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The Response to One Cycle of Divergent Selection for Abdominal Fat in Broilers Raised Under Different Conditions1
BREEDING AND GENETICS The Response to One Cycle of Divergent Selection for Abdominal Fat in Broilers Raised Under Different Conditions1 A. CAHANER and M. KRINSKY The Hebrew University of Jerusalem, Faculty of Agriculture, P.O. Box 12, Rehovot 76100, Israel ZAFRIRA NITSAN Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel (Received for publication November 26, 1984) ABSTRACT One cycle of divergent sib selection, for or against abdominal fat weight, combined with within-family selection for body weight, was performed in a population of a commercial female grandparent line (White Rock type). Chicks from the first generation ( S ^ , ) of high-fat (HF) and low-fat (LF) selection lines were raised in three different environmental and climatic conditions. A second generation (S, G 2 ) of the selection lines was obtained by within-line selection for body weight only. The HF line had about 60% more abdominal fat than the LF line in all three environments. The realized heritability of this trait was calculated to be .73, almost identical to the heritability estimated from the base population. The LF line mean body weight was between 1% and 5% lower than that of the HF line. This difference was not significant and it was almost abolished in S, G 2 , after both lines were selected for body weight only. The difference between lines in abdominal fat weight was increased to 80% in S, G 2 . It can be concluded that reduction in abdominal fat weight was not associated with a decrease in body weight. Selection for body weight, in a line previously selected against abdominal fat, did not reverse the reduction in fat. The difference between the HF and LF lines had the same magnitude in both sexes and under different environmental conditions that increase or decrease fat deposition. (Key words: divergent selection, realized heritability, body weight, genotype X environment interaction, abdominal fat) 1985 Poultry Science 64:1813-1820 INTRODUCTION T h e increasing a m o u n t of excessive fat in broiler carcasses is widely recognized as o n e of t h e i n d u s t r y ' s primary p r o b l e m s . It is also generally accepted t h a t this p r o b l e m will b e resolved mainly t h r o u g h genetic and breeding efforts, r a t h e r t h a n n u t r i t i o n and m a n a g e m e n t . A b d o m i n a l and whole carcass fat were s h o w n t o have m o d e r a t e t o high heritability and to b e positively correlated, b o t h genetically and phenotypically, with b o d y weight (Siegel, 1 9 8 4 ) . Becker et al. ( 1 9 8 4 ) concluded t h a t a b d o m i n a l fat could b e reduced b y selection b u t with a correlated r e d u c t i o n in b o d y weight. Results of a n o t h e r s t u d y r e p o r t e d b y Becker et al. ( 1 9 8 2 ) and theorectical calculations b y Cahaner and Nitsan ( 1 9 8 5 ) indicate t h a t b y
using selection index o r i n d e p e n d e n t culling levels, o n e can r e d u c e a b d o m i n a l fat and simultaneously increase b o d y weight. A fat line and a lean line were established b y divergent selection for high or low a b d o m i n a l fat (Leclercq et al, 1 9 8 0 ; Leclercq, 1 9 8 3 ) . After seven generations of selection, t h e fat-line had a b o u t four t i m e s m o r e a b d o m i n a l fat t h a n t h e leanline, while b o d y weight w a s almost equal in t h e t w o lines. T h e p u r p o s e s of t h e present s t u d y w e r e t o c o m p a r e t h e p r e d i c t e d vs. t h e actual response of divergent selection o n a b d o m i n a l fat, t o d e t e r m i n e w h e t h e r this selection affects males and females differently, a n d t o ascertain w h e t h e r genetic response is expressed similarily a t different locations, housing, and seasons. MATERIALS AND METHODS
1
Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 1263-E, 1984 series.
A commercial female g r a n d p a r e n t line (White R o c k t y p e ) was used as a base population for t h e present s t u d y . A fully pedigreed
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flock was studied in two consecutive generations. Experimental procedures and several genetic and phenotypic parameters of the base population have been reported by Cahaner and Nitsan (1985). Sib selection was practiced in the second generation of the base population (S 0 ). There were 800 birds in the flock, about 45 in each of the 18 sire families which constituted the population; they were weighed at 7 weeks of age. At 9 weeks of age, eight males and eight females were taken at random from each sire family. They were slaughtered 10 hr after feed removal and their live body and abdominal fat weights were recorded. Abdominal fat was removed from the cloaca to the gizzard, without the fat adhering to the gizzard. The data from the slaughtered birds were used to rank the sire families according to the amount of abdominal fat. The heaviest two males and six females from each of the six families that had the- highest average of abdominal fat were used as parents of the first generation of the highfat (HF) line. Similarly, the heaviest two males and six females from each of the six families that had the lowest average of abdominal fat were used as parents of the first generation of the low-fat (LF) line. The mean body weight of the birds selected as parents of the two lines, for each OA the Si HOCKS, uiu not uiner signiiicantly. Within each line, three random females were assigned to each of the 12 males and matings were performed by artificial insemination; half-sib and full-sib matings were avoided. In June, 1981, about 300 chicks were hatched from each of the two lines of the first selected generation, designated S t G j . Four hundred and eighty chicks, representing all families in the two lines, were raised at the Akko Farm (Flock A). A total of 120 chicks, the offspring of six sires and 24 dams from each line, were grown at the Rehovot Farm (Flock B). The birds on the two farms were exposed to different environmental conditions. On the Akko farm, the chicks were brooded in conventional floor pens where they were raised until 8 weeks of age. On the Rehovot farm, the chicks were brooded in batteries; at 4 weeks of age they were transferred to individual cages in the open shade where they were kept until 8 weeks of age. The birds in the floor pen (Flock A) suffered more than the birds in the cages (Flock B) from the hot and humid weather,
which is typical for that growing season. A standard broiler feeding regime was applied to the chicks on both farms. Birds on the two farms were slaughtered at 8 weeks of age after overnight feed removal. Live body and abdominal fat weights were recorded for about 300 random birds of Flock A and 100 random birds in Flock B. Four months later, in October 1981, another hatch of SiGi chicks was obtained. Some sires and dams that had been used in the first hatch had died and some were not fertile; they were replaced by reserves kept from the selected families. Altogether, 7 sires and 21 dams from each line were mated and about 170 chicks were hatched from each line. All the chicks were raised on the 'Akko Farm (Flock C) in the same rearing system as described above but under better climatic conditions. The birds were weighed at 7 weeks of age, after food was removed for overnight. All the females, about 75 per line, were kept to reproduce the lines. The 25 heaviest males within each line were kept for reproduction while the rest of the males were slaughtered at 8 weeks of age and their body and abdominal fat weights were recorded. The offspring of the above SiG! males and females (SiG 2 ) were obtained without additional selection for or against abdominal fat. All the chicks, about 400 per line, were grown on the Akko Farm under normal conditions. At 8 weeks of age, five males and five females from each sire family were taken at random, and their live body and abdominal fat weights were determined as in all the earlier flocks. Because divergent selection was practiced, without a control line, the response to selection (R) was calculated from the difference between the means of the HF and LF lines in S i G ^ Similarly, selection differentials (S) were calculated from the difference between the mean abdominal fat weight of the sibs of the selected sires and dams of the two lines. Values for S were calculated separately from the actual sires and dams of each of the three Si Gx flocks and each was weighted by the number of offspring. Realized heritability was calculated from the R/S ratio for each flock in Si G : . The potential existence of an interaction between genotypes (HF and LF lines) and farms or hatches was tested by two-way analysis of variances of mean abdominal fat weight of the families represented in the two flocks.
DIVERGENT SELECTION FOR ABDOMINAL FAT IN BROILERS
1815
TABLE 1. Means, coefficients of variation (CV), and heritability (b*) of body weight and abdominal fat weight of males (M) and females (F) of the base population (S0 generation)
Trait
Weeks of age
Body weight, g
SE (h2)
Sex
Mean
CV
7
M F
1767 1492
9 9
.47 .48
.20 .20
Body weight, g
9
M F
2203 1817
7 8
.43 .52
.19 .21
Abdominal fat, g
9
M F
30 30
.73 .86
.26 .29
Abdominal fat, %
9
M F
30 28
.77 .90
.27 .29
(%)
1
40.0 41.8 1.91 2.30
SE = Standard error.
RESULTS
abdominal fat weight were much larger than those of body weight. Variation and heritability of percent abdominal fat (g/100 g body weight) were very similar to those of the absolute weight. Similar estimates were obtained for males and for females in all traits. The results of one cycle of divergent selection for and against abdominal fat are summarized in Table 2. Means of abdominal fat and body weight of the HF and LF lines were calculated separately for the three flocks of the SiGi generation. The differences in abdominal
Means, coefficients of variation, and estimates of heritability by full-sib analysis of variance (ANOVA) (Cahaner and Nitsan, 1985) were calculated from the So for body weight and abdominal fat weight and are presented in Table 1. The body weight means and the amount of abdominal fat were similar to those of a commercial "pure" line at the time. At 9 weeks of age, the coefficients of variation of abdominal fat were four times greater than those of body weight. Heritability estimates of
TABLE 2. Mean body weight and abdominal fat weight of8-week-old males and females of the high-fat (HF) and low-fat (LF) lines, and HF/LFratios in the first CSiG,^ generation (Flocks A, B and C) Males Generation
Flock
HF
Females HF/LF 1
LF r,-
S,G, S,G, S,G,
A B C
1835 2159 2046
SiG, S,G, S,G,
A B C
32.7 52.9 44.2
S,G, S,G, S,G,
A B C
1.8 2.5 2.2
J
1797 2098 2046
19.4 39.0 26.4
•
HF
LF
HF/LF
1628 1873
1547 1833
1.05 1.02
1 .. / %
1.02 1.03 1.00
1.68 1.36 1.67
36.7 51.8
22.0 34.1
1.67 1.52
2.3 2.8
1.4 1.9
1.64 1.47
iminal fat (g/100 g body weight) 1.1 1.8 1.3
1.64 1.39 1.70
1 In each flock, the difference between lines in abdominal fat was significant at the .01 level. The differences in body weight were not significant.
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CAHANER ET AL.
fat weight between lines were significant in all flocks. The HF line had between 1.36 and 1.70 times more abdominal fat than the LF line. No significant difference was found in body weight between the lines. The mean body weight of the HF line was, at the most, 1.05 times larger than that of the LF line (Table 2). Mean body weight of the selected parents of the two lines were also similar. The mean abdominal fat weights of the parents of the HF and LF lines of each of the three flocks of SiG x are given in Table 3. The differences between these means represent the S due to the divergent selection. The selection intensity of the females, which was lower than that of males because a larger number of dams than sires was needed for reproduction, resulted in smaller S values. The R were the differences between the means of abdominal fat weights of the HF arid LF lines (Table 2). Because S and R values were obtained from flocks raised in different years, locations, and environmental conditions, each was standardized by dividing it by the corresponding standard deviation. The ratios between the standardized R and S values
represent the realized heritabilities, which ranged from .56 to .86 with a mean of .73 (Table 3). Positive phenotypic correlations between body and abdominal fat weights were found in most of the flocks under study, but their magnitude could have been affected by selection. After one cycle of selection, the r values in the HF line, in two consecutive generations, remained similar to those of So, while in the LF line they tended to be lower in both sexes (Table 4). The mean body and abdominal fat weights of S1G2 are given in Table 5. While S1G1 males and females of the LF line had lower body weight than those of their HF counterparts (Table 2), in SiG 2 these differences were almost abolished. On the other hand, the difference between the two lines in abdominal fat weight, absolute or relative to body weight, was not decreased. Although no selection for or against abdominal fat was practiced between the two generations, the HF/LF ratios were about 1.67 in S ^ (Table 2) and 1.80 in SiG 2 (Table 5).
TABLE 3. Mean abdominal fat weight of birds selected from the base population (S0 ) as parents of the high-fat (HF) and low-fat (LF) lines, the difference between these means (S = selection differential, weighted by the number of offspring), the difference between HF and LF means1 (R = genetic response), and realized heritability (R/S) calculated separately for Flocks A, B, and C of the SJGi generation Abdominal fat weight of selected parents HF
LF
Sx lines difference
Standardized 2
S
R
R
S
R/S
19.8 13.7 16.8
13.3 14.7 14.0
1.2
1.4
.86
24.5 15.5 20.0
13.9 17.7 15.9
.9
1.6
.56
24.5 14.5 19.5
17.8 17.8
1.2
1.6
.75
18.7
15.6
1.1
1.5
.73
(g) S ^ , , Flock A Males Females Mean
50.7 49.5
S,G„, Flock B Males Females Mean
56.4 50.5
S,G,, Flock C Males Females Mean
58.4 53.6
S,G, means Males + females 1 2
30.5 35.2
31.3 35.4
33.3 38.6
Means of abdominal fat weight of HF and LF lines are given in Table 2.
S and R values were divided by the corresponding standard deviations of S0 (parents), and of Flocks A, B a n d C o f S , G , (offspring).
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DIVERGENT SELECTION FOR ABDOMINAL FAT IN BROILERS
TABLE 4. Phenotypic correlations between body weight and abdominal fat in 8^week-old males and females of the base population (S0), the first selection generation (SlG1) Flocks A and C, and the subsequent one (S1G2) Females
Males
Generation and flock
HF 1
S,G!, A S,G,,C
.47** .37**
.28* .28*
.59*
.34**
S,G 2
.30*
.17
.51*
.39**
LF
HF
LF
.36*
1
.56*
HF = High-fat line, LF = low-fat line.
*P<.05. **P<.01.
Variances and coefficients of variation of body and abdominal fat weights in all three generations are presented in Table 6. Body weight in the LF line was slightly more variable than in the HF line, but the differences in variances between the lines were not significant. The variation in body weight in both lines was similar to that of the base population (Table 6). Variances in abdominal fat weight of the LF line were significantly smaller than those of the HF line. The variability of abdominal fat weight in the HF line was similar to that of the base population, but it was reduced in the LF line. However, relative to line averages, both selection lines retained a high level of variation in abdominal fat, with coefficients of variation (CV) values around 30% (HF) and 40% (LF) in the SiG 2 generation (Table 6). The differences in abdominal fat weight between the HF and LF lines were similar in Flocks A and B, although birds in Flock B had between 12 and 20 g more abdominal fat than their full sibs in Flock A (Table 7). The inter-
action term in the two-way ANOVA of these data was not significant, indicating that there was no interaction between the HF-LF lines and the different locations and systems of rearing. Flock C males were about 250 g heavier and deposited about 15 g more abdominal fat than Flock A males (Table 2), but the difference between the HF and LF lines in the two hatches was quite similar (Table 7) and the absence of line X hatch interaction was indicated by two-way ANOVA. The ratios HF/LF for abdominal fat weights (Tables 2 and 5) or variances (Table 6) were very similar for males and females in all flocks.
DISCUSSION
The estimates of heritability of abdominal fat weight were very high (Table 1), but as they were based on ANOVA of full-sib families (Cahaner and Nitsan, 1985), they might contain nonadditive genetic variance or maternal effects and hence lead to an exaggerated prediction of
TABLE 5. Mean body weight and abdominal fat weight of8-week-old males and females of the high-fat (HF) and low-fat (LF) lines, and HF/LF ratios in the second selection (SlGi) generation Males
Body weight, g Abdominal fat, g Abdominal fat, g/100 g body wt
Females
HF
LF
HF/LF 1
HF
LF
HF/LF
1887 33.9 1.8
1865 18.9 1.0
1.01 1.79 1.80
1540 36.4 2.3
1538 20.1 1.3
1.00 1.81 1.77
' T h e differences between lines in abdominal fat were significant (P<.01). The differences in body weight were not significant.
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CAHANER ET AL.
TABLE 6. Variances and coefficients of variation (CV) of body and abdominal fat weights of8-v>eek-old male (M) and females (F) of the base population (S0), the selection generation fS, G , j Flocks A and C, and the subsequent one (S1G2) CV
Variances Generation
Flock
Sex
HF
LF
F ratio
HF
LF • (%)
M F
So
•
7 8
23409 21025
S,G,
A A
M F
34596 38416
36100 39204
1.04 1.02
10 12
11 13
S,G,
C
M
21458
24655
1.15
7
8
16499 15178
20667 19160
1.25 1.26
7 8
8 9
S,G,
M F
s.
M F
144 157
30 30
S,G,
A A
M F
146 194
60 112
2.43* 1.73*
37 38
40 48
S,G,
C
M
213
101
2.11*
35
38
M F
118 137
52 81
2.27* 1.69*
32 32
38 45
S,G2
1
HF = High-fat line, LF = low-fat line.
*The differences between variances were significant (P<.05) by two-tailed F-test.
genetic response. Although a substantial response to divergent selection for high or low abdominal fat was reported by Leclercq et al. (1980), it was not compared with what could be expected from the heritability and the selection differentials. In the present study, after one cycle of divergent selection for or against abdominal fat, combined with selection for body weight, the HF line had about 60% more abdominal fat than the LF line (Table 2). This response was obtained in three flocks of SjGi and it agrees closely with results reported by Leclercq et al. (1980). The realized heritability of abdominal fat weight, calculated from actual selection differentials and responses in three replicates, was .73 (Table 3). This value substantiates the estimates obtained by ANOVA of full-sib families, hence, it supports the additive nature of the genetic variance of abdominal fat and the reliability of prediction of response to selection against it. Because selection was based on sibs, the R/S values in Table 3 represent realized heritability of half-sib family means. The expression for the heritability of sib (h^) means is:
h4=h2nr/[l+(n-l)t] where:
h n r t
(Falconer, 1981)
= heritability of individual values. = number of sibs measured. = coefficient of relationship. = intraclass correlation between family members.
In the present study, n=16 (8 males and 8 females were measured from each sire family in the base population), r=.25 for half sibs, and t=.2 (intraclass correlation of abdominal fat weight calculated from sire families). The substitution of these values in the expression of sib heritability reveals that, in this case, it is equal to individual heritability, as: (16 X .25)/(l+15 X .2)=1. Body weight and abdominal fat weight were positively correlated in the base population. It was expected, therefore, that changes in abdominal fat would be associated with changes in body weight. However, after one cycle of selection, the HF and LF lines had almost the
DIVERGENT SELECTION FOR ABDOMINAL FAT IN BROILERS
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TABLE 7. The differences between mean abdominal fat weight of the high-fat (HF) and low-fat (LF) lines in different locations ('Akko and Rehovot) and hatches (Flocks A and C)
Generation
Flock
Location
HF
Abdominal fat LF
13.3 13.9 17.8
14.7 17.7
S^,1 S,G, S,G,
A B C
'Akko Rehovot 'Akko
32.7 52.9 44.2
(g) Males 19.4 39.0 26.4
S^, S,G!
A B
'Akko Rehovot
36.7 51.8
Females 22.0 34.1
1
HF-LF
S, G, First selection generation.
same body weight although they differed dramatically in their abdominal fat weights. In a previous study, the genetic correlation between body and abdominal fat weights in the base population was found to be only .26 (Cahaner and Nitsan, 1985). Friars et al. (1983) reported a genetic correlation of —.06 between body and abdominal fat weights in a population where the corresponding phenotypic correlation was .38. A lack of genetic correlation might be one explanation for the similar mean body weight of the two selection lines. Another possibility might be the smaller correlations between body and abdominal fat weights found in the LF line, compared with the HF line or the base population (Table 4). The difference in abdominal fat between the HF and LF lines was maintained, or even increased from 67% to 80%, following additional generation of selection within each line for body weight only ( S ^ , Table 5). These results support the assumption that among birds that are leaner and heavier than the population mean, larger body weight is not associated with more abdominal fat. The independence of body and abdominal fat weights was also expressed by the variances of the two traits in the selection lines. The selection against abdominal fat reduced the variability of this trait in the LF line, while variances of body weight were similar to, or larger than, those of the base population or of the HF line, respectively (Table 6). It can be concluded that selection against abdominal fat will not hamper further selection for body weight.
Females deposit more abdominal fat than males (relative to body weight), and they were reported to respond differently to selection based on this trait (Leclercq, 1983). However, in the present selection experiment, similar responses were obtained from both sexes. Furthermore, sire families within lines ranked almost identically when measured by male or by female offspring; the calculated Spearman rank correlations were >.95 in all the flocks. Environmental conditions are also known to increase or decrease fat deposition. Interactions between genotypes, sex, and system of rearing (floor vs. cages) were reported to affect abdominal fat (Deaton et al, 1974; Friars et al, 1979; Merkley et al, 1973). However, the genetic difference in abdominal fat, which was achieved between the HF and LF lines in the present study, was found to be consistent in different levels of fattening and in both sexes. The lack of genotype-by-environmental interaction in abdominal fat, as well as the other results reported here, are advantageous for practical broiler breeding.
ACKNOWLEDGMENTS We thank R. Hermann, S. Garlitzer, S. Litmann, and V. Boimann of the Akko Farm for their excellent technical assistance. REFERENCES Becker, W. A., J. V. Spencer, L. W. Mirosh, and J. A. Verstrate, 1982. Selection of broilers for large carcass weight and low abdominal fat. Poultry
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Sci. 61:1415. (Abstr.) Becker, W. A., J. V. Spencer, L. W. Mirosh, and J. A. Verstrate, 1984. Genetic variation of abdominal fat, body weight, and carcass weight in a female broiler line. Poultry Sci. 6 3 : 6 0 7 - 6 1 1 . Cahaner, A., and Z. Nitsan, 1985. Evaluation of simultaneous selection for live body weight and against abdominal fat in broilers. Poultry Sci. 64:1257-1263. Deaton, J. W., L. F. Kubena, T. C. Chen, and F. N. Reece, 1974. Factors influencing the quantity of abdominal fat in broilers. 2. Cage versus floor rearing. Poultry Sci. 53:574. Falconer, D. S., 1981. Page 215 in Introduction to Quantitative Genetics. 2nd ed. Longman, London. Friars, G. W., C. Y. Lin, and D. L. Patterson, 1979. Strain-cross and sex interactions with systems of rearing broiler chickens. Genetics 91(Suppl.):35.
Friars, G. W., C. Y. Lin, D. L. Patterson, and L. N. Irwin, 1983. Genetic and phenotypic parameters of fat deposition and associated traits in broilers. Poultry Sci. 62:1425. (Abstr.) Leclercq, B., 1983. The influence of dietary protein content on the performance of genetically lean or fat growing chickens. Br. Poult. Sci. 24:581—587. Leclercq, B., J. C. Blum, and J. P. Boyer, 1980. Selecting broilers for low or high abdominal fat: initial observations. Br. Poult. Sci. 2 1 : 1 0 7 - 1 1 3 . Merkley, J. W., L. H. Littlefield, and G. W. Chalpouka, 1973. Abdominal fat, skin and subcutaneous fat from six broiler strains raised on the floor and in cages. Poultry Sci. 52:2064. (Abstr.) Siegel, P. B., 1984. Factors influencing excessive fat deposition in meat poultry. Pages 51—52 in 1. Genetics. Proc. XVII-th World's Poult. Congr., Helsinki.
using selection index o r i n d e p e n d e n t culling levels, o n e can r e d u c e a b d o m i n a l fat and simultaneously increase b o d y weight. A fat line and a lean line were established b y divergent selection for high or low a b d o m i n a l fat (Leclercq et al, 1 9 8 0 ; Leclercq, 1 9 8 3 ) . After seven generations of selection, t h e fat-line had a b o u t four t i m e s m o r e a b d o m i n a l fat t h a n t h e leanline, while b o d y weight w a s almost equal in t h e t w o lines. T h e p u r p o s e s of t h e present s t u d y w e r e t o c o m p a r e t h e p r e d i c t e d vs. t h e actual response of divergent selection o n a b d o m i n a l fat, t o d e t e r m i n e w h e t h e r this selection affects males and females differently, a n d t o ascertain w h e t h e r genetic response is expressed similarily a t different locations, housing, and seasons. MATERIALS AND METHODS
1
Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 1263-E, 1984 series.
A commercial female g r a n d p a r e n t line (White R o c k t y p e ) was used as a base population for t h e present s t u d y . A fully pedigreed
1813
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CAHANER ET AL.
flock was studied in two consecutive generations. Experimental procedures and several genetic and phenotypic parameters of the base population have been reported by Cahaner and Nitsan (1985). Sib selection was practiced in the second generation of the base population (S 0 ). There were 800 birds in the flock, about 45 in each of the 18 sire families which constituted the population; they were weighed at 7 weeks of age. At 9 weeks of age, eight males and eight females were taken at random from each sire family. They were slaughtered 10 hr after feed removal and their live body and abdominal fat weights were recorded. Abdominal fat was removed from the cloaca to the gizzard, without the fat adhering to the gizzard. The data from the slaughtered birds were used to rank the sire families according to the amount of abdominal fat. The heaviest two males and six females from each of the six families that had the- highest average of abdominal fat were used as parents of the first generation of the highfat (HF) line. Similarly, the heaviest two males and six females from each of the six families that had the lowest average of abdominal fat were used as parents of the first generation of the low-fat (LF) line. The mean body weight of the birds selected as parents of the two lines, for each OA the Si HOCKS, uiu not uiner signiiicantly. Within each line, three random females were assigned to each of the 12 males and matings were performed by artificial insemination; half-sib and full-sib matings were avoided. In June, 1981, about 300 chicks were hatched from each of the two lines of the first selected generation, designated S t G j . Four hundred and eighty chicks, representing all families in the two lines, were raised at the Akko Farm (Flock A). A total of 120 chicks, the offspring of six sires and 24 dams from each line, were grown at the Rehovot Farm (Flock B). The birds on the two farms were exposed to different environmental conditions. On the Akko farm, the chicks were brooded in conventional floor pens where they were raised until 8 weeks of age. On the Rehovot farm, the chicks were brooded in batteries; at 4 weeks of age they were transferred to individual cages in the open shade where they were kept until 8 weeks of age. The birds in the floor pen (Flock A) suffered more than the birds in the cages (Flock B) from the hot and humid weather,
which is typical for that growing season. A standard broiler feeding regime was applied to the chicks on both farms. Birds on the two farms were slaughtered at 8 weeks of age after overnight feed removal. Live body and abdominal fat weights were recorded for about 300 random birds of Flock A and 100 random birds in Flock B. Four months later, in October 1981, another hatch of SiGi chicks was obtained. Some sires and dams that had been used in the first hatch had died and some were not fertile; they were replaced by reserves kept from the selected families. Altogether, 7 sires and 21 dams from each line were mated and about 170 chicks were hatched from each line. All the chicks were raised on the 'Akko Farm (Flock C) in the same rearing system as described above but under better climatic conditions. The birds were weighed at 7 weeks of age, after food was removed for overnight. All the females, about 75 per line, were kept to reproduce the lines. The 25 heaviest males within each line were kept for reproduction while the rest of the males were slaughtered at 8 weeks of age and their body and abdominal fat weights were recorded. The offspring of the above SiG! males and females (SiG 2 ) were obtained without additional selection for or against abdominal fat. All the chicks, about 400 per line, were grown on the Akko Farm under normal conditions. At 8 weeks of age, five males and five females from each sire family were taken at random, and their live body and abdominal fat weights were determined as in all the earlier flocks. Because divergent selection was practiced, without a control line, the response to selection (R) was calculated from the difference between the means of the HF and LF lines in S i G ^ Similarly, selection differentials (S) were calculated from the difference between the mean abdominal fat weight of the sibs of the selected sires and dams of the two lines. Values for S were calculated separately from the actual sires and dams of each of the three Si Gx flocks and each was weighted by the number of offspring. Realized heritability was calculated from the R/S ratio for each flock in Si G : . The potential existence of an interaction between genotypes (HF and LF lines) and farms or hatches was tested by two-way analysis of variances of mean abdominal fat weight of the families represented in the two flocks.
DIVERGENT SELECTION FOR ABDOMINAL FAT IN BROILERS
1815
TABLE 1. Means, coefficients of variation (CV), and heritability (b*) of body weight and abdominal fat weight of males (M) and females (F) of the base population (S0 generation)
Trait
Weeks of age
Body weight, g
SE (h2)
Sex
Mean
CV
7
M F
1767 1492
9 9
.47 .48
.20 .20
Body weight, g
9
M F
2203 1817
7 8
.43 .52
.19 .21
Abdominal fat, g
9
M F
30 30
.73 .86
.26 .29
Abdominal fat, %
9
M F
30 28
.77 .90
.27 .29
(%)
1
40.0 41.8 1.91 2.30
SE = Standard error.
RESULTS
abdominal fat weight were much larger than those of body weight. Variation and heritability of percent abdominal fat (g/100 g body weight) were very similar to those of the absolute weight. Similar estimates were obtained for males and for females in all traits. The results of one cycle of divergent selection for and against abdominal fat are summarized in Table 2. Means of abdominal fat and body weight of the HF and LF lines were calculated separately for the three flocks of the SiGi generation. The differences in abdominal
Means, coefficients of variation, and estimates of heritability by full-sib analysis of variance (ANOVA) (Cahaner and Nitsan, 1985) were calculated from the So for body weight and abdominal fat weight and are presented in Table 1. The body weight means and the amount of abdominal fat were similar to those of a commercial "pure" line at the time. At 9 weeks of age, the coefficients of variation of abdominal fat were four times greater than those of body weight. Heritability estimates of
TABLE 2. Mean body weight and abdominal fat weight of8-week-old males and females of the high-fat (HF) and low-fat (LF) lines, and HF/LFratios in the first CSiG,^ generation (Flocks A, B and C) Males Generation
Flock
HF
Females HF/LF 1
LF r,-
S,G, S,G, S,G,
A B C
1835 2159 2046
SiG, S,G, S,G,
A B C
32.7 52.9 44.2
S,G, S,G, S,G,
A B C
1.8 2.5 2.2
J
1797 2098 2046
19.4 39.0 26.4
•
HF
LF
HF/LF
1628 1873
1547 1833
1.05 1.02
1 .. / %
1.02 1.03 1.00
1.68 1.36 1.67
36.7 51.8
22.0 34.1
1.67 1.52
2.3 2.8
1.4 1.9
1.64 1.47
iminal fat (g/100 g body weight) 1.1 1.8 1.3
1.64 1.39 1.70
1 In each flock, the difference between lines in abdominal fat was significant at the .01 level. The differences in body weight were not significant.
1816
CAHANER ET AL.
fat weight between lines were significant in all flocks. The HF line had between 1.36 and 1.70 times more abdominal fat than the LF line. No significant difference was found in body weight between the lines. The mean body weight of the HF line was, at the most, 1.05 times larger than that of the LF line (Table 2). Mean body weight of the selected parents of the two lines were also similar. The mean abdominal fat weights of the parents of the HF and LF lines of each of the three flocks of SiG x are given in Table 3. The differences between these means represent the S due to the divergent selection. The selection intensity of the females, which was lower than that of males because a larger number of dams than sires was needed for reproduction, resulted in smaller S values. The R were the differences between the means of abdominal fat weights of the HF arid LF lines (Table 2). Because S and R values were obtained from flocks raised in different years, locations, and environmental conditions, each was standardized by dividing it by the corresponding standard deviation. The ratios between the standardized R and S values
represent the realized heritabilities, which ranged from .56 to .86 with a mean of .73 (Table 3). Positive phenotypic correlations between body and abdominal fat weights were found in most of the flocks under study, but their magnitude could have been affected by selection. After one cycle of selection, the r values in the HF line, in two consecutive generations, remained similar to those of So, while in the LF line they tended to be lower in both sexes (Table 4). The mean body and abdominal fat weights of S1G2 are given in Table 5. While S1G1 males and females of the LF line had lower body weight than those of their HF counterparts (Table 2), in SiG 2 these differences were almost abolished. On the other hand, the difference between the two lines in abdominal fat weight, absolute or relative to body weight, was not decreased. Although no selection for or against abdominal fat was practiced between the two generations, the HF/LF ratios were about 1.67 in S ^ (Table 2) and 1.80 in SiG 2 (Table 5).
TABLE 3. Mean abdominal fat weight of birds selected from the base population (S0 ) as parents of the high-fat (HF) and low-fat (LF) lines, the difference between these means (S = selection differential, weighted by the number of offspring), the difference between HF and LF means1 (R = genetic response), and realized heritability (R/S) calculated separately for Flocks A, B, and C of the SJGi generation Abdominal fat weight of selected parents HF
LF
Sx lines difference
Standardized 2
S
R
R
S
R/S
19.8 13.7 16.8
13.3 14.7 14.0
1.2
1.4
.86
24.5 15.5 20.0
13.9 17.7 15.9
.9
1.6
.56
24.5 14.5 19.5
17.8 17.8
1.2
1.6
.75
18.7
15.6
1.1
1.5
.73
(g) S ^ , , Flock A Males Females Mean
50.7 49.5
S,G„, Flock B Males Females Mean
56.4 50.5
S,G,, Flock C Males Females Mean
58.4 53.6
S,G, means Males + females 1 2
30.5 35.2
31.3 35.4
33.3 38.6
Means of abdominal fat weight of HF and LF lines are given in Table 2.
S and R values were divided by the corresponding standard deviations of S0 (parents), and of Flocks A, B a n d C o f S , G , (offspring).
1817
DIVERGENT SELECTION FOR ABDOMINAL FAT IN BROILERS
TABLE 4. Phenotypic correlations between body weight and abdominal fat in 8^week-old males and females of the base population (S0), the first selection generation (SlG1) Flocks A and C, and the subsequent one (S1G2) Females
Males
Generation and flock
HF 1
S,G!, A S,G,,C
.47** .37**
.28* .28*
.59*
.34**
S,G 2
.30*
.17
.51*
.39**
LF
HF
LF
.36*
1
.56*
HF = High-fat line, LF = low-fat line.
*P<.05. **P<.01.
Variances and coefficients of variation of body and abdominal fat weights in all three generations are presented in Table 6. Body weight in the LF line was slightly more variable than in the HF line, but the differences in variances between the lines were not significant. The variation in body weight in both lines was similar to that of the base population (Table 6). Variances in abdominal fat weight of the LF line were significantly smaller than those of the HF line. The variability of abdominal fat weight in the HF line was similar to that of the base population, but it was reduced in the LF line. However, relative to line averages, both selection lines retained a high level of variation in abdominal fat, with coefficients of variation (CV) values around 30% (HF) and 40% (LF) in the SiG 2 generation (Table 6). The differences in abdominal fat weight between the HF and LF lines were similar in Flocks A and B, although birds in Flock B had between 12 and 20 g more abdominal fat than their full sibs in Flock A (Table 7). The inter-
action term in the two-way ANOVA of these data was not significant, indicating that there was no interaction between the HF-LF lines and the different locations and systems of rearing. Flock C males were about 250 g heavier and deposited about 15 g more abdominal fat than Flock A males (Table 2), but the difference between the HF and LF lines in the two hatches was quite similar (Table 7) and the absence of line X hatch interaction was indicated by two-way ANOVA. The ratios HF/LF for abdominal fat weights (Tables 2 and 5) or variances (Table 6) were very similar for males and females in all flocks.
DISCUSSION
The estimates of heritability of abdominal fat weight were very high (Table 1), but as they were based on ANOVA of full-sib families (Cahaner and Nitsan, 1985), they might contain nonadditive genetic variance or maternal effects and hence lead to an exaggerated prediction of
TABLE 5. Mean body weight and abdominal fat weight of8-week-old males and females of the high-fat (HF) and low-fat (LF) lines, and HF/LF ratios in the second selection (SlGi) generation Males
Body weight, g Abdominal fat, g Abdominal fat, g/100 g body wt
Females
HF
LF
HF/LF 1
HF
LF
HF/LF
1887 33.9 1.8
1865 18.9 1.0
1.01 1.79 1.80
1540 36.4 2.3
1538 20.1 1.3
1.00 1.81 1.77
' T h e differences between lines in abdominal fat were significant (P<.01). The differences in body weight were not significant.
1818
CAHANER ET AL.
TABLE 6. Variances and coefficients of variation (CV) of body and abdominal fat weights of8-v>eek-old male (M) and females (F) of the base population (S0), the selection generation fS, G , j Flocks A and C, and the subsequent one (S1G2) CV
Variances Generation
Flock
Sex
HF
LF
F ratio
HF
LF • (%)
M F
So
•
7 8
23409 21025
S,G,
A A
M F
34596 38416
36100 39204
1.04 1.02
10 12
11 13
S,G,
C
M
21458
24655
1.15
7
8
16499 15178
20667 19160
1.25 1.26
7 8
8 9
S,G,
M F
s.
M F
144 157
30 30
S,G,
A A
M F
146 194
60 112
2.43* 1.73*
37 38
40 48
S,G,
C
M
213
101
2.11*
35
38
M F
118 137
52 81
2.27* 1.69*
32 32
38 45
S,G2
1
HF = High-fat line, LF = low-fat line.
*The differences between variances were significant (P<.05) by two-tailed F-test.
genetic response. Although a substantial response to divergent selection for high or low abdominal fat was reported by Leclercq et al. (1980), it was not compared with what could be expected from the heritability and the selection differentials. In the present study, after one cycle of divergent selection for or against abdominal fat, combined with selection for body weight, the HF line had about 60% more abdominal fat than the LF line (Table 2). This response was obtained in three flocks of SjGi and it agrees closely with results reported by Leclercq et al. (1980). The realized heritability of abdominal fat weight, calculated from actual selection differentials and responses in three replicates, was .73 (Table 3). This value substantiates the estimates obtained by ANOVA of full-sib families, hence, it supports the additive nature of the genetic variance of abdominal fat and the reliability of prediction of response to selection against it. Because selection was based on sibs, the R/S values in Table 3 represent realized heritability of half-sib family means. The expression for the heritability of sib (h^) means is:
h4=h2nr/[l+(n-l)t] where:
h n r t
(Falconer, 1981)
= heritability of individual values. = number of sibs measured. = coefficient of relationship. = intraclass correlation between family members.
In the present study, n=16 (8 males and 8 females were measured from each sire family in the base population), r=.25 for half sibs, and t=.2 (intraclass correlation of abdominal fat weight calculated from sire families). The substitution of these values in the expression of sib heritability reveals that, in this case, it is equal to individual heritability, as: (16 X .25)/(l+15 X .2)=1. Body weight and abdominal fat weight were positively correlated in the base population. It was expected, therefore, that changes in abdominal fat would be associated with changes in body weight. However, after one cycle of selection, the HF and LF lines had almost the
DIVERGENT SELECTION FOR ABDOMINAL FAT IN BROILERS
1819
TABLE 7. The differences between mean abdominal fat weight of the high-fat (HF) and low-fat (LF) lines in different locations ('Akko and Rehovot) and hatches (Flocks A and C)
Generation
Flock
Location
HF
Abdominal fat LF
13.3 13.9 17.8
14.7 17.7
S^,1 S,G, S,G,
A B C
'Akko Rehovot 'Akko
32.7 52.9 44.2
(g) Males 19.4 39.0 26.4
S^, S,G!
A B
'Akko Rehovot
36.7 51.8
Females 22.0 34.1
1
HF-LF
S, G, First selection generation.
same body weight although they differed dramatically in their abdominal fat weights. In a previous study, the genetic correlation between body and abdominal fat weights in the base population was found to be only .26 (Cahaner and Nitsan, 1985). Friars et al. (1983) reported a genetic correlation of —.06 between body and abdominal fat weights in a population where the corresponding phenotypic correlation was .38. A lack of genetic correlation might be one explanation for the similar mean body weight of the two selection lines. Another possibility might be the smaller correlations between body and abdominal fat weights found in the LF line, compared with the HF line or the base population (Table 4). The difference in abdominal fat between the HF and LF lines was maintained, or even increased from 67% to 80%, following additional generation of selection within each line for body weight only ( S ^ , Table 5). These results support the assumption that among birds that are leaner and heavier than the population mean, larger body weight is not associated with more abdominal fat. The independence of body and abdominal fat weights was also expressed by the variances of the two traits in the selection lines. The selection against abdominal fat reduced the variability of this trait in the LF line, while variances of body weight were similar to, or larger than, those of the base population or of the HF line, respectively (Table 6). It can be concluded that selection against abdominal fat will not hamper further selection for body weight.
Females deposit more abdominal fat than males (relative to body weight), and they were reported to respond differently to selection based on this trait (Leclercq, 1983). However, in the present selection experiment, similar responses were obtained from both sexes. Furthermore, sire families within lines ranked almost identically when measured by male or by female offspring; the calculated Spearman rank correlations were >.95 in all the flocks. Environmental conditions are also known to increase or decrease fat deposition. Interactions between genotypes, sex, and system of rearing (floor vs. cages) were reported to affect abdominal fat (Deaton et al, 1974; Friars et al, 1979; Merkley et al, 1973). However, the genetic difference in abdominal fat, which was achieved between the HF and LF lines in the present study, was found to be consistent in different levels of fattening and in both sexes. The lack of genotype-by-environmental interaction in abdominal fat, as well as the other results reported here, are advantageous for practical broiler breeding.
ACKNOWLEDGMENTS We thank R. Hermann, S. Garlitzer, S. Litmann, and V. Boimann of the Akko Farm for their excellent technical assistance. REFERENCES Becker, W. A., J. V. Spencer, L. W. Mirosh, and J. A. Verstrate, 1982. Selection of broilers for large carcass weight and low abdominal fat. Poultry
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Sci. 61:1415. (Abstr.) Becker, W. A., J. V. Spencer, L. W. Mirosh, and J. A. Verstrate, 1984. Genetic variation of abdominal fat, body weight, and carcass weight in a female broiler line. Poultry Sci. 6 3 : 6 0 7 - 6 1 1 . Cahaner, A., and Z. Nitsan, 1985. Evaluation of simultaneous selection for live body weight and against abdominal fat in broilers. Poultry Sci. 64:1257-1263. Deaton, J. W., L. F. Kubena, T. C. Chen, and F. N. Reece, 1974. Factors influencing the quantity of abdominal fat in broilers. 2. Cage versus floor rearing. Poultry Sci. 53:574. Falconer, D. S., 1981. Page 215 in Introduction to Quantitative Genetics. 2nd ed. Longman, London. Friars, G. W., C. Y. Lin, and D. L. Patterson, 1979. Strain-cross and sex interactions with systems of rearing broiler chickens. Genetics 91(Suppl.):35.
Friars, G. W., C. Y. Lin, D. L. Patterson, and L. N. Irwin, 1983. Genetic and phenotypic parameters of fat deposition and associated traits in broilers. Poultry Sci. 62:1425. (Abstr.) Leclercq, B., 1983. The influence of dietary protein content on the performance of genetically lean or fat growing chickens. Br. Poult. Sci. 24:581—587. Leclercq, B., J. C. Blum, and J. P. Boyer, 1980. Selecting broilers for low or high abdominal fat: initial observations. Br. Poult. Sci. 2 1 : 1 0 7 - 1 1 3 . Merkley, J. W., L. H. Littlefield, and G. W. Chalpouka, 1973. Abdominal fat, skin and subcutaneous fat from six broiler strains raised on the floor and in cages. Poultry Sci. 52:2064. (Abstr.) Siegel, P. B., 1984. Factors influencing excessive fat deposition in meat poultry. Pages 51—52 in 1. Genetics. Proc. XVII-th World's Poult. Congr., Helsinki.