Genetic Control of Canalization for Twinning Rates in Polypay and Rambouillet Sheep Using Threshold or Double...

In most sheep production systems, the ideal scenario is for every ewe to lamb and successfully raise twins. However, selection for increased number of lambs born (NLB) is often accompanied by a higher incidence of triplet and higher-order births, which increases management challenges and producer reluctance to intensify selection for this trait. Therefore, there is a need to identify animals who not only exhibit a high twinning rate, but also canalized reproductive performance, characterized by consistent expression of twin births and reduced variability toward extreme litter sizes. In this context, the double hierarchical generalized linear model (DHGLM) provides a mechanism to evaluate both the mean and the dispersion components of a trait, enabling the evaluation of the heteroskedastic residual variance for each animal, and thereby select animals with lower residual variances. Therefore, the main objectives of this study were to estimate (co)variance components and genetic parameters for the mean and dispersion traits related to number of lambs born and twinning rate and to develop a strategy to identify rams whose daughters consistently produce twins. Data from the US Sheep Experiment Station (USSES; Dubois, ID, USA) Polypay flock recorded from 1999 to 2025 included 8061 litters (3115 dams; 293 sires) and the Rambouillet flock included 5948 litters (2333 dams; 228 sires) recorded from 2005 to 2025. The proportion of twin litters have significantly decreased in the Polypay flock while remaining unchanged (p = 0.13) in the Rambouillet flock. For both breeds, the proportion of single lamb litters have decreased while triplets have increased. For sires with a minimum of 15 daughter litters, the average litter size was 1.53 ± 0.55 for 1-year-old ewes and 2.29 ± 0.75 for mature ewes for the Polypay and 1.24 ± 0.43 for 1-year-old ewes and 1.93 ± 0.64 for mature ewes for the Rambouillet. A DHGLM was used to estimate both the mean and dispersion of the NLB3 considering 3 classes (1, 2 or 3+) and NLB for all classes (1 to 5 for Polypay and 1 to 4 for Rambouillet) traits using the DHGLMF90 package. A bivariate animal model was fitted each for NLB3 and NLB, and their associated dispersions (vNLB3 and vNLB) with contemporary group as a concatenation of ewe age category and birth year fitted as a categorical fixed effect and animal (ewe), permanent environment, and the residual as random effects for a model including data from all ewes (ALL) and excluding phenotypic records from 1-year-old ewes (MATURE). The same mean traits and twinning (TWIN; 1 for non-twins and 2 for twins) were evaluated using a threshold model with the THRGIBBSF90 package. Heritability estimates ranged from 0.024 ± 0.012 for vNLB3 analysed using the DHGLM model to 0.110 ± 0.020 for NLB3 analysed using the threshold model for the ALL Polypay dataset and from 0.016 ± 0.014 for vNLB3 analysed using the DHGLM model to 0.094 ± 0.022 for NLB3 analysed using the threshold model for the MATURE Polypay dataset. For the Rambouillet, heritability estimates ranged from 0.020 ± 0.013 for TWIN analysed using the threshold model to 0.150 ± 0.022 for NLB analysed using the threshold model for the ALL dataset and from 0.021 ± 0.012 for vNLB3 analysed using the DHGLM model to 0.151 ± 0.023 for NLB analysed using the threshold model for the MATURE dataset. The estimated breeding values (EBV) were filtered if lower than the square root of heritability, or the equivalent of a single phenotypic record and correlations of EBV among animals were compared. Correlations among all NLB approaches were high (0.80 to 1.00) for both breeds, indicating high agreement between NLB considering differing numbers of classes and also for different models (DHGLM and threshold). Theoretical accuracies were evaluated for all animals and sires with more than 15 daughter litters. The TWIN trait analysed using a threshold model and the vNLB trait analysed using the DHGLM model were selected to represent twinning rate using the MATURE dataset for both breeds. Of the sires, 9.9% of Polypay and 32.1% of Rambouillet had both positive TWIN EBV and negative vNLB EBV, suggesting that progress can be achieved for both increased twins and canalization of number of lambs born. Extension of twinning rate to national genetic evaluation is warranted to meet the needs of US sheep producers.