romanoff (1967) made the same observation for chicks. Sklan and Noy (2000) reported that the mechanisms for the utilization of lipids are present in the developing embryo, so that little change is necessary in the digestive tract of the posthatch poult to be able to utilize lipid absorption. One would therefore expect that utilization of lipids from the yolk would be an efficient mechanism to meet the energy requirement of the posthatch ostrich chick. Several authors have suggested that the composition of the yolk only provides maintenance requirements for chicks and poults, whereas exogenous energy sources are utilized for growth (Romanoff, 1960; Thaxton and Parkhurst, 1976; Chamblee et al., 1992; Sklan and Noy, 2000). Chamblee et al. (1992), however, suggested that yolk fat is necessary for early growth, as dietary fat is only effectively utilized 10 d after hatching. As glucose is used to provide energy for hatching activities of chicks (John et al, 1998; Christensen et al., 2001), Hoiby et al. (1987) reported that glycolysis would be preferred above fatty acid oxidation as oxygen is limited during hatching. It may therefore be a possibility that ostrich chicks preferably synthesize glucose from protein rather than from fat during posthatch development. However, this would not favor sustainable early growth and stronger chicks.
Conclusion Thaxton and Parkhurst (1976) reported that adding carbohydrate and amino acids to the diet in the first 2 wk posthatching is essential for the initiation of growth of chicks. This is especially important as dietary fat is only utilized effectively 10 d posthatch in broilers (Chamblee et al, 1992). In both the trials conducted on ostrich chicks, protein was assimilated from the yolk sacs, while it appeared that fat was absorbed at a much slower rate. The changes observed in fatty acid composition on d 7 posthatching may indicate that ostrich chicks have the ability to withdraw certain fatty acid components, especially MUFA from the yolk content. It would be interesting to see whether this apparent ability to withdraw certain fatty acids is repeated with exogenous feed over the first weeks posthatching. Moran and Reinhart (1980) and Reidy et al. (1998) indicated that poults have the ability for selective withdrawal from the yolk, whereas poults and chicks display differences in metabolism of various lipid fractions. Even though this study indicated that yolk absorption is faster in fasted than fed ostrich chicks, the authors agree with previous studies on chicks and poults (Bierer and Eleazer, 1965; Chamblee et al., 1992; Noy et al., 1996; Noy and Sklan, 1998; Dibner, 2000; Smirnov et
EGG YOLK ABSORBED BY FASTED AND FED OSTRICH CHICKS al., 2003; Uni, 2003), which suggest that ostrich chicks should be fed immediately posthatch. FYirther studies should be performed with prestarter diets that include added amino acids, especially histidine and valine, as well as MUEA during the first few weeks posthatching, due to the fact that a preference for these nutrients from the yolk content was indicated in this trial.
REFERENCES AOAC. 1995. Official Methods of Analysis. 16th ed. Assoc. Off. Anal. Chem., Washington, DC. Bertram, B. C. R., and A. E. Burger. 1981. Aspects of incubation in ostriches. Ostrich 52:36-43. Bierer, B. W., and T. H. Eleazer. 1965. Effect of feed and water deprivation on yolk utilization in chicks. Poult. Sci. 44:1608-1609. Chamblee, T. N., J. D. Brake, C. D. Schultz, and J. P. Thaxton. 1992. Yolk sac absorption and initiation of growth in broilers. Poult. Sci. 71:1811-1816. Christensen, V. L., M. J. Wineland, C. M. Fasenko, and W. E. Donaldson. 2001. Egg storage effects on plasma glucose and supply and demand tissue glycogen concentrations of broiler embryos. Poult. Sci. 80:1729-1735. Deeming, D. C. 1995. Possible effect of microbial infection on yolk utilization in ostrich chicks. Vet. Rec. 136:270-271. Dibner, J. 2000. Early nutrition in young poultry. Pages 73-88 in Recent Advances in Animal Nutrition. P. C. Garnsworthy and J. Wiseman, ed. University of Nottingham, Nottingham, UK. Escribano, F., B. E. Rahn, and J. L. Sell. 1988. Development of lipase activity in yolk membrane and pancreas of young turkeys. Poult. Sci. 67:1089-1097. Heywang, B. W. 1940. The effect of cold drinking water on chick growth and yolk absorption. Poult. Sci. 19:201-204. Hejrwang, B. W., and M. A. Jull. 1930. The influence of starving and feeding mash and scratch grain, respectively, at different times on yolk absorption in chicks. Poult. Sci. 9:291-295. H0iby, M., A. Aulie, and P. O. Bjonnes. 1987. Anaerobic metabolism in fowl embryos during normal incubation. Comp. Biochem. Physiol. A 86:91-94. Iji, P. A., J. G. Van der Walt, T. S. Brand, E. A. Boomker, and D. Booyse. 2003. Development of the digestive tract in the ostrich {Struthio camelus). Arch. Tierenahr 57:217-228. John, T. M., J. C. George, and E. T. Moran Jr.. 1988. Metabolic changes in pectoral muscles and liver of turkey embryos in relation to hatching: Influence of glucose and antibiotic-treatment of eggs. Poult. Sci. 67:463-469. Jull, M. A., and B. W. Heywang. 1930. Yolk assimilation during the embryonic development of the chick. Poult. Sci. 9:393-404. Keffen, R. H., and M. J. F. Jarvis. 1984. Some measurements relating to ostrich eggs. Ostrich 55:182-187. Levene, H. 1960. Robust test in the equality of variance. In Contributions to Probability and Statistics. Iolkin Palo Alto, ed. Stanford University Press, Palo Alto, CA. Moran, E. T., and B. S. Reinhart. 1980. Poult yolk sac amount and composition upon placement: Effect of breeder age, egg weight.
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sex and subsequent change with feeding or fasting. Poult. Sci. 59:1521-1528. Morrison, W. R., and M. L. Smith. 1964. Preparation of fatty acid methyl esters and dimethylacetals from lipids with boron fluoride methanol. J. Lipid Res. 5:600-608. Mushi, E. Z., M. G. Binta, and R. G. Chabo. 2004. Yolk sac utilization in ostrich {Struthio camelus) chicks. Onderstepoort J. Vet. Res. 71:247-249. Noble, R. C, D. Ogunyemi, and S. G. TuUett. 1988. Understanding the chick embryo. V: A close view of the uptake of yolk fat. Poult. Misset Int. 4:32-33. Noj', Y., and D. Sklan. 1998. Yolk utilisation in the newly hatched poult. Br. Poult. Sci. 39:446-451. Noy, Y., and D. Sklan. 1999. Energy utilization in newly hatched chicks. Poult. Sci. 78:1750-1756.' Noy, Y., and D. Sklan. 2001. Yolk and exogenous feed utilization in the post-hatch chick. Poult. Sci. 80:1490-1495. Noy, Y., and D. Sklan. 2002. Nutrient use in chicks during the first week posthatch. Poult. Sci. 81:391-399. Noy, Y., Z. Uni, and D. Sklan. 1996. Routes of yolk utilisation in the newlj'-hatched chick. Br. Poult. Sci. 37:987-995. Ott, R. L. 1998. An Introduction to Statistical Methods and Data Analysis. Duxbury Press, Belmont, CA. Reid}', T. R., J. L. Atkinson, and S. Leeson. 1998. Size and components of poult yolk sacs. Poult. Sci. 77:639-643. Roberts, R. E. 1928. When should chicks be given flrst feed? Poult. Sci. 7:220-224. Romanoff, A. C. 1960. The Avian Embryo. MacMillan Publishing Co., New York, NY. Romanoff, A. C , and A. J. Romanoff. 1967. Biochemistry of the Avian Embryo. Interscience Publishers, New York, NY. SAS Institute. 2000. SAS User's Guide. Version 8.1 ed. SAS Inst. Inc., Cary, NC. Shapiro, S. S., and M. B. Wilk. 1965. An analysis of variance test for normality (complete samples). Biometrika 52:591-611. Sklan, D. 2003. Fat and carbohydrate use in post-hatch chicks. Poult. Sci. 82:117-122. Sklan, D., and Y. Noy. 2000. Hydrolysis and absorption in the intestines of newly hatched chicks. Poult. Sci. 79:1306-1310. Smirnov, A., Z. Uni, and D. Sklan. 2003. Role of mucin in the chicken gastrointestinal tract—The influence of starvation. Pages 231232 in 14th Eur. Symp. Poult. Nutr. World's Poult. Sci. Assoc, Lillehamnier, Norway. Surai, P. F., B. K. Speake, R. G. Noble, and M. Mezes. 1999. Speciesspecific differences in the fatty acid profiles of the lipids of the yolk and of the hver of the chick. J. Sci. Food Agrie. 79:733-736. Thaxton, J. P., and C. R. Parkhurst. 1976. Growth efficiency and livability of newly hatched broilers as influenced by hydration and intake of sucrose. Poult. Sci. 55:2275-2279. Uni, Z. 2003. Methods for early nutrition and their potential. Pages 254-260 in 14th Eur. Symp. Poult. Nutr. Lillehammer, Norway. Wijtten, P. J. A., E. Hangoor, J. K. W. M. Spai'la, and M. W. A. Verstegen. 2010. Dietary amino acid levels and feed restriction affect small intestinal development, mortalit}', and weight gain of male broilers. Poult. Sci. 89:1424-1439.
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Fasted ostrich chicks absorbed the yolk content at a rate of 28.9 g/d, compared with 22.3 g/d over the first 8 d and 16.3 g/d over the 16 d for fed ostrich chicks. The CP content of the yolk decreased by 6.84 g daily in fed ostrich chicks, whereas fat content of the yolk increased by 1.39% daily, although total yolk fat weight decreased by 6.61 g daily. Yolk weight and total CP decreased faster over the first 7 d in the fasted ostrich chicks compared with the fed ostrich chicks, which indicated that the decrease in yolk weight could be attributed to absorption of protein from the yolk. Fat content decreased faster over the first 8 d from the yolk of the fed ostrich chicks compared with that from the yolk of the fasted ostrich chicks, which could indicate that external feed has a positive influence on the absorption of fat from the yolk content.
yolk, fasted, fed, ostrich chick 2012 Poultry Science 91:1342-1349 http://dx.doi.org/10.3382/ps.2011-01833
INTRODUCTION During the early part of the 20th century, studies were conducted to determine the effect of feeding on yolk assimilation in chickens (Roberts, 1928; Heywang and Jull, 1930). Chicks were fasted for approximately 48 to 72 h posthatching to allow the yolk to be completely absorbed and prevent possible bowel problems that were thought to manifest when yolk assimilation occurred simultaneously with exogenous feed intake (Heywang and Jull, 1930). Since then, various studies have investigated the effect of feeding and starvation on yolk utilization and the development of the digestive tract and enzyme activity in poultry (Bierer and ©2012 Poultry Science Association Inc. Received August 31, 2011. Accepted January 22, 2012. ^Corresponding author: [email protected]
Eleazer, 1965; Chamblee et al., 1992; Noy et al., 1996; Noy and Sklan, 1998; Dibner, 2000; Smirnov et al, 2003; Uni, 2003). It was concluded that yolk utilization was more efficient in chicks that received feed and water after hatching, and that starvation has a negative effect on the development of the digestive tract and digestive enzymes. Much less work has been done on yolk utilization in ostrich chicks (Bertram and Burger, 1981; Deeming, 1995; Mushi et al., 2004), especially with regard to the influence of feeding and starvation on yolk utilization. To establish whether yolk utihzation in ostrich chicks was similar to that reported in chickens, 2 separate trials were conducted. In the first trial, the chemical composition of the yolk of fasted ostrich chicks from 1 to 7 d posthatching was determined. The aim of the second trial was to establish the chemical composition of the yolk of ostrich chicks fed a broiler prestarter diet
1342
EGG YOLK ABSORBED BY FASTED AND FED OSTRICH CHICKS
1343
Table 1. Mean hatching and slaughter weights (± SE) of fasted ostrich chicks as well as the yolk weight and pH of the yolk on 7 consecutive days after hatching-' Slaughter age (d) 1 2 3 4 5 6 7 LSD
Hatching weight (g) 833 ± 884 ± 836 ± 835 ± 836 ± 768 ± 824 ± 116
102* 81.6" 99.6" 101" 84.4* 86.6* 57.3*
Slaughter weight (g) 833 ± 838 ± 739 ± 782 ± 749 ± 657 ± 661 ± 119
102* 91.9" 148*'' 79.4" 83.2"'' 67.1'' 46.0''
Yolk weight (g) 230 ± 249 ± 206 ± 164 ± 109 ± 78.0 ± 102 ± 78.8
104*'' 74.9" 39.9"'' 76.5'"^ 51.8«' 12.0'' 7.3«'
DM (%)
Yolk pH
48.4 ± 6.23* 45.3 ± 5.60*'' 45.9 ± 2.48*'' 44.0 ± 2.95"'' 36.6 ± 4.98' 48.0 ± 2.41" 41.1 ± 8.22'"^ 2.05
7.34 ± 0.22* 7.18 ± 0.35*'' 7.00 ± 0.28'"^ 6.98 ± 0.18'"= 6.81 ± 0.80<= 6.46 ± 0.30^ 6.69 ± 0.30<=<' 2.05
" ''Column means with different superscripts differ significantly at P < 0.05, n = 5. 'Student's i-test least significant difference (LSD) was used for separation of treatment means.
that could conceivably affect the uptake of the yolk and stimulate growth and development of the digestive tract and digestive enzymes. The 2 trials are not compared, as there were too many variables in the methodology of the trials. The project had ethical approval from the Animal Use and Care Committee (Protocol 36-5-623) of the Faculty of Veterinary Science, University of Pretoria, South Africa.
MATERIALS AND METHODS
Birds The first trial involved 35 ostrich chicks. Fertilized eggs were obtained from the Oudtshoorn Experimental farm of the Department of Agriculture, Western Cape Province, South Africa and transported to the Faculty of Veterinary Science of the University of Pretoria, Onderstepoort, South Africa. Eggs were moved on d 36 of incubation. The eggs were packed by a professional ostrich egg transport company in suitable containers that eliminated excessive shaking and cooling of the eggs during transport, but allowed sufficient ventilation. Eggs were hatched in a poultry incubator (Octagon 100, Brinsea Products Inc., Titusville, FL) at 36°C with the humidity set at 24%. Eggs that did not hatch within the first 2 d of arrival at Onderstepoort were manually cracked to assist with hatching. Assisted chicks were slaughtered first, as they tended to be weaker than the others (Zanell Brand, Little Karoo Agricultural Development Centre, Oudtshoorn, South Africa, personal communication). To make up for losses sustained, 1-dold chicks (n = 8) were obtained from a nearby ostrich farm to maintain the required number of birds for the trial. Chicks were housed in a clean, disinfected room that was kept cool and dark. Noise and human contact were restricted to the minimum to limit stress. Chicks were provided with clean drinking water but no food. Five chicks were slaughtered each day for 7 consecutive days, starting from the day of hatching (day 1). The mean hatching and slaughter weights of the fasted chicks, as well as the yolk weight and pH, are presented in Table 1.
During the second trial, 6 chicks were slaughtered every second day (except on d 8 when 7 chicks were slaughtered) over a period of 16 d. The first group of chicks was thus 2 d old when killed, and a total of 49 chicks was slaughtered. The rationale for starting the trial on d 2 was based on the observation that feed and water does not appear to affect weight or yolk sac utilization of broilers during the first day posthatch (Chamblee et al., 1992). The chicks were obtained from the same source as those used for the first trial and were similarly handled and transported to the Faculty of Veterinary Science at Onderstepoort. The average weight of the 1-d-old chicks was 839 g. The trial continued for 16 d, as it had been reported that yolk was still present in 12-d-old chicks (Mushi et al., 2004). Chicks were kept under similar conditions to those used in the first trial, except for the provision of a poultry prestarter diet (AFCRI Animal Feeds, South Africa). The minimum specifications for the commercial prestarter diet were protein 24.5%, moisture 11.5%, energy 2,866 kcal/kg of feed, fat 6.7%, crude fiber 3.6%, and ash 6.2%. The mean hatching and slaughter weights for the fed chicks are presented in Table 2.
Measurements and Calculations In both trials, chicks were euthanized with CO2 in a closed container. The yolk sacs were immediately removed from the carcasses, weighed, and pH values determined using a Thermo Orion pH meter (Orion Research Inc., Beverly, MA). As much yolk as possible was taken from the yolk sac for proximate analysis and immediately frozen at —20°C. Yolk samples from both trials were freeze-dried and analyzed for DM (method 930.15), CP content (method 976.05), and fat content (method 920.39) according to AOAC (1995). Yolk samples from the first trial (the fasted chicks) were further analyzed for amino acids, fatty acids, and glucose composition. As the same tendencies were observed for CP and fat values in both trials, the analyses for amino acids, fatty acids, and glucose composition were not repeated for the second trial due to financial constraints and were assumed to be similar.
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VILJOEN ET AL.
Table 2. Mean hatching and slaughter weights (± SE) of fed ostrich chicks, as well as the yolk weight and pH on 16 d posthatching'^ Slaughter age (d) 2 4 6 *8 10 12 14 16 LSD
Hatching weight (g)
870 ± 132* 858 ± 65.0* 925 ± 129* 827 ± 129* 767 ± 136* 881 ± 44.2* 829 ± 89.0* 831 ± 116* 127
Slaughter weight (g)
772 ± 787 ± 853 ± 801 ± 902 ± 1,040 ± 1,192 ± 1.342 ± 181
118'' 38.3'' 123'' 167'' 107'='' 170'"= 273*'' 145*
Yolk weight (g)
DM weight (g)
229 ± 61.1* 160 ± 29.6'' 135 ± 42.4'' 75.2 ± 36.7<= 68.6 ± 36.0"= 51.6 ± 20.1'='' 18.4 ± 28.5''" 4.56 ± 4.0"= 41.4
51.7 db 0.16* 51.5 ± 0.33* 51.4 ± 0.29* 51.3 ± 0.54* 51.6 ± 0.28* 51.7 ± 0.28* — — 1.02
Yolk pH
7.28 7.03 6.81 7.01 7.49 7.04 7.40
± 0.16*'' ± 0.17'"= ± 0.16"= ± 0.19'"= ± 0.40* ± 0.21'"= ± 0.25* — 0.30
* "Column means with different superscripts differ significantly at P < 0.05, n = 6, *n = 7. '^Student's i-test least significant difference (LSD) was used for separation of treatment means.
Amino acids were determined on freeze-dried samples by ion-exchange chromatography of the acid-hydrolyzed protein. Samples were hydrolyzed (AOAC, 1995) with 6 M HCl in a sealed tube under N2 for 22 h in an oil bath at 110°C and then stored at -20°C. On the day the analyses were done, the samples were thawed to room temperature. Each sample was then mixed by vortex for 5 to 10 s and centrifuged at 15,000 x 5 for 5 min at room temperature in a Hermle bench centrifuge (HERMLE Labortechnik GmbH, Wehingen). The supernatant (25 fxL) was placed in a glass hydrolysis tube and dried under vacuum for 1 h. The pH was adjusted to pH = 7 by adding 20 |J,L of methanol:water:triethylamine, 2:2:1, and the samples were redried for 1 h. Each sample was derivatised by adding 20 |a,L of derivatising solution [methanol:wate r:triethylamine:phenyhsothiocyanate (PITC), 7:1:1:1]. The mixture was incubated at room temperature for 10 min and then dried under vacuum for a minimum of 1 h and a maximum of 3 h until completely dry. The derivatised dried sample was dissolved in 400 \xL of Picotag sample diluent (Waters, Milford, MA), filtered through a 0.45-|j,m filter and 16 |J-L of sample was subjected to HPLC using a standard method for PTC-amino acid chromatography. Data was collected and analyzed using Breeze software (Waters, Millford, MA). The percentage recovery of standards was determined by analyzing standards of a known quantity. For each batch of samples, at least 2 standards were dried and treated under the exact conditions as the samples. Eatty acid methyl esters (FAME) were prepared according to the method of Morrison and Smith (1964). The EAME were analyzed with a gas-liquid Chromatograph: Varian model 3300, equipped with flame ionization detection and two 30-m fused silica mega bore DB-225 columns of 0.53-mm internal diameter (J & W Scientific, Eolsom, CA). Gas flow rates were hydrogen, 25 mL/min; air, 250 mL/min; and nitrogen (carrier gas), 5 to 8 mL/min. Temperature programming was linear at 4°C/min; initial temperature, 160°C; final temperature, 220°C held for 10 min; injector temperature, 240°C; and detector temperature, 250°C. The EAME were identified by comparison of the retention times to those of a standard EAME mixture (Nu-Chek-
Prep Inc., Elysian, MN) and the milhgrams of fatty acid per gram of tissue sample was calculated. Glucose content was determined enzymatically using glucose oxidase within the ACE clinical chemistry system (Alfa Wassermann Inc., West Caldwell, NJ) according to manufacturer instructions.
Statistics The experimental design for the first trial entailed the selection of 5 ostrich chicks each day from hatching up to 7 d, with daily measurement of the yolk content parameters, with the exception of amino acids and fatty acids that were only analyzed on d 1, 3, 5, and 7. Amino acid and fatty acid composition was subjected to one-way ANOVA using SAS version 9.2 (SAS Institute, 2000). Levene's test was performed to test for homogeneity of daily ANOVA (Levene, 1960). The Shapiro-Wilk test was performed to test for nonnormality (Shapiro and Wilk, 1965). Student's t- test least significant difference was calculated at the 5% confidence level to compare means for age in days (Ott, 1998). The experimental design for the second trial entailed random selection of chicks, with BW and yolk content analysis being made on 6 chicks every second day from hatching up to 16 d. Linear and quadratic regression functions were fitted to weight and yolk variables over both trial periods to quantify changes over time using Proc Reg SAS version 9.2 (SAS Institute, 2000). Regressions are presented in Tables 3 and 4, respectively. In the case of certain variables, the pattern of change showed a linear tendency to decrease or increase, whereas other variables showed a tendency that was more quadratic. The decision of best fit was based on the R^ criteria.
RESULTS AND DISCUSSION
Fasted-Chlcks Triai (Triai 1) There was no difference in the hatching weight between individual chicks {P = 0.56; Table 1) although the average hatching weight was much lower than that reported in previous studies that ranged from 1.4 to 1.6 kg (Keffen and Jarvis, 1984; Mushi et al, 2004). This
EGG YOLK ABSORBED BY FASTED AND FED OSTRICH CHICKS
1345
Table 3. Regression statistics for the fasted ostrich chick trial Item
Equation
R2
Slaughter weight (g) Yolk weight (g) CP (%) CP content (g) Fat content (%) Fat content (g)
y y y y y y
0.36 0.55 0.48 0.57 0.48 0.48
= = = = = =
878 - 31.3x 3.1x2 _ ;)5.2x + 317 0.07x2 _ 2.21x + 46.8 1.58x2 - 27.2x + 142 0.12x2 _ 0.89x + 40.1 1.35x2 - 21.2x + 129
discrepancy may be due to the inherent characteristics of the particular eggs that were sourced for the trial. There was a decrease {P < 0.05) in slaughter weight from 1 to 7 d posthatching (Table 1). The chicks lost an average of 31.3 g of BW over the first 7 d posthatching (Table 3). This was expected as the chicks did not receive any feed and utilized 44% of their yolk over the trial period. Yolk weight decreased linearly (P < 0.05) over the trial period. On d 1, the average yolk weight was 230 g and decreased daily by 28.9 g to reach an average weight of 102 g on d 7 posthatching (Table 1). Yolk weight, as a percentage of BW, was 28% for 1-d-old and 2-d-old posthatch ostrich chicks and decreased to 12% of total BW on d 7. Romanoff (1960) and Sklan and Noy (2000) reported yolk weight as 20% of BW for chicks immediate posthatch, and Noy and Sklan (1998) reported 13 to 15% for poults at hatch. Only 44.4% of the ostrich chick yolk however, was assimilated over the 7 d posthatching period. Bierer and Eleazer (1965) reported that chickens deprived of feed for the first 7 d posthatching utilized 88.8% of their yolk. This difference could be due to the production cycle of ostriches being much longer than that of poultry, resulting in the utilization of the yolk over a longer period of time (Iji et al., 2003). The total DM percentage of the yolk declined {P < 0.05) from 1 to 7 d posthatching, with a significant drop in DM percentage on d 5, followed by a significant spike on d 6 (Table 1). The average pH of the yolk of chicks slaughtered on d 1 posthatching was 7.34. Yolk pH showed a linear dechne (P < 0.05) of 0.10 daily over the trial period, with an average pH of 6.69 on d 7 (Table 1). No literature could be found on the pH of yolk sac contents posthatch in any poultry. However, the decline in pH could be attributed to the changing composition of the yolk, as it is utilized during the first week posthatch. It has also been reported that a wide range of enzymes has been detected in the yolk membrane of the avian embryo during yolk digestion (Escribano et al., 1988).
P-value <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
Crude protein and fat content as a percentage of yolk composition, as well as total crude protein and fat content in grams is presented in Table 5. Crude protein content of the yolk decreased {P < 0.05) by 1.5% daily from d 1 (44.3%) to d 7 (32.5%) posthatching. This accounts for a daily decrease of 13.2 g in protein content. Fat content, on the other hand, increased (P < 0.05) by 1.77% daily from d 1 (41.2%) to 7 (48.9%) posthatching. Total yolk fat weight, however, decreased daily by 8.91 g from 125 g to 48.2 g. The increase in yolk fat percentage, therefore, simply refiected an increase in concentration of the component as a percentage of total yolk weight. The crude protein content of 43.8% for 2-d-old ostrich chicks was shghtly less than that of fasted poults that reflected a yolk crude protein content of 49.8% two days posthatch (Moran and Reinhart, 1980). Yolk fat content for ostrich chicks 2 d posthatching (42.4%) was also less than that reported for poults (50.6%) 2 d posthatching (Moran and Reinhart, 1980). Chicks reportedly showed a decrease in yolk protein and fat content after 2 d of fasting (Noy and Sklan, 1999). Although no difference (P < 0.05) in the glucose content over the entire trial period (P = 0.36) was observed, the glucose level in the yolk increased (P < 0.05) from d 3 (8.48 mmol/L) to d 7 posthatching (11.8 mmol/L). The same conclusion can be made as for yolk fat percentage; that is, that the apparent increase in glucose content within the yolk was only due to an increase in concentration of glucose as a percentage of total yolk volume. This was probably due to the high protein removal from the yolk. Noy and Sklan (1998) reported that the absorption of glucose did not change with age in poults, but although the absorption of glucose is low in chicks close to hatch, it increased slightly over the first 7 d posthatch (Noy and Sklan, 2001; Sklan, 2003). No literature could be found that referred to the glucose content of the yolk in either fasted or fed birds. Prom this study, it seemed that little glucose, or lipids, was utilized from the yolk as an energy source for the fasted ostrich chick. The fasted ostrich chick
Table 4. Regression statistics for the fed ostrich chick trial Item
Equation
R2
Slaughter weight (g) Yolk weight (g) CP (%) CP content (g) Fat content (%) Fat content (g)
y y y y y y
0.64 0.81 0.47 0.74 0.48 0.73
= = = = = =
3.88x2 - 30.2x + 835 0.82x2 - 29.8x + 278 46.2 - 1.20x 97.4 - 6.84x 38.7 + 1.39x 99.3 - 6.61x
P-value <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
VILJOEN ET AL.
1346 Table 5. Crude protein and fat content 1 to 7 d posthatching-' Slaughter age (d) 1 2 3 4 5 6 7 LSD
I of total yolk content; ± SE) of fasted ostrich chicks from
n
CP (%)
CP(g)
n
Fat (%)
Fat (g)
5 5 5 5 5 4 4
44.3 ± 1.98" 43.8 ± 1.76"'' 41.0 ± 3.81"'' 40.1 ± 2.82'' 36.0 ± 1.88'= 35.4 ± 1.66'= 32.5 ± 4.62'= 3.77
134 ± 23.4'^ 93.0 ± 23.1'' 73.7 ± 26.1'' 69.8 ± 37.9'"= 39.0 ± 17.7'='' 34.3 ± 3.10'' 30.8 ± 11.8'' 31.3
5 5 5 5 5 5 3
41.2 ± 2.49'= 42.4 ± 2.01'= 43.6 ± 4.67'"= 43.5 ± 6.04'= 50.1 ± 1.90" 52.8 ± 2.38" 48.9 ± 6.58"'' 5.32
125 ± 29.1" 90.1 ± 23.0'' 76.5 ± 17.2'"= 70.1 ± 22.7'"= 55.1 ± 26.5'= 49.7 ± 6.02'= 48.2 ± 4.31'= 28.9
" ''Column means with different superscripts differ significantly at P < 0.05. •'Student's t-test least significant difference (LSD) was used for sepai-ation of treatment means.
would therefore have had to use yolk protein as an energy source, which could also contribute to the weight loss observed over the 7-d trial period, as less protein was then available for growth. Amino acid composition of yolk from fasted ostrich chicks is displayed in Table 7. Of the 18 amino acids analyzed, only 7 amino acids showed any differences {P < 0.05) in percentage contribution to the total amount of protein in the yolk over the 1 to 7 d posthatching trial period. Glycine and threonine showed a similar pattern over the 7-d period, with an initial decline in content from d 1 to 3 (although not significant; P > 0.05), followed by an increase {P < 0.05) from d 3 to 5 and declining again on d 7, but not significantly so. Methionine and serine showed the same tendencies, with first a slight increase from d 1 to 3, a decline {P < 0.05) from d 3 to 5, and a shght decrease from d 5 to 7. Histidine declined {P < 0.05) from d 1 to 5, whereas valine decreased {P < 0.05) from d 1 to 5 and then increased {P < 0.05) again from d 5 to 7. No information is available in the literature on the uptake of amino acids from the posthatch yolk contents in either poultry or ratites. It has been reported that amino acids from the yolk are used both as an energy source and as building blocks for the development of the small intestine (Wijtten et al., 2010). Noy and Sklan (2002) reported that amino acids are not easily absorbed from the rich yolk medium, but that absorption increases with age and with development of the hydrophilic conditions in the small
of total yolk content; ± SE) of fed ostrich chicks 16 d
Table 6. Crude protein and fat content posthatching'^ Slaughter age (d) 2 4 6 8 10 12 14 16 LSD
n 6 6 6 6 5 6 1
CP (%) 44.8 39.3 39.7 37.1 32.0 31.9
± 3.77" ± 6.78"'' ± 3.14"'' ± 6.43"'' ± 4.52'"= ± 4.85'"= — — 7.96
intestine (Noy and Sklan, 1999). The results of this trial may indicate that the absorption of histidine and valine is favored by ostrich chicks and that the inclusion of these amino acids in prestarted ostrich chick diets should be considered. Fatty acid composition of the yolk is given in Table 8 and is expressed as percentage contribution to the total amount of fat in the yolk. Saturated fatty acid (SFA) content remained constant from d 1 to 5 posthatching and then increased {P < 0.05) from d 5 to 7. The main contributors to this increase were palmitic acid (16:0), stearic acid (18:0), arachidic acid (20:0), and lignoceric acid (24:0). Results obtained in a study conducted in poults (Reidy et al., 1998) indicated that stearic acid (18:0) also increased posthatch from 10.4 to 11.3% 24 h posthatch. Monounsaturated fatty acids (MUFA) also remained constant over the first 5 d posthatching, but decreased {P < 0.05) from d 5 to 7. This decrease was due to similar decreases {P < 0.05) in oleic acid (18:ln9c) and nervonic acid (24:1). Reidy et al. (1998), however, reported an increase in oleic acid (18:ln9c) from 45.0 to 46.9%. There were no differences in the polyunsaturated fatty acid (PUFA) contents from 1 to 7 d posthatching, regardless of differences {P < 0.05) in fatty acid content within the group, mostly from 5 to 7 d posthatching. The SFA:UFA ratio also remained constant over the first 5 d, but then increased {P < 0.05) from d 5 to 7. This is the result of the large increase in SFA content
CP(g) 103 63.9 53.7 28.3 22.1 16.8 1.25
± 31.5" ± 18.0'' ± 17.4'' ± 17.8'= ± 13.8'= ± 8.04'='' ± 3.05'' — 41.4
n 6 6 6 5 4 6 1
Fat (%) 40.9 46.3 46.5 48.5 51.7 55.9
± ± ± ± ± ±
5.27'' 6.67'='' 3.74'='' 7.70'"='' 1.23'"= 6.56"'' — — 9.22
Fat (g) 93.3 73.7 63.1 30.4 30.2 28.4 2.65
" ''Column means with different superscripts differ significantly aX P < 0.05. ^Student's i-test least significant difference (LSD) was used for separation of treatment means.
± 25.5» ± 14.2"'' ± 21.8'' ± 23.3'= ± 24.3'= ± 9.85'= ± 6.49'' — 21.0
EGG YOLK ABSORBED BY EASTED AND EED OSTRICH CHICKS
1347
Table 7. Amino acid composition (% of total yolk protein content) of yolk from fasted ostrich chicks from 1 to 7 d posthatching^ Item n Alanine Arginine Aspartate Gy steine Glutamate Glycine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tyrosine Valine
Id
3d
5d
7d
LSD
5 7.18 3.24 8.54 1.80 11.9 5.64»'' 1.84» 4.58 9.50 5.86 2.90*'' 3.78 4.94 12.0"'' 6.49'^'^ 3.51 6.09"
4 7.26 3.15 8.49 1.81 11.7 5.63'' 1.78"'' 4.50 9.53 6.15 2.97" 3.87 4.91 12.1« 6.46= 3.53 5.81''
4 7.23 3.21 8.64 1.84 11.7 5.78" 1.68<= 4.47 9.52 5.98 2.89'' 3.88 4.96 11.8'' 6.69" 3.61 5.85''
3 7.27 3.09 8.53 1.84 12.0 5.75"'' 1.7l'"= 4.58 9.69 5.97 2.95"'' 3.89 4.99 12.0"'' 6.62"'' 3.58 6.00"
0.21 0.22 0.16 0.19 0.34 0.15 0.08 0.22 0.24 0.53 0.08 0.15 0.13 0.29 0.16 0.12 0.11
means with different superscripts differ significantly a t f < 0.05. ^Student's i-test least significant difference (LSD) was used for separation of t r e a t m e n t means.
from d 5 (46.2) to d 7 (77.9). Reidy et al. (1998) reported that poults and chicks also differ in fatty acid utilization from the yolk, and that poults prefer palmitoleic acid (16:1) over oleic acid (18:ln9c), whereas Noble et al. (1988) reported a decrease in oleic acid and an increase in palmitoleic acid with age in chicks. Surai et al.
(1999) reported a marked difference between the fatty acid profiles of the yolk lipids between 4 avian species (chicken, turkey, duck, and goose), even though all the birds in the trial received very similar dietary fatty acids. The authors suggested that the fatty acid profile of layer diets should be designed to accommodate the par-
Table 8. Fatty acid composition {% of total yolk fat content) of yolk from fasted ostrich chicks from 1 to 7 d posthatching^ Item n Palmitic acid Steai'ic acid Arachidic acid Heneicosanoic acid Behenic acid Lignoceric acid Palmitoleic acid Elaidic acid Gleic acid Gondoic acid Erucic acid Nervonic acid Linolelaidic acid Linoleic acid Eicosadienoic acid Brassic acid 7-Linolenic acid a-Linolenic acid Dihomo-"/-linolenic acid Eicosatrienoic acid Arachidonic acid Eicosapentaenoic acid Docosapentaenoic acid Docosahexaenoic acid
G:D2 16:0 18:0 20:0 21:0 22:0 24:0 SFA 16:1 18:ln9t 18:ln9c 20:1 22:ln9 24:1 MUFA 18:2n6t 18:2n6c 20:2 22:2 18:3n6 18:3n3 20:3n6 20:3n3 20:4n6 20:5n3 22:5n3 22:6n3 PUFA UFA SFA:UFA
Id 5 32.8'' 9.82'' 0.07 0.02 0.02 0.18 42.9'' 7.43"'' 0.22 34.1" 0.26 0.03 0.13 42.2" 0.02 6.02" 0.22 0.05 0.22 3.05" 0.12 3.24" 0.02 0.15 0.13 1.15"'' 14.9" 57.1" 0.76
3d 4 33.3'' 9.87'' 0.08 0.03 0.02 0.27 43.6'' 8.47"'' 0.30 33.3" 0.26 0.04 0.09 42.4" 0.03 5..36" 0.18 0.04 0.2lt 2.25"'' 0.15t 3.79" 0.05 0.11 0.12 1.02'' 14.0" 56.4" 0.78
5d
7d
LSD
4 35.2'' 10.4'' 0.08 0.02 0.22 0.27 46.2'' 6.39'' 0.35 36.8" 0.26 0.03 0.10 43.9" 0.03 1.99" 0.15 0.02 0.19 2.78" 0.15 3.18" 0.02 0.18 0.16 1.06'' 9.92" 53.8" 0.87
2 58.6" 18.5" 0.15 0.03 0.04 0.40 78.0" 8.98"* 0.34* 0.07'' 0.59 0.05* 0.10* 10.8'' 0.05* 0.09"* 0.10* 0.03* 0.30* 1.14'' 0.20* 0.22'' 5.19 0.30 0.35 1.63"* 10.3"* 22.1'' 3.65
3.53 3.56 0.02 0.02 0.04 0.11 6.09 2.46 0.07 5.99 0.08 0.03 0.04 6.73 0.03 9.67 0.09 0.02 0.09 1.59 0.07 1.15 0.22 0.07 0.14 0.50 8.54 6.09 0.54
"•''Row means with different superscripts differ significantly at P < 0.05. t' Golumn means with different superscripts have n = 3. ^Student's i-test least significant difference (LSD) was used for separation of treatment means. 2G:D = lipid number (G = number of atoms in a fatty acid: D = number of double bonds in the fatty acid); SFA = saturated fatty acids, i\'IUFA ; monounsaturated fatty acids, PUFA = polyunsaturated fatty acids, UFA = unsaturated fatty acids.
1348
VILJOEN ET AL.
ticular metabolic features of each species with regard to the specific fatty acids of the yolk that are essential for the embryonic development of each individual species. No literature could be found on the fatty acid content of ostrich yolk posthatching, especially with regard to possible preference for specific fatty acid withdrawal from hatching until the yolk has been totally absorbed. The results of this trial may indicate that the ostrich chick has a greater ability to absorb MUFA, especially oleic acid (18:ln9c). The inclusion of oleic acid in prestarter ostrich chick diets should be considered in future studies, as energy from fatty acids may be a better source of energy for sustainable early growth than glucose derived from protein, as is discussed later in this paper.
Fed-Chicks Trial (Trial 2) There was an increase (P < 0.05) in slaughter weight from 1 to 16 d posthatching (Table 2). Yolk weight decreased (P < 0.05) by 16.3 g daily from d 2 (229 g) to d 16 (4.56 g) posthatching (Table 2). Yolk content of 2-d-old fed ostrich chicks in this trial was 26% of BW, which was more than reported for poults (10.1%; Moran and Reinhart, 1980) and broilers (16.7%; Chamblee et al., 1992) at hatch. Several studies on newly hatched chicks reported that the yolk is absorbed during the first 3 d posthatching (Heywang and Jull, 1930; Jull and Heywang, 1930; Heywang, 1940; Chamblee et al., 1992). In contrast, this trial indicated that ostrich chicks only absorb 30% of the yolk over the first 4 d posthatch, 67% after 8 d posthatch, and only deplete the yolk reserve after 14 d posthatch. This observation is supported by the study of Mushi et al. (2004) on ostrich chicks that suggested yolk reserves were considered retained beyond 13 d posthatch. Crude protein as a percentage of yolk composition, total crude protein content in grams, fat content as a percentage of yolk composition, and total fat content in grams are presented in Table 6. Both constituents showed similar tendencies to those reported for crude protein and fat content in the yolks of fasted ostrich chicks. Crude protein content decreased (P < 0.05) by 1.2% daily (Table 4) from d 2 (44.8%) to d 14 (28.7%) posthatching, which amounts to a daily decrease of 6.84 g (Table 4). This result corresponds with work reported by other authors. Noy and Sklan (1999) reported that yolk protein content decreased posthatch for chicks and was depleted 4 d posthatch. Similar results were obtained for poults by Moran and Reinhart (1980), who reported that the yolk protein content decreased from 51.3% 1 d posthatch to 45.3% 2 d posthatch. Fat content of the yolk increased (P < 0.05) by 1.39% daily (Table 4) from d 2 (40.8%) to d 14 (61%) posthatch, although total yolk fat weight decreased by 6.61 g daily (Table 4) from 93.75 g at 2 d of age to 11.25 g at 14 d of age. There was too little yolk left to measure crude protein and fat content at 16 d of age. The
increase in yolk fat percentage could therefore be attributed to fat being slowly assimilated from the yolk content, hence increasing as a percentage of total yolk concentration. These results were in direct contrast to results obtained for chicks and poults. Moran and Reinhart (1980) reported that lipids were utilized faster than protein from the yolk of poults, whereas Romanoff and Romanoff (1967) made the same observation for chicks. Sklan and Noy (2000) reported that the mechanisms for the utilization of lipids are present in the developing embryo, so that little change is necessary in the digestive tract of the posthatch poult to be able to utilize lipid absorption. One would therefore expect that utilization of lipids from the yolk would be an efficient mechanism to meet the energy requirement of the posthatch ostrich chick. Several authors have suggested that the composition of the yolk only provides maintenance requirements for chicks and poults, whereas exogenous energy sources are utilized for growth (Romanoff, 1960; Thaxton and Parkhurst, 1976; Chamblee et al., 1992; Sklan and Noy, 2000). Chamblee et al. (1992), however, suggested that yolk fat is necessary for early growth, as dietary fat is only effectively utilized 10 d after hatching. As glucose is used to provide energy for hatching activities of chicks (John et al, 1998; Christensen et al., 2001), Hoiby et al. (1987) reported that glycolysis would be preferred above fatty acid oxidation as oxygen is limited during hatching. It may therefore be a possibility that ostrich chicks preferably synthesize glucose from protein rather than from fat during posthatch development. However, this would not favor sustainable early growth and stronger chicks.
Conclusion Thaxton and Parkhurst (1976) reported that adding carbohydrate and amino acids to the diet in the first 2 wk posthatching is essential for the initiation of growth of chicks. This is especially important as dietary fat is only utilized effectively 10 d posthatch in broilers (Chamblee et al, 1992). In both the trials conducted on ostrich chicks, protein was assimilated from the yolk sacs, while it appeared that fat was absorbed at a much slower rate. The changes observed in fatty acid composition on d 7 posthatching may indicate that ostrich chicks have the ability to withdraw certain fatty acid components, especially MUFA from the yolk content. It would be interesting to see whether this apparent ability to withdraw certain fatty acids is repeated with exogenous feed over the first weeks posthatching. Moran and Reinhart (1980) and Reidy et al. (1998) indicated that poults have the ability for selective withdrawal from the yolk, whereas poults and chicks display differences in metabolism of various lipid fractions. Even though this study indicated that yolk absorption is faster in fasted than fed ostrich chicks, the authors agree with previous studies on chicks and poults (Bierer and Eleazer, 1965; Chamblee et al., 1992; Noy et al., 1996; Noy and Sklan, 1998; Dibner, 2000; Smirnov et
EGG YOLK ABSORBED BY FASTED AND FED OSTRICH CHICKS al., 2003; Uni, 2003), which suggest that ostrich chicks should be fed immediately posthatch. FYirther studies should be performed with prestarter diets that include added amino acids, especially histidine and valine, as well as MUEA during the first few weeks posthatching, due to the fact that a preference for these nutrients from the yolk content was indicated in this trial.
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