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Updated: Aug 6, 2026
14:40
Instrumentation of Near-term Fetal Sheep for Multivariate Chronic Non-anesthetized Recordings
Published on: October 25, 2015
Genetic Control of Canalization for Twinning Rates in Polypay and Rambouillet Sheep Using Threshold or Double
Carrie S Wilson1, J Bret Taylor1, Luiz F Brito2
1Range Sheep Production Efficiency Research Unit, ARS, USDA, Dubois, Idaho, USA.
Summary
Selecting sheep for consistent twinning involves evaluating both the number of lambs born (NLB) and its variability. This study used the double hierarchical generalized linear model (DHGLM) to identify rams that produce daughters with predictable twin births, improving sheep production efficiency.
Area of Science:
- Animal Genetics and Breeding
- Reproductive Biology in Livestock
- Quantitative Genetics
Background:
- Increasing the number of lambs born (NLB) in sheep is desirable but often leads to undesirable higher-order litters, increasing management complexity.
- There is a need to select for animals with high twinning rates and consistent reproductive performance, minimizing extreme litter sizes.
- The double hierarchical generalized linear model (DHGLM) allows for the evaluation of both mean and dispersion of traits, enabling selection for reduced variability.
Purpose of the Study:
- To estimate (co)variance components and genetic parameters for the mean and dispersion of NLB and twinning rate in Polypay and Rambouillet sheep.
- To develop a strategy for identifying rams whose daughters consistently produce twin births.
- To assess the potential for genetic improvement in twinning rate and litter size consistency.
Main Methods:
- Utilized data from Polypay and Rambouillet flocks (1999-2025 and 2005-2025, respectively) including thousands of litters.
- Employed DHGLM to estimate mean and dispersion for NLB and number of lambs born in three classes (NLB3).
- Applied a threshold model to evaluate twinning rate (TWIN) and NLB, fitting bivariate animal models for mean and dispersion traits.
Main Results:
- Heritability estimates for NLB and its dispersion varied, with some traits showing low heritability (e.g., vNLB3 ~0.02).
- Correlations between different NLB approaches and models were high (0.80-1.00), indicating model agreement.
- A proportion of sires (9.9% Polypay, 32.1% Rambouillet) showed desirable genetic potential for both increased twins (positive TWIN EBV) and reduced variability (negative vNLB EBV).
Conclusions:
- Genetic selection can simultaneously improve twinning rates and canalize litter size, reducing variability towards extreme litter sizes.
- The DHGLM approach effectively identifies animals with consistent reproductive performance.
- Incorporating twinning rate into national genetic evaluations is recommended to meet US sheep producer needs.
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