# Genetic Analysis of Litter Size Across Parities in Prolific and Conventional Populations of Tunisian Barbarine Sheep Using a Random Regression Model

## TL;DR

This study uses a random regression model to analyze litter size in two lines of Tunisian Barbarine sheep, finding that genetic parameters vary across parities and suggesting breeding programs should treat litter size as a changing trait.

## Contribution

The study introduces the use of a random regression model to analyze litter size in Barbarine sheep, revealing varying genetic parameters across parities.

## Key findings

- Genetic parameters for litter size differ across parities in both prolific and conventional lines of Barbarine sheep.
- Heritability estimates range from 0.04 to 0.18 in the prolific line and 0.17 to 0.39 in the conventional line.
- Genetic correlations within litter size across parities are not uniform, ranging from 0.25 to 0.96 in the prolific line and zero to 0.93 in the conventional line.

## Abstract

The Barbarine is an autochthonous sheep breed originating from Tunisia and raised in low-input production systems. This study was conducted to estimate the genetic parameters of litter size in two lines of this Barbarine breed using a random regression model (RRM). An RRM is recommended to better model the change in variance structure over time and to obtain a more accurate genetic evaluation of traits that are repeatedly measured over time. Estimates of variance components were not constant across parities in either line. In addition, within-trait genetic correlations across parities were different from unity. Thus, litter size should be treated as a distinct trait throughout the life of a female in the Barbarine breed. This study provided valuable insights for optimising breeding programmes for these two lines to accelerate genetic progress, which is crucial in low-input production systems.

Litter size records from two lines of Tunisian Barbarine sheep were analysed across parities using an RRM. A total of 2751 and 2562 litter records from the first to the sixth parity from the prolific and the conventional lines, respectively, were included in the analysis. The total number of animals in the pedigree was 1277 for the prolific line and 1102 for the conventional line. The estimation of genetic parameters was based on Bayesian inference under categorical distribution. Fixed effects included the year and month of lambing and a fixed quadratic regression coefficient for the lambing number with Legendre polynomials. The random additive and permanent environmental effects were modelled by second-order Legendre polynomials. Heritability ranged from 0.04 to 0.18 for the prolific line and from 0.17 to 0.39 for the conventional line. Genetic correlations within trait through parities showed a wide range of values, from 0.25 to 0.96 for the prolific line and from zero to 0.93 for the conventional line. Due to the changes in the variances and the genetic correlations different from unity across parities, the use of an RRM is recommended to analyse litter size in the Barbarine sheep.