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Shell Quality in Poultry as Seen From the Breeder’s Viewpoint
Shell Quality in Poultry as Seen From the Breeder's Viewpoint 1. Improvement Reached After Four Years of Selection and the Effect on Productivity W. F. vanTIJEN Spelderholt Institute for Poultry Research, Beekbergen, The Netherlands (Received for publication September 7, 1976)
INTRODUCTION In recent years shell quality has come more and more under stress of high productivity. Not only higher peak productions can be observed but also higher persistencies. The latter leads to the situation where a relative high number of eggs with poor shell quality is produced in the final months of the laying cycle. The use of wire nest floors and battery cages also gives more cracked eggs than the traditional nesting system. As a result, a percentage of broken shells between 10 and 20 at the end of the laying period is estimated to be more the rule than the exception. There is, therefore, no doubt that the quality of the shell of the market eggs deserves constant attention. Weak shells are the cause of a considerable loss for the egg producer as well as a source of frustration for the housewife. The producer is being faced with cracks and leakers which, if not entirely lost, are degraded and soil neighbouring eggs in the pack. The housewife, when boiling eggs with haircracks, encounters the nuisance of boiled-out eggs with white coming through the cracked shell. In the years 1967—71 inclusive, selection experiments with the object to improve the quality of the egg shell by means of breeding have been carried out at the Spelderholt Institute for Poultry Research at Beekbergen, The Netherlands. Estimates of heritabilities and genetic correlations which had been obtained on the same strain led to the conclusion that a) it should be possible to improve the shell quality by means of breeding and b) this improvement could be reached without detri-
ment to the productivity of the flock (van Tijen, 1973a,b, 1974; van Tijen and Kuit, 1970). The above mentioned experiment was set up in order to test these conclusions under conditions of high selection pressure for shell quality. Moreover, the selected lines were crossed in all combinations in search for a heterosis effect which could occur as a result of the difference in gene frequencies for the various traits, and heritabilities were estimated to see if any change had occurred due to the selection. The response obtained during the first three years of selection was compared with what could be expected in view of the heritabilities and the selection differential. LITERATURE Reports on genetic parameters—heritabilities and genetic correlations—for shell quality are not very hard to find in the literature. Summaries of these can be found in the publications of Kinney (1969) and van Tijen and Kuit (1970). Few papers, however, have been published which deal with actual selection for shell quality. Quinn et al. (1945) selected for shell quality on the basis of weight loss during incubation. They found this to be a heritable trait and a satisfactory guide for improvement by selection and breeding. King and Hall (1955) investigated the relative importance of strain— and breed differences in egg quality factors during four years of random sample testing at the New York Random Sample station. They found significant differences in shell thickness
1107
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
ABSTRACT A comparative selection experiment has shown that it is very well possible to improve shell quality by means of breeding. These stronger shells are acquired however at the cost of a part of productivity. The gain in eggs delivered will have to be weighed against the loss in productivity (egg mass in grams per hen). It is pointed out that in the case of market eggs, the shell should be strong enough to withstand the stress of normal transport. One overshoots ones purpose, however, by trying to select for shells that are stronger than necessary for this purpose. Poultry Science 56:1107-1114, 1977
73.7 34.1 13.2 81.5 5.01
143 71.5 7721 58.9 53.8
73.1 34.5 13.4 82.1 5.09
Egg quality 60 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
72.0 35.3 13.6 88.4 6.30
149 73.8 8035 59.1 54.1
71.9 35.5 13.5 88.8 6.32
Egg quality 31 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
152 78.4 8515 60.2 56.0
170
73.1 36.4 12.9 83.6 4.77
73.8 37.6 12.2 91.8 6.05
155 79.5 8746 60.7 56.5
169
74.3 35.1 13.9 80.8 5.14
74.7 36.3 12.9 89.7 6.49 74.4 37.5 13.5 83.5 5.39
74.3 36.7 13.2 89.2 6.53
141 70.9 7691 59.2 54.7
165
156 76.5 8777 61.8 56.4
160
Rhode Island Red
73.0 39.2 12.2 87.1 4.89
72.7 38.5 11.6 93.4 5.81
154 74.8 8868 63.4 57.6
149 73.0 8394 62.2 56.3
155 77.5 8787 64.1 57.0
152 75.6 8561 62.9 56.6
143 67.8 7786 61.9 54.5
164
144 68.2 7851 62.2 54.5
158
160
164
163
Prod.
153
Second shell
Prod.
Selected g First shell
Prod.
162
153
Age at first egg Prod, per hen present 18-52 weeks Number of eggs Production percentage Grams of egg mass Egg weight 52 weeks Average egg weight 18-52 weeks
Age at first egg Prod, per hen present 18-52 weeks Number of eggs Production percentage Grams of egg mass Egg weight 52 weeks Average egg weight 18-52 weeks
Shell
Trait
White Leghorn Starting generation
TABLE 1.—Productivity and shell quality data during four generations of se in two divergent directions, (shell quality and productivity)
from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
EGG SHELL QUALITY
in O O »n f"*
r* O O TH O
Tj- N i n H ON w i-i irl t% in t*s rO rH tN
^Om
O N N
00 N ^O H >t rh
q N *-j rs •* v i m ^ O ^O
r*- q q
t** m i-i 00
CO
t^. tf\ y-t t>
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o^ (S o
r*. •«*•
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t*. rti
o
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n m 0\N\o (S TH vd -
H <) O^ I s ; N in *-H <> co ^d fs m r-t t**
in fS t-N rft in N r i vdmifi r^. m I-H r -
O O J3
^^ B
6
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§1
as „*s ^ US
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2 o o D ifl I
number of eggs and egg weight. Furthermore, there is a clear trend in the sense that the P-line, which was slightly selected for quality of the albumen is better in this respect than the S-line where this trait was completely ignored. The frequency distributions of the shell quality traits—deformation, specific gravity and shell thickness—are also illustrative of the response to selection for shell quality. They are given in Fig. 1 (a, b and c). The polygons for the deformation show a skewed distribution with a tail to the high values. In the course of the years they have—as a result of the selection pressure—shifted to the lower values, while the variation has become smaller. (For this trait the low figures mean the better quality shells). For the most part, the tail has disappeared, which means that after four years of selection there are no—or at least very few—producers of poor quality shells left in the population. The polygons for the specific gravity and for the shell thickness are of a more normal character. The selection pressure has caused them to shift to the higher values and the variation has remained more or less the same. Furthermore it should be noted that in the graphs for 60 weeks of age the distribution polygons of R (S) 1970 and W (S) 1967 for the traits deformation and specific gravity nearly coincide. It should be remarked that the rising trends which were observed seem to level off during the last year of the selection. We are unable to say—on the basis of the above data—whether this is only a temporary phenomenon or that a plateau has been reached. For a definite conclusion in this respect it would have been necessary to continue the experiment for some more years. Thus, it can be concluded that a few generations of selection are sufficient to achieve a considerable improvement in shell quality. This is the same conclusion as has been reached by Rodda on theoretical grounds. Our experiment, however, indicates that this improvement is acquired at the cost of a part of the productivity. Once again this is clear from the figures in Table 3 in which the relative differences between the two strains for the traits shell thickness at the ages of 30 and 60 weeks, respectively, and the productivity—expressed as egg mass—are given. In the fourth selected generation an improvement in shell thickness of about 3 percent stands against a loss of produc-
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
O w> Q)
1109
1110
VAN TIJEN
MATERIAL AND METHODS The experiment was started with a strain of White Leghorns and a strain of Rhode Island Reds from the Institute's breeding unit. Both these strains were purchased in 1959 and there were sound reasons to believe that they had been bred as a closed group for years before they were obtained by the Institute. In 1967 the above mentioned strains were divided up in two genetically equivalent substrains. In order to achieve this, the hens in a breeding pen which were mated to an individual cock were two-by-two full sisters of each other. The progeny of the one sister was allotted to one substrain and of the other sister to the second. Except in the first generation, each substrain consisted of 750 hens obtained from 20 sires and 120—160 dams and hatched from three settings spaced two weeks apart. In the years thereafter—1968/71, inclusive— the one substrain was exclusively selected for shell quality, while for the other a more traditional system was followed, stressing mainly productivity and egg weight—and to some extent egg quality. The substrains will hencef o r t h be d e s i g n a t e d as (S)hell- and (P)roduction line, respectively. The production level was established at two fixed times: at the age of 3 and 72 weeks. Egg quality was determined twice during the laying period, viz. at 30 and at 60 weeks of age. Selection for shell strength as well as for productivity was carried out with the help of an index. In the shell quality index three factors, viz. shell thickness, deformation and specific
1 Shell thickness was measured in 0.01 of a mm., deformation in 0.001 of a mm.
gravity of the entire egg, were each represented on an equal basis 1 . The index which formed the basis for the selection of the productivity lines contained four characteristics: production percentage, egg weight, shell quality and internal quality. Each represented on the basis of 50, 30, 20 and 10 percent, respectively, of the total selection pressure. The formulae for both indices are given below. Shell quality index (SI) = 50 — [(log deformation — av. log. deformation)/(
RESULTS AND DISCUSSION The complete data over the five generations are given in Table 1. Table 2 gives the differences as they arose during the course of time. The figures indicate that the divergent selection pressures—for shell quality and for productivity—have had clear results and that from each of the original strains two clearly distinguishable lines have emerged. The eggs of the S-line hens have—in comparison to those of the P-line—acquired a stronger shell as a result of the exclusive selection for this trait. The productivity however—number of eggs, production percentage and egg weight—appears to have remained behind in the S-line as compared to the P-line, in which 80 percent of the selection pressure was placed on laying percentage and egg weight. This also manifests itself in the trait "grams of egg mass per hen present", which in fact is a combination of
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
between breeds but not between strains within breeds. The Rhode Island Reds appeared to have the thinnest, and the Barred Rocks the thickest, shell. Rodda (1972) showed that in order to improve the shell quality at the end of the laying period, selection could take place at an early age, thus improving the progress per year by shortening the generation interval. He also concluded that lack of any improvement inend- of year shell strength was probably due to lack of selection pressure and not to low heritability, adverse genetic correlations or inaccurate measuring methods.
-0.1 +0.2 -0.1 +0.4 +0.02
-0.6 +0.4 -0.1 +0.6 +0.08
Egg quality 60 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
0
+0.3
65
+0.4
+1
Starting generation
Egg quality 31 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
Age at first egg Prod, per hen present 18-52 weeks Number of eggs Production percentage Grams of egg mass Egg weight 52 weeks Average egg weight 18-52 weeks
Trait
-1.2 +1.3 -1.0 +2.8 -0.37
-0.9 +1.3 -0.7 +2.1 -0.44
-1.9 -226 -1.2 -0.4
-3
First
-1.4 +1.7 -1.3 + 3.6 -0.50 -1.8 +2.0 -1.8 +4.9 -0.76
-1.9 +2.1 -1.4 +5.1 -1.13
-•.8 -872 -2.0 -2.0
-1.6 +1.8 -1.6 +4.2 -0.72
-10
-1.8 -474 -1.2 -1.3
Third
-5
+2
Second
Selected generation
White Leghorn
-1.3 +1.2 -0.9 +2.6 -0.89
-869 -1.8 -1.7
-11
Fourth
+0.4 -0.1 -0.2 -0.9 -0.06
+0.3 -0.4 +0.5 -1.0 -0.11
+2.3 +314 +0.2 +0.3
+6
Starting generation
TABLE 2.—Differences (S-P) in the five consecutive generations
from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
VAN TIJEN
1112
(30 weeks of age)
15
20
25
30
i
r
i
I
60
70
80
90
25
30
100 110
35 (60 weeks of age) RS WS
(60 weeks of age) RS
20
~i
I
ft
35
FIG. la. Frequency polygons for shell quality in four strains of laying stock — deformation.
60
70
80
90
100 110
FIG. l b . Frequency polygons for shell quality in four strains of laying stock - specific gravity.
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
10
(30 weeks of age)
1113
EGG SHELL QUALITY
(30weeksofage) RS
"35 O
H<
c o
*rtn 1)
c
so
SO
d oo os
O OS O 00
T-l
T3
.b
.C f-H
T-i
'-J
• *
o dOs o *-4
O
r-t
:lec
*-> T3C
C/5
§
w
(fl
t^
o ort
4-»
e .2
o o
25
T 30
•*
'""' *
t~^ Os
O tx O Os
o o
O
•*
O
o o
o o
O fn O O
O
20
SO
o ^
O 00
r——r——r 1 35 AO A5 50 Os
(60 weeks of age) RS WS
o <*> O r-n o o O O
O
tn O
Ow O O
T»
O
tn
o o o o BO T3
8S
O
w
o o
o »^ o o
o o o oo
25
30
35
AO
A5 50
FIG. l c . Frequency polygons for shell quality in four strains of laying stock - shell thickness.
O O
O Os
a* si
cu c/5
3
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
;rs
• *
Os
VAN TIJEN
1114
REFERENCES King, S. C , and G. O. Hall, 1955. Egg quality studies at the New York Random Sample Test. Poultry Sci. 34:799-809.
Kinney Jr., T. B., 1969. A summary of reported estimates of heritabilities and of genetic and phenotypic correlations for traits of chickens. Washington, U.S. Dept. of Agric. Agricultural Handbook No. 363. Quinn, J. P., C. D. Gordon and A. B. Godfrey, 1945. Breeding for egg shell quality as indicated by weight loss. Poultry Sci. 24:399-403. Rodda, D. D., 1972. Breeding for late egg shell quality in the domestic hen. Brit. Poultry Sci. 13:45—60. Tijen, W. F. van, 1973a. The consequences of selection for shell quality. Ann. Genet. Sel. Anim. 5:403^-10. Tijen, W. F. van, 1973b. The influence of selection procedures on heterosis for productivity and shell quality in the chicken. In: Proc. 4th Europ. Poultry Conf. London, Sept. 1972. Tijen, W. F. van, 1974. Predicted and realized response during three years of selection for shell quality. In •. Proceedings 1st World Congress on Genetics Applied to Livestock Production, Madrid, October 1974. Tijen, W. F. van, and A. R. Kuit, 1970. The heritability of characteristics of egg quality, their mutual correlation and the relationship with productivity. Arch. Gefliigelk. 34:201-210.
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
tivity of a b o u t 10 t o 11 percent in b o t h strains. In t h e second selected generation t h e t w o R h o d e Island Red lines showed an absolute difference of +0.12 m m . against a negative difference of m o r e t h a n 1 kg. egg mass per hen present at the age of 52 weeks. T h e decreased loss t h r o u g h damage as a result of a stronger shell will t h u s have t o be weighed against t h e fewer eggs t h a t are gathered due t o a lower p r o d u c t i o n . Taking t h e above into consideration, o n e has t o bear in mind t h a t in t h e case of m a r k e t eggs, t h e shell has t o be strong enough t o withstand t h e stress of n o r m a l t r a n s p o r t , b u t t h a t it m a k e s n o sense t o t r y t o obtain b y m e a n s of breeding shells t h a t are stronger t h a n necessary for this p u r p o s e .
INTRODUCTION In recent years shell quality has come more and more under stress of high productivity. Not only higher peak productions can be observed but also higher persistencies. The latter leads to the situation where a relative high number of eggs with poor shell quality is produced in the final months of the laying cycle. The use of wire nest floors and battery cages also gives more cracked eggs than the traditional nesting system. As a result, a percentage of broken shells between 10 and 20 at the end of the laying period is estimated to be more the rule than the exception. There is, therefore, no doubt that the quality of the shell of the market eggs deserves constant attention. Weak shells are the cause of a considerable loss for the egg producer as well as a source of frustration for the housewife. The producer is being faced with cracks and leakers which, if not entirely lost, are degraded and soil neighbouring eggs in the pack. The housewife, when boiling eggs with haircracks, encounters the nuisance of boiled-out eggs with white coming through the cracked shell. In the years 1967—71 inclusive, selection experiments with the object to improve the quality of the egg shell by means of breeding have been carried out at the Spelderholt Institute for Poultry Research at Beekbergen, The Netherlands. Estimates of heritabilities and genetic correlations which had been obtained on the same strain led to the conclusion that a) it should be possible to improve the shell quality by means of breeding and b) this improvement could be reached without detri-
ment to the productivity of the flock (van Tijen, 1973a,b, 1974; van Tijen and Kuit, 1970). The above mentioned experiment was set up in order to test these conclusions under conditions of high selection pressure for shell quality. Moreover, the selected lines were crossed in all combinations in search for a heterosis effect which could occur as a result of the difference in gene frequencies for the various traits, and heritabilities were estimated to see if any change had occurred due to the selection. The response obtained during the first three years of selection was compared with what could be expected in view of the heritabilities and the selection differential. LITERATURE Reports on genetic parameters—heritabilities and genetic correlations—for shell quality are not very hard to find in the literature. Summaries of these can be found in the publications of Kinney (1969) and van Tijen and Kuit (1970). Few papers, however, have been published which deal with actual selection for shell quality. Quinn et al. (1945) selected for shell quality on the basis of weight loss during incubation. They found this to be a heritable trait and a satisfactory guide for improvement by selection and breeding. King and Hall (1955) investigated the relative importance of strain— and breed differences in egg quality factors during four years of random sample testing at the New York Random Sample station. They found significant differences in shell thickness
1107
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
ABSTRACT A comparative selection experiment has shown that it is very well possible to improve shell quality by means of breeding. These stronger shells are acquired however at the cost of a part of productivity. The gain in eggs delivered will have to be weighed against the loss in productivity (egg mass in grams per hen). It is pointed out that in the case of market eggs, the shell should be strong enough to withstand the stress of normal transport. One overshoots ones purpose, however, by trying to select for shells that are stronger than necessary for this purpose. Poultry Science 56:1107-1114, 1977
73.7 34.1 13.2 81.5 5.01
143 71.5 7721 58.9 53.8
73.1 34.5 13.4 82.1 5.09
Egg quality 60 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
72.0 35.3 13.6 88.4 6.30
149 73.8 8035 59.1 54.1
71.9 35.5 13.5 88.8 6.32
Egg quality 31 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
152 78.4 8515 60.2 56.0
170
73.1 36.4 12.9 83.6 4.77
73.8 37.6 12.2 91.8 6.05
155 79.5 8746 60.7 56.5
169
74.3 35.1 13.9 80.8 5.14
74.7 36.3 12.9 89.7 6.49 74.4 37.5 13.5 83.5 5.39
74.3 36.7 13.2 89.2 6.53
141 70.9 7691 59.2 54.7
165
156 76.5 8777 61.8 56.4
160
Rhode Island Red
73.0 39.2 12.2 87.1 4.89
72.7 38.5 11.6 93.4 5.81
154 74.8 8868 63.4 57.6
149 73.0 8394 62.2 56.3
155 77.5 8787 64.1 57.0
152 75.6 8561 62.9 56.6
143 67.8 7786 61.9 54.5
164
144 68.2 7851 62.2 54.5
158
160
164
163
Prod.
153
Second shell
Prod.
Selected g First shell
Prod.
162
153
Age at first egg Prod, per hen present 18-52 weeks Number of eggs Production percentage Grams of egg mass Egg weight 52 weeks Average egg weight 18-52 weeks
Age at first egg Prod, per hen present 18-52 weeks Number of eggs Production percentage Grams of egg mass Egg weight 52 weeks Average egg weight 18-52 weeks
Shell
Trait
White Leghorn Starting generation
TABLE 1.—Productivity and shell quality data during four generations of se in two divergent directions, (shell quality and productivity)
from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
EGG SHELL QUALITY
in O O »n f"*
r* O O TH O
Tj- N i n H ON w i-i irl t% in t*s rO rH tN
^Om
O N N
00 N ^O H >t rh
q N *-j rs •* v i m ^ O ^O
r*- q q
t** m i-i 00
CO
t^. tf\ y-t t>
m ^- m ^t" f» VJ T^ Tt ' t ^ N M H 00
o^ (S o
r*. •«*•
co MD in n r-i
00 (N -^- CO ^t-
C0# O ^
t*. rti
o
-4- ci \d oC \d
n m 0\N\o (S TH vd -
H <) O^ I s ; N in *-H <> co ^d fs m r-t t**
in fS t-N rft in N r i vdmifi r^. m I-H r -
O O J3
^^ B
6
a
fJ3sO- d . §
§1
as „*s ^ US
"* 2 S I S3
6 o .ti
2 &•* .— u c > -s
bo<<
.2 S « El a.=s o ^ W T3 <« BOX J3 «i
w
III
2 o o D ifl I
number of eggs and egg weight. Furthermore, there is a clear trend in the sense that the P-line, which was slightly selected for quality of the albumen is better in this respect than the S-line where this trait was completely ignored. The frequency distributions of the shell quality traits—deformation, specific gravity and shell thickness—are also illustrative of the response to selection for shell quality. They are given in Fig. 1 (a, b and c). The polygons for the deformation show a skewed distribution with a tail to the high values. In the course of the years they have—as a result of the selection pressure—shifted to the lower values, while the variation has become smaller. (For this trait the low figures mean the better quality shells). For the most part, the tail has disappeared, which means that after four years of selection there are no—or at least very few—producers of poor quality shells left in the population. The polygons for the specific gravity and for the shell thickness are of a more normal character. The selection pressure has caused them to shift to the higher values and the variation has remained more or less the same. Furthermore it should be noted that in the graphs for 60 weeks of age the distribution polygons of R (S) 1970 and W (S) 1967 for the traits deformation and specific gravity nearly coincide. It should be remarked that the rising trends which were observed seem to level off during the last year of the selection. We are unable to say—on the basis of the above data—whether this is only a temporary phenomenon or that a plateau has been reached. For a definite conclusion in this respect it would have been necessary to continue the experiment for some more years. Thus, it can be concluded that a few generations of selection are sufficient to achieve a considerable improvement in shell quality. This is the same conclusion as has been reached by Rodda on theoretical grounds. Our experiment, however, indicates that this improvement is acquired at the cost of a part of the productivity. Once again this is clear from the figures in Table 3 in which the relative differences between the two strains for the traits shell thickness at the ages of 30 and 60 weeks, respectively, and the productivity—expressed as egg mass—are given. In the fourth selected generation an improvement in shell thickness of about 3 percent stands against a loss of produc-
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
O w> Q)
1109
1110
VAN TIJEN
MATERIAL AND METHODS The experiment was started with a strain of White Leghorns and a strain of Rhode Island Reds from the Institute's breeding unit. Both these strains were purchased in 1959 and there were sound reasons to believe that they had been bred as a closed group for years before they were obtained by the Institute. In 1967 the above mentioned strains were divided up in two genetically equivalent substrains. In order to achieve this, the hens in a breeding pen which were mated to an individual cock were two-by-two full sisters of each other. The progeny of the one sister was allotted to one substrain and of the other sister to the second. Except in the first generation, each substrain consisted of 750 hens obtained from 20 sires and 120—160 dams and hatched from three settings spaced two weeks apart. In the years thereafter—1968/71, inclusive— the one substrain was exclusively selected for shell quality, while for the other a more traditional system was followed, stressing mainly productivity and egg weight—and to some extent egg quality. The substrains will hencef o r t h be d e s i g n a t e d as (S)hell- and (P)roduction line, respectively. The production level was established at two fixed times: at the age of 3 and 72 weeks. Egg quality was determined twice during the laying period, viz. at 30 and at 60 weeks of age. Selection for shell strength as well as for productivity was carried out with the help of an index. In the shell quality index three factors, viz. shell thickness, deformation and specific
1 Shell thickness was measured in 0.01 of a mm., deformation in 0.001 of a mm.
gravity of the entire egg, were each represented on an equal basis 1 . The index which formed the basis for the selection of the productivity lines contained four characteristics: production percentage, egg weight, shell quality and internal quality. Each represented on the basis of 50, 30, 20 and 10 percent, respectively, of the total selection pressure. The formulae for both indices are given below. Shell quality index (SI) = 50 — [(log deformation — av. log. deformation)/(
RESULTS AND DISCUSSION The complete data over the five generations are given in Table 1. Table 2 gives the differences as they arose during the course of time. The figures indicate that the divergent selection pressures—for shell quality and for productivity—have had clear results and that from each of the original strains two clearly distinguishable lines have emerged. The eggs of the S-line hens have—in comparison to those of the P-line—acquired a stronger shell as a result of the exclusive selection for this trait. The productivity however—number of eggs, production percentage and egg weight—appears to have remained behind in the S-line as compared to the P-line, in which 80 percent of the selection pressure was placed on laying percentage and egg weight. This also manifests itself in the trait "grams of egg mass per hen present", which in fact is a combination of
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
between breeds but not between strains within breeds. The Rhode Island Reds appeared to have the thinnest, and the Barred Rocks the thickest, shell. Rodda (1972) showed that in order to improve the shell quality at the end of the laying period, selection could take place at an early age, thus improving the progress per year by shortening the generation interval. He also concluded that lack of any improvement inend- of year shell strength was probably due to lack of selection pressure and not to low heritability, adverse genetic correlations or inaccurate measuring methods.
-0.1 +0.2 -0.1 +0.4 +0.02
-0.6 +0.4 -0.1 +0.6 +0.08
Egg quality 60 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
0
+0.3
65
+0.4
+1
Starting generation
Egg quality 31 weeks Shape index (width/length X 100) Shell thickness (0.01 mm.) Deformation (0.001 mm.) Specific gravity (1.0...) Height of thick white (mm.)
Age at first egg Prod, per hen present 18-52 weeks Number of eggs Production percentage Grams of egg mass Egg weight 52 weeks Average egg weight 18-52 weeks
Trait
-1.2 +1.3 -1.0 +2.8 -0.37
-0.9 +1.3 -0.7 +2.1 -0.44
-1.9 -226 -1.2 -0.4
-3
First
-1.4 +1.7 -1.3 + 3.6 -0.50 -1.8 +2.0 -1.8 +4.9 -0.76
-1.9 +2.1 -1.4 +5.1 -1.13
-•.8 -872 -2.0 -2.0
-1.6 +1.8 -1.6 +4.2 -0.72
-10
-1.8 -474 -1.2 -1.3
Third
-5
+2
Second
Selected generation
White Leghorn
-1.3 +1.2 -0.9 +2.6 -0.89
-869 -1.8 -1.7
-11
Fourth
+0.4 -0.1 -0.2 -0.9 -0.06
+0.3 -0.4 +0.5 -1.0 -0.11
+2.3 +314 +0.2 +0.3
+6
Starting generation
TABLE 2.—Differences (S-P) in the five consecutive generations
from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
VAN TIJEN
1112
(30 weeks of age)
15
20
25
30
i
r
i
I
60
70
80
90
25
30
100 110
35 (60 weeks of age) RS WS
(60 weeks of age) RS
20
~i
I
ft
35
FIG. la. Frequency polygons for shell quality in four strains of laying stock — deformation.
60
70
80
90
100 110
FIG. l b . Frequency polygons for shell quality in four strains of laying stock - specific gravity.
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
10
(30 weeks of age)
1113
EGG SHELL QUALITY
(30weeksofage) RS
"35 O
H<
c o
*rtn 1)
c
so
SO
d oo os
O OS O 00
T-l
T3
.b
.C f-H
T-i
'-J
• *
o dOs o *-4
O
r-t
:lec
*-> T3C
C/5
§
w
(fl
t^
o ort
4-»
e .2
o o
25
T 30
•*
'""' *
t~^ Os
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o o
O
•*
O
o o
o o
O fn O O
O
20
SO
o ^
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r——r——r 1 35 AO A5 50 Os
(60 weeks of age) RS WS
o <*> O r-n o o O O
O
tn O
Ow O O
T»
O
tn
o o o o BO T3
8S
O
w
o o
o »^ o o
o o o oo
25
30
35
AO
A5 50
FIG. l c . Frequency polygons for shell quality in four strains of laying stock - shell thickness.
O O
O Os
a* si
cu c/5
3
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
;rs
• *
Os
VAN TIJEN
1114
REFERENCES King, S. C , and G. O. Hall, 1955. Egg quality studies at the New York Random Sample Test. Poultry Sci. 34:799-809.
Kinney Jr., T. B., 1969. A summary of reported estimates of heritabilities and of genetic and phenotypic correlations for traits of chickens. Washington, U.S. Dept. of Agric. Agricultural Handbook No. 363. Quinn, J. P., C. D. Gordon and A. B. Godfrey, 1945. Breeding for egg shell quality as indicated by weight loss. Poultry Sci. 24:399-403. Rodda, D. D., 1972. Breeding for late egg shell quality in the domestic hen. Brit. Poultry Sci. 13:45—60. Tijen, W. F. van, 1973a. The consequences of selection for shell quality. Ann. Genet. Sel. Anim. 5:403^-10. Tijen, W. F. van, 1973b. The influence of selection procedures on heterosis for productivity and shell quality in the chicken. In: Proc. 4th Europ. Poultry Conf. London, Sept. 1972. Tijen, W. F. van, 1974. Predicted and realized response during three years of selection for shell quality. In •. Proceedings 1st World Congress on Genetics Applied to Livestock Production, Madrid, October 1974. Tijen, W. F. van, and A. R. Kuit, 1970. The heritability of characteristics of egg quality, their mutual correlation and the relationship with productivity. Arch. Gefliigelk. 34:201-210.
Downloaded from http://ps.oxfordjournals.org/ at East Tennessee State University on June 1, 2015
tivity of a b o u t 10 t o 11 percent in b o t h strains. In t h e second selected generation t h e t w o R h o d e Island Red lines showed an absolute difference of +0.12 m m . against a negative difference of m o r e t h a n 1 kg. egg mass per hen present at the age of 52 weeks. T h e decreased loss t h r o u g h damage as a result of a stronger shell will t h u s have t o be weighed against t h e fewer eggs t h a t are gathered due t o a lower p r o d u c t i o n . Taking t h e above into consideration, o n e has t o bear in mind t h a t in t h e case of m a r k e t eggs, t h e shell has t o be strong enough t o withstand t h e stress of n o r m a l t r a n s p o r t , b u t t h a t it m a k e s n o sense t o t r y t o obtain b y m e a n s of breeding shells t h a t are stronger t h a n necessary for this p u r p o s e .