Genetic implications of a bivariate threshold model for litter size components

Area of Science:

• Animal Genetics
• Reproductive Biology
• Quantitative Genetics

Background:

• Litter size is a key economic trait in sheep production.
• Understanding genetic correlations between ovulation rate and embryonic survival is crucial for breeding programs.
• Previous models have not fully integrated these complex relationships.

Purpose of the Study:

• To develop a bivariate threshold model for ovulation rate and embryonic survival.
• To derive genetic relationships among ovulation rate, embryonic survival, and litter size.
• To evaluate different selection criteria for increasing litter size in Lacaune sheep.

Main Methods:

• A bivariate threshold model was applied to nulliparous Lacaune sheep data.
• Heritabilities for ovulation rate, embryonic survival, and litter size were assumed.
• Genetic correlations between ovulation rate and embryonic survival were analyzed.
• A six-generation selection scheme simulation was conducted using different selection criteria.

Main Results:

• A linear index combining ovulation rate and embryonic survival offered short-term advantages over direct selection.
• The superiority of the index was not sustained over multiple generations.
• Indirect selection on ovulation rate alone was less effective than index or direct selection.
• Litter size appeared close to an optimal 'natural index' in the studied population.

Conclusions:

• Direct selection for litter size is effective for long-term genetic gain in Lacaune sheep.
• Breeding strategies should consider the complex genetic interplay between ovulation rate and embryonic survival.
• The developed model provides insights into optimizing reproductive traits in sheep.