# Unravelling Twin Births in German Holstein Cows: Phenotypic Associations, Genetic Analysis and Potential Underlying Genes and Hormones

## TL;DR

This study explores the genetic and phenotypic factors influencing twin births in German Holstein cows, linking traits like milk yield and fertility to potential genes and hormones.

## Contribution

The study identifies genetic associations and hormonal pathways related to twin births in dairy cattle, revealing interactions between genotype and environment.

## Key findings

- Twin births are phenotypically linked to milk production and timing of successful insemination.
- Genome-wide association studies identified candidate genes LHCGR and FSHR on BTA11 related to estrus hormones.
- Significant genetic associations on BTA5 and BTA25 were found for calving numbers 2 and 3.

## Abstract

Twin births in dairy cattle are rare but present significant challenges for animal welfare, as both the health of the cow and the calves are affected. This causes economic losses, which prompts breeders to select against twin births and identify associated risk factors. This study examines the phenotypic relationship between milk yield, fertility traits and twin births in German Holstein cattle using a large, population‐wide dataset. GEBV correlations for twin births, milk production and fertility traits were estimated. Genome‐wide association studies (GWAS) were conducted for calving numbers 1–3 in order to explore the genetic background in more detail. The twin birth rate showed a strong phenotypic association with milk production and a moderate phenotypic association with the timing of successful insemination. However, GEBV correlations were low: 0.04 with milk yield and −0.10 to 0.01 with fertility traits. GWAS revealed two potential candidate genes on BTA11: LHCGR and FSHR, which encode receptors for LH and FSH, two hormones crucial to estrus. In contrast to the first calving, significantly associated regions on BTA5 and BTA25 were found in calving numbers 2 and 3. This study demonstrates the interaction between genotype and environment, concluding that a genetic predisposition for twin births, in combination with a favourable endocrine state (environment), increases the likelihood of twin births.