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GENETIC PARAMETERS AND TRENDS IN MERINO LINES DIVERGENTLY SELECTED FOR MULTIPLE REARING ABILITY
S.W.P. Cloete1,2, A.R. Gilmour3, J.J.Olivier4 and J.B. van Wyk1
1University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
2Elsenburg Agricultural Centre, Private Bag X1, Elsenburg 7607, South Africa
3NSW Agriculture, Orange Agricultural Institute, Forest Road, Orange 2800, NSW, Australia
4ARC Animal Improvement Institute, Private Bag X5013, Stellenbosch 7599, South Africa
SUMMARY
Reproduction, greasy fleece weight (GFW) and live weight (LW) were analysed for Merino ewes that were divergently selected for (H line) or against (L line) multiple rearing ability. Estimates of h² were 0.10±0.02 for number of lambs born (NLB), 0.04±0.02 for number of lambs weaned (NLW), 0.04±0.02 for weight of lamb weaned (TWW), 0.57±0.06 for GFW and 0.49±0.06 for LW. Respective ewe permanent environment (PE) variance ratios were 0.07±0.03, 0.11±0.03, 0.12±0.03, 0.14±0.05 and 0.25±0.06. Genetic correlations of reproduction traits with GFW were low and generally negative.
Genetic correlations of reproduction with LW were positive and high for TWW. PE correlations of NLW and TWW with GFW and LW weight were negative in sign, but not significant. Genetic trends (expressed as percentage of overall means) in the H line increased annually with 1.3% for NLB, 1.5% for NLW, 1.8% for TWW and 0.06% for LW. Corresponding trends in the L line were –0.6%, –1.0%, –1.3%
and –0.17% per year. Substantial genetic progress in lamb output was attainable, despite low h² estimates.
Keywords: Ewe productivity, lamb output, live weight, repeated records, weight of lamb weaned
INTRODUCTION
It is possible to increase the efficacy of production per ewe by an enhanced net reproduction rate, a shorter production cycle, and optimum fibre production (Olivier 1999). Net reproduction was defined as total weight of lamb weaned per breeding ewe (TWW). It depends on the number (quantity) of lambs weaned, as well as the weight (quality) of the lamb(s). Heritability estimates of TWW and genetic correlations with other traits are scarce in the literature (Fogarty 1995). Available estimates involve TWW totalled over parities, and related to live weight or wool traits at hogget age (Snyman et al. 1998;
Cloete et al. 2002b). A sound breeding strategy for sheep depends on knowledge of such (co)variances.
Realised responses from selection experiments with TWW as an objective are scarce in the sheep breeding literature. Genetic improvement of reproduction is challenging, as expression is sex-limited, computational difficulties owing to the discrete nature of data are encountered, while low levels of genetic variation are frequently observed (Purvis and Hillard 1997). High coefficients of variation, however, allowed substantial gains based on selection. The objective of this study was to obtain (co)variance estimates for traits of economic importance in a resource population that were divergently selected for maternal multiple rearing ability (Cloete and Scholtz 1998).
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MATERIALS AND METHODS
Two lines of Merino sheep were divergently selected from the same base population since 1986. Details of selection procedures can be found in the literature (Cloete and Scholtz 1998). Ewe and ram progeny of ewes rearing more than 1 lamb per joining (i.e. reared twins at least once) were preferred as replacements in the High (H) line. Replacements in the Low (L) line were preferably descended from ewes rearing fewer than 1 lamb per joining (i.e. barren or lost all lambs born at least once). The H line was augmented by 28 ewes from a multiple ovulation and embryo transfer program, that were born during 1991 and 1992. The two lines were maintained as a single flock, under the same level of husbandry (Cloete &
Scholtz 1998). Lamb weaning weight was recorded at roughly 3.5 months, corrected for age, sex and birth year, and used to calculate total weight of lamb weaned (TWW) for 2955 parities of 809 individual ewes. Reproduction records included number of lambs born (NLB), number of lambs weaned (NLW) and TWW. Live weight (LW) and greasy fleece weight (GFW) were recorded at joining and shearing.
ASREML (Gilmour et al. 1999) was used to derive variance components in univariate analyses. The random terms of animal, ewe permanent environment (PE) and service sire (for reproduction traits) were included. GFW were corrected for past reproduction by the inclusion of NLW as covariate. All analyses included the full pedigree file, consisting of 4351 individuals, the progeny of 216 sires and 1107 dams.
Genetic parameters were estimated using the parsimonious model for each trait, based on Likelihood ratio tests (LRT) after the sequential fitting of random terms. Two-trait animal models were fitted, to obtain direct and PE correlations between traits. Breeding values were obtained and averaged within birth years.
RESULTS AND DISCUSSION
Estimates of h² amounted to 0.10 for NLB and 0.04 for both NLW and TWW (Table 1). Respective ewe PE estimates were 0.07, 0.11 and 0.12. GFW and LW were highly heritable, with ewe PE estimates of roughly 0.20. The h² estimates for NLB and NLW accorded with the literature (Brash et al. 1994a;
1994b; Bromley et al. 2000; Swan et al. 2001; Cloete et al. 2002c). Our estimates for TWW accorded with estimates ranging from 0.02 to 0.11 (Bromley et al. 2001; Cloete et al. 2002c). Estimates for ewe PE also corresponded with those in the literature (Bromley et al. 2000; 2001; Swan et al. 2001; Cloete et al. 2002c). The inclusion of service sire tended (P<0.10) to improve the LRT for TWW. The ratio amounted to 0.011±0.007 as proportion of the phenotypic variance. Service sire accounted for 0 to 3 % of the variation in TWW (Bromley et al. 2001), but has a limited influence on additive variances, breeding values and ranking of individual ewes (Burfening and Davis 1996). Correlations among reproduction traits were high and consistent with the literature (Bromley et al. 2000; Swan et al. 2001; Cloete et al.
2002c). Genetic correlations of reproduction traits with GFW were low and variable. No comparable estimates from repeated records models were found, but averaged correlations with GFW were –0.49 for NLB and –0.10 for NLW (Fogarty 1995). When hogget clean fleece weight were correlated with TWW totaled across parities, genetic correlations were positive (Snyman et al. 1998; Cloete et al. 2002b). Ewe PE correlations of NLB and NLW with GFW were negative, without exceeding twice their standard errors. Considering the metabolic demands of reproduction on fleece weight (Corbett 1979; Lee and Atkins 1995), the direction of these correlations is not surprising. Genetic correlations of reproduction with LW were positive and high for TWW. When hogget LW was correlated with TWW across parities,
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high genetic correlations were estimated (Snyman et al. 1998; Cloete et al., 2002b). The positive genetic correlation of wool weight with LW (Table 2) is well-established (Cloete et al. 2002a).
Table 1 Parameters for the various traits. Estimates of heritability (upper value) and ewe PE (lower value) are given on the diagonal in bold. Genetic (upper value) and ewe PE (lower value) correlations are given above the diagonal, with phenotypic (upper value) and environmental (lower value) correlations below the diagonal.
Trait NLB NLW TWW GFW LW
NLB NLW TWW GFW LW
0.10±0.02 0.07±0.03 0.61±0.01 0.57±0.01 0.55±0.01 0.50±0.02 0.01±0.02 0.05±0.02 0.09±0.02 0.12±0.02
0.91±0.13 0.87±0.11 0.04±0.02 0.11±0.03 0.95±0.01 0.95±0.01 -0.01±0.02
0.03±0.02 0.06±0.02 0.12±0.02
0.77±0.15 0.85±0.13 0.91±0.05 0.98±0.01 0.04±0.02 0.12±0.03 -0.00±0.02 0.03±0.02 0.11±0.02 0.11±0.02
-0.13±0.15 0.16±0.27 0.01±0.22 -0.19±0.20 0.08±0.17 -0.29±0.23 0.57±0.06 0.14±0.05 0.46±0.03 0.40±0.02
0.08±0.15 0.12±0.21 0.36±0.26 -0.27±0.16
0.79±0.21 -0.38±0.18
0.41±0.05 0.99±0.20 0.49±0.06 0.25±0.06
Genetic trends for the H and L lines were divergent (P<0.01) for reproduction traits and LW. Expressed as percentage of the overall means, breeding values in the H line increased annually with 1.3 % for NLB, 1.5 % for NLW and 1.8 % for TWW (Table 2). Corresponding trends in the L line were –0.6 %, –1.0 % and –1.3 % respectively. Genetic trends in TWW amounted to between 1.0 and 3.4 % per year in the study of Ercanbrack and Knight (1998). Atkins (1980) also reported marked responses in lamb output after selection for twinning and against barrenness and rearing failure in Merinos.
Table 2 Details of linear regressions of averaged predicted breeding values on year of birth for ewe reproduction and LW in the H and L lines. Correlation coefficients are given in brackets.
Trait H line L line
NLB NLW TWW LW
0.0185a ± 0.0004 (0.99) 0.0158 a ± 0.0006 (0.96) 0.392 a ± 0.018 (0.93) 0.032 a ± 0.015 (0.37)
-0.0096 b ± 0.0012 (0.66) -0.0105 b ± 0.0007 (0.84) -0.271 b ± 0.018 (0.81) -0.094 b ± 0.023 (0.53)
a,b Denote significant (P<0.05) divergence between lines for a specific trait
Breeding values for LW in the H line tended (P<0.10) to increase at 0.06% of the overall mean per year (Table 2). The corresponding change in the L line amounted to –0.17%. This change is consistent with
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estimates of genetic correlations of live weight with reproduction in sheep (Snyman et al. 1998; Olivier et al. 2001, Cloete et al. 2002b). A correlated response in TWW amounted to 67 % of that attainable through direct selection when selection was based on yearling LW (Ercanbrack and Knight 1998).
CONCLUSIONS
Additive variance ratios were low for lamb output, as reflected by NLW or TWW. Substantial genetic changes were nevertheless achieved, indicating that the genetic improvement of a composite trait like TWW is feasible, should it be desired in the breeding strategy. An improved reproduction is essential for economic success when meat production contributes markedly to income. Genetic improvement of reproduction is thus stressed in the South African Small Stock Improvement Scheme (Olivier 1999), since meat constitutes an important source of revenue for sheep farmers in this country. This objective appears to be attainable without marked unwanted correlated responses in GFW and LW.
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