# Reproductive Neutrality of the A2 β-Casein Variant in Holstein Cows

## TL;DR

This study shows that selecting for the A2 β-casein variant in Holstein cows does not negatively impact their fertility under commercial dairy conditions.

## Contribution

The study provides robust evidence that A2 β-casein selection does not compromise fertility in Holstein cows under intensive production systems.

## Key findings

- Differences in fertility traits among CSN2 genotypes were small and biologically negligible.
- Reproductive performance was primarily influenced by parity and year, not CSN2 genotype.
- Selection for A2 β-casein can be implemented without adverse effects on fertility.

## Abstract

Demand for A2 milk has led to the rapid adoption of A2-oriented selection in dairy cattle breeding. This selection is based on variation in the CSN2 gene encoding the milk protein β-casein. However, concerns remain that focusing on A2 milk production could unintentionally affect important functional traits such as fertility, especially in intensive commercial systems. In this study, reproductive performance was evaluated in Holstein cows with different CSN2 genotypes using a large dataset from a commercial dairy herd. Fertility traits included days open, number of services per conception, calving interval, first-service conception rate, and pregnancy by 100 days in milk. Differences among CSN2 genotypes were consistently small and not biologically meaningful. Reproductive performance was mainly influenced by parity and year, rather than CSN2 genotype. These results indicate that selection for the A2 β-casein variant does not compromise fertility under intensive production conditions and can be implemented without adverse effects on reproductive performance.

The CSN2 gene encoding β-casein has gained increasing attention in dairy cattle breeding due to the global adoption of A2-oriented selection strategies. However, robust large-scale evidence assessing potential unintended effects on functional traits, particularly fertility, under intensive commercial conditions remains limited. This study evaluated whether selection for the CSN2 A2 β-casein variant is associated with biologically relevant differences in fertility traits in Holstein cows. Reproductive and genomic data from 7826 lactation records of 2773 Holstein cows collected between 2022 and 2025 in a large commercial dairy herd were analyzed. Fertility indicators included days open, number of services per conception, calving interval, first-service conception rate, and pregnancy by 100 days in milk. Mixed-effects models accounting for repeated lactations and cow- and sire-level clustering were applied, and predefined equivalence margins were used to distinguish statistical non-significance from biological irrelevance. Across all evaluated fertility traits, differences among CSN2 genotypes (A1A1, A1A2, and A2A2) were consistently small, biologically negligible, and well within predefined equivalence margins. Differences in days open were within ±2 days, and effect sizes for count and binary traits were close to unity. Parity and calving year significantly influenced reproductive performance, whereas no CSN2 genotype × parity interactions were detected. These findings indicate that selection for the CSN2 A2 β-casein variant does not compromise reproductive performance under intensive commercial management conditions. From a breeding and industry perspective, the results support the implementation of A2-oriented selection strategies without biologically meaningful adverse effects on fertility.