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

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 (sigma 2a_LW = 28.02 for IF and sigma 2a_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).