J. Anim Sci. 2008. 86:804-814. doi:10.2527/jas.2007-0544
© 2008 American Society of Animal Science
Reproductive performance and genetic parameters in first cross ewes from different maternal genotypes1
R. A. Afolayan*,
N. M. Fogarty*,2,
A. R. Gilmour*,
V. M. Ingham*,3,
G. M. Gaunt
and
L. J. Cummins
* The Australian Sheep Industry Cooperative Research Centre, NSW Department of Primary Industries, Orange Agricultural Institute, Orange, New South Wales 2800, Australia; and
and
Department of Primary Industries, Primary Industries Research, Rutherglen, Victoria 3685; and
Hamilton, Victoria 3300, Australia
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Abstract
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The reproduction of 2,846 crossbreed ewes with 7,899 records is reported. The ewes were progeny of mainly Merino dams and 91 sires from several maternal sire breeds including Border Leicester, East Friesian, Finnsheep, Coopworth, Corriedale, Booroola Leicester, and several others. There were 3 cohorts of ewes at each of 3 sites that were bred naturally to meat-type rams for each of 3 yr to evaluate reproduction and lamb production. At 2 sites, the ewes were mated in the autumn, first at 7 mo of age, and at 2 sites the ewes were mated in the spring, first at 14 or 17 mo of age. The cohorts of ewes and sites were genetically linked by 3 common sires. Mixed linear models were used to analyze ultrasound scanned pregnancy rate, fetal number, fertility (ewes lambing), litter size, lamb survival, number of lambs born (NLBj), number of lambs weaned (NLWj), and total weight of lamb weaned (TWWj) per ewe bred. Fixed effects included sire breed (1 to 10), environment (1 to 4, site and season of breeding: autumn, spring), breeding (1 to 3), cohort (1 to 3), and their interactions. The REML procedures were used to estimate (co)variance components. Ewe sire breed effects were significant (P < 0.01) for all the reproductive traits and breed means ranged from 0.75 to 0.96 for fertility, 1.22 to 2.08 for litter size, 0.70 to 0.90 for lamb survival, 0.99 to 1.66 for NLBj, 0.87 to 1.26 for NLWj, and 22.9 to 33.8 kg for TWWj, with the ranking of sire breeds varying for different traits. For all traits except lamb survival, the contrast between breeding 1 vs. 2 and 3 was considerably greater than the contrast between breeding 2 vs. 3, with significant environment x breeding interactions (P < 0.01). Estimates of heritability for the components of reproduction ranged from 0.03 ± 0.02 for lamb survival to 0.19 ± 0.05 for litter size, and those for the composite traits were 0.17 ± 0.04 for NLBj, 0.13 ± 0.04 for NLWj, and 0.17 ± 0.04 for TWWj, with repeatability ranging from 0.10 to 0.19. Genetic and phenotypic correlations among the traits are reported. The significant variation among sire breeds of the crossbred ewes can be used to improve reproduction, although there was a change in the rank of the breeds for the various traits. There was considerable overlap between the breeds, and additional improvement could be achieved by exploiting the genetic variation between sires within breeds for all the ewe reproductive traits.
Key Words: ewe • heritability • genetic correlation • maternal breed • lamb production
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INTRODUCTION
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There is extensive use of crossbreeding in Australian lamb production with over 5 million crossbred ewes mated annually to meat rams, and their progeny compose more than 30% of the national lamb slaughter for meat production. Crossbreeding utilizes diverse breed resources and heterosis. It can exploit genetic differences between breeds for economically important production traits as well as complementarity for growth and carcass, reproduction, and wool traits in different sheep breeds. The terminal crossbreeding systems complement maternal genetics by exploiting genetic and heterosis effects for reproductive traits in crossbred ewes to enhance the efficiency of lamb production. Because all the genes for reproduction and maternal characteristics are expressed in the ewe, exploiting the variation in lambing performance among diverse crossbred ewes could improve lamb productivity. The number of lambs or carcasses sold, carcass weight and fat levels, and wool production all contribute to the productivity of the ewe flock in Australia. Lamb weaning and final turnoff rate are the major profit drivers with lamb growth rate and carcass fat levels also contributing (Fogarty et al., 2006
). Significant breed differences in reproductive traits for many purebred and crossbred ewes have been reported in Australia (Spiker et al., 1987), New Zealand (McMillan and McDonald, 1983; Moore et al., 1983), and the United States (Fogarty et al., 1984; Mohd-Yusuff et al., 1992; Gallivan et al., 1993). The Border Leicester is the major maternal sire breed of crossbred ewes in Australia with several other breeds becoming available to the industry in recent years. There is little comparative information on these breeds for reproductive traits or genetic variation and the potential for improvement.
This study reports on the reproduction of crossbred ewes by sires from a range of maternal breeds and provides estimates of genetic parameters for ewe reproduction and productivity traits.
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MATERIALS AND METHODS
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Animals
The project was conducted under approval of the relevant state Departmental Animal Ethics Committee at each site.
The data were from 2,846 crossbred ewes with 7,899 records that were the progeny of 91 maternal sires mated in the national maternal sire central progeny test (MCPT) that was described in detail by Fogarty et al. (2005a). The sires were mated using laparoscopic AI and thawed-frozen semen at 3 sites (Cowra, Hamilton, and Struan) over 3 yr to Merino ewes (and Corriedale ewes at Hamilton). The 91 sires were entered by breeders from several breeds including Border Leicester (n = 18), East Friesian and crosses (n = 12), Finnsheep and crosses (n = 12), Coopworth (n = 9), White Suffolk (n = 7), Corriedale (n = 6), Booroola Leicester (n = 6), and Hyfer (n = 4).
The other 17 sires were from several breeds and because their ewe progeny varied considerably in reproduction, they were assigned for analysis to Other High or Other Low sire breeds on the basis of mean litter size of their ewe progeny. The sire breeds in the Other High group were: Poll Dorset (n = 2), and 1 sire from each of Cheviot, SAMM, Texel, White Dorper, and Wilt-shire Horn; and in the Other Low group were English Leicester (n = 2), Gromark (n = 2), Merino (n = 2), Romney (n = 2), and 1 sire from each of South Hampshire Down and a Texel-Coopworth composite. The White Suffolk is a composite breed derived from the Suffolk and bred to remove the black face and points. The Booroola Leicester was derived from crossing the Booroola Merino with the Border Leicester and subsequent backcrossing to the Border Leicester and selection to retain the high ovulation rate Fec(B) genotype (Meyer et al., 1994a). The Hyfer is a composite of Dorset and high reproduction strains of Merino (Fogarty et al., 1994). The SAMM is derived from the South African Mutton Merino, and the Gromark is a Border Leicester/Corriedale composite.
The sites and years of birth for the crossbred ewes were the Center for Sheep Meat Development, Cowra, on the central slopes of NSW from 1997 to 1999; the Pastoral and Veterinary Institute, Hamilton, in western Victoria from 1997 to 1999; and the Struan Research Center in southeastern South Australia from 1998 to 2000. To provide genetic links for the evaluation, 3 of the sires, one from each of Border Leicester, Coopworth, and Finnsheep breeds, were used at all sites and in all years, except that only 2 of the link sires were used at Struan in 1998. The crossbred ewes born at Struan were transferred to Rutherglen, Victoria, at approximately 12 mo of age for evaluation of their adult performance. The dams of the crossbred ewes were mature medium wool Merino ewes at Cowra, fine wool Merino ewes and Corriedale ewes at Hamilton, and broad wool Merino ewes at Struan. Further details on the design of the MCPT, the genetic merit of the sires, and the source of the base ewes have been reported by Fogarty et al. (2005a).
Management
The crossbred ewes were bred naturally to groups of meat-type rams at each site, generally within their age cohort, for 3 yr to evaluate their lamb production. The term bred (breeding) is used to define the grazing of ewes and rams together for breeding. The crossbred ewes at Cowra were randomly split after they were weaned into 2 groups that were bred in either the autumn (February to March) or spring (October to November) season. At Hamilton all ewes were bred in autumn (March to April), and at Rutherglen all ewes were bred in late spring (November to December). The autumn-bred ewes at Cowra and Hamilton were first bred at approximately 7 mo of age. The first spring breeding at Cowra was at 14 mo of age, whereas that at Ruther-glen was at 17 mo age. The ewes at Hamilton in 1999 experienced vibriosis prelambing, which resulted in some abortion loss, and these cohorts (1997 and 1998) were bred for an additional year. In 2002, the ewes at Hamilton succumbed to perennial ryegrass toxicosis during breeding, which resulted in low fertility (Cummins et al., 2002). Therefore, the data from Hamilton for 1999 and 2002 breedings were excluded from these analyses (Table 1). There were a total of 7,899 breeding records, with the numbers of ewes bred for each cohort and year shown in Table 1.
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Table 1. Number of maternal sires and first cross ewes from cohorts bred in various years at Cowra, Hamilton, and Rutherglen
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The meat-type rams bred to the crossbred ewes were Poll Dorset at Cowra, composite breed at Hamilton, and White Suffolk at Rutherglen. The ewes were scanned in mid pregnancy by experienced ultrasound operators to assess pregnancy rate and fetal number. The second crossbred lamb progeny were ear tagged within 15 h of birth, and their birth weight, dam identification, sex, birth type (number of lambs in the litter), and date were recorded. Subsequently, at marking the lambs were vaccinated and had their tails docked, and the males were castrated. Lambs were weaned at approximately 12 wk of age and weighed. Rearing type is the number of lambs in the litter at weaning. Growth and carcass performance of the second cross lambs have been reported by Afolayan et al. (2007).
Statistical Analysis
Ewe reproductive traits and overall ewe productivity were analyzed with mixed linear models using ASReml (Gilmour et al., 2006). Records for all crossbred ewes that were bred and alive at lambing were included in the analyses. The ewe reproductive traits analyzed were pregnancy rate (ultrasound scanned pregnant), fetal number (ultrasound scanned number of fetuses for pregnant ewes), fertility (ewes lambing), litter size (number of lambs born for ewes lambing), lamb survival as a trait of the ewe (ratio of lambs weaned to lambs born for lambing ewes), lambs born (NLBj, number of lambs born for all ewes bred), and lambs weaned (NLWj, number of lambs weaned for all ewes bred). The total weight of lamb weaned (TWWj, for all ewes bred) was also analyzed as a measure of the overall lamb productivity of the crossbred ewes. For each ewe in each year, TWWj was the sum of individual lamb weaning weights in the naturally reared litter, adjusted to 100 d of age and male equivalent within site and season. Ewes that did not lamb or lambed and failed to rear any lambs to weaning were given a value of zero for NLBj, NLWj, and TWWj.
Fixed effects included in the model were environment (Cowra autumn breeding, Cowra spring breeding, Hamilton autumn breeding, Rutherglen spring breeding), which encompasses the site and season of breeding effects, breeding (1 to 3), ewe birth year cohort (1 to 3), sire breed (1 to 10), and ewe type of birth and rearing (11, 21, 22). Because the 1999 and 2002 data were excluded at Hamilton, the breedings for the 1997 cohort ewes were classified as 1 for 1998, 2 for 2000, and 3 for 2001, and for the 1998 cohort ewes as 2 for 2000 and 3 for 2001 (Table 1
). For all traits analyzed, the 2- and 3-way interactions were included in the initial model. Interactions that were not significant (P > 0.05) were subsequently removed from the final model. Common significant interactions in the final model for all traits were environment x breeding, environment x sire breed, environment x ewe type of birth and rearing. Sire, ewe, and ewe x breeding were included as random effects for the repeated measures analysis. Under this model, the sire breed fixed effect was tested relative to the between sire variance.
A univariate sire model was used to estimate variance components for all the reproductive traits by REML procedures using ASReml (Gilmour et al., 2006), with similar fixed effects as described above. The ewe x breeding variance component was used to estimate repeatability of the traits. Bivariate analyses were conducted between pairs of reproductive traits to estimate genetic and phenotypic correlations. Estimation of correlations between fetal number, litter size, and lamb survival with fertility, NLBj, NLWj, and TWWj were possible because repeated records for the ewes were available. The data for variance component estimation were restricted to 74 sires of 2,456 ewes and 6,824 records from the 8 breeds with 4 or more sires as the 17 Other sires were from several divergent breeds with generally only 1 or 2 sires per breed, which may have inflated the variances.
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RESULTS
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The means for the reproductive traits of the 2,846 crossbred ewes within each of the cohorts over their 3 breedings in each environment are shown in Table 2. There was considerable variation among the cohort groups at each location for prebreeding weights. This is a reflection of the age at first breeding with the autumn bred ewes being younger than the spring-bred ewes and being only 7 mo of age at their first breeding. The means for the 1998 cohort of ewes at Hamilton were based on breedings 2 and 3 and the 1999 cohort on breedings 1 and 2 rather than breedings 1 to 3 for all the other ewe groups. This affects their means for prebreeding BW and lambing performance. From the 7,899 ewe breedings recorded over 3 yr, the average performance for the reproductive traits across ages and years were fertility 0.83, litter size 1.58, and lamb survival 0.84. This resulted in overall average performance of 1.32 lambs born, 1.07 lambs weaned, and a total weight of lamb weaned of 29.1 kg per ewe bred.
Fixed Effects
The levels of significance and the incremental F-values, with adjustment for all other terms, for the fixed effects and interactions fitted in the models for the various lambing performance traits of the crossbred ewes are presented in Table 3. The variance ratio due to breeding was considerably greater than any other factor with the contrast of breeding 1 vs. 2, 3 greater than 2 vs. 3 for all traits except lamb survival. The variation due to sire breed of the ewes was significant for all traits (P < 0.01), as was the environment x breeding interaction for all traits (P < 0.01), except lamb survival.
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Table 3. Incremental F-values and significance for fixed effects in the models for reproductive traits of crossbred ewes
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The predicted means of lambing performance are reported for sire breed of the crossbred ewes in Table 4. The means for ultrasound scan pregnancy rate were slightly higher than fertility with generally similar differences across the sire breeds. Scanned fetal number was also similar to litter size born. The Corriedale cross-bred ewes had the lowest levels of reproduction for all the traits except lamb survival, which is not adjusted for litter size. Fertility was higher for the Hyfer- and Finnsheepsired ewes than the other crossbred ewes. The Booroola Leicester and Finnsheep cross ewes had higher litter size than the other crossbred ewes and were 70 and 50%, respectively, higher than the Corriedale cross ewes. The Hyfer cross ewes had the highest lamb survival, whereas the Booroola Leicester and Finnsheep cross ewes with high litter size had the lowest lamb survival. These differences in component traits resulted in sire breed being highly significant (P < 0.001) for the composite traits NLBj, NLWj, and TWWj. There was a range in NLBj of 0.7 lambs born per ewe bred between the highest Booroola Leicester and Finnsheep and the lowest Corriedale cross ewes. For NLWj the Hyfer weaned 0.4 more lambs per ewe bred than the Corriedale cross ewes. In the overall measure of reproduction and lamb growth to weaning, TWWj, which accounts for differences in fertility, litter size, lamb survival, and average weaning weight of lambs, the East Friesian weaned 10.9 kg or 48% more weight of lamb than the Corriedale cross ewes.
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Table 4. Predicted means (± SE) for reproductive traits over 3 breedings of crossbred ewes by various sire breeds
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There were significant interactions of sire breed and environment for pregnancy and fertility (P < 0.01), and litter size and TWWj (P < 0.05). The interaction for fertility was mainly due to the East Friesian cross ewes having relatively low fertility from the spring breedings and high fertility from the autumn breedings compared with the other cross ewes. At Cowra the East Friesian ewes had fertility of 0.94 ± 0.06 from autumn and 0.82 ± 0.06 from spring breedings, with similar trends for the Hamilton autumn and Rutherglen spring breedings, which were 0.91 ± 0.05 and 0.80 ± 0.06, respectively. The relatively low fertility of the Corriedale cross ewes from the autumn compared with the spring breedings and a similar trend for the White Suffolk cross ewes also contributed to the interaction. For example, at Cowra the fertility from autumn and spring breedings was 0.67 ± 0.07 and 0.86 ± 0.07, respectively, for the Corriedale cross ewes and 0.79 ± 0.06 and 0.88 ± 0.06, respectively, for the White Suffolk cross ewes. For litter size the interaction was largely due to very low litter size of 1.08 ± 0.10 for Corriedale cross ewes at Hamilton and relatively low litter size for Coopworth ewes in the spring compared with autumn breeding. For TWWj the interaction was mainly due to the high level for the East Friesian cross ewes from the autumn relative to the spring breeding (e.g., at Cowra 40.2 ± 3.8 vs. 33.9 ± 3.8 kg, respectively).
The interaction of environment and breeding was significant for all the reproductive traits (P < 0.01) except for lamb survival, with most variance due to breeding 1 vs. 2, 3 rather than breeding 2 vs. 3 (Table 3). The predicted means for the 3 breedings in each environment are shown in Table 5. Fertility at the first breeding was relatively lower than for later breedings for the autumn compared with spring breeding environments; the autumn ewes were first bred at 7 mo of age compared with 14 to 17 mo for the spring-bred ewes. Similarly, litter size was relatively lower for the first autumn breedings than for the spring breedings. For example at Cowra, litter size increased by 0.47 from the first to the second autumn breeding, whereas the increase was only 0.23 for the similar spring breedings. These effects in the component traits resulted in significant interactions for similar reasons in the composite traits of NLBj, NLWj, and TWWj. In addition there were highly significant interactions for environment and breedings (2 vs. 3) for NLBj (P < 0.01), NLWj (P < 0.001), and TWWj (P < 0.001). These were caused by substantial increases from the second to the third breeding at Hamilton and Rutherglen, whereas there was little increase at Cowra because they were already at a high level at the second breeding. For TWWj the increases from second to third breeding were 11.5 kg at Hamilton and 5.1 kg at Rutherglen, whereas at Cowra they were 2.1 and 0.6 kg for the autumn and spring breedings, respectively.
There were significant differences between the environments for lamb survival (P < 0.001). The lamb survival for the spring bred ewes was 8 to 14% higher than for the autumn-bred ewes, which reflects the more benign climatic conditions usually experienced at lambing (Donnelly, 1984) and the generally lower litter size following spring breeding compared with autumn breeding. There was a significant (P < 0.001) environment x cohort interaction for lamb survival. This was largely due to the low performance of 0.59 ± 0.03 for the 1999 drop cohort of ewes at Hamilton compared with the 1997 and 1998 cohorts, which were 0.82 ± 0.02 and 0.81 ± 0.03, respectively. Although there is confounding between lambing years and genetic effects between the cohorts that have link sires, these differences between cohorts were not expected and were not apparent in the other environment groups.
Ewe prebreeding weight affected all the reproductive traits as well as the total weight of lamb weaned. There were significant positive regressions for prebreeding weight for fertility of 0.003 ± 0.001/kg (P < 0.01), litter size of 0.012 ± 0.001/kg (P < 0.001), NLBj of 0.005 ± 0.001/kg (P < 0.001), NLWj of 0.003 ± 0.001/kg (P < 0.001), and TWWj of 0.27 ± 0.02 kg/kg (P < 0.001). However, the regression for lamb survival of –0.003 ± 0.001/kg (P < 0.001) was significantly negative.
Genetic Parameters
Parameter estimates from the univariate analyses for all the reproductive traits are shown in Table 6. Litter size had a moderate estimate of heritability, which was similar to fetal number, with fertility and particularly lamb survival being lower. The NLBj and TWWj had higher heritability estimates than NLWj. The repeatability estimates were moderate for pregnancy, NLBj, and NLWj and lower for lamb survival. The high phenotypic standard deviations for the various traits indicated a range in the coefficient of variation of 34 to 40% for the component traits and over 60% for NLWj and TWWj.
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Table 6. Estimates of heritability (± SE), repeatability (± SE), and phenotypic standard deviation ( p) for reproductive traits in crossbred ewes
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The estimates of genetic and phenotypic correlations among the reproductive traits are presented in Table 7. Fertility and pregnancy had a very high genetic correlation of 0.99 ± 0.02, which was the same as that for litter size and scanned fetal number. There was also a very high genetic correlation of 0.92 ± 0.04 between NLBj and NLWj, as well as for these traits with each of the component traits of fertility and litter size. As expected, lamb survival had negative genetic correlations of –0.36 ± 0.24 with fetal number, of –0.29 ± 0.26 with litter size, and of –0.19 ± 0.28 with NLBj, although it had a positive correlation of 0.66 ± 0.14 with NLWj. These standard errors were generally high due to the very low genetic variance for lamb survival. The phenotypic correlations between the reproductive traits were generally lower than the corresponding genetic correlations.
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Table 7. Estimates of genetic (below diagonal) and phenotypic (above diagonal) correlations (± SE) among reproductive traits
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The TWWj had high genetic correlations with all the other reproductive traits, which ranged from 0.73 to 0.94 (Table 8). The phenotypic and environmental correlations for TWWj with the reproductive traits were similar and were generally lower than the corresponding genetic correlations.
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Table 8. Estimates of genetic, phenotypic, and environmental correlations (± SE) between TWWj1 and reproductive traits for crossbred ewes
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DISCUSSION
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The MCPT is the largest across-breed evaluation of maternal genetics for crossbreeding available to the Australian sheep industry. The sires that were progeny tested are regarded as largely representative of their breeds. The sires were selected and entered by their owners and had above average breeding values for weight although they were generally about breed average for reproduction (number of lambs weaned) and carcass traits as described in detail by Fogarty et al. (2005a). There were highly significant differences between the sire breeds of the crossbred ewes for all of the reproductive traits analyzed. The generally high performance of the Finnsheep cross ewes, especially for litter size, which reflects high ovulation rate, is consistent with other reports that have evaluated different sheep breeds and crosses (Quirke et al., 1988; Mohd-Yusuff et al., 1992; Casas et al., 2004, 2005). The Booroola Leicester cross ewes also had very high litter size indicating a high frequency of the Booroola FecB gene (Piper et al., 1985), and the results are consistent with other studies involving Booroola cross ewes (Ponzoni et al., 1985; Meyer et al., 1994a,b; Fogarty and Hall, 1995), including having higher ovulation rates than Finnsheep cross ewes (Young and Dickerson, 1991). Although the higher litter size resulted in more lambs born per ewe bred for the crossbred ewes by these 2 sire breeds, their lamb survival was lower, particularly for the Booroola Leicester cross ewes, which meant their advantage over some of the other sire breeds in lambing rate was largely lost by weaning. Booroola Merino cross ewes have been reported to have low embryo survival (Meyer et al., 1994a) and low lamb survival especially among higher litter size categories with lower birth weight lambs (Meyer et al., 1994b; Fogarty and Hall, 1995). The East Friesian cross ewes had the highest TWWj (13% higher than the average and 48% higher than the Corriedale cross ewes), although they were not significantly different from several of the other ewe breed crosses. The high growth rate of their second cross lambs (Afolayan et al., 2007) contributed to this, which is due to a combination of high milk production from the ewes (Morgan et al., 2006) and high genetic merit for growth of the East Friesian sires of these crossbred ewes (Fogarty et al., 2005a).
Total weight of lamb weaned (TWWj) is a measure of the overall productivity of the ewe and all the components contribute (i.e., fertility, litter size, lamb survival, and lamb growth). The results show that the different breed crosses varied in performance for the various components, which meant that the ranking of the breeds changed for NLBj, NLWj, and TWWj. This is illustrated by the improvement in ranking of the East Friesian and White Suffolk cross ewes and the decline in ranking of the Finnsheep when lamb growth is included with net reproduction in TWWj as a measure of overall ewe productivity. However, the differences in TWWj were not significant for several of the high ranking breeds and further consideration of the environment, production system, and other factors (such as availability, price, differential management requirements, and personal preference) is required as well as the genetic variation within breeds that can be exploited, before choosing a particular breed cross.
Significant breed x breeding season interactions have been reported where breeds change rankings for fertility and litter size and some composite traits when bred at different seasons (Mohd-Yusuff et al., 1992; Freking et al., 2000; Casas et al., 2004, 2005). Our results also showed significant sire breed x environment interactions for pregnancy, fertility, litter size, and TWWj, although the interactions were small. The major contributor to the interactions was the relatively lower fertility of the East Friesian cross ewes when bred in the spring compared with the autumn. This is due to a combination of higher lambing performance than the other crosses when they were bred in autumn at 7 mo of age (Fogarty et al., 2007) and lower fertility in the spring or out of season breedings.
There was a very large increase in performance from the first to the subsequent breedings for all traits except lamb survival. Whereas lamb survival may be expected to be lower among 1 yr old than older ewes they had a lower litter size and fewer multiple births resulting in counter balancing higher lamb survival. The increase in performance for all the other traits was greater for the autumn breeding (Cowra and Hamilton) environments where ewes were first bred at 7 mo of age, than for the spring breeding (Cowra and Rutherglen) environments where they were older at first breeding (14 and 19 mo, respectively). There was a 33 to 48% improvement in fertility from first to second breeding for the autumn bred ewes with no change from breeding 2 to 3. For litter size among the same ewes there was an increase of 0.34 to 0.47 from first to second breeding and a further increase of 0.14 to 0.15 to the third breeding. The magnitude of these differences is very close to those reported by Casas et al. (2004) for crossbred ewes of similar ages. For the older spring bred ewes there was a 10% improvement in fertility from first to second breeding with no further increase to the third breeding and an increase in litter size of 0.17 to 0.19 from first to second breeding with a further increase of 0.08 to 0.22 to the third breeding.
There was considerable variance among the sires within each of the breeds such that for most traits there was an overlap of sire progeny groups of crossbred ewes across the breeds (Fogarty et al., 2005b). This led to a high level of sire genetic variance within breeds and resulted in significant estimates of heritability for all traits except lamb survival. The estimates of heritability were somewhat higher (except for lamb survival) than the means of literature estimates reviewed by Fogarty (1995) and Safari et al. (2005), although they are within the range of the estimates from the literature covered in the reviews. More recently estimates of heritability for litter size ranging from 0.04 to 0.13 have been reported in various breeds (Hanford et al., 2003, 2005, 2006; Casellas et al., 2007; Maxa et al., 2007; Safari et al., 2007a; Vanimisetti et al., 2007). Other estimates for NLBj (0.05 to 0.11) and NLWj (0.02 to 0.05) have also recently been reported (Snowder et al., 2004; Safari et al., 2007a). Our estimates of heritability are derived from crossbred ewes, and there is a tendency for some of the higher estimates from the reviews to be also from crossbred or composite breed populations [e.g., 0.19 for litter size in the Hyfer composite (Fogarty et al., 1994)]. The reproductive performance of the crossbred ewes in our data would be expressing maximum heterosis (100% heterozygosity for almost all sire groups), which may have contributed to the increased genetic variance. Average levels of heterosis of 35% for lambs weaned and 39% for weight of lamb weaned and even higher levels for first lambing (47% for lambs weaned) have been reported by Pitchford (1993) with similar levels of heterosis reported by Mohd-Yusuff et al. (1992). Maternal effects could not be accounted for in our model and may also have contributed to the high genetic variance, although maternal effects were zero for reproductive traits in Merino sheep (Safari et al., 2007a), which have a lower level of reproduction than our crossbred ewes. The phenotypic variation in our results was similar to the mean levels for most traits reported in the reviews (Fogarty, 1995; Safari et al., 2005).
The genetic correlations among the various reproductive traits showed a similar pattern to the corresponding mean correlations reported in the review by Safari et al. (2005) and the recent estimates for Merino ewes (Safari et al., 2007b), although they are generally slightly higher in magnitude. Lamb survival as a trait of the ewe is the trait that has some exceptions to this generalization. The means from the review were based on only a few reports in the literature that varied considerably, which meant the means had wide confidence intervals. In particular, the mean genetic correlation between lamb survival, which is called ewe rearing ability in the review, and NLBj was 0.52 with a 95% confidence interval of –0.99 to 1.00 based on 4 widely differing estimates (Safari et al., 2005). Our negative estimate of this genetic correlation (–0.19 ± 0.28) would seem to be more appropriate, despite its high standard error, and it is close to that reported by Rosati et al. (2002) for a composite population (–0.16).
There is a general perception in the sheep industry that because heritabilities of reproductive traits are low and selection is likely to be difficult, only slow response can be achieved. However, response to selection is dependent on a function of heritability, phenotypic variation, and selection intensity. Whereas the heritabilities of reproductive traits are generally lower than other production traits such as growth, carcass and wool production, and quality, there is much greater phenotypic variation to compensate (Safari et al., 2005). A major limitation in direct selection for reproduction is that males need to be mainly selected on the performance of their female relatives and females do not express lambing traits until at least 1 or 2 yr of age. However, repeated records of lambing performance with the level of repeatability found here will approximately double the accuracy of selection or heritability with the use of 3 lambing records, although phenotypic variation may be reduced slightly (Fogarty et al., 1994). There are several examples of selection for reproductive traits achieving over 1% per annum response in lambs weaned (Fogarty, 1994; Ercanbrack and Knight, 1998; Cloete et al., 2004). In addition the screening of large populations of sheep can impose a high selection intensity and has been used to successfully establish high performing flocks (McEwan et al., 1992; Davis et al., 1998). Higher levels of flock reproduction also allow greater selection intensity to be achieved in breeding programs.
There is significant variation among the sire breeds of crossbred ewes, and these breed effects can be utilized to improve reproduction and ewe productivity traits (e.g., litter size). However, breeders need to be cognizant of their environment and production system when assessing breeds for their enterprise. Overall productivity needs to be considered because high litter size may not necessarily result in higher ewe productivity. Other traits may also have a significant impact on the enterprise, such as early puberty (Fogarty et al., 2007), out of season breeding ability, and growth rate of lamb progeny (Afolayan et al., 2007). Having chosen a particular breed cross, it is important that the best animals are selected within the breed to exploit the considerable genetic variation available for ewe reproduction and other traits (Ingham et al., 2007). The genetic merit of breeding animals needs to be evaluated through a combination of traits that can be achieved by a selection index approach using available genetic parameter estimates. This study provides estimates of genetic parameters for reproductive traits among crossbred progeny, which adds to the limited information available in the literature for these traits (Safari et al., 2005) and contributes to the accuracy of the parameter matrix for genetic evaluation (Brown et al., 2006).
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Footnotes
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1 Additional financial support was provided by Meat and Livestock Australia and the Commonwealth of Australia through the Australian Sheep Industry Cooperative Research Centre. We also thank J. Morgan, L. McLeod, K. Lees, T. Markham, M. Arnold, K. Groves, T. Pollard, G. Seymour, T. Phillips, and P. Curran for technical support. 
3 Current address: Agrisearch Services Pty. Ltd., Orange, NSW 2800, Australia.
2 Corresponding author: neal.fogarty{at}dpi.nsw.gov.au
Received for publication August 29, 2007.
Accepted for publication December 10, 2007.
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LITERATURE CITED
|
|---|
Afolayan, R. A., N. M. Fogarty, V. M. Ingham, A. R. Gilmour, G. M. Gaunt, L. J. Cummins, and T. Pollard. 2007. Genetic evaluation of crossbred lamb production. 3. Growth and carcass performance of second-cross lambs. Aust. J. Agric. Res. 58:457–466.[CrossRef]
Brown, D. J., A. J. Ball, A. E. Huisman, A. A. Swan, K. D. Atkins, H. U. Graser, R. G. Banks, P. Swan, and R. R. Woolaston. 2006. Sheep Genetics Australia: A national genetic evaluation system for Australian sheep. Proceedings of the 8th World Congress on Genetics Applied to Livestock Production, Belo Horizonte, Brazil, August, 2006CD-ROM Communication 05–03.
Casas, E., B. A. Freking, and K. A. Leymaster. 2004. Evaluation of Dorset, Finnsheep, Romanov, Texel, and Montadale breeds of sheep: II. Reproduction of F-1 ewes in fall mating seasons. J. Anim. Sci. 82:1280–1289.[Abstract/Free Full Text]
Casas, E., B. A. Freking, and K. A. Leymaster. 2005. Evaluation of Dorset, Finnsheep, Romanov, Texel, and Montadale breeds of sheep: V. Reproduction of F-1 ewes in spring mating seasons. J. Anim. Sci. 83:2743–2751.[Abstract/Free Full Text]
Casellas, J., G. Caja, A. Ferret, and J. Piedrafita. 2007. Analysis of litter size and days to lambing in the Ripollesa ewe. II. Estimation of variance components and response to phenotypic selection on litter size. J. Anim. Sci. 85:625–631.[Abstract/Free Full Text]
Cloete, S. W. P., A. R. Gilmour, J. J. Olivier, and J. B. van Wyk. 2004. Genetic and phenotypic trends and parameters in reproduction, greasy fleece weight and liveweight in Merino lines divergently selected for multiple lamb survival. Aust. J. Exp. Agric. 44:745–754.[CrossRef]
Cummins, L. J., N. M. Fogarty, J. Hocking-Edwards, N. Edwards, J. Stafford, and G. Gaunt. 2002. The economic return from first-cross ewes is dependent on the genetic value of their sire. Wool Technol. Sheep Breed. 50:602–607.
Davis, G. H., C. A. Morris, and K. G. Dodds. 1998. Genetic studies of prolificacy in New Zealand sheep. Anim. Sci. 67:289–297.
Donnelly, J. R. 1984. The productivity of breeding ewes grazing on lucerne or grass and clover pastures on the tablelands of southern Australia. III. Lamb mortality and weaning percentage. Aust. J. Agric. Res. 35:709–721.[CrossRef]
Ercanbrack, S. K., and A. D. Knight. 1998. Responses to various selection protocols for lamb production in Rambouillet, Targhee, Columbia, and Polypay sheep. J. Anim. Sci. 76:1311–1325.[Abstract/Free Full Text]
Fogarty, N. M. 1994. Response to selection for lamb production in an 8-monthly system. Proc. 5th World Congr. Genet. Appl. Livest. Prod., Guelph, Canada 18: 79–82.
Fogarty, N. M. 1995. Genetic parameters for live weight, fat and muscle measurements, wool production and reproduction in sheep: A review. Anim. Breed. Abstr. 63:101–143.
Fogarty, N. M., L. D. Brash, and A. R. Gilmour. 1994. Genetic parameters for reproduction and lamb production and their components and liveweight, fat depth and wool production in Hyfer sheep. Aust. J. Agric. Res. 45:443–457.[CrossRef]
Fogarty, N. M., G. E. Dickerson, and L. D. Young. 1984. Lamb production and its components in pure breeds and composite lines. II. Breed effects and heterosis. J. Anim. Sci. 58:301–311.[Abstract/Free Full Text]
Fogarty, N. M., and D. G. Hall. 1995. Performance of crossbred progeny of Trangie Fertility Merino and Booroola Merino rams and Poll Dorset ewes. 3. Reproduction, liveweight and wool production of adult ewes. Aust. J. Exp. Agric. 35:1083–1091.[CrossRef]
Fogarty, N. M., V. M. Ingham, A. R. Gilmour, R. A. Afolayan, L. J. Cummins, J. E. Hocking Edwards, and G. M. Gaunt. 2007. Genetic evaluation of crossbred lamb production. 5. Age of puberty and lambing performance of yearling crossbred ewes. Aust. J. Agric. Res. 58:928–934.[CrossRef]
Fogarty, N. M., V. M. Ingham, A. R. Gilmour, L. J. Cummins, G. M. Gaunt, J. Stafford, J. E. Hocking Edwards, and R. G. Banks. 2005a. Genetic evaluation of crossbred lamb production. 1. Breed and fixed effects for birth and weaning weight of first-cross lambs, gestation length, and reproduction of base ewes. Aust. J. Agric. Res. 56:443–453.[CrossRef]
Fogarty, N. M., V. M. Ingham, L. McLeod, G. M. Gaunt, and L. J. Cummins. 2006. Genetic resources to increase the profitability of crossbred lamb production. Aust. J. Exp. Agric. 46:799–802.[CrossRef]
Fogarty, N. M., V. M. Ingham, L. McLeod, J. E. Morgan, and G. M. Gaunt. 2005b. Dynamic dams for lamb production: More $$$s from crossbred ewes with the right genetics. Technical Bulletin 50, NSW Department of Primary Industries, Orange, Australia. http://www.dpi.nsw.gov.au/agriculture/livestock/sheep/breed-select/meat/dams Accessed Feb. 1, 2008.
Freking, B. A., K. A. Leymaster, and L. D. Young. 2000. Evaluation of Dorset, Finnsheep, Romanov, Texel, and Montadale breeds of sheep: 1. Effects of ram breed on productivity of ewes of two crossbred populations. J. Anim. Sci. 78:1422–1429.[Abstract/Free Full Text]
Gallivan, C., R. A. Kemp, Y. M. Berger, and L. D. Young. 1993. Comparison of Finnish Landrace and Romanov as Prolific Breeds in a Terminal-Sire Crossbreeding System. J. Anim. Sci. 71:2910–2918.[Abstract]
Gilmour, A. R., B. J. Gogel, B. R. Cullis, and R. Thompson. 2006. ASReml User Guide Release 2.0. VSN International Ltd., Hemel Hempstead, UK.
Hanford, K. J., L. D. Van Vleck, and G. D. Snowder. 2003. Estimates of genetic parameters and genetic change for reproduction, weight, and wool characteristics of Targhee sheep. J. Anim. Sci. 81:630–640.[Abstract/Free Full Text]
Hanford, K. J., L. D. Van Vleck, and G. D. Snowder. 2005. Estimates of genetic parameters and genetic change for reproduction, weight, and wool characteristics of Rambouillet sheep. Small Rumin. Res. 57:175–186.[CrossRef]
Hanford, K. J., L. D. Van Vleck, and G. D. Snowder. 2006. Estimates of genetic parameters and genetic trend for reproduction, weight, and wool characteristics of Polypay sheep. Livest. Sci. 102:72–82.[CrossRef]
Ingham, V. M., N. M. Fogarty, A. R. Gilmour, R. A. Afolayan, L. J. Cummins, G. M. Gaunt, J. Stafford, and J. E. Hocking Edwards. 2007. Genetic evaluation of crossbred lamb production. 4. Genetic parameters for first cross animal performance. Aust. J. Agric. Res. 58:839–846.[CrossRef]
Maxa, J., E. Norberg, P. Berg, and J. Pedersen. 2007. Genetic parameters for growth traits and litter size in Danish Texel, Shropshire. Oxford Down and Suffolk. Small Rumin. Res. 68:312–317.[CrossRef]
McEwan, J. C., G. H. Davis, K. G. Dodds, P. F. Fennessy, J. N. Clarke, G. D. Bruce, and M. G. Hishon. 1992. Comparison of growth, hogget fleece weight and reproductive traits of three sheep breeds from a flock selected for prolificacy and a Romney control flock. Proc. N. Z. Soc. Anim. Prod. 52:211–215.
McMillan, W. H., and M. F. McDonald. 1983. Reproduction in ewe lambs and its effect on 2-year-old performance. N. Z. J. Agric. Res. 26:437–442.
Meyer, H. H., R. L. Baker, T. G. Harvey, and S. M. Hickey. 1994a. Effects of Booroola Merino breeding and the Fec(B) gene on performance of crosses with longwool breeds. 2. Effects on reproductive performance and weight of lamb weaned by young ewes. Livest. Prod. Sci. 39:191–200.[CrossRef]
Meyer, H. H., L. R. Piper, B. M. Bindon, and R. R. Woolaston. 1994b. Litter size and uterine efficiency of Booroola Merinos, Control Merinos and their crosses with Border Leicester and Dorset. Livest. Prod. Sci. 38:217–223.[CrossRef]
Mohd-Yusuff, M. K., G. E. Dickerson, and L. D. Young. 1992. Reproductive rate and genetic and genetic variation in composite and parental populations: Experimental results in sheep. J. Anim. Sci. 70:673–688.[Abstract]
Moore, R. W., R. M. W. Sumner, J. J. Baas, and H.-U. P. Hockey. 1983. Hogget lambing and its effect on the subsequent two-tooth performance of 3 breeds. Proc. N. Z. Soc. Anim. Prod. 43:21–24.
Morgan, J. E., N. M. Fogarty, S. Nielsen, and A. R. Gilmour. 2006. Milk yield and milk composition from grazing primiparous non-dairy crossbred ewes. Aust. J. Agric. Res. 57:377–387.[CrossRef]
Piper, L. R., B. M. Bindon, and G. H. Davis. 1985. The single gene inheritance of the prolificacy of the Booroola Merino. Pages 115–125 in Genetics of Reproduction in Sheep. L. R. Piper and D. W. Robinson, ed. Butterworths, London, UK.
Pitchford, W. S. 1993. Growth and lambing performance of ewes from crosses between the Dorset Horn. Merino and Corriedale. Livest. Prod. Sci. 33:127–139.[CrossRef]
Ponzoni, R. W., S. K. Walker, J. R. W. Walkley, and M. R. Fleet. 1985. The productivity of Bungaree, Booroola x Bungaree and Trangie Fertility x Bungaree Merino ewes in South Australia. Pages 127–137 in Genetics of Reproduction in Sheep. L. R. Piper and D. W. Robinson, ed. Butterworths, London, UK.
Quirke, J. F., G. H. Stabenfeldt, and G. E. Bradford. 1988. Year and season effects on oestrus and ovarian activity in ewes of different breeds and crosses. Anim. Reprod. Sci. 16:39–52.[CrossRef]
Rosati, A., E. Mousa, L. D. van Vleck, and L. D. Young. 2002. Genetic parameters of reproductive traits in sheep. Small Rumin. Res. 43:65–74.[CrossRef]
Safari, E., N. M. Fogarty, and A. R. Gilmour. 2005. A review of genetic parameter estimates for wool, growth, meat and reproduction traits in sheep. Livest. Prod. Sci. 92:271–289.[CrossRef]
Safari, E., N. M. Fogarty, A. R. Gilmour, K. D. Atkins, S. I. Mortimer, A. A. Swan, F. D. Brien, J. C. Greeff, and J. H. J. van der Werf. 2007a. Across population genetic parameters for wool, growth, and reproduction traits in Australian Merino sheep. 2. Estimates of heritability and variance components. Aust. J. Agric. Res. 58:177–184.
Safari, E., N. M. Fogarty, A. R. Gilmour, K. D. Atkins, S. I. Mortimer, A. A. Swan, F. D. Brien, J. C. Greeff, and J. H. J. van der Werf. 2007b. Genetic correlations among and between wool, growth and reproduction traits in Merino sheep. J. Anim. Breed. Genet. 124:65–72.[CrossRef][Medline]
Snowder, G. D., J. N. Stellflug, and L. D. Van Vleck. 2004. Genetic correlation of ram sexual performance with ewe reproductive traits of four sheep breeds. Appl. Anim. Behav. Sci. 88:253–261.[CrossRef]
Spiker, S. A., L. J. Cummins, and M. A. Brockhus. 1987. Performance of purebred and crossbred ewes for prime lamb production. Proc. Aust. Assoc. Anim. Breed. Genet. 6:297–299.
Vanimisetti, H. B., D. R. Notter, and L. A. Kuehn. 2007. Genetic (co) variance components for ewe productivity traits in Katahdin sheep. J. Anim. Sci. 85:60–68.[Abstract/Free Full Text]
Young, L. D., and G. E. Dickerson. 1991. Comparison of Booroola Merino and Finnsheep: Effects on productivity of mates and performance of crossbred lambs. J. Anim. Sci. 69:1899–1911.[Abstract]
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Abstract
INTRODUCTION
