(1)| Livestock Research for Rural Development 23 (1) 2011 | Notes to Authors | LRRD Newsletter | Citation of this paper |
This study investigated the sources of variation and the genetic profile of the spring fertility of 18 months old Barbarine maiden ewes. The trait was coded as 1 or 0 for maiden ewes that did, or did not, lamb in fall, respectively. Data were 3314 maiden ewe records daughters of 408 sires taken over 12 consecutive years and 9 flocks kept at the pilot sheep farm of Jebibina and Saouaf in semi arid Tunisia. Full pedigree was also available for most of the sires and dams of the maiden ewes. Average spring fertility of Barbarine maiden ewes in the studied environment is 0.85±0.35 with significant effects of the year, flock and the average daily growth between 10 and 30 days (ADG 10-30). These sources of variation were included in the linear animal model to estimate genetic parameters.
Heritability estimate of spring fertility was 0.05±0.02 while genetic correlation between spring fertility and ADG 10-30 was -0.29. There was a very close and positive (P < 0.001) relationship between the sires’ EBV for the studied trait and the average fertility of their daughters. The 20 % top ranked sires had a mean 0.97 spring fertility of their daughters in comparison to only 0.75 for the 20 % bottom ranked sires. The results obtained in this study show that direct selection on spring fertility of maiden ewes would generate negligible genetic progress. They also highlight the importance of considering the sires’ EBV’s in the development of a breeding plan for the Barbarine sheep breed.
Keywords: Barbarine Maiden ewes, estimated breeding value, heritability, reproduction
The main sheep mating system in Tunisia is the one where the ewes are mated in spring outside the breeding season and therefore lamb in autumn. For those ewe lambs that are selected as replacement stock, they attain puberty when they are about 10 to 12 months of age corresponding to late summer-early autumn and then elapse into anoestrus before they are traditionally mated for the first time at 18 months together with adult ewes when photoperiod is not favourable (Khaldi 1984). This practice is intended to allow maiden ewes time to achieve a sufficient growth rate and target body weight before first mating and relies on a good response of local breeds to the ram effect (Rekik et al 2005). In spite of this delayed start of the reproductive life, conception rate of 4263 maiden ewes aged 18 months, representing 35 commercial flocks and 5 consecutive years, averaged 71%; varying between 38 and 86% according to several management and environmental factors (Ben Salem et al 2005). Live weight and metabolic (as assessed by plasma insulin and leptin levels) changes prior to first mating of maiden ewes of the Barbarine breed were shown to not affect levels of reproductive performance (Ben Salem et al 2009).
Possibly genetic differences can account for individual differences of fertility of maiden ewes when first mated at the age of 18 months. In this respect, heritability of spontaneous cyclic ovarian activity which represents a good indicator of sensitivity to seasonal anoestrous (Rosa and Bryant 2002) was reported to be around 20% with large individual differences (Hanocq et al 1999). Moreover, apparent genetic responses to selection for spring fertility, defined as ability to lamb following exposure to rams in May and June for crossbred Dorset, Rambouillet and Finnsheep ewes was reported by Al-Shorepy and Notter (1997) and this resulted in a shorter duration of anoestrus for ewes with a high fertility EBV (Estimated Breeding Value) in comparison to counterparts with low fertility EBV (Vincent et al 2000).
In the particular case of the
currently studied production system, the ability of the Barbarine maiden ewes to
conceive at 18 months of age can be regarded as the aptitude
to conceive at a young age and the aptitude to breed out-of-season. The
objective of this study was to estimate environmental and genetic trends for
spring fertility of maiden Barbarine ewes raised under marginal semi arid
conditions.
This experiment used data generated between 1993 and 2004 at the Office de l’Elevage et des Pâturages farm at Jebibina and Saouaf (Central Tunisia) at 35° north latitude. The region is semi-arid with an average annual rainfall of 390 mm. The area experiences a Mediterranean-type climate with cool winters and hot dry summers. The farm has 9 flocks of the Barbarine breed each comprising between 250 and 280 ewes. The farm is known for the production of improved ram lambs of the Barbarine breed that are selected on their growth performances and are sold to other flocks in the National Sheep Recording Scheme. The flocks are housed outdoors year-round and are grazed on natural and improved rangeland but are supplemented with hay and concentrate particularly during fall and winter seasons. Annually, mating season for all the flocks takes place from the first week of May until mid July by the introduction of rams of the same breed at a ratio of 6 rams: 100 ewes. All the ewes are weighed prior to the start of the mating season and no hormonal treatment is involved. As a general practice for the studied breed, the shepherd assistance is required to lift the tail at the time of copulation-this allowed a full record of oestrous occurrences for each individual female and also paternity (Rekik et al 2005). Replacement ewe lambs are annually selected when they are 6 months old based on their growth performances, pedigree and conformation. From 6 to 18 months of age, they are run in a separate flock and then return to their original flock for first mating at 18 months. Between 30 and 40 maiden ewes are annually introduced in each flock. Ewes, including maiden ewes, are culled as necessary for physical unsoundness or for being barren two consecutive years. The rams’ population comprises individuals that originate from the same farm but also individuals with known pedigree purchased from outside the farm. Prior to mating, rams are randomly assigned to flocks with the restriction that males originating from the farm are never assigned to the original flock where they were born.
Data were extracted from the National Performance Recording Database (Office de l’Elevage et des Pâturages, Tunisian Ministry of Agriculture). A total of 3314 maiden ewe records daughters of 408 sires were used for the analysis of fertility. Full pedigree was also available for most of the sires and dams of the maiden ewes. Trait considered in the present study was fertility at spring mating of maiden ewes aged 18 months. The trait was coded as 1 or 0 for maiden ewes that did, or did not, lamb in fall, respectively. CATMOD procedure of SAS (2005) was used to assess the non genetic effects on fertility of maiden ewes. Fixed effects due to year of mating (12 levels from 1993 to 2004), flock (9 levels), average daily growth between 10 and 30 days (ADG 10-30) and live weight at mating (LWM) of the maiden ewe that were reconsidered as categorical variables with 5 classes on the basis of the mean and standard error after checking for their normality using the UNIVARIATE NORMAL PLOT procedure of SAS (SAS 2005). The five classes were defined by means of 129±19.4, 166±8.5, 199±11.5, 234±8.9 and 271±16.2 g. In preliminary analyses, LWM and year of mating by flock interaction were assessed and found to be generally unimportant (P > 0.05); therefore those terms were assumed to be negligible and only year of mating, flock and ADG 10-30 were fitted to the model for the estimation of genetic parameters.
Genetic parameters were estimated with an animal model from variance components using the Derivative-Free Restricted Maximum Likelihood Method (DF-REML) as described by Neumeir and Groeneveld (1998) applying the VCE 4.2.5 package. The linear mixed model can be represented in matrix notation as:
Y = X b + Za a + Zp p + e
where Y is a N*1 vector of records, b denotes the fixed effects in the model with association matrix X, a is the vector of the direct genetic effects with association matrix Za, p is the vector of permanent environmental effect with association matrix Zp and e denotes the vector of residual.
Estimate of heritability of fertility was produced by the following linear model (1) which allows taking into account the largest quantity of pedigree information on the animals via an animal model (Hanocq et al. 1999). In addition, it was possible using this model to separately include the individual animal effect and all available genetic relationships. The fitted model was:
(1)
where:
Y = spring fertility of the maiden ewe (0, 1);
µ = overall mean;
my = fixed effect of the ith year of mating;
flock = fixed effect of the jth flock of the maiden ewe;
ADG = fixed effect of the kth class of ADG 10-30 of the maiden ewe;
animal = random effect of the lth maiden ewe (Variance = Va);
e = random residual effect (Variance = Ve).
Estimates of heritability of ADG 10-30 and LWM were produced by the following linear model (2):
(2)
Where:
Y = ADG 10-30 or LWM of the maiden ewe;
µ = overall mean;
my*flock = fixed effect of ith modality of interaction between year of mating and flock;
animal = random effect of the jth maiden ewe (Variance = Va);
e = random residual effect (Variance = Ve).
For both models (1 and 2), all random factors were assumed to be normally distributed. Heritability (h2) was calculated from estimated variance components as follows:
h2 = Va / Va + Ve
The best
linear unbiased estimate (BLUE) for the fixed and predictor (BLUP) for the
random effects of models (1) and (2) were computed using the variance components
from the REML analysis. Variance and covariance components for genetic
correlations between fertility and ADG 10-30 were estimated according to
bi-character model 2.
The average spring fertility of Barbarine maiden ewes in the flocks of the Jebibina and Saouaf farm was 0.85±0.35. All the fixed effects included had an important effect (P<0.05) on the studied trait. Between years, large variation of spring fertility of the maiden ewe was recorded. The highest figure (0.93±0.26) was recorded in 1998 and the lowest (0.70±0.46) was obtained in 1993. For 5 consecutive years (1996 to 2000) average spring fertility of the maiden ewes in the studied flocks was above 0.90. With regards to the flock effect, the highest fertility mean (0.91±0.29) is attributed to flock 9, while the lowest figure (0.78±0.41) is recorded in flock 2. Mean figures of ADG 10-30 and LWM were 192±45.2 g and 38.9±5.28 kg respectively. Interaction between year of mating and flock had a very high significant effect (P<0.001) on ADG 10-30 and LWM. The significant (P<0.05) effect of the maiden ewe ADG 10-30 on the spring fertility is shown in Fig. 1. Maiden ewes with higher growth rates between 10 and 30 days of age had, in general, higher fertilities. The largest difference observed was 0.09 between maiden ewes in the 129 and the 234 g classes.
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Figure 1. Best linear unbiased estimates for the effect of average daily growth between 10 and 30 days (ADG 10-30) on spring fertility of maiden ewes (adjusted to the phenotypic mean) |
Inbreeding coefficients among the studied population of maiden ewes were negligible except for 12 individuals for which this coefficient was higher than 25%. These 12 individuals occurred in different flocks and different years.
Heritability estimates were 0.05±0.02, 0.05±0.02 and 0.017±0.02 for respectively spring fertility, ADG 10-30 and LWM of maiden ewes. Phenotypic and genetic correlations between spring fertility and ADG 10-30 of the maiden ewes were -0.05 and -0.29 respectively. The relationship between the sires’ EBV for the studied trait and the average fertility of their daughters was investigated by targeting only the 220 sires that had at least 5 daughters in the database. There was a very close and positive (P<0.001) relationship between the two variables (Fig. 2). In addition, the 20 % top ranked sires on the basis of EBV had a mean 0.97 spring fertility of their daughters in comparison to only 0.75 for the 20 % bottom ranked sires. For intermediate sires, mean fertility of their daughters was 0.9.
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Figure 2. Mean spring fertility of Barbarine maiden ewes according to the EBV of their respective sires |
The average spring fertility of maiden Barbarine ewes in this study is considered satisfactory in comparison to levels of fertility in the production flocks of this breed that was reported to vary between 84 and 98% in 25 flocks over a period of 15 years (Khaldi 1989). It is also higher than the 71% fertility rate for maiden ewes of the same breed in other commercial flocks of the south of Tunisia (Ben Salem et al 2005). A better basic management of the flocks in the studied farm can account for these differences. This study has also revealed important effects of the year and the flock on fertility of the maiden ewes; a finding that is consistent with results of Matos et al (1997) who reported a highly significant effect of the year on fertility of Rambouillet ewes and those of Gaskins et al (2005) who showed that year of mating, flock and their interaction were very important sources of variation of conception rate of young female sheep. When considering the particular arid context of this study, the relationship between spring fertility of the maiden ewes and their growth rate during the first month of their life is interesting with a gain of 9 points in fertility between the slow and the fast growing females. Such relationship was also true in the study of Rekik and Gharbi (1999) working on the early breeding ability of ewe lambs of the Barbarine and Queue Fine de l’Ouest breeds in a similar environment to the one of this study and also to what has been reported by Dickerson and Laster (1975) that an early accelerated growth of the lambs has a positive impact on their subsequent reproductive performance. Our results on the phenotypic relationship between growth rate and fertility are not backed by the negative genetic correlation between the two traits (-0.29) suggesting a strong environmental effect on the expression of this trait. It is however surprising that live weight at mating of maiden ewes had no effect on their spring fertility; a finding that is contradictory to reports by Gaskins et al (2005) who pointed out that for young female sheep, fertility increases when their live weight at mating is higher. Furthermore and even though not measuring conception rate, Avdi et al (2003) demonstrated that live weight had a positive effect on spontaneous out-of-season ovulatory activity of Chios sheep, being more frequent with larger weights.
The calculated heritability estimate of spring fertility is low (0.05) most likely as a result of the strong influence of environment and the extremely low genetic variability of reproductive traits in sheep (Rosati et al 2002). Such a statement has led Shrestha and Heaney (1987) to postulate that selection on fertility of the young female sheep would generate negligible genetic progress and suggest that farmers should not cull yearling females based on their fertility. A similar conclusion was reached by Vatankhah et al (2008) for the Lori-Bakhtiari sheep who stated that because the heritability estimate is quite low, the potential for genetic improvement of conception rate by within flock selection would be difficult even though this trait has great economical importance.
The existence
of a large range between sires in the spring fertility of their daughters is a
similar result to those by Starbuck and Hocking Edwards (2004) for Merino ewe
lambs in Australia and Hanocq et al (1998) for Mérinos d’Arles sheep mated out
of season. The latter have shown that on the basis of their EBV, the top and the
bottom ranked 20% of sires had respectively 50 and 11% of their daughters
spontaneously ovulating out-of-season. Our results on the relationship between
the sires EBV and spring fertility of their daughters indicate that the mode of
inheritance of the trait deserves further investigation in an attempt to improve
designs of selection schemes of this breed.
Spring fertility of Barbarine maiden ewes is likely to be a resultant of the ability of these animals to breed at a young age and also their aptitude to successfully conceive out-of-season in an arid, harsh environment where other constraints (feeding availability, sanitary status) can be a major impediment to improvement of reproductive efficiency.
Valorisation of the relationship between the sires EBV and fertility of their daughters would cater to the needs of the farmers and market demands to make available genetically more efficient sires.
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Received 14 September 2010; Accepted 9 December 2010; Published 5 January 2011
