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Relationship of Embryo Weight as a Percent of Egg Weight to Efficiency of Feed Utilization in the Hatched Chick
Relationship of Embryo Weight as a Percent of Egg Weight to Efficiency of Feed Utilization in the Hatched Chick K . W . WASHBURN AND R. A . G U I L L 1
Department of Poultry Science, University of Georgia, Athens, Georgia 30602 (Received for publication July 16, 1973)
POULTRY SCIENCE 53: 766-769, 1974
A
NUMBER of studies have compared the embryonic growth rate of relatively fast and slow growing strains or breeds of chickens. Henderson (1930) concluded there was no difference in 4-20 day embryo growth rate between White Leghorn and Dark Cornish breeds. Byerly (1930) found differences in embryonic growth between White Leghorn and Rhode Island Reds from 10-20 days of incubation. The embryos of the heavier breed and the hybrids were slightly heavier than those of the lighter breed from the tenth day of incubation to hatching time. However, these differences had disappeared by hatching due to limitations of food. McNary et al. (1960), using White Leghorn and Rhode Island Red inbred lines and their reciprocal crosses, studied the effect of genetic variation ingrowth rate on embryonic growth. Heterosis for embryonic growth was apparent at all stages, but the effect was partially obscured by a maternal growth depressant transmitted by one line. Egg size had little effect on embryo size in this study. Embryos
from the heavy-weight lines were consistently larger than those from the Leghorn lines at 38 hours, one week and two weeks of incubation. The effect of egg size on embryonic growth was studied earlier by Byerly (1932) who concluded that embryonic growth is dependent on an inherent growth rate, identical for all breeds, which is modified during incubation in direct proportion to the egg size. Byerly et al. (1938) reported that during the 2-20 day incubation period the small and large embryos from corresponding egg sizes grew at practically identical rates. If inherent genetic differences in growth or efficiency of feed utilization were present in an individual, it may be expressed at an early embryonic age. The relationship between efficiency of embryonic utilization and efficiency of utilization of feed by the hatched chick was studied by comparing 10-day embryo weight as a percentage of egg weight to the feed conversion ratio of their full sibs. MATERIALS AND METHODS
1. Present address: Pilch, Inc., Troutman, North Carolina 28166
766
of
Six eggs from each dam of twenty families the Athens-Canadian Randombred
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
ABSTRACT The relationship between efficiency of embryonic utilization and efficiency of utilization of feed by the hatched chick was studied by comparing the 10-day embryo weight as a percentage of egg weight (percent embryo weight) to feed conversion ratio of their full-sibs from 1-9 weeks of age. There was no relationship between embryonic efficiency (as measured by percent 10-day embryo weight) and early (1-5 week) chick efficiency (as measured by feed conversion ratio). The correlations of .19 and .12 obtained for percent 10-day embryo weight and 5-7, 7-9 feed conversion also indicated a lack of relationship between embryo efficiency and latter chick efficiency. Similar low correlations between percent 10-day embryo weight and feed conversion (.12 and .16) and embryo weight with growth weight (0 and -.06) were obtained. A negative correlation of - .46 between percent embryo weight and egg weight resulted because of the rather constant embryo weight over varying egg weights.
767
EMBRYO WEIGHT AND CHICK EFFICIENCY
Group feed conversion ratios were obtained for the periods of 1-3 and 3-5 weeks of age. The chicks were classified into 17 groups (with means ranging from 3.8-5.6 percent) on the basis of percent embryo weight of their full sibs (Table 1). Individual feed conversion ratios and growth rate were
obtained for the period from 5-7 and 7-9 weeks of age. The methods used in obtaining the individual feed conversion ratios have been described in detail by Guill and Washburn (1972). The degree of association between the traits was measured by correlations between traits. RESULTS AND DISCUSSION The relationship of embryonic efficiency (as measured by percent 10-day embryo weight) to 1-5 week efficiency of feed utilization (measured as feed conversion ratio) is shown in Table 1. For the determination of 1-5 week feed conversion ratios, chicks were grouped on the basis of the percent 10-day embryonic weight of their full-sibs and the ratios for 1-3 and 3-5 weeks of age compared with the mean percent embryo weight for that group. The pen conversion ratios ranged from 1.57-2.25 from 1-3 weeks; however, these differences did not appear to be related to percent 10-day embryo weight of full-sibs. The regression line of group feed conversion on mean percent embryo weight was not significantly different from zero and no trends toward improved efficiency of feed utilization (lower feed conversion ratio) with better embryonic efficiency (larger percent 10-day
TABLE 1.—Feed conversion ratios of chicks grouped on basis of their full-sib percent 10-day embryo weight Range 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
3.52-4.00 4.02-4.18 4.19-4.24 4.26-4.34 4.35-4.38 4.39-4.46 4.47-4.56 4.57-4.62 4.63-4.66 4.68-4.75 4.76-4.83 4.84-4.94 4.95-5.09 5.10-5.13 5.14-5.25 5.27-5.43 5.48-5.84
% 10-day embryo wt. X No. chicks 3.80 4.09 4.22 4.29 4.35 4.43 4.50 4.60 4.64 4.71 4.80 4.90 4.98 5.12 5.18 5.34 5.60
17 15 15 15 15 15 14 15 14 16 15 18 16 14 14 13 12
Feed conv ersion ratio 1-3 wks.
3-5 wks.
1.85 1.80 1.85 1.74 1.69 1.89 1.68 1.82 1.60 1.57 2.12 2.02 1.74 1.61 2.25 1.68 1.91
2.65 2.60 2.38 2.50 2.12 3.04 2.53 3.16 2.54 2.62 2.09 2.32 2.55 2.64 2.27 2.52 2.22
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
(ACRB) population were obtained. The eggs were weighed at collection, stored at 10° C. (50° F.) for up to 10 days and again weighed before incubation to determine if the weight loss in storage was reflected in percent embryo weight or in percent hatch-weight. After incubation, 10-day embryo weights of two eggs were obtained from each dam. The embryos were removed from the yolk, excess moisture removed and weighed in hundredths of a gram. The embryo weight as a percentage of egg weight was calculated to determine the percent embryo weight. The chicks obtained from the remaining eggs were used to determine hatch weight, chick weight as a percentage of egg weight (% hatch weight), growth rate and feed conversion ratio. The mean weight and percent embryo weight of the ten-day embryos which were full sibs to the hatched chicks was used as a measure of the embryo weight and percent embryo weight of the chicks allowed to hatch.
768
K. W. WASHBURN AND R. A. GUILL
TABLE 2.—Correlations between egg, 10-day embryo and chick parameters
(1) Egg wt. 1 (2) Egg wt. 2 (3) Hatch wt. (4) % embryo wt. (5) Embryo wt. (6) % Hatch wt. (of egg wt. 1) (7) % Hatch wt. (of egg wt. 2) (8) Feed conversion (5-7 weeks) (9) Feed conversion (7-9 weeks) (10) Growth (5-7) (11) Growth (7-9)
(4)
(5)
1.0 .62
1.0
.44
-.11
-.01
(2) (3) (1) 1.0 .99 1.0 .94 .94 1.0 -.46 -.46 -.45 .25 .25 .22 .14
.14
(6)
(7)
(8)
(9)
(10)
(H)
1.0
.13
.13
.44
-.11
-.02
.99
-.07
-.07
-.05
.19
.12
.05
.06 .21 .08
.06 .21 .08
.04 .25 .12
.12 .16 -.13 .00 -.12 -.06
-.01 .15 .12
1.0 .05
1.0
-.02 .33 1.0 .14 - . 5 8 - . 0 6 .12 - . 3 7 - . 6 0
1.0 .65
1.0
TABLE 3.—Mean values for egg weight, embryo weight, and percent weight, chick weight and percent weight, feed conversion and growth Egg weight (g.) Before After Storage 49.8 49.6
Embryo Wt. (g.) 2.14
% Wt. 4.66
Chick Wt. (g.) 34.2
% Wt. 68.7
Feed conversion
Growth
5-7 Wk. 7-9 Wk. 5-7 Wk. 7-9 Wk. 2.44 3.54 344g 322g
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
and feed conversion were found in the present study. Correlations for 5-7 and 7-9 week conversion with hatch weight as a percentage of egg weight at time of laying and time of incubation ranged from - . 0 1 to .05 (Table 2). There were also low correlations between the percent hatch weights and 5-7 or 7-9 week growth rate. The average weight loss of the eggs in storage in the present experiment was 0.2 gms (Table 3). The correlation between weight of egg when laid and when set was very high (.99). There were no differences in correlations of 10-day embryo weight or hatch weight with egg weight before or after storage. Thus, the small amount of weight loss in this study did not change the degree of association between the traits measured. A very high correlation (.94) between egg weight and hatch weight was observed. This is similar to the findings of numerous previous studies. Egg weight was not correlated with 5-7 or 7-9 week conversion. There was some
embryonic weight) were noted. For the period from 5-9 weeks, individual feed conversion ratios were determined and correlated with full-sib percent 10-day embryo weights. The mean feed conversion ratios for the 5-7 week and 7-9 week periods (Table 3) were 2.44 and 3.54, respectively. The correlation of .19 and .12 obtained for percent embryo weight and 5-7, 7-9 week feed conversion (Table 2) shows a lack of relationship between embryonic efficiency and 5-9 week efficiency of feed utilization. There was also a low correlation between percent embryo weight and 5-7 or 7-9 week growth rate (-.13 and —.12). Similar low correlations between 10-day embryo weight and feed conversion ratio (. 12 and . 16) and growth rate (0 and - .06) were obtained. Previous results (Guill and Washburn, 1973) have shown a lack of relationship between percent hatch weight and feed conversion for 5-9 weeks of age. A similar lack of association between percent hatch weight
769
EMBRYO WEIGHT AND CHICK EFFICIENCY
TABLE 4.—Analysis of variance for dam effect on egg weight, embryo weight and percent hatch weight
Source Total Bet. Dams Error
d.f.
Mean squares Egg wt. Embryo wt.
189 94 95
26.3** 3.5
.064 .052
% Embryo wt. .435* .218
*P < .05 **P s .01
ed to egg size (r = .25). Bray and Iton (1962) indicated that the effect of the size of the egg on strain differences began after 11 days of incubation and increased gradually to a maximum at hatching time. Thus, after 10 days of age one would expect an increasing masking of genetic differences in growth (and possible efficiency) by the effect of egg size. However, the lack of correlation between 10-day embryonic efficiency and subsequent chick efficiency and the uniformity of embryo weight indicates that the genes present for differences in growth and efficiency are not active in the embryonic state.
REFERENCES Bray, D. F., and E. L. Iton, 1962. The effect of egg weight on strain differences in embryonic and post-embryonic growth in the domestic fowl. Brit. Poultry Sci. 3: 175-188. Byerly, T. C , 1930. The effect of breed on the growth of the chick embryo. J. Morph. Physiol. 50: 341-359. Byerly, T. C , 1932. Growth of the chick embryo in relation to its food supply. J. Exp. Biol. 9: 15-44. Byerly, T. C , W. G. Helsel and J. P. Quinn, 1938. Growth in weight and cell number. Genetic effects in the chick embryo and chick. J. Exp. Zool. 78: 185-203. Guill, R. A., and K. W. Washburn, 1972. Cages and feed troughs for individual broiler feed consumption experiments. Poultry Sci. 51: 1047-1048. Guill, R. A., and K. W. Washburn, 1973. Relationship between percent hatch weight and feed conversion ratio in broilers. Poultry Sci. 52: 1641-1646. Henderson, E. W., 1930. Growth and development. The influence of temperature and breeding upon the rate of growth of chick embryos. Missouri Agri. Exp. Sta. Res. Bull. 149: 5-47. McNary, H. W., A. E. Bell and C. H. Moore, 1960. The growth of inbred and hybrid chicken embryos. Poultry Sci. 39: 378-384.
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
degree of association between egg weight and 5-7 week growth (correlation of .21) but none to 7-9 week growth. An interesting negative correlation (-.46) between egg size and percent embryo weight was observed. Since percent embryo weight is a function of embryo weight and egg weight, a correlation between percent embryo weight with embryo and/or egg weight would be expected. Although the variation in egg weight was as great as expected in a randombred population, the variation in embryo weights was quite small (s = .25, C.V. = .56). Over the range of egg weights measured in this study the 10-day embryo weight appeared relatively constant compared to hatching weight which had a high correlation (.94) with egg weight. The negative correlation between percent embryo weight and egg weight resulted because of the variation in egg weight over relatively constant embryo weights. The analysis of dam effect on variation in egg weights, embryo weights and percent hatch weight (Table 4) support this conclusion. There was a significant difference between dams in percent embryo weight, but there were no significant differences between dams in embryo weight, indicating the difference in percent embryo weight was due to the differences in egg weight. Byerly et al. (1938) stated that growth in embryonic weight is conditioned by the limited food supply in the egg so that the weight of the chick at hatching time is proportional to the weight of the egg. However, the results of this experiment indicate that the 10-day embryo weight was only moderately correlat-
Department of Poultry Science, University of Georgia, Athens, Georgia 30602 (Received for publication July 16, 1973)
POULTRY SCIENCE 53: 766-769, 1974
A
NUMBER of studies have compared the embryonic growth rate of relatively fast and slow growing strains or breeds of chickens. Henderson (1930) concluded there was no difference in 4-20 day embryo growth rate between White Leghorn and Dark Cornish breeds. Byerly (1930) found differences in embryonic growth between White Leghorn and Rhode Island Reds from 10-20 days of incubation. The embryos of the heavier breed and the hybrids were slightly heavier than those of the lighter breed from the tenth day of incubation to hatching time. However, these differences had disappeared by hatching due to limitations of food. McNary et al. (1960), using White Leghorn and Rhode Island Red inbred lines and their reciprocal crosses, studied the effect of genetic variation ingrowth rate on embryonic growth. Heterosis for embryonic growth was apparent at all stages, but the effect was partially obscured by a maternal growth depressant transmitted by one line. Egg size had little effect on embryo size in this study. Embryos
from the heavy-weight lines were consistently larger than those from the Leghorn lines at 38 hours, one week and two weeks of incubation. The effect of egg size on embryonic growth was studied earlier by Byerly (1932) who concluded that embryonic growth is dependent on an inherent growth rate, identical for all breeds, which is modified during incubation in direct proportion to the egg size. Byerly et al. (1938) reported that during the 2-20 day incubation period the small and large embryos from corresponding egg sizes grew at practically identical rates. If inherent genetic differences in growth or efficiency of feed utilization were present in an individual, it may be expressed at an early embryonic age. The relationship between efficiency of embryonic utilization and efficiency of utilization of feed by the hatched chick was studied by comparing 10-day embryo weight as a percentage of egg weight to the feed conversion ratio of their full sibs. MATERIALS AND METHODS
1. Present address: Pilch, Inc., Troutman, North Carolina 28166
766
of
Six eggs from each dam of twenty families the Athens-Canadian Randombred
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
ABSTRACT The relationship between efficiency of embryonic utilization and efficiency of utilization of feed by the hatched chick was studied by comparing the 10-day embryo weight as a percentage of egg weight (percent embryo weight) to feed conversion ratio of their full-sibs from 1-9 weeks of age. There was no relationship between embryonic efficiency (as measured by percent 10-day embryo weight) and early (1-5 week) chick efficiency (as measured by feed conversion ratio). The correlations of .19 and .12 obtained for percent 10-day embryo weight and 5-7, 7-9 feed conversion also indicated a lack of relationship between embryo efficiency and latter chick efficiency. Similar low correlations between percent 10-day embryo weight and feed conversion (.12 and .16) and embryo weight with growth weight (0 and -.06) were obtained. A negative correlation of - .46 between percent embryo weight and egg weight resulted because of the rather constant embryo weight over varying egg weights.
767
EMBRYO WEIGHT AND CHICK EFFICIENCY
Group feed conversion ratios were obtained for the periods of 1-3 and 3-5 weeks of age. The chicks were classified into 17 groups (with means ranging from 3.8-5.6 percent) on the basis of percent embryo weight of their full sibs (Table 1). Individual feed conversion ratios and growth rate were
obtained for the period from 5-7 and 7-9 weeks of age. The methods used in obtaining the individual feed conversion ratios have been described in detail by Guill and Washburn (1972). The degree of association between the traits was measured by correlations between traits. RESULTS AND DISCUSSION The relationship of embryonic efficiency (as measured by percent 10-day embryo weight) to 1-5 week efficiency of feed utilization (measured as feed conversion ratio) is shown in Table 1. For the determination of 1-5 week feed conversion ratios, chicks were grouped on the basis of the percent 10-day embryonic weight of their full-sibs and the ratios for 1-3 and 3-5 weeks of age compared with the mean percent embryo weight for that group. The pen conversion ratios ranged from 1.57-2.25 from 1-3 weeks; however, these differences did not appear to be related to percent 10-day embryo weight of full-sibs. The regression line of group feed conversion on mean percent embryo weight was not significantly different from zero and no trends toward improved efficiency of feed utilization (lower feed conversion ratio) with better embryonic efficiency (larger percent 10-day
TABLE 1.—Feed conversion ratios of chicks grouped on basis of their full-sib percent 10-day embryo weight Range 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
3.52-4.00 4.02-4.18 4.19-4.24 4.26-4.34 4.35-4.38 4.39-4.46 4.47-4.56 4.57-4.62 4.63-4.66 4.68-4.75 4.76-4.83 4.84-4.94 4.95-5.09 5.10-5.13 5.14-5.25 5.27-5.43 5.48-5.84
% 10-day embryo wt. X No. chicks 3.80 4.09 4.22 4.29 4.35 4.43 4.50 4.60 4.64 4.71 4.80 4.90 4.98 5.12 5.18 5.34 5.60
17 15 15 15 15 15 14 15 14 16 15 18 16 14 14 13 12
Feed conv ersion ratio 1-3 wks.
3-5 wks.
1.85 1.80 1.85 1.74 1.69 1.89 1.68 1.82 1.60 1.57 2.12 2.02 1.74 1.61 2.25 1.68 1.91
2.65 2.60 2.38 2.50 2.12 3.04 2.53 3.16 2.54 2.62 2.09 2.32 2.55 2.64 2.27 2.52 2.22
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
(ACRB) population were obtained. The eggs were weighed at collection, stored at 10° C. (50° F.) for up to 10 days and again weighed before incubation to determine if the weight loss in storage was reflected in percent embryo weight or in percent hatch-weight. After incubation, 10-day embryo weights of two eggs were obtained from each dam. The embryos were removed from the yolk, excess moisture removed and weighed in hundredths of a gram. The embryo weight as a percentage of egg weight was calculated to determine the percent embryo weight. The chicks obtained from the remaining eggs were used to determine hatch weight, chick weight as a percentage of egg weight (% hatch weight), growth rate and feed conversion ratio. The mean weight and percent embryo weight of the ten-day embryos which were full sibs to the hatched chicks was used as a measure of the embryo weight and percent embryo weight of the chicks allowed to hatch.
768
K. W. WASHBURN AND R. A. GUILL
TABLE 2.—Correlations between egg, 10-day embryo and chick parameters
(1) Egg wt. 1 (2) Egg wt. 2 (3) Hatch wt. (4) % embryo wt. (5) Embryo wt. (6) % Hatch wt. (of egg wt. 1) (7) % Hatch wt. (of egg wt. 2) (8) Feed conversion (5-7 weeks) (9) Feed conversion (7-9 weeks) (10) Growth (5-7) (11) Growth (7-9)
(4)
(5)
1.0 .62
1.0
.44
-.11
-.01
(2) (3) (1) 1.0 .99 1.0 .94 .94 1.0 -.46 -.46 -.45 .25 .25 .22 .14
.14
(6)
(7)
(8)
(9)
(10)
(H)
1.0
.13
.13
.44
-.11
-.02
.99
-.07
-.07
-.05
.19
.12
.05
.06 .21 .08
.06 .21 .08
.04 .25 .12
.12 .16 -.13 .00 -.12 -.06
-.01 .15 .12
1.0 .05
1.0
-.02 .33 1.0 .14 - . 5 8 - . 0 6 .12 - . 3 7 - . 6 0
1.0 .65
1.0
TABLE 3.—Mean values for egg weight, embryo weight, and percent weight, chick weight and percent weight, feed conversion and growth Egg weight (g.) Before After Storage 49.8 49.6
Embryo Wt. (g.) 2.14
% Wt. 4.66
Chick Wt. (g.) 34.2
% Wt. 68.7
Feed conversion
Growth
5-7 Wk. 7-9 Wk. 5-7 Wk. 7-9 Wk. 2.44 3.54 344g 322g
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
and feed conversion were found in the present study. Correlations for 5-7 and 7-9 week conversion with hatch weight as a percentage of egg weight at time of laying and time of incubation ranged from - . 0 1 to .05 (Table 2). There were also low correlations between the percent hatch weights and 5-7 or 7-9 week growth rate. The average weight loss of the eggs in storage in the present experiment was 0.2 gms (Table 3). The correlation between weight of egg when laid and when set was very high (.99). There were no differences in correlations of 10-day embryo weight or hatch weight with egg weight before or after storage. Thus, the small amount of weight loss in this study did not change the degree of association between the traits measured. A very high correlation (.94) between egg weight and hatch weight was observed. This is similar to the findings of numerous previous studies. Egg weight was not correlated with 5-7 or 7-9 week conversion. There was some
embryonic weight) were noted. For the period from 5-9 weeks, individual feed conversion ratios were determined and correlated with full-sib percent 10-day embryo weights. The mean feed conversion ratios for the 5-7 week and 7-9 week periods (Table 3) were 2.44 and 3.54, respectively. The correlation of .19 and .12 obtained for percent embryo weight and 5-7, 7-9 week feed conversion (Table 2) shows a lack of relationship between embryonic efficiency and 5-9 week efficiency of feed utilization. There was also a low correlation between percent embryo weight and 5-7 or 7-9 week growth rate (-.13 and —.12). Similar low correlations between 10-day embryo weight and feed conversion ratio (. 12 and . 16) and growth rate (0 and - .06) were obtained. Previous results (Guill and Washburn, 1973) have shown a lack of relationship between percent hatch weight and feed conversion for 5-9 weeks of age. A similar lack of association between percent hatch weight
769
EMBRYO WEIGHT AND CHICK EFFICIENCY
TABLE 4.—Analysis of variance for dam effect on egg weight, embryo weight and percent hatch weight
Source Total Bet. Dams Error
d.f.
Mean squares Egg wt. Embryo wt.
189 94 95
26.3** 3.5
.064 .052
% Embryo wt. .435* .218
*P < .05 **P s .01
ed to egg size (r = .25). Bray and Iton (1962) indicated that the effect of the size of the egg on strain differences began after 11 days of incubation and increased gradually to a maximum at hatching time. Thus, after 10 days of age one would expect an increasing masking of genetic differences in growth (and possible efficiency) by the effect of egg size. However, the lack of correlation between 10-day embryonic efficiency and subsequent chick efficiency and the uniformity of embryo weight indicates that the genes present for differences in growth and efficiency are not active in the embryonic state.
REFERENCES Bray, D. F., and E. L. Iton, 1962. The effect of egg weight on strain differences in embryonic and post-embryonic growth in the domestic fowl. Brit. Poultry Sci. 3: 175-188. Byerly, T. C , 1930. The effect of breed on the growth of the chick embryo. J. Morph. Physiol. 50: 341-359. Byerly, T. C , 1932. Growth of the chick embryo in relation to its food supply. J. Exp. Biol. 9: 15-44. Byerly, T. C , W. G. Helsel and J. P. Quinn, 1938. Growth in weight and cell number. Genetic effects in the chick embryo and chick. J. Exp. Zool. 78: 185-203. Guill, R. A., and K. W. Washburn, 1972. Cages and feed troughs for individual broiler feed consumption experiments. Poultry Sci. 51: 1047-1048. Guill, R. A., and K. W. Washburn, 1973. Relationship between percent hatch weight and feed conversion ratio in broilers. Poultry Sci. 52: 1641-1646. Henderson, E. W., 1930. Growth and development. The influence of temperature and breeding upon the rate of growth of chick embryos. Missouri Agri. Exp. Sta. Res. Bull. 149: 5-47. McNary, H. W., A. E. Bell and C. H. Moore, 1960. The growth of inbred and hybrid chicken embryos. Poultry Sci. 39: 378-384.
Downloaded from http://ps.oxfordjournals.org/ at Florida Atlantic University on November 23, 2014
degree of association between egg weight and 5-7 week growth (correlation of .21) but none to 7-9 week growth. An interesting negative correlation (-.46) between egg size and percent embryo weight was observed. Since percent embryo weight is a function of embryo weight and egg weight, a correlation between percent embryo weight with embryo and/or egg weight would be expected. Although the variation in egg weight was as great as expected in a randombred population, the variation in embryo weights was quite small (s = .25, C.V. = .56). Over the range of egg weights measured in this study the 10-day embryo weight appeared relatively constant compared to hatching weight which had a high correlation (.94) with egg weight. The negative correlation between percent embryo weight and egg weight resulted because of the variation in egg weight over relatively constant embryo weights. The analysis of dam effect on variation in egg weights, embryo weights and percent hatch weight (Table 4) support this conclusion. There was a significant difference between dams in percent embryo weight, but there were no significant differences between dams in embryo weight, indicating the difference in percent embryo weight was due to the differences in egg weight. Byerly et al. (1938) stated that growth in embryonic weight is conditioned by the limited food supply in the egg so that the weight of the chick at hatching time is proportional to the weight of the egg. However, the results of this experiment indicate that the 10-day embryo weight was only moderately correlat-