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