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Research Article
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Estimation of Genetic Parameters and Environmental Factors on Early Growth Traits for Lori Breed Sheep Using Single Trait Animal Model |
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A. Lavvaf
and
A. Noshary
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ABSTRACT
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The effects of different environmental factors and estimation of genetic parameters on early growth traits for Lori breed sheep including birth weight, weaning weight and body weight at 6 months of age using 19960 records from 35 herds of Lorestan Jahad Agriculture Organization were studied in the cities of Aleshtar, Khorramabad and Poldokhtar from 1995 to 2003. The effect of herd, sex of lambs, dam age and birth year on all traits and birth type had significant effect only on weaning weight. Different single trait animal models estimated the components of direct additive genetic variance, maternal genetic variance and maternal permanent environment variance through restricted maximum likelihood using environmental factors as a fixe effect and different random effects. The results showed that direct additive genetic effect had additionally significant effect on all traits moreover maternal additive genetic and maternal permanent environment effects. Results also revealed that the maternal permanent environment variance for all traits is higher than maternal genetic variance. Also the direct heritability for all traits was higher than maternal heritability. Estimation of the direct heritability from the birth to 6 months of age showed a reducing trend that could arise from high dependence of birth and weaning weight on maternal environment conditions as compared with the age conditions afterward. The genetic assessment of growth traits in Lori breed sheep without inclusion of maternal effect in animal model causes decreased selection accuracy and incorrect genetic assessment of the lambs.
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INTRODUCTION
Lori is a fat-tailed and white wool sheep, which accounts
for one of the heavy weight breeds in Iran as the dominating breed of
Lorestan province. It has a population more than 4.8 million animal units
and is considered to have superiority over other breeds in terms of resistance
against disease and tolerance of hard environmental conditions. Breeding
of this sheep highly depends on the pastures and its environmental conditions
(Tavakkolian, 2000).
Nowadays, body weight is one of the major selection traits
in sheep population (Szwaczkowski et al., 2006). Also studies have
shown that in hard environmental conditions profitability of sheep husbandry
industry depends on the total weight of the lambs born by each ewe in
each year of pregnancy when they stop feeding on milk (Olivier et al.,
2000). The importance of this characteristic is due to different features
of reproduction (such as ovulation number, fecundity, longevity of the
lamb, number of lambs born and weaned), maternal potentials, lactation
of ewes and growth capacity of the lamb (Vatankhah and Moradi, 2002; Ligda
et al., 2000).
In order to have a successful breeding plans and relative
increase in economic value of traits, we should know responsible environmental
and genetic factors and their impact on the function of each trait. In
consequence, maternal effects are defined as any influence from dam to
progeny, excluding the effects of directly transmitted genes (Nejad Sajjadi
et al., 2007; Szwaczkowski et al., 2006). Due to the important
of maternal effects on the growth of lambs many of researchers such as
Nejad Sajjadi et al. (2007), Nasholm (2004), Hassen et al.
(2003), Van Vleck et al. (2003) and Vatankhah and Moradi (2002)
shown that the important of including it in various genetic assay.
Biological mechanisms for explaining of maternal effects include cytoplasm
inheritance, nutritional condition during pregnancy and neonatal stages,
antibodies, pathogenic factors transfer from dam to progeny and maternal
behavior (Vatankhah and Moradi, 2002). In fact the mother other than transmitted
half of total genes but also reveal part of its genes as maternal effects
in the phenotype of lamb (Nejad Sajjadi et al., 2007)
The objective of this research was to study the effect
of different environmental factors on early growth traits including birth
weight, weaning weight, body weight at 6 months of age and estimation
of the direct genetic parameters as well as indirect (maternal) genetic
effects on this traits and their importance in selection programs of Lori
breed sheep. MATERIALS AND METHODS The
effect of environmental factors and estimation of genetic parameters for
early growth traits including birth weight, weaning weight and body weight
at 6 months of age were studied by using 19960 records collected from
35 herds from 1995 to 2003 by Lorestan Jahad Agriculture Organization.
The environmental factors including dam age, sex of lambs, birth type,
birth year and geographical region were studied by using a proper statistical
method.
The statistical analysis of data for study of environmental
effects of understudied traits was done by general linear model method
and the comparison of least squares means using SPSS software and following
statistical model.
Where: |
Yijklmn |
= |
Observation of each trait |
μ |
= |
Mean of trait |
b i |
= |
Ith lamb sex of fixed effect (male and female) |
Cj |
= |
Jth birth type of fixed effect (single or
twin) |
Hk |
= |
The random animal effect |
Psl |
= |
Lth birth year of fixed effect (1995 to 2003) |
Rm |
= |
Mth geographical region (Aleshtar, Khorramabad,
Poldokhtar) of fixed effect |
e ijklmn |
= |
Residual random effect |
Study of the random effects including direct additive
genetic effect, maternal additive genetic effect and maternal permanent
environmental effect as well as variance components estimation for the
understudied traits were done using DFREML software (Meyer, 1997) and
derivative free algorithm restricted maximum likelihood method. In order
to study genetic and maternal environment effects on understudied trait,
analysis of data was done by fitting six different animal models as follows:
Y |
= |
Xb + Z1a + e |
Meyer 1 |
Y |
= |
Xb + Z1a + Z3Pe + e |
Meyer 2 |
Y |
= |
Xb + Z1a + Z2m + e (cova,m =
0) |
Meyer 3 |
Y |
= |
Xb + Z1a + Z2m + e (cova,m #0) |
Meyer 4 |
Y |
= |
Xb + Z1a + Z2m + Z3Pe + e (cova,m
=0) |
Meyer 7 |
Y |
= |
Xb + Z1a + Z2m + Z3Pe + e (cova,m
#0) |
Meyer 8 |
Where: |
Y |
= |
Observation vector |
b |
= |
Fixed effect vector |
a |
= |
Random direct additive genetic effect vector |
m |
= |
Random maternal additive genetic effect vector |
Pe |
= |
Random maternal permanent environment effect
vector |
e |
= |
Residual random effect vector |
X, Z1, Z2,
Z3 |
= |
Incidence matrices relating records to b,
a, m and Pe vectors, respectively |
In order to determine best model for processing of data
under study, the likelihood ratio test method and comparison of significant
increase in logL in Meyer 1 to Meyer8 models were used and if -2 logL
higher than chi-square distribution with one degree of freedom its use
as a criteria for maintaining an effect in the model. RESULTS
AND DISCUSSION
Estimation of environmental factors: The least square means and
standard errors for birth weight, weaning weight and body weight at 6
months of age by geographical region, lamb sex, dam age, birth type and
birth year as well as criterion of variation (CV) for each trait have
been shown in Table 1.
Results show that the effect of geographical region on
all traits was significant. The birth weight of the lambs of Aleshtar
was the highest and that of the Khorramabad was the lowest. The lambs
of Aleshtar had the least weaning weight, however their weights at the
end of 6 months of age were higher than other regions which reveals that
environmental difference in a small region of area could have a significant
effect on economic traits, which arises from the inferior maternal environmental
effect in Aleshtar region. However, the lambs of this region due to the
compensatory growth at the end of 6 months of age, have shown a higher
weaning weight than other regions. Khezdozi et al. (2007) shown
that a difference between two type of field and alpine Kordi breed sheep
for birth weight, weaning weight, body
Table 1: |
Least square means and standard errors
for early growth traits |
 |
ns: non significant, **: p<0.01,***:
p<0.001 |
weight at the 12 months of age and milk yield of ewes
because of distinctive environmental condition in two region.
Results of Table 1 shows that the effect of lamb sex,
dam age and birth year on all traits are significant that correspond with
many of study so Khezdozi et al. (2007), Dashti et al.
(2007), Tibbo (2006), Al-Saigh and Al-Khouzai (1991), Buvanendran
et al. (1992) and Naikar et al. (1990) have proven that the
weight of male lambs at time of birth, weaning and older ages is significantly
higher than the female lambs, which is compatible with the results of
this study and may be attributed to the physiological differences of two
sex.
Kulkarni and Deshpande (1991), Al-Saigh and Al-Khouzai
(1991), Buvanendran et al. (1992) and Oltoff and Boylan (1991)
have reported the birth and weaning weight of lambs born by the 2 years
old ewes to be significantly less than the lambs born by ewes of other
age groups (3 to 7 years old) that correspond to result of this study
can be caused by higher body condition in older ewes.
Makarechian et al. (1977) have point out in their
studies that the lambs born by the 4 and 6 years old ewes would have more
birth and weaning weight as compared with the lambs born by the 2 years
old ewes which is due to the sufficient feeding. At the same time there
is decreasing effect of dam age on growing traits after weaning time due
to less dependence of lambs to dams at the older ages which conforms the
results of this study.
Results of Table 1 show that the effect of the birth
type is significant only for weaning weight. The number of lambs per ewe
in parturition has an impact on birth weight and growth of lambs before
the end of suckling period and the rate of growth of singles at this stage
is quicker than twin lambs (Khezdozi et al., 2007; Al-Saigh and
Al-Khouzai, 1991; Dimsoski et al., 1999; Bourfia and Touchberry,
1993) that opposite with results of this study.
Generally, with the increase in litter size and due to
the competition between the embryos to receive nutrients from dam, birth
weight of lambs decreases, however the lack of significant difference
between the birth weight of the single and twin lambs in this study may
arise from the high capacity of this breed and provision of suitable maternal
environmental effect at the time of pregnancy.
As increase in litter size and due to the competition
for the satisfaction of needs through milk consumption, weaning weight
in twin lambs decreases as compared with singles according to Shelton
et al. (1991) and Vaez Torshizi et al. (1992) and conform the results
of this study.
The results show that there is no significant difference
in the body weight at 6 months of age between single and twin lambs which
may be attributed to the independence of lambs on the maternal environmental
effects in older ages and compensatory growth of the twin lambs.
As shown by Tibbo (2006) and Yazdi et al. (1998)
the effect of birth year on early growth traits is significant which correspond
the results of this study and may be caused by difference in raining,
humidity and temperature which effect the quantity and quality of the
pastures and this is predictable in breeding of Lori sheep because of
it dependency to the condition of pastures.
Estimation of genetic factors: Variance components and genetic
parameters of studied traits in this research have been estimated with
six different animal models and by using the likelihood ratio test, the
model which showed a significantly increase in the likelihood logarithm
was used as the most suitable model for the estimation of the variance
components. Elements of this model include direct additive genetic variance,
maternal additive genetic variance and maternal permanent environment
variance, which were derived by using the Meyer 7 model (Table
2).
The birth weight is the first growth index and it`s phenotypic
expression in progeny is influenced by the ability of the dam to provide
a suitable environment in the form of better nourishment. Thus the dam
contributes to the performance of the progeny in two ways: Firstly, through
her direct genetic effects passed to the progeny and secondly, through
her ability to provide a suitable environment. The ability of dam is partly
genetic and partly environmental (Mrode, 1996). Thus, the genotype of
lamb and maternal ability has the highest effect on the birth weight (Bourdon,
1997). The birth weight has a high correlation to other weight related
traits, so that the lambs had heavier birth weight would have a higher
weight in the subsequent stages of growth as compared to the lambs with
lower birth weight.
Upon awareness of the genetic and environmental components, which affect
a trait, suitable decisions can be made in breeding management and methods.
Results of the Table 2 show that the direct additive
genetic variance for birth weight is relatively high, thus confirming
the high potential of this breed in response to the selection for this
traits and whereas Lori breed is considered as a heavy weight breed, the
high potential of this breed for high direct additive genetic effect for
birth weight is predictable.
Despite the relatively high direct additive genetic variance,
the maternal additive genetic variance and maternal permanent environmental
effect of the birth
Table 2: |
Estimation of
variance components and genetic parameters for early growth traits |
 |
σ2a=
Direct additive genetic variance, σ2m
= Maternal genetic variance, σ2Pe=
Maternal permanent environmental variance, σ2e
= Residual variance, σ2P = Phenotype
variance, h2a = σ2a
/ σ2P, h2m =
σ2m / σ2P,
h2Pe = σ2Pe /
σ2P |
weight is significant and the portion of the maternal
additive genetic variance and maternal permanent environmental variance
in the total phenotype variance for the birth weight is relatively higher
than the weaning weight and body weight at 6 months of age, which shows
that ignoring these effects in genetic assessment of the lambs will cause
biases and inaccuracy of estimation.
The results reported on the effect of maternal factors
on the growth traits are much varied. The present study shows that both
additive genetic and maternal permanent environment effect had a considerable
part of the phenotype variance of the understudied traits. Safari
et al. (2005) by using the six different animal models on Turkish
merino sheep have reported that in addition to the direct additive genetic
effect, both of maternal additive genetic and maternal environment effect
have a significant effect on the birth and weaning weights. The effect
of these factors decreased for the weight related traits after weaning
time, although it has been significant effect. Moreover, the significant
effects of maternal additive genetic variance have been reported in the
many studies such as Vaez Torshizi et al. (1996) and Yazdi et
al. (1998).
On the other hand, selection for increased birth weight
ignoring of the potential of the ewe would be accompanied by the hard
delivery. However the number and structure of date to be used may influence
the results of the estimation and suitable model (Hanford et al.,
2003). The genetic parameters and the heritability is different by herd
condition, number of records, the number of fixed or random effects as
well as the type of the software (Sadeghi and Yazdanshenace, 2007). However,
estimation of genetic parameters in this research was in the range of
other study.
The weaning weight is one of the most important economic
trait in sheep, which is influenced by the birth weight and the growth
rate of the lamb before the end of suckling period. Results of the Table
2 show that the portion of the maternal genetic variance and maternal
permanent environment variance in the phenotypic variance (h2Pe
and h2m) for weaning weight decreases as compared
to the birth weight and this portion decreases once again in body weight
at 6 months of age due to the decreased dependence of the lambs on the
environment conditions of the ewes in the after weaning times. Nejad Sajjadi
et al. (2007) has pointed to the reduction of the maternal effects
on growth traits of the Kermari lambs with the increase of age. Sadeghi
and Yazdanshenace (2007) shown that the decreasing trend of direct heritability
from 6 to 12 months of age in Sanjabi breed sheep that similar to results
of this study. Wilson and Reale (2006) show that mean standardized variance
components decreased with age, consistent with compensatory growth. Phenotypic
convergence among adult sheep occurs through decreasing environmental
and maternal genetic variation. Maternal genetic effects are thus reduced
with age.
The results show that there is a significant trend in
heritability that result from declining maternal and environmental components
rather than from changing additive variation and correspond with study
of Wilson and Reale (2006).
CONCLUSIONS
Studies have shown that ignoring of maternal effects
in animal model for estimation of variance components will cause decrease
in direct genetic variance components and add in maternal genetic components
(Elfadili et al., 2000), therefore, if in the traits that influenced
by the maternal ability, the only random animal effect in the model was
mentioned, the variance component resulting from other random effects
would be insert in the direct additive genetic variance component and
will cause up estimate of direct heritability (Nejad Sajjadi et al.,
2007). The likelihood ratio test shows that insert of maternal effect
in the model for estimation of the variance components is better than
condition that only random animal effect inserts in the model. Therefore,
maternal effects are considered as a major source of variety in the growth
traits. In the present study the maternal genetic heritability of birth
weight, weaning weight and body weight at 6 months of age are as much
as 0.375, 0.312 and 0.252 times of direct genetic heritability, respectively,
which reveal the reduction of the role of maternal effects for growth
traits with grow up animals and the highest rate is related to the birth
weight which shows the very high maternal effects on the birth weight.
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