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BAYESIAN ESTIMATION OF GENETIC PARAMETERS OF
GROWTH TRAITS IN ZANDI SHEEP
Beigi Nassiri Mohammad Taghi*,A. Asefi **,M. Karami***,J. Fayazi ****
*Department of animal science, Ramin Agriculture, and Natural Resources
University,Iran
**Department of Animal science, Ramin Agriculture, and Natural Resources
University,Iran
*** Department of Animal science, Ramin Agriculture, and Natural Resources
University,Iran
****Department of Animal science, Ramin Agriculture, and Natural Resources
University,Iran
Key word: Zandi Sheep, Genetic Parameter, Bayesian.
ABSTRACT
This study was conducted at Ramin agricultural and natural resources university in
Khuzestan of Iran in 2012 to 2013. In order to estimate the (co) variance components and
genetic parameters of growth traits in Zandi sheep, It was used a total of 6188 records of birth
weight (BW), 5170 records of weaning weight(WW), 2994 records of 6 month weight
(6MW), 2283 records of 9 month weight(9 MW) respectively which collected in the Khajir
animal breeding station from Tehran city during 1994-2010. The SAS statistical software was
used to determine the environmental factors that affect these traits and MTGSAM software
was used to determine genetic parameters of growth traits under Bayesian method.
Environmental factors include year of birth, lamb sex, type of birth and age of dam had a
significant difference on all traits. It is entered the age of animal in to the model as covariate
during the weigh. It is estimated the heritability and variance components of each trait with
Bayesian method under the uni-trait animal model. By including or ignoring maternal genetic
effector or common environmental effects due to dam to direct additive genetic effects of
animal, six different model of analysis were fitted into each trait. To find the best model for
each trait, it was considered the minimum residual variance. Mean and standard deviation of
BW, WW,6MW and 9 MW were 4.24±0.72, 21.48±3.79, 30.98±4.7, 32.8±4.53 respectively.
Results showed that for BW, model was included direct additive genetic effects, maternal
additive genetic effects and maternal permanent environmental effects without considering
the covariance between them. For WW, model was included direct additive genetic effects,
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maternal additive genetic effects with considering the covariance between them. For 6 MW,
model was included direct additive genetic effects, maternal additive genetic effects and
maternal permanent environmental effects with considering the covariance between them.
For 9 MW, model was included direct additive genetic effects, maternal additive genetic
effects and maternal permanent environmental effects without considering the covariance
between them. The direct estimated heritability of BW,WW, 6MW with the best model were
0.124, 0.169, 0.258 and 0.163 respectively.
INTRODUCTION
In the world of agriculture, sheep breeding is one of the most important branches of
livestock in terms of the number of animals and the value of the products. Sheep are
important due to having several desirable features such as compromises in different
environmental conditions, low demand for food, and value of sheep products (6). Sheep
products constitute an important component of livestock production in Iran. There are nearly
50 million sheep with more than 20 breeds in Iran (31). The aims of breeding programs are to
maximize the rate of genetic progress for economic traits of sheep. One of the most important
breeds of Iranian sheep is Zandi sheep. Mutton is a traditional source of protein in Iran but
meat production from the sheep does not cover the increasing consumer demand. In this case
Yazdi et al. (34) pointed out that the improvement in efficiency of any sheep production
enterprises can be achieved by enhancing economically important traits such as litter size of
ewe and body weight of lamb. To determine optimal breeding strategies for increase the
efficiency of sheep production, knowledge of genetic parameters for weight traits at various
ages and also the genetic relationships between the traits are needed (5). Numerous studies
have demonstrated that live body weight and growth rate of lambs of different breeds
considerably are affected by maternal as well as the direct genetic effects (34; 21; 1; 5; 23;
19). Most of these studies concluded that ignoring maternal effects in genetic analysis of
these traits, especially for pre-weaning ones, resulted in upward biases in estimation of direct
heritability. Hence, to achieve optimum genetic progress in a selection program both the
direct and maternal components should be taken into account (14; 12). Furthermore, it is
important to try to characterize genetically indigenous breeds. Genes affecting polygenic
traits and characterizing milk or meat productions are difficult to identify. However, several
potential candidate genes have been recognized. They may be selected on the basis of a
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known relationship between physiological or biochemical processes and production traits,
and could be tested as quantitative trait loci (QTLs) (28). The most important trait is body
weight and also there was no information regarding (co)variance components and genetic
parameters for such important traits in Zandi sheep. Thus, the objective of the present study
was to estimate the genetic parameters of body weight traits in Zandi sheep.
MATERIALS AND METHODS
In general, animals were managed following semi-intensively. Natural pasture is the main
source of feed. The quantity and quality of the pasture varies considerably during the year.
With the dry season, the quantity and quality of the pasture decreases and supplemental
feeding comprising dried alfalfa and barley grains has to be provided especially at the time of
the flushing and late pregnancy. A controlled mating strategy was designed during mating
period (Early September to mid-November) and ewes were mated to fertile rams at the rate of
20 ewes per ram. Lambing was in January and March. At birth time and / or within 24 h
afterwards lambs were weighted and ear-tagged. Lambs were kept indoors from mid January
to late April and manually fed afterwards lambs were grazed on pastures of low quality and
productivity. The lambs were weaned about 3 months of age. The female lambs were exposed
to the rams about 18 months of age.
Using pedigree information and body weight records which collected from 1994 to2010 at
Zandi sheep breeding station (khojir station). Studied traits were birth weight (bw,n=6188),
weaning weight (ww, n=5170), 6-month weight (n=2994) and 9-month weight (n=2283).The
maximum of data which were available for analysis included lamb records born from 245
sires and 1919 dams (Table 1). Traits investigated were body weight at birth (BW), weaning
(WW), six months of age (6MW), nine months of age (9MW). All body weights, except BW,
were pre-adjusted for the effect of weighing age assuming a linear growth rate and weighing
ages of 120, 180, 270 and 365 days for WW, 6MW and 9MW respectively. The structure of
Pedigree information given in Table 1 and the records used in the analysis is given in Table 2.
Table1. Pedigree information
Item Number
N. of records 6917
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N. of sires 245
N. of animal with progeny 2164
N. of granddams 959
N. of dams 1919
N. of animalwithout progeny 4753
Table 2. The records used in the analysis
Fixed effect BW WW 6MW 9MW
Birth year
1994 227 206 197 -
1995 187 116 103 -
1996 372 252 191 128
1997 307 259 130 115
1998 309 260 118 116
1999 243 194 185 156
2000 272 205 213 185
2001 361 260 201 191
2002 391 230 244 224
2003 365 273 159 116
2004 369 307 247 159
2005 494 432 293 260
2006 418 371 214 181
2007 451 426 198 175
2008 493 472 301 277
2009 406 406 - -
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2010 523 501 - -
Birth type
Single 4876 4113 2426 1894
Twin 1312 1057 568 389
Sex
Male 3131 2599 1626 1321
Female 3057 2571 1368 962
Ewe age
2 1358 1092 652 483
3 1294 1088 637 493
4 1117 959 517 425
5 1023 880 534 371
6 760 648 363 271
7 433 344 217 169
8 203 159 74 71
BW: birth weight, WW: weaning weight (three-month weight), 6MW: six-month weight and 9MW: nine-month.
The model accounting for environmental (fixed) effects were included year of lambing
(1994-2010), sex of lamb (male and female), type of birth (single and twin) and age of ewe
(2-8 years old).Test of significance for the fixed effects carried out using GLM procedure of
SAS program(27). The interactions between the fixed effects were not significant and
therefore excluded from the model. The SAS statistical software was used to determine the
environmental factors that affect these traits and MTGSAM software was used to determine
genetic parameters of growth traits under Bayesian method. Birth year, lamb’s sex, type of
birth and dam age had a significant effect on all traits(p<0.001). It is entered the age of
animal in to the model as covariate during the weigh. It is estimated the heritability and
variance components of each trait with Bayesian method under the uni-trait animal model. By
including or ignoring maternal genetic effect or common environmental effects due to dam to
direct additive genetic effects of animal, six different model of analysis were fitted into each
trait. To find the best model for each trait, It was considered the minimum residual variance:
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y= Xb + Zaa + e Model 1
y= Xb + Zaa + Zpepe + e Model 2
y= Xb + Zaa + Zmm + e Model 3
Cov(a, m)= 0
y= Xb + Zaa + Zmm + e Model 4
Cov(a, m)= Aσam
y= Xb + Zaa + Zmm + Zpepe + e Model 5
Cov(a, m)= 0
y= Xb + Zaa + Zmm + Zpepe + e Model 6
Cov(a, m)= Aσam
Where:
y: vector of records.
b: vector of fixed effects.
a: vector of direct additive genetic effects.
m: vector of maternal additive genetic effects.
pe: vector of permanent environmental effects due to ewe. X, Za, Zm and Zpe: corresponding
design matrices relating the fixed effects, direct additive genetic effects, maternal additive
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genetic effects and permanent environmental effects due to ewe to vector of y, respectively.
e: vector of residual effects.
Cov(a, m): covariance between direct additive genetic and maternal additive genetic effects.
It is assumed that:
It was assumed that the direct additive genetic effects, maternal additive genetic effects,
permanent environmental effects due to ewe and residual effects to be normally distributed
with mean 0 and variance respectively.
: direct additive genetic variance, maternal additive genetic variance,
permanent environmental variance due to ewe and residual variance, respectively.
A: additive numerator relationship matrix.
Id and In: identity matrices that have order equal to the number of ewes and number of
records, respectively.
σam: covariance between direct additive genetic and maternal additive genetic effects.
Total heritabilities were estimated according to formula of Willham (33):
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In univariate analysis, the log likelihood values were applied to choose the most appropriate
model for each trait (14). Estimation of genetic and phenotypic correlations was
accomplished using multi-trait analysis applying the most appropriate model which was
determined in univariate analysis. The fixed effects included in the multi-trait animal models
were those in single-trait analyses.
RESULTS
Least square means for studied traits are shown in Table 3. The result of variance analysis
showed that the year of birth had significant effects on all studied traits (p<0.01).Sex of lamb
had significant effect on all traits (p<0.01). The significant effect of fixed factors in these
characters could be assigned partly to the differences in the endocrine system of female and
male lambs. Also, age of dam had significant effect on BW, 3MW, 6MW and
9MW(p<0.05).Type of birth had a significant effect on weight changes in all traits (p<0.01).
Single born lambs had higher body weights and pre-weaning growth rate than twins and
triplets.
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Table 3. Lease square means and standard errors for the studied traits
Fixed effect BW WW 6MW 9MW
Birth year
1994 3.94J±0.72 21.19f±3.79 34.46a±4.7 -
1995 4.27de±0.72 24.17a±3.79 31.12cd±4.7 -
1996 4.01jl±0.72 19.46g±3.79 32.12b±4.7 31.80c±4.53
1997 4.35cd±0.72 17.85h±3.79 29.52fe±4.7 28.99f±4.53
1998 4.16gfh±0.72 17.17l±3.79 29.23ef±4.7 30.16ed±4.53
1999 4.08glh±0.72 21.87e±3.79 32.0cd±4.7 30.72d±4.53
2000 3.70k±0.72 22.53cbd±3.79 30.84d±4.7 32.99b±4.53
2001 4.1glh±0.72 21.14f±3.79 30.38ed±4.7 29.40ef±4.53
2002 4.04hl±0.72 22.23ced±3.79 29.68fe±4.7 34.14a±4.53
2003 4.05hl±0.72 19.16g±3.79 27.57g±4.7 32.49bc±4.53
2004 4.16gf±0.72 21.08f±3.79 30.99d±4.7 34.17a±4.53
2005 4.22fe±0.72 23.03b±3.79 31.29cbd±4.7 34.33a±4.53
2006 4.32cde±0.72 22.59cbd±3.79 32.07b±4.7 34.43a±4.53
2007 4.29de±0.72 22.38cdel±3.79 31.09cd±4.7 34.0a±4.53
2008 4.47b±0.72 21.92e±3.79 30.84d±4.7 34.43a±4.53
2009 4.67a±0.72 22.75cb±34.79 - -
2010 4.39bc±0.72 22.14de±3.79 - -
Birth type
Single 4.39a±0.72 22.01a±3.79 31.46a±4.7 33.09a±4.53
Twin 3.68b±0.72 19.47b±3.79 28.94b±4.7 31.45b±4.53
Sex
Male 4.10b±0.72 20.01b±3.79 29.55b±4.7 31.45b±4.53
Female 4.37a±0.72 22.28a±3.79 32.68a±4.7 34.68a±4.53
Ewe age
2 4.10c±0.72 21.08c±3.79 30.47bc±4.7 32.63ab±4.53
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3 4.26b±0.72 21.63ab±3.79 30.99abc±4.7 32.94a±4.53
4 4.39a±0.72 22.0a±3.79 31.16ab±4.7 33.04a±4.53
5 4.30b±0.72 21.58b±3.79 31.38a±4.7 32.88a±4.53
6 4.32ab±0.72 21.51b±3.79 31.44a±4.7 33.19a±4.53
7 4.02b±0.72 20.85c±3.79 30.54bc±4.7 32.0b±4.53
8 4.01b±0.72 20.70c±3.79 30.29c±4.7 31.91b±4.53
The means within the same column with at least one common letter, do not have significant difference(P>0.01).
BW: birth weight, WW: weaning weight (three-month weight), 6MW: six-month weight, 9MW: nine-month
weight.
Table 4. (Co)variance components, genetic parameters estimates for the studied traits with different
models
Traits Models σ2
a σ2
m σ2
am σ2
e σ2
p h2*
a c2* h2*
m
BW 1 0.098 - - 0.278 0.376 0.261 - -
BW 2 0.0529 - - 0.269 0.370 0.142 0.127 -
BW 3 0.044 0.065 - 0.273 0.372 0.119 - 0.176
BW 4 0.045 0.066 -0.012 0.273 0.372 0.121 - 0.178
BW 5 0.046 0.044 - 0.268 0.370 0.124 0.054 0.121
BW 6 0.042 0.045 -0.010 0.273 0.371 0.115 0.053 0.121
WW 1 1.88 - - 7.438 9.318 0.201 - -
WW 2 1.603 - - 7.332 9.278 0.172 0.036 -
WW 3 1.52 0.570 - 7.371 9.314 0.163 - 0.061
WW 4 1.578 0.665 -0.246 7.327 9.325 0.169 - 0.071
WW 5 1.458 0.542 - 7.397 9.300 0.156 0.003 0.058
WW 6 1.497 0.605 -0.194 7.368 9.306 0.160 0.003 0.065
6MW 1 4.420 - - 12.618 17.039 0.258 - -
6MW 2 3.823 - - 12.193 16.924 0.225 0.05 -
6MW 3 4.442 1.863 - 11.909 17.0338 0.260 - 0.109
6MW 4 4.470 1.885 -1.211 11.893 17.038 0.261 - 0.110
6MW 5 4.382 1.591 - 11.731 17.036 0.256 0.027 0.093
6MW 6 4.408 1.577 -1.168 11.679 17.042 0.258 0.031 0.092
9MW 1 2.631 - - 10.913 13.544 0.193 - -
9MW 2 2.345 - - 10.541 13.515 0.173 0.046 -
9MW 3 2.188 1.414 - 10.493 13.620 0.160 - 0.103
9MW 4 2.187 1.441 -0.487 10.484 10.626 0.160 - 0.105
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9MW 5 2.23 1.109 - 10.360 13.608 0.163 0.025 0.081
9MW 6 2.210 1.168 -0.471 10.428 13.628 0.161 0.021 0.085
σ2
a: direct genetic variance, σ2
m: maternal additive genetic variance σ2
e: residual variance, σ2
p: phenotypic
variance, σ2
am: covariance between direct genetic and maternal additive genetic, h2
a: direct heritability, h2
m:
maternal heritability, c2: ratio of maternal permanent environmental effect to phenotypic variance.
BW: birth weight, WW: weaning weight (three-month weight), 6MW: six-month weight and 9MW: nine-month
weight. The most appropriate model for each trait is shown in bold face.
DISCUSSION
Estimates of phenotypic variance using different models were generally similar for all
considered traits. Residual variance was also similar in models 1 to 6. The most appropriate
models for BW and 3MW were Model 5 and 4 respectively. The most appropriate models for
6MW and 9MW were Model 6and 5 respectively. Maternal permanent environmental effects
had a considerable impact on variation for BW, 3MW, 6MW and 9MW. In general, the
values observed in this study are in agreement with the estimates reported by the other
researchers (38; 16).The significant influence of lambing year can be described by the
variation in the climate conditions and dependence of sheep to pastures, management and
breeding conditions of mothers and lambs feeding in various years. Significant effects of year
on reproductive traits have been reported by several authors (32; 16; 4; 18).According to the
previous reports, the growth rate of female lambs was slower than in male lambs, and thus
their weight was less, respectively (17). Also, competition for milk consumption can be
effective between twins and triplets particularly in pre-weaning period, which was consistent
with other reports (22). Including of birth age as a correlated variable into the statistical
model (covariate) had a significant effect on all traits (p<0.01).The estimate of direct
heritability for BW in the current study (0.12) is lower than in the report of Mohammadi et al.
(17) (0.15). Lower heritability of birth weight compared to the other weights is related to the
following reasons. Fetal growth is influenced by genetic and environmental factors such as
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the placenta and the fetal nutrition by a dam. Therefore, environmental factors affecting dam
growth, especially the quality and quantity of food and the storage of food for dam can
influence the growth of the embryo. In the present research the estimate of direct heritability
for 3MW (0.16) corresponds to the data of Jafaroghli et al. (11). Higher estimate (17; 0.19)
have also been reported. The reason for low heritability is that the lambs are more affected by
breast milk during infancy. The estimate of direct heritability for 6MW in this study (0.25) is
higher than the estimate by Mohammadi et al. (17; 0.21) and is lower than by Ghafouri-Kesbi
et al. (10). Also, the estimate of direct heritability for 9MW in this study (0.16) is
approximately compatible with previous results in the Shal breed by Mohammadi et al. (17;
0.18). As it is explicit, direct heritability has had upward trend, which has been proved by
different researchers. Also, maternal heritability for 6MW was estimated to be 0.07 (2),
whilst in our study this parameter was estimated to be 0.092. The estimate of maternal
heritability for 9MW in the present study (0.085) is higher than the estimate published by
(10) -0.05. In addition, c2 for 6MW was estimated to be 0.03,that was lower than the results
reported by others researches (17; 0.06). The rate of c2 for 9MW was estimated to be 0.02,
which is in accordance with results of others researches (10; 0.02) (Table 4). The results
indicate that maternal additive genetic effects, which regard to the growth of fetus, could
have some beneficial effect on the post-natal growth traits too. In the other words, body
weight from birth to 6MW of age is partly influenced by similar genes of the dam in terms of
maternal genetic effects.
Several reports have been published on the contribution and importance of the maternal
genetic variance, permanent environmental variance and direct-maternal genetic covariance
in improving the fit of models for growth performance in sheep (30; 12; 1; 5; 23 ; 15). Based
on the genetic parameters estimate fitting different models for body weight traits, direct
heritability estimates with best models for body weight of lambs were relatively low to
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medium ranging from 0.12 for BW to 0.25 for 9MW. The direct additive heritability estimate
(0.12) of BW in present study is low, but within the range reported by others. The range of
direct heritability estimates for BW varies substantially from 0.04 (23) to 0.46 (9). The results
in the present study were similar to the results reported by Mohammadi et al. (15) for Iranian
Sanjabi lambs. Safari et al. (25), reported estimates of 0.19 and 0.15 for direct heritability of
BW in dual-purpose and meat type breeds of sheep, respectively. These estimates were
higher than our obtained value in the present study. The maternal additive genetic variances
were low. Estimates of maternal heritability with appropriate models for BW and WW were
0.12 and 0.07, respectively. The estimated values for the maternal heritability of BW were
well consistent with some of the published values (12; 5; 23). Safari et al. (25) reported
weighted mean of the maternal heritability estimated for BW of 0.18 in dual-purpose and
0.24 in meat type. Corresponding value for meat type ones was in general agreement with our
estimated value. Birth weight is a trait of economical importance mainly due to its effect on
pre-weaning growth of lambs and accordingly on economic success of lamb production
(3). Estimate of direct heritability for WW (0.16) obtained in the present study was within the
range of those published in the literature, which varied from 0.09 (20) to 0.33 (29). A
decreasing trend in the maternal effects from birth to later ages has shown. Estimate of direct
heritability for 6MW (0.25) obtained in the current study werein the range the estimates of
Bahreini Behzadi et al. (5), Eskandarinasab et al. (7) and Mohammadi et al. (15). The direct
heritability estimate of 9MW (0.16) was in the range of 0.03 (19) to 0.59 (29). The low
estimates of maternal heritability for 6MW and 9MW were expected, because at these ages
individuals do not depend on their mother and their weights should reflect only the direct
effect of the genes on growth except for carry over maternal effects from before weaning. For
animals raised on pasture, the length of time from birth to yearling is probably not enough
that compensatory gain could buffer completely the maternal effect existing at birth. Robison
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(1981) suggested that even if maternal effects tend to diminish with age, some adult traits will
nevertheless contain this source of variation. In general, different estimates of the direct and
maternal heritabilities of body weight traits in various studies can be due to model of
analysis, sheep breed, data structure, different management of herds and different breeding
strategies in sheep. The relatively low heritability estimates for the studied traits can be
perhaps explained by the low nutritional management, low quality of pastures and harsh
climatic conditions, which result in a high environmental variance. Sizeable effects of
maternal influences on BW and WW traits suggest that these effects need to be considered in
selection programs and exclusion of them may lead to biased estimations of direct
heritability. When maternal effects are of high importance, total heritability values are more
efficient than direct heritabilities for estimation of selection response based on phenotypic
values (1).
CONCLUSION
The estimates of genetic parameters reported for the Zandi sheep were in general
agreement with those reported in the literature. Maternal effects were significant sources of
variation for BW and WW traits in Zandi sheep. Therefore, effects of genetic maternal need
to be accounted for estimate the best linear unbiased predicted value (BLUP) of Zandi lambs.
The estimates of direct heritability tended to increase from birth to weaning. These results
indicated that selection for body weight traits on WW will be effective. The estimates of
direct genetic and additive genetic maternal correlations between body weight traits were
positive and high. So selection for any of these traits could result in genetic progress for the
other traits.
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