Genetic control of canalization for twinning rates in Polypay and Rambouillet sheep using double hierarchical generalized linear models

Authors: Wilson, Carrie; Taylor, Joshua; BRITO, LUIZ - Purdue University

Submitted to: Journal of Animal Breeding and Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2026
Publication Date: 7/16/2026
Citation: Wilson, C.S., Taylor, J.B., Brito, L.F. 2026. Genetic control of canalization for twinning rates in Polypay and Rambouillet sheep using double hierarchical generalized linear models. Journal of Animal Breeding and Genetics. 1-14. https://doi.org/10.1111/jbg.70060.
DOI: https://doi.org/10.1111/jbg.70060

Interpretive Summary: In most sheep production systems, the ideal scenario is for every ewe to successfully raise twins. However, selection for increased number of lambs born is often accompanied by ewes having triplets or more, which is undesirable and can erode profits. We set out to identify genetic traits specifically related to twinning only, no more no less. Our focus was on key rams whose daughters consistently produced twins. Based on our findings, 35.8% of Polypay and 51.5% of Rambouillet rams had favorable Estimated Breeding Values (an estimate of genetic merit for a trait), suggesting that progress can be achieved for selecting ewes that will most likely twin, rather than having costly triplet births or more.

Technical Abstract: 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 variance components and genetic parameters for the mean and dispersion traits related to twinning rate and to develop a strategy to identify rams whose daughters consistently produce twins. Data from the U.S. Sheep Experiment Station (USSES; Dubois, ID, USA) Polypay flock recorded from 1999 to 2025 included 8,061 litters (3,115 dams; 293 sires) and the Rambouillet flock included 5,948 litters (2,333 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 NLB (1, 2, or 3+), triplet (TRIP; 2 for singles or twins and 3 for triplets and higher order), and twinning (TWIN; 1 for singles or triplets and higher order and 2 for twins) traits using the DHGLMF90 package. A bivariate animal model was fitted each for NLB, TRIP, and TWIN, and their associated dispersions (vNLB, vTRIP, and vTWIN) 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 1-year-old ewes (ALL) and excluding phenotypic records from 1-year-old ewes (MATURE). Heritability estimates ranged from 0.00008 < 0.001 for TRIP to 0.169 ± 0.033 for vTWIN for the ALL Polypay dataset and from 0.030 ± 0.026 for vNLB to 0.094 ± 0.034 for vTWIN for the MATURE Polypay dataset. For the Rambouillet, heritability estimates ranged from 0.00003 < 0.001 for TRIP to 0.166 ± 0.035 for vTRIP for the ALL dataset and from 0.0000003 < 0.001 for TRIP to 0.109 ± 0.040 for vTWIN for the MATURE dataset. Based on extremely low additive genetic variance and heritability estimates for all except the Polypay MATURE dataset, TRIP was eliminated as an option to represent twinning rate, leaving NLB and TWIN and their dispersions as potential indicator traits of twinning rate. Theoretical accuracies, EBV correlations, and sire rankings among the top 5, 10, and 20% of EBV for the remaining traits and datasets were evaluated. The TWIN and vTWIN traits were selected to represent twinning rate using the MATURE dataset for both breeds. Of the sires, 35.8% of Polypay and 51.5% of Rambouillet had both positive TWIN EBV and negative vTWIN EBV, suggesting that progress can be achieved for both increased twins and canalization of twin litters. Extension of twinning rate to national genetic evaluation is warranted to meet the needs of U.S. sheep producers.