Proc. Assoc. Advmt. Anim. Breed. Genet. 17: 491-494
491
SELECTION INDEXES FOR CROSSBRED EWE REPRODUCTION AND PRODUCTIVITY R.A. Afolayan, A.R. Gilmour and N.M. Fogarty
The Australian Sheep Industry CRC and NSW Department of Primary Industries, Orange Agricultural Institute, Orange, NSW 2800
SUMMARY
The higher value of meat in recent years has increased interest in genetic improvement of ewe productivity and use of multi-trait index selection to optimise reproduction and meat traits in both Merino and maternal meat breeds. Total weight of lamb weaned of all ewes joined (TWWj) is an overall composite trait of several reproduction and growth traits that is a useful measure of ewe productivity. Evaluation of an optimal index for TWWj involving its major components (fertility, litter size, rearing ability and average lamb weight weaned (AWWj) was undertaken using genetic parameters estimated from the Maternal sire Central Progeny Test data on lambing performance of 2846 ewes over 3 joinings. The results show predicted response to direct selection for TWWj of 3.17 kg/generation, with selection for litter size alone achieving 80% of this predicted response. The relative responses in TWWj from selection on the other component traits alone were: fertility 64%, rearing ability 31% and AWWj 13%. An optimum index of the 4 component traits was predicted to result in 17% improved response in TWWj over direct selection for TWWj.
INTRODUCTION
Ewe productivity is an important component of lamb enterprise economic performance and need to be included in genetic improvement programs for lamb production. Total weight of lamb weaned per ewe joined (TWWj) is a measure of ewe productivity that encompasses net reproduction and lamb growth and is proposed as a biological index for improving overall flock productivity. The higher value of meat in recent years has increased interest in improving reproduction traits for both Merino and maternal meat breeds. Among reproductive traits, litter size has most often been used as a selection criterion. This is because litter size is relatively easy to measure and report, and heritability estimates for litter size are generally higher than those of other reproductive traits such as fertility or lamb survival. However, TWWj provides an overall indication of ewe fertility, litter size, maternal performance, and rearing ability as well as lamb survival and growth (Falconer and Mackay 1996).
Improvement of TWWj could either select directly for TWWj or through the development of an optimum index from its component traits. It is expected that combining traits into an appropriate selection index might achieve more efficient overall gains per generation or per annum. However, the ease and costs of measurement for individual component traits determines the profit margin realized for such efforts. The aim of this study was to evaluate an optimum index of TWWj with its component ewe reproduction and growth traits, namely, fertility, litter size, rearing ability and average weight of lamb weaned (AWWj). The study used estimates of genetic parameters derived from the Maternal sire Central Progeny Test data (Afolayan et al. 2007). The relative effectiveness of alternative selection procedures for improving TWWj were also evaluated.
Breeding Objectives
492 MATERIALS AND METHODS
The data are from the lambing performance of 2846 crossbred ewes that were the progeny of 91 maternal breed rams (Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, Booroola Leicester, Hyfer, and several other breeds) and Merino ewes that were born at each of 3 sites over 3 years (Afolayan et al. 2007). Three common sires were used in each year at each site to provide genetic linkage across all groups. The crossbred ewes were joined naturally to groups of terminal sire rams to evaluate their lambing performance over 3 years at 3 sites (Cowra, NSW; Hamilton and Rutherglen, Vic). The production systems for evaluation of the crossbred ewes were: Cowra – ewes split to autumn and spring joining, first joined at 7 and 14 months of age respectively; Hamilton – autumn joined, first at 7 months of age; Rutherglen – spring joined, first at 17 months. The ewe reproduction traits analysed were: fertility (number of ewes lambing per ewe joined), litter size (number of lambs born per ewe lambing), ewe rearing ability (ratio of lambs weaned to lambs born for lambing ewes), average weight weaned (AWWj, average weaning weight of lambs weaned per ewe joined independent of birth and rearing effects), and the total weight of lamb weaned per ewe joined (TWWj). TWWj for each ewe was the sum of adjusted weaning weights of all lambs reared each year (adjusted to 100 days of age and to male equivalent). Records for all crossbred ewes that were joined and present at lambing were included in the analyses except the data from Hamilton for the 1999 joining which was affected by abortion loss due to vibriosis and the 2002 joining in which the ewes were affected by ryegrass staggers. The total number of ewes joined and mean performance over their 3 joinings for the reproduction traits are shown in Table 1.
Table 1. Number of crossbred ewes and mean (s.d.) performance from 3 joinings in four environments
Environment Ewes
(n) Fertility Litter size Rearing
ability AWWj
(kg) TWWj
(kg) Cowra Autumn
Cowra Spring 626
616 0.79 (0.41)
0.86 (0.34) 1.78 (0.71)
1.58 (0.64) 0.79 (0.34)
0.88 (0.27) 30.0 (5.8)
28.5 (5.2) 30.9 (23.1) 32.1 (19.4) Hamilton Autmn 724 0.80 (0.40) 1.50 (0.57) 0.77 (0.38) 30.1 (6.0) 26.1 (21.7) Rutherglen Sprg 880 0.87 (0.34) 1.51 (0.56) 0.88 (0.27) 25.6 (6.4) 27.7 (16.8) Overall 2846 0.83 (0.38) 1.58 (0.63) 0.84 (0.31) 28.1 (6.2) 29.1 (20.2) The variance components for the traits were estimated using a univariate sire model by restricted maximum likelihood using ASReml (Gilmour et al. 2006). Bivariate analyses were conducted among the component traits and with TWWj using a sire model with environmental dam and cohort (environment×year) terms to estimate covariances and the derived genetic correlations are shown in Table 2. These parameters were used to form the genetic (G) and phenotypic (P) matrices to calculate individual and optimum indexes using R (2007). The efficiency of an index based on a set of the component traits relative to direct selection for TWWj was calculated as 100√(g’P-1g/σ2)/h2 where g is the vector of covariances between the index traits and TWWj, P is the phenotypic variance matrix for the index traits, σ2 is the phenotypic variance of TWWj and h2 is the heritability of TWWj. The efficiency was calculated for all 15 combinations of component traits. Direct response to selection (per generation) for TWWj and the expected correlated responses in TWWj from selection for each component trait was also calculated from the genetic and phenotypic (co)variances.
Proc. Assoc. Advmt. Anim. Breed. Genet. 17: 491-494
493 RESULTS AND DISCUSSION
The effectiveness of the selection indexes for a combination of traits is largely achieved through favourable correlations among component traits. While most of these component traits (except AWWj) were highly correlated genetically with TWWj, there were large differences in heritability estimates with litter size being the highest (0.19) and rearing ability being lowest (0.03) (see Table 2).
The relatively low heritability estimate for TWWj (0.17) is typical of many reproductive traits.
Moreover, because litter size at weaning is a multiplicative component of TWWj, a large environmental variation is expected in its expression which was evidenced by its large residual variance (not presented). It is pertinent to note that heritability estimates for TWWj, although relatively small, were greater than the estimates for most of the component traits.
Table 2. Phenotypic variance (σ2p), direct heritability (h2), and genetic correlations (rg) among component traits and with total weight weaned (TWWj)
Fertility Litter size Rearing ability AWWj TWWj
h2 0.11 0.19 0.03 0.12 0.17
σ2p 0.12 0.32 0.09 33.2 347
Genetic correlations (rg)
Litter size 0.63
Rearing ability 0.30 -0.29
AWWj -0.09 -0.25 0.32
TWWj 0.79 0.76 0.73 0.16
The relative contributions of the various components to genetic improvement in ewe productivity (TWWj) vary. Using one of the components instead of total weight weaned gave relative efficiencies ranging between 13% for AWWj to 80% for litter size (Table 3). Therefore when scales for weight measurement are not available, selection for litter size is clearly a good alternative to improve TWWj.
However, direct selection for TWWj is better than direct selection for litter size. This is supported by a selection experiment with mice where direct selection for total litter weight weaned was 3 times as effective as selection for litter size (Luxford and Beilharz 1990). The relatively poor predicted response in TWWj from selection on AWWj is largely due to the low genetic correlation (0.16) between the traits, whereas the poor response from rearing ability is largely due to its very low heritability (0.03).
A multiple trait selection index of the 4 component traits is predicted to result in 17% greater response in TWWj than direct selection for TWWj (Table 3). Litter size was the most important trait to include in the index as the relative response was over 90% for all indexes that included litter size.
The use of rearing ability and AWWj in an index resulted in only 34% of predicted response in TWWj because of their low genetic correlation (0.32) and the negative genetic correlations of each trait with litter size (-0.29 and -0.25 respectively).
CONCLUSIONS
Total weight of lamb weaned per ewe joined could be used as a selection criterion to improve ewe productivity as it includes components of reproduction and lamb growth. Litter size is the most important component and its selection alone is predicted to result in 80% of the response in TWWj
Breeding Objectives
494
that could be achieved from direct selection. Use of an optimum index of the 4 component traits of TWWj, is predicted to result in 17% higher efficiency of selection for TWWj than direct selection.
Table 3. Direct response in TWWj (kg/generation) and relative responses in TWWj to selection on component traits and various indexes
Trait Relative response Response in TWWj
TWWj Fertility (Fert) Litter size (LS) Rearing ability (RA) AWWj
Index Fert + LS Fert + RA Fert + AWWj LS + RA LS + AWWj RA + AWWj Fert + LS + RA Fert + LS + AWWj Fert + RA + AWWj LS + RA + AWWj Fert + LS + RA + AWWj
100 64 80 31 13
92 72 65 95 93 34 105 102 74 110 117
3.17 1.99 2.55 0.95 0.43
ACKNOWLEDGMENTS
The MCPT was run by NSW Department of Primary Industries, Department of Primary Industries Vic. and SA Research & Development Institute, with support from Meat and Livestock Australia and the Australian Sheep Industry CRC. The support of breeders who entered sires and provided semen is appreciated. Dr Kevin Atkins provided useful contributions on the derivation of the proportion of various component traits of the index.
REFERENCES
Afolayan, R.A., Fogarty, N.M., Gilmour, A.R., Ingham, V.M., Gaunt, G.M. and Cummins, L.J.
(2007) Aust. J. Agric. Res. (in preparation)
Falconer, D.S. and Mackay, T.F.C. (1996) “Introduction to Quantitative Genetics” Longman, New York, NY.
Gilmour, A.R., Gogel, B.J., Cullis, B.R., and Thompson, R. (2006). “ASREML User Guide Release 2.0” VSN International Ltd: Hemel Hempstead, HP1 1ES, UK.
Luxford, B.G., and Beilharz, R.G. (1990). Theor. App. Genet. 80:625.
R Development Core Team (2007). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org.
