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Changes in Shell Conductance, Pores, and Physical Dimensions of Egg and Shell During the First Breeding Cycle of Turkey Hens1,2
Changes in Shell Conductance, Pores, and Physical Dimensions of Egg and Shell During the First Breeding Cycle of Turkey Hens h2 H. RAHN Department of Physiology, Schools of Medicine and Dentistry, State University of New York at Buffalo, Buffalo, New York 14214 V. L. CHRISTENSEN
3
and F. W. EDENS
Department of Poultry Science, North Carolina State University, Raleigh, North Carolina 2 7650 (Received for publication February 17,1981) ABSTRACT Measurements of changes in eggs, shells, and the functionality of the shell in conducting vital gases to the chorioallantois in embryos were made three times during the first laying cycle of the same turkey breeder hens. Normal increases in egg size were accompanied by concomitant increases in all physical dimensions of eggs and shells measured. Conductance of the egg shells, however, would allow smaller fractional water losses in late cycle eggs than in early cycle eggs, indicating different environmental incubation conditions may be needed with turkey eggs produced by hens in different stages of the reproductive cycle. (Key words: pore geometry, conductance, turkey eggs) 1981 Poultry Science 60:2536-2541 INTRODUCTION In the past, m u c h emphasis has been given to the physical egg shell qualities, i.e., deformation (Brunson and Godfrey, 1 9 5 3 ; Cherms and Wolff, 1 9 6 8 ; Ehlhardt, 1974) and egg density (Payne and McDaniel, 1 9 5 8 ) , b u t little attention has been given t o t h e functional egg shell qualities when expressed in terms of egg shell c o n d u c t a n c e t o gases. C o n d u c t a n c e , in this c o n t e x t , is a measure of t h e ease with which a gas diffuses across t h e pores of t h e shell and is (by Fick's law) d e t e r m i n e d by the n u m b e r and g e o m e t r y of the individual pores (Paganelli, 1980). Thus, t h e a m o u n t of water t h a t is lost from a t u r k e y egg during i n c u b a t i o n ( a b o u t 15 liters of water vapor), which d e t e r m i n e s p o u l t e m b r y o n i c livability (MacLaury and Insko, 1953), d e p e n d s n o t o n l y u p o n t h e t e m p e r a t u r e and h u m i d i t y of t h e i n c u b a t o r b u t also u p o n the shell c o n d u c t a n c e . F u r t h e r m o r e , t h e 0 2
'Paper No. 6780 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27650. 2 The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the product named nor criticism of similar ones not mentioned. 3 To whom correspondence and requests for reprints should be directed.
and C 0 2 c o n c e n t r a t i o n s of t h e air space beneath t h e shell just prior t o pipping attain optimal p r e h a t c h i n g values of a b o u t 14 and 6%, respectively, which are d e t e r m i n e d n o t only by t h e metabolic rate of the e m b r y o (Visschedijk, 1968a,b,c) b u t also b y t h e shell c o n d u c t a n c e ( R a h n , 1981). Thus, u n d e r a given i n c u b a t o r c o n d i t i o n and with normal d e v e l o p m e n t and metabolism it is t h e shell c o n d u c t a n c e which is largely responsible for the total water loss and t h e precise 0 2 and C 0 2 c o n c e n t r a t i o n s t o which the chorioallantois is e x p o s e d prior to hatching. Measurements of the shell conductance, pore g e o m e t r y , and egg dimensions as t h e y change during the first breeding cycle of t u r k e y hens were o b t a i n e d in this s t u d y .
MATERIALS AND METHODS Eggs from Large White t u r k e y s were obtained on t h e d a y of oviposition from t h e same hens three times during the laying cycle. A total of 4 2 eggs were collected during t h e 1st (early), 10th (mid), and 20th (late) week of t h e cycle. F o u r t e e n eggs from each collection period were placed in desiccators containing silica gel and were m a i n t a i n e d in c o n s t a n t temperature cabinets at 25 C. Daily weighing of t h e eggs established their water vapor shell c o n d u c t a n c e (see below). Later t h e eggs were weighed in air and water t o determine their volume and
2536
EGG AND SHELL CHANGES FROM FIRST BREEDING
subsequently weighed again after the air cell gas had been displaced by water to estimate the initial egg mass upon laying. Following these procedures the contents were removed, the shells internally washed and dried in a desiccator for weighing, and finally measured for shell thickness and pore counting. All gravimetric measurements were made to six significant figures. Shell Conductance. Five eggs were placed in each of three large desiccators which contained silica gel and were maintained at 25 C for 6 days. These eggs were weighed once a day. The water vapor pressure inside the egg is saturated and in the desiccator it is essentially zero. Thus, by dividing the daily mass loss by the saturation vapor pressure of 23.86 torr (25 C) the water vapor conductance expressed in mg d a y - 1 t o r r - 1 is obtained. This value must be corrected to a barometric pressure of 1 atmosphere as described by Ar et al. (1974). The daily values obtained for each egg were then averaged. Volume Determination. A specially designed egg holder for clamping eggs replaced the scale pan of a Mettler balance. With this device eggs were weighed dry and again while submersed in a beaker of distilled water (making appropriate tare corrections for egg holder in air and water). From the difference in mass divided by the density of water at the water temperature the volume was obtained by Archimedes' principle. Initial Egg Mass. Eggs were candled to visualize the air cell, and the gas was replaced by puncturing the shell with a hypodermic needle and injecting distilled water. The gas escaped through another puncture hole in the shell made just prior to injection. This method has been used recently with chicken eggs for obtaining initial egg mass at the time of laying (Ar andRahn, 1980). Shell Weights. Eggs were emptied, internally washed with water to remove all albumen, and then dried for several days before weighing. Shell weights include the shell membrane. Shell Thickness. Shells were broken to obtain representative areas except at the pointed end. With ball-point calipers twentyfour areas in each egg were measured to four significant figures and averaged. Shell thickness also includes the membrane thickness. Shell Volume. This was calculated from the relationship Vol = A X L where A = surface area of the egg, cm 2 , and L = thickness of the shell, cm.
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Pore Counts. Representative sections of the shells were boiled in 5% NaOH for 5 to 10 min to remove cuticle and membranes. After washing and drying these pieces were etched for 10 to 15 sec in concentrated H N 0 3 and again washed and dried. The shell sections were painted on the inside with a concentrated aqueous solution of methylene blue. The solution taken up by the pores then could be seen easily with the aid of a dissecting microscope. The pores of .25 cm 2 area were counted. Twenty areas for each egg were used to establish a mean value, which was multiplied by the total surface area of the egg to estimate the total number of pores per egg. Surface Area of Egg. This was estimated from the allometric relationship where area (cm 2 ) 4.835 W * 6 ' ! and W = initial egg mass (Paganelli et al, 1974). Total Pore Area. This was calculated from the relationship Ap = .447 G X L where Ap = total effective pore area (mm 2 ), G = water vapor conductance, mg d a y - t o r r ~ ' , L = length of the pore or shell thickness (mm), and .447 is a constant at 25 C based on the diffusion coefficient of water vapor, the gas constant R, and the absolute temperature T (Rahn^aZ., 1976). Pore Radius. This was calculated from the relationship R = 1000 \/Ap/(N x IT) where R = effective pore radius, assuming that the pores have a constant diameter (£im), Ap = total effective pore area, mm 2 , N = number of pores in egg, 7T = 3.14, and 1000 = constant to adjust the dimensions from millimiter to micrometer. RESULTS Tables 1 and 2 show the mean dimensions and standard errors of 14 eggs and shells ob-
tained during each of the early, middle, and late portions of the laying cycle. The bottom line of each table shows the change expressed as a ratio of the late cycle to early cycle value. Initial egg mass and egg volume increased significantly (P<.01) by 28%. The surface area of the eggs increased significantly (P<.01) by 18%, but the egg density remained constant (Table 1). If these changes are analyzed allometrically, it becomes apparent that the changes are proportional. Volume, surface area, and length (L) are related exponentially, i.e., for a cube the surface area = L 2 and volume = L 3 . Therefore, 28% increase in egg volume is exactly proportional to the observed 18% increase in egg surface area, or 1.28-6 = 1.18. The length and width of the eggs also increased significantly (P<.01) in proportion to the other dimensions. Since 1 . 2 8 3 3 = 1.086, the observed increases in length and width of 1.088 and 1.084, respectively, are also proportional (Table 1). As the egg mass increased across the laying cycle there were no significant changes in shell thickness (Table 2) or shell density. Therefore, the change in shell mass was due primarily to a change in shell surface area. Shell surface area did not account for all the increase in shell mass since shell weight increased 22% whereas shell surface area increased by 18%. Mean values and standard errors of water vapor conductance, pore length (shell thickness), total pore area, number of pores, and pore radius for the same eggs are given in Table 3. The conductance of the turkey egg shells increased by 17% from the beginning to the end of the lay cycle. Since there was no alteration in pore length, the total effective pore area (Area = .447 G X L, see methods) increased by
TABLE 2. Physical dimensions (mean + SE) of eggshells produced during a laying cycle by turkey hens Time of cycle1
W (g)
Thickness (cm)
Vol (cm 3 )
Density (g cm" 3 )
Early Mid Late
7.26 ± .17 c 8.02 + . 1 3 b 8.87 + .15 a
.041 + .0006 .041 + .0005 .041 ± .0006
3.53 + .08C 3.83 ± .06 b 4.22 ± .07 a
2.06 ± .01 2.09 ± .01 2.10 + .01
Late/early
1.22
1.00
1.20
1.02
a,b,cMeans in the same column with different superscripts differ significantly (P<.05). 1 Early = First week of the cycle. Mid = 10VS weeks into the cycle. Late = 20 weeks into the cycle.
EGG AND SHELL CHANGES FROM FIRST BREEDING
a similar amount (19%). The number of pores increased significantly (P<.01) by 20%, which very closely approximates the 19% significant (P<.01) increase in total effective pore area, and thus the effective pore radius remained constant.
Bi 3
DISCUSSION
MacLaury and Insko (1953) speculated that embryos in smaller turkey eggs respond better to higher humidity during incubation. They also noted that higher humidities generally gave lower overall mortality and usually lowered mortality on the 28th day of incubation, which accounts for much of the loss in modern-type turkeys (Christensen, 1978). Lundy (1969) has reviewed much of the information on incubational egg weight losses of the fowl, and Reinhart and Moran (1979) and Ehlhardt (1974) observed weight losses during incubation of eggs from Small White turkey breeder hens. However, little is known about shell conductance and pore geometry of turkey eggs. Ehlhardt (1974) related incubation weight losses to number of pores in turkey eggs, but he indicated a possible failure to identify all pores during counting. The experiment reported here is the first study of which the authors are aware exploring the possibility that conductance of turkey eggshells changes with increasing egg mass during the course of the first laying cycle of turkey hens (Nestor et ai, 1972; Moran and Reinhart, 1979). Cyclic declines in hatchability have been observed during the course of the first lay cycle (Horn and Pereyni, 1974); these declines may have been due to such changes. From the observations and calculations made in this study, it is concluded that the individual pore dimensions did not change as egg mass increased during a 20 week breeding cycle, and the increase in total number of pores was similar to the increase in the surface area of the shell. The mean interpore distance also remained the same and averaged 1.07 mm in this study. Thus each pore serves an area of the chorioallantois equal to .9 mm 2 in both early and late cycle turkey eggs. An estimate of the total water loss of turkey eggs produced at different times of the reproductive cycle can be predicted using the conductance values obtained in this study. Since water vapor conductance increased by 17%, the absolute loss of water of a late cycle egg would be 17% greater than an early cycle
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RAHN ET AL.
egg if b o t h were exposed t o the same i n c u b a t o r h u m i d i t y and t e m p e r a t u r e . However, t h e late cycle eggs were 28% heavier, and thus, die fractional w a t e r loss of t h e late cycle egg during incubation should be less. F o r example, assuming an i n c u b a t o r h u m i d i t y where t h e water pressure difference between t h e inside and outside of t h e egg is 30 torr, t h e daily water loss of an early cycle egg would be 12.22 X 3 0 = 366.6 m g d a y - 1 . T h e late cycle egg would lose 14.24 X 30 = 4 2 7 . 2 m g d a y - 1 . Multiplying these values by 28 days of incubation yields a total water loss of 10.26 and 11.96 g for early and late cycle eggs, respectively. T h e former value divided by the initial egg mass of 78.76 g yields a fractional mass loss of 13%, and t h e late cycle egg of 100.56 g would sustain a fractional mass loss of 12%. These correspond well t o t h e weight losses in Small White t u r k e y eggs observed b y R e i n h a r t a n d Moran ( 1 9 7 9 ) . T o achieve in late cycle eggs a fractional w a t e r loss equal t o t h e loss for early cycle eggs, t h e relative h u m i d i t y in die i n c u b a t o r would have t o be lowered a p p r o x i m a t e l y 7% for late cycle eggs ( R a h n and Ar, 1974). A typical avian egg loses 15% of its initial mass during natural incubation (Ar and R a h n , 1980). Artificially incubated turkey eggs lose a b o u t 12 to 13% of their initial mass ( R e i n h a r t and Moran, 1979). T h e weight loss of t u r k e y eggs during natural incubation is u n k n o w n and the lower weight loss of t u r k e y eggs c o m p a r e d t o o t h e r avian eggs may be due t o species differences or incorrect h u m i d i t y settings in t h e incubators. MacLaury and Insko ( 1 9 5 3 ) suggested t h a t less e m b r y o n i c m o r t a l i t y would result if higher h u m i d i t y were used during artificial i n c u b a t i o n . More e x p e r i m e n t s are needed to evaluate h a t c h e r y practices used to i n c u b a t e t u r k e y eggs t o d e t e r m i n e which of these possibilities is true. ACKNOWLEDGMENTS It is a pleasure t o acknowledge in this study t h e skill and help of Phyllis Parisi.
REFERENCES Ar, A., C. V. Paganelli, R. B. Reeves, D. G. Green, and H. Rahn, 1974. The avian egg: Water vapor conductance, shell thickness, and functional pore area. The Condor 76:153-158. Ar, A., and H. Rahn, 1980. Water in the avian egg; overall budget of incubation. Amer. Zool. 20: 373-384. Branson, C. C , and G. F. Godfrey, 1953. The re-
lationship of egg shape, egg weight, specific gravity and 21-day incubation weight-loss to hatchability of broad-breasted bronze turkey eggs. Poultry Sci. 32:846-849. Cherms, F. L., and E. Wolff, 1968. The incidence of physically abnormal turkey hatching eggs and their relationship to hatchability. Poultry Sci. 47:760-765. Christensen, V. L., 1978. Physiological parameters limiting hatchability in the domestic turkey. Ph.D. thesis, University of Missouri, Columbia, MO. Ehlhardt, D. A., 1974. The influence of shell characteristics on fertility and hatchability of turkey eggs. Arch. Geflugelkd 38:50-56. Horn, P., and M. Pereyni, 1974. The effects of time in production and season of lay on hatchability parameters of artificially inseminated turkeys. Pages 14—15 m Proc. XV World's Poultry Congr. Lundy, H. 1969. A review of the effects of temperature, humidity, turning and gaseous environment in the incubator on the hatchability of the hen's egg. In The fertility and hatchability of the hen's egg. T. C. Carter and B. M. Freeman, ed. Oliver and Boyd, Edinburgh. MacLaury, D. W., and W. M. Insko, Jr., 1953. A study of turkey hatchability by means of embryo mortality curves. Kentucky Agr. Exp. Sta. Bull. 603. Moran, E. T., Jr., and B. S. Reinhart, 1979. Physical and chemical characteristics of Small White turkey eggs from older commercial breeder flocks with an examination of changes due to weight and time after lay. Poultry Sci. 58:341-349. Nestor, K. E., K. I. Brown, and S. A. Touchburn, 1972. Egg quality and poult production in turkeys. 1. Variation during a seven month lay period. Poultry Sci. 51:341-349. Paganelli, C. V., 1980. The physics of gas exchange across the avian eggshell. Amer. Zool. 20: 329— 338. Paganelli, C. V., A. Olszowka, and A. Ar, 1974. The avian egg: surface area, volume, and density. The Condor 76:319-325. Payne, L. F., and G. R. McDaniel, 1958. Shell thickness as related to "shuck-outs" in turkey eggs. Poultry Sci. 37:825-828. Rahn, H., 1981. Gas exchange of avian eggs with special reference to turkey eggs. Poultry Sci. 60:1971-1980. Rahn, H., and A. Ar, 1974. The avian egg: incubation time and water loss. The Condor 76:147-152. Rahn, H., C. V. Paganelli, I.C.T. Nisbet, and G. C. Whittow, 1976. Regulation of incubation water loss in eggs of seven species of terns. Physiol. Zool. 49:245-259. Reinhart, B. S., and E. T. Moran, Jr., 1979. Incubation characteristics of eggs from older Small White turkeys with emphasis on effects due to egg weight. Poultry Sci. 58:1599-1605. Visschedijk, A.H.J., 1968a. The air space and embryonic respiration. 1. The pattern of gaseous exchange in the fertile eggs during the closing stages of incubation. Brit. Poultry Sci. 9 : 1 7 3 182. Visschedijk, A.H.J., 1968b. The air space and embryonic respiration. 2. The times of pipping and
EGG AND SHELL CHANGES FROM FIRST BREEDING hatching as influenced by an artificially changed permeability of the shell over the air space. Brit. Poultry Sci. 9:183-196. Visschedijk, A.H.J., 1968c. The air space and em-
2541
bryonic respiration. 3. The balance between oxygen and carbon dioxide in the air space of the incubating chicken egg and its role in stimulating pipping. Brit. Poultry Sci. 9:197-210.
3
and F. W. EDENS
Department of Poultry Science, North Carolina State University, Raleigh, North Carolina 2 7650 (Received for publication February 17,1981) ABSTRACT Measurements of changes in eggs, shells, and the functionality of the shell in conducting vital gases to the chorioallantois in embryos were made three times during the first laying cycle of the same turkey breeder hens. Normal increases in egg size were accompanied by concomitant increases in all physical dimensions of eggs and shells measured. Conductance of the egg shells, however, would allow smaller fractional water losses in late cycle eggs than in early cycle eggs, indicating different environmental incubation conditions may be needed with turkey eggs produced by hens in different stages of the reproductive cycle. (Key words: pore geometry, conductance, turkey eggs) 1981 Poultry Science 60:2536-2541 INTRODUCTION In the past, m u c h emphasis has been given to the physical egg shell qualities, i.e., deformation (Brunson and Godfrey, 1 9 5 3 ; Cherms and Wolff, 1 9 6 8 ; Ehlhardt, 1974) and egg density (Payne and McDaniel, 1 9 5 8 ) , b u t little attention has been given t o t h e functional egg shell qualities when expressed in terms of egg shell c o n d u c t a n c e t o gases. C o n d u c t a n c e , in this c o n t e x t , is a measure of t h e ease with which a gas diffuses across t h e pores of t h e shell and is (by Fick's law) d e t e r m i n e d by the n u m b e r and g e o m e t r y of the individual pores (Paganelli, 1980). Thus, t h e a m o u n t of water t h a t is lost from a t u r k e y egg during i n c u b a t i o n ( a b o u t 15 liters of water vapor), which d e t e r m i n e s p o u l t e m b r y o n i c livability (MacLaury and Insko, 1953), d e p e n d s n o t o n l y u p o n t h e t e m p e r a t u r e and h u m i d i t y of t h e i n c u b a t o r b u t also u p o n the shell c o n d u c t a n c e . F u r t h e r m o r e , t h e 0 2
'Paper No. 6780 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27650. 2 The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the product named nor criticism of similar ones not mentioned. 3 To whom correspondence and requests for reprints should be directed.
and C 0 2 c o n c e n t r a t i o n s of t h e air space beneath t h e shell just prior t o pipping attain optimal p r e h a t c h i n g values of a b o u t 14 and 6%, respectively, which are d e t e r m i n e d n o t only by t h e metabolic rate of the e m b r y o (Visschedijk, 1968a,b,c) b u t also b y t h e shell c o n d u c t a n c e ( R a h n , 1981). Thus, u n d e r a given i n c u b a t o r c o n d i t i o n and with normal d e v e l o p m e n t and metabolism it is t h e shell c o n d u c t a n c e which is largely responsible for the total water loss and t h e precise 0 2 and C 0 2 c o n c e n t r a t i o n s t o which the chorioallantois is e x p o s e d prior to hatching. Measurements of the shell conductance, pore g e o m e t r y , and egg dimensions as t h e y change during the first breeding cycle of t u r k e y hens were o b t a i n e d in this s t u d y .
MATERIALS AND METHODS Eggs from Large White t u r k e y s were obtained on t h e d a y of oviposition from t h e same hens three times during the laying cycle. A total of 4 2 eggs were collected during t h e 1st (early), 10th (mid), and 20th (late) week of t h e cycle. F o u r t e e n eggs from each collection period were placed in desiccators containing silica gel and were m a i n t a i n e d in c o n s t a n t temperature cabinets at 25 C. Daily weighing of t h e eggs established their water vapor shell c o n d u c t a n c e (see below). Later t h e eggs were weighed in air and water t o determine their volume and
2536
EGG AND SHELL CHANGES FROM FIRST BREEDING
subsequently weighed again after the air cell gas had been displaced by water to estimate the initial egg mass upon laying. Following these procedures the contents were removed, the shells internally washed and dried in a desiccator for weighing, and finally measured for shell thickness and pore counting. All gravimetric measurements were made to six significant figures. Shell Conductance. Five eggs were placed in each of three large desiccators which contained silica gel and were maintained at 25 C for 6 days. These eggs were weighed once a day. The water vapor pressure inside the egg is saturated and in the desiccator it is essentially zero. Thus, by dividing the daily mass loss by the saturation vapor pressure of 23.86 torr (25 C) the water vapor conductance expressed in mg d a y - 1 t o r r - 1 is obtained. This value must be corrected to a barometric pressure of 1 atmosphere as described by Ar et al. (1974). The daily values obtained for each egg were then averaged. Volume Determination. A specially designed egg holder for clamping eggs replaced the scale pan of a Mettler balance. With this device eggs were weighed dry and again while submersed in a beaker of distilled water (making appropriate tare corrections for egg holder in air and water). From the difference in mass divided by the density of water at the water temperature the volume was obtained by Archimedes' principle. Initial Egg Mass. Eggs were candled to visualize the air cell, and the gas was replaced by puncturing the shell with a hypodermic needle and injecting distilled water. The gas escaped through another puncture hole in the shell made just prior to injection. This method has been used recently with chicken eggs for obtaining initial egg mass at the time of laying (Ar andRahn, 1980). Shell Weights. Eggs were emptied, internally washed with water to remove all albumen, and then dried for several days before weighing. Shell weights include the shell membrane. Shell Thickness. Shells were broken to obtain representative areas except at the pointed end. With ball-point calipers twentyfour areas in each egg were measured to four significant figures and averaged. Shell thickness also includes the membrane thickness. Shell Volume. This was calculated from the relationship Vol = A X L where A = surface area of the egg, cm 2 , and L = thickness of the shell, cm.
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Pore Counts. Representative sections of the shells were boiled in 5% NaOH for 5 to 10 min to remove cuticle and membranes. After washing and drying these pieces were etched for 10 to 15 sec in concentrated H N 0 3 and again washed and dried. The shell sections were painted on the inside with a concentrated aqueous solution of methylene blue. The solution taken up by the pores then could be seen easily with the aid of a dissecting microscope. The pores of .25 cm 2 area were counted. Twenty areas for each egg were used to establish a mean value, which was multiplied by the total surface area of the egg to estimate the total number of pores per egg. Surface Area of Egg. This was estimated from the allometric relationship where area (cm 2 ) 4.835 W * 6 ' ! and W = initial egg mass (Paganelli et al, 1974). Total Pore Area. This was calculated from the relationship Ap = .447 G X L where Ap = total effective pore area (mm 2 ), G = water vapor conductance, mg d a y - t o r r ~ ' , L = length of the pore or shell thickness (mm), and .447 is a constant at 25 C based on the diffusion coefficient of water vapor, the gas constant R, and the absolute temperature T (Rahn^aZ., 1976). Pore Radius. This was calculated from the relationship R = 1000 \/Ap/(N x IT) where R = effective pore radius, assuming that the pores have a constant diameter (£im), Ap = total effective pore area, mm 2 , N = number of pores in egg, 7T = 3.14, and 1000 = constant to adjust the dimensions from millimiter to micrometer. RESULTS Tables 1 and 2 show the mean dimensions and standard errors of 14 eggs and shells ob-
tained during each of the early, middle, and late portions of the laying cycle. The bottom line of each table shows the change expressed as a ratio of the late cycle to early cycle value. Initial egg mass and egg volume increased significantly (P<.01) by 28%. The surface area of the eggs increased significantly (P<.01) by 18%, but the egg density remained constant (Table 1). If these changes are analyzed allometrically, it becomes apparent that the changes are proportional. Volume, surface area, and length (L) are related exponentially, i.e., for a cube the surface area = L 2 and volume = L 3 . Therefore, 28% increase in egg volume is exactly proportional to the observed 18% increase in egg surface area, or 1.28-6 = 1.18. The length and width of the eggs also increased significantly (P<.01) in proportion to the other dimensions. Since 1 . 2 8 3 3 = 1.086, the observed increases in length and width of 1.088 and 1.084, respectively, are also proportional (Table 1). As the egg mass increased across the laying cycle there were no significant changes in shell thickness (Table 2) or shell density. Therefore, the change in shell mass was due primarily to a change in shell surface area. Shell surface area did not account for all the increase in shell mass since shell weight increased 22% whereas shell surface area increased by 18%. Mean values and standard errors of water vapor conductance, pore length (shell thickness), total pore area, number of pores, and pore radius for the same eggs are given in Table 3. The conductance of the turkey egg shells increased by 17% from the beginning to the end of the lay cycle. Since there was no alteration in pore length, the total effective pore area (Area = .447 G X L, see methods) increased by
TABLE 2. Physical dimensions (mean + SE) of eggshells produced during a laying cycle by turkey hens Time of cycle1
W (g)
Thickness (cm)
Vol (cm 3 )
Density (g cm" 3 )
Early Mid Late
7.26 ± .17 c 8.02 + . 1 3 b 8.87 + .15 a
.041 + .0006 .041 + .0005 .041 ± .0006
3.53 + .08C 3.83 ± .06 b 4.22 ± .07 a
2.06 ± .01 2.09 ± .01 2.10 + .01
Late/early
1.22
1.00
1.20
1.02
a,b,cMeans in the same column with different superscripts differ significantly (P<.05). 1 Early = First week of the cycle. Mid = 10VS weeks into the cycle. Late = 20 weeks into the cycle.
EGG AND SHELL CHANGES FROM FIRST BREEDING
a similar amount (19%). The number of pores increased significantly (P<.01) by 20%, which very closely approximates the 19% significant (P<.01) increase in total effective pore area, and thus the effective pore radius remained constant.
Bi 3
DISCUSSION
MacLaury and Insko (1953) speculated that embryos in smaller turkey eggs respond better to higher humidity during incubation. They also noted that higher humidities generally gave lower overall mortality and usually lowered mortality on the 28th day of incubation, which accounts for much of the loss in modern-type turkeys (Christensen, 1978). Lundy (1969) has reviewed much of the information on incubational egg weight losses of the fowl, and Reinhart and Moran (1979) and Ehlhardt (1974) observed weight losses during incubation of eggs from Small White turkey breeder hens. However, little is known about shell conductance and pore geometry of turkey eggs. Ehlhardt (1974) related incubation weight losses to number of pores in turkey eggs, but he indicated a possible failure to identify all pores during counting. The experiment reported here is the first study of which the authors are aware exploring the possibility that conductance of turkey eggshells changes with increasing egg mass during the course of the first laying cycle of turkey hens (Nestor et ai, 1972; Moran and Reinhart, 1979). Cyclic declines in hatchability have been observed during the course of the first lay cycle (Horn and Pereyni, 1974); these declines may have been due to such changes. From the observations and calculations made in this study, it is concluded that the individual pore dimensions did not change as egg mass increased during a 20 week breeding cycle, and the increase in total number of pores was similar to the increase in the surface area of the shell. The mean interpore distance also remained the same and averaged 1.07 mm in this study. Thus each pore serves an area of the chorioallantois equal to .9 mm 2 in both early and late cycle turkey eggs. An estimate of the total water loss of turkey eggs produced at different times of the reproductive cycle can be predicted using the conductance values obtained in this study. Since water vapor conductance increased by 17%, the absolute loss of water of a late cycle egg would be 17% greater than an early cycle
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RAHN ET AL.
egg if b o t h were exposed t o the same i n c u b a t o r h u m i d i t y and t e m p e r a t u r e . However, t h e late cycle eggs were 28% heavier, and thus, die fractional w a t e r loss of t h e late cycle egg during incubation should be less. F o r example, assuming an i n c u b a t o r h u m i d i t y where t h e water pressure difference between t h e inside and outside of t h e egg is 30 torr, t h e daily water loss of an early cycle egg would be 12.22 X 3 0 = 366.6 m g d a y - 1 . T h e late cycle egg would lose 14.24 X 30 = 4 2 7 . 2 m g d a y - 1 . Multiplying these values by 28 days of incubation yields a total water loss of 10.26 and 11.96 g for early and late cycle eggs, respectively. T h e former value divided by the initial egg mass of 78.76 g yields a fractional mass loss of 13%, and t h e late cycle egg of 100.56 g would sustain a fractional mass loss of 12%. These correspond well t o t h e weight losses in Small White t u r k e y eggs observed b y R e i n h a r t a n d Moran ( 1 9 7 9 ) . T o achieve in late cycle eggs a fractional w a t e r loss equal t o t h e loss for early cycle eggs, t h e relative h u m i d i t y in die i n c u b a t o r would have t o be lowered a p p r o x i m a t e l y 7% for late cycle eggs ( R a h n and Ar, 1974). A typical avian egg loses 15% of its initial mass during natural incubation (Ar and R a h n , 1980). Artificially incubated turkey eggs lose a b o u t 12 to 13% of their initial mass ( R e i n h a r t and Moran, 1979). T h e weight loss of t u r k e y eggs during natural incubation is u n k n o w n and the lower weight loss of t u r k e y eggs c o m p a r e d t o o t h e r avian eggs may be due t o species differences or incorrect h u m i d i t y settings in t h e incubators. MacLaury and Insko ( 1 9 5 3 ) suggested t h a t less e m b r y o n i c m o r t a l i t y would result if higher h u m i d i t y were used during artificial i n c u b a t i o n . More e x p e r i m e n t s are needed to evaluate h a t c h e r y practices used to i n c u b a t e t u r k e y eggs t o d e t e r m i n e which of these possibilities is true. ACKNOWLEDGMENTS It is a pleasure t o acknowledge in this study t h e skill and help of Phyllis Parisi.
REFERENCES Ar, A., C. V. Paganelli, R. B. Reeves, D. G. Green, and H. Rahn, 1974. The avian egg: Water vapor conductance, shell thickness, and functional pore area. The Condor 76:153-158. Ar, A., and H. Rahn, 1980. Water in the avian egg; overall budget of incubation. Amer. Zool. 20: 373-384. Branson, C. C , and G. F. Godfrey, 1953. The re-
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EGG AND SHELL CHANGES FROM FIRST BREEDING hatching as influenced by an artificially changed permeability of the shell over the air space. Brit. Poultry Sci. 9:183-196. Visschedijk, A.H.J., 1968c. The air space and em-
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bryonic respiration. 3. The balance between oxygen and carbon dioxide in the air space of the incubating chicken egg and its role in stimulating pipping. Brit. Poultry Sci. 9:197-210.