Elsevier

Livestock Science

Volume 250, August 2021, 104596
Livestock Science

Genetic parameters of litter weight, an alternative criterion to prolificacy and pre-weaning weight for selection of French meat sheep

https://doi.org/10.1016/j.livsci.2021.104596Get rights and content

Highlights

  • The economic objective of meat sheep farmers is the kg of lambs produced per ewe.

  • The estimated genetic parameters of the litter weight are close to those of litter size.

  • Litter weight is positively and highly correlated with litter size.

  • By selecting on the synthetic maternal index, litter weight has also been improved.

Abstract

Weight of lambs produced per ewe is an important economic-related trait for French meat sheep farmers. Consequently, genetic improvement of their ewes is based on selecting maternal traits using a global index, called MAT, which combines estimated breeding values for litter size at lambing (LS), weight at 30 days of age (W30D) and viability over the same period. The frame of this study is to compare the official approach to genetic evaluation of maternal traits which combine several estimated breeding values with a direct genetic evaluation of a proxy trait of the farmer's objective. The weight of lambs produced per ewe and per lambing was chosen as this proxy of meat production potential of ewes. In a first step, we estimated the genetic parameters of this alternative criterion, the litter weight (LW), which is the sum of the W30Ds of lambs of the same litter. In a second step, we compared these parameters with those of LS, W30D and MAT, which are routinely used. Datasets comprising 2006–2018 records of 190,883 and 271,963 litters of the Ile de France (IF) and Blanche du Massif Central (BMC) breeds respectively, were analysed. The genetic evaluations were performed using Asreml software according to BLUP animal models, which were the closest to the models used in routine evaluation. Two models are presented: a two-trait model LS and W30D and a two-trait model LS and LW. For LS and LW, records are linked to ewes. For W30D, both direct and maternal effects were considered. Direct animal variance (σ2a_LW = 28.02 for IF and σ2a_LW = 16.55 for BMC) and heritability (h2a_LW = 0.06 for IF and h2a_LW = 0.04 for BMC) of LW suggest it is possible to select based on this trait while simultaneously improving LS (rga_LS/a_LW = 0.78 for IF and rga_LS/a_LW = 0.67 for BMC). Moreover, the genetic progress curves of MAT and LW indicate that the selection based on MAT gave a positive correlated response on LW. Highly correlations between MAT and LW breeding values were estimated (rg = 0.85 for IF and BMC breeds).

Introduction

French meat sheep breeders want to simultaneously improve both maternal and meat traits (Ménissier and Bouix, 1992). To reach this goal, synthetic indexes are available combining the predicted genetic values for traits by weighting them according to their economic importance. Based on a bio-economic model, Cheype et al. (2013) derived the weight of traits and showed that maternal traits play a major role in the selection objective for meat sheep. For example, in the Blanche du Massif Central breed, based on economics, the selection objective is composed of 71% maternal traits and 29% meat traits. Among the maternal traits taken into account, 21% are attributed to prolificacy, 29% to the combination of pre-weaning weight and lamb viability and 21% to fertility. In France, the genetic evaluation of maternal traits is based on data (pedigrees, litter information and lamb weight) collected from the flock. Two maternal traits are under selection: litter size at lambing (LS) and maternal abilities, which combine weight at 30 days of age (W30D) and viability of lambs over the same period. First, estimated breeding values (EBVs) are predicted using a BLUP animal model. Breeding values for LS are estimated based on a two-trait model that considers LS after natural oestrus and LS after induced oestrus, as two different but genetically linked traits. For W30D and viability, both maternal and direct genetic effects are evaluated (Tiphine et al., 2011). Second, two indexes are computed: an "LS" index that mixes EBVs for LS after natural and induced oestrus, and a “maternal ability” index which is a linear combination of direct and maternal EBVs of W30D and of viability. Both “LS” and “maternal ability” indexes are provided to breeders. Third, a synthetic maternal index named MAT is computed using a linear combination of the "LS" and the "maternal ability" indexes according to coefficients based on the breeding goal defined by the breeder societies. In this study, we have estimated EBV_LS and EBV_W30D that were combined to compute EBV_ MAT. The relationships between the elementary components are complex, particularly those between the dam and her lambs from birth to weaning (Petit and Liénard, 1988; Ménissier, 1976); therefore we have considered an alternative criterion in line with one farmer's objective: the weight of lambs produced per ewe. This new criterion is the litter weight (LW), and is defined as the sum of W30Ds of the lambs of the same litter. Although it is not the net margin resulting of each lambing, it can be considered as a proxy trait to meat production potential of ewes. The assumption is that the use of the EBV_ LW could be a selection criterion as is the use of a linear combination of the EBV of elementary components. For any new selection criterion, the first step is the estimation of its genetic parameters (Vanimisetti et al., 2007), this is the purpose of this study for LW. Research on such an alternative criterion has already been conducted but with lamb weights at weaning. Duguma et al. (2002) suggested combining the number of lambs weaned and the total weight of lambs weaned per ewe per year, while Bromley et al. (2001) suggested using LW at weaning alone and reported the heritability of this criterion to range from 0.02 to 0.11. The objective of the present study was to estimate the genetic parameters of such a criterion adapted to the French context of the sheep on-farm recording where individual lamb weight is not recorded at weaning but at 30 days of age. At this age, lambs have only been fed by their mother, W30D thus allows an effective estimation of maternal traits with no bias related to the transition to solid food.

Section snippets

Description of the dataset

This study was based on two French meat sheep breeds, Ile de France (IF) and Blanche du Massif Central (BMC). The first breed is a national breed mainly raised indoors throughout France, while the second is a local breed mainly raised outdoors and is common in the central part of France. Records from 2006 to 2018 were extracted from the official national genetic database for analysis. Records with outliers or missing data were removed from the dataset as were categories with low numbers such as

Main performances of the two breeds

Descriptive statistics for each trait and each breed are presented in Table 3. IF ewes and lambs performed better than BMC animals. LS (+ 0.13 points) and W30D (+ 0.93 kg) were higher in IF animals than in BMC. The LW of IF animals was also 2.85 kg heavier than that of BMC animals.

Genetic parameter estimates for LS

All genetic parameter estimates for LS, the trait common to both two-trait models (LS/W30D and LS/LW models), matched regardless of the model and the breed (Table 4). The additive genetic variances were between 0.012

Discussion

LS repeatability and heritability were low but in agreement with results in the literature (Maxa et al., 2007; Lee et al., 2000; Janssens et al., 2004). However, these values were lower than those used in the French genetic evaluation (Poivey, unpublished data) but similar to more recent estimates (David et al., 2011). Although LS heritability was low in the two breeds, genetic variances were relatively high and led to a wide range of genetic values (± 0.3 lambs).

The variances of the direct

Conclusion

In France, meat breed ewes are selected for maternal traits based on a synthetic index, MAT, which is a linear combination of EBVs for LS, W30D and viability at 30 days of age. As we question the linearity of the relationship between these three traits in assessing the maternal traits of meat sheep, we have identified a new potential criterion: LW, which represents an important economic-related trait for the farmers i.e. the weight of lambs produced per ewe and per lambing. Before any

Author statement

All authors have seen and approved the final version of the submitted manuscript. This article is the authors'original work, it has not been previously published and it is not intended for publication elsewhere.

Research grants

This research did not receive any specific grant from funding agencies in the public, commercial,or not-for-profit sectors.

Declaration of Competing Interest

Authors declare that there are no known conflicts of interest, which could have affected the outcome of this publication.

Acknowledgments

Sincere thanks to the breeder societies of both breeds for providing data, to Flavie Tortereau and Ingrid David for their methodological support, and to Daphne Goodfellow for her proofreading in English. We would also like to thank the reviewers for their constructive comments.

References (32)

  • E. Safari et al.

    A review of genetic parameter estimates for wool, growth, meat and reproduction traits in sheep

    Livest. Prod. Sci.

    (2005)
  • O.T. Zishiri et al.

    Genetic parameters for live weight traits in South African terminal sire sheep breeds

    Small Rumin. Res

    (2014)
  • I. Boujenane et al.

    Estimates of (co) variance due to direct and maternal effects for body weights in Timahdite sheep

    Anim. Sci.

    (2002)
  • C.M. Bromley et al.

    Genetic correlations for litter weight weaned with growth, prolificacy, and wool traits in Columbia, Polypay, Rambouillet, and Targhee sheep

    J. Anim. Sci.

    (2001)
  • A. Cheype et al.

    Economical weighting of breeding objectives and definition of total merit indexes in BMC sheep breed

  • I. David et al.

    Heterogeneity of variance components for preweaning growth in Romane sheep due to the number of lambs reared

    Genet. Sel. Evol.

    (2011)
  • Cited by (0)

    View full text