# The Effect of Heat Stress During the Insemination Period on the Conception Outcomes of Dairy Cows

## TL;DR

This study shows that heat stress around the time of insemination, especially three weeks before and the day of, significantly lowers pregnancy success in dairy cows.

## Contribution

The study identifies specific time frames around insemination most sensitive to heat stress and highlights genetic variability in cow resilience.

## Key findings

- The three weeks before insemination and the day of insemination are most sensitive to heat stress.
- Mild heat stress on the day of insemination reduces pregnancy success with a critical THI threshold of 60.
- Genetic variability in heat resilience suggests breeding for heat resistance could improve fertility.

## Abstract

Heat stress can negatively affect the ability of dairy cows to become pregnant after insemination. This study looked at over 47,000 insemination records from more than 6000 cows to find out which periods around insemination are most sensitive to heat. The objective was to understand when cows are most at risk to enable farmers and researchers to take action to improve fertility. This study focused on different time frames before and after insemination. The results showed that the day of insemination and the three weeks before it were the most sensitive periods. Even mild heat during that time reduced the chance of successful pregnancy. Some effects were also seen after insemination when the heat was more extreme, while other time frames were not affected. This study also found that some cows may cope better with heat than others, suggesting that breeding cows for heat resistance could improve fertility. These findings can help farmers make better decisions about cow care during hot weather and guide future breeding programmes to improve success rates, especially as climate change leads to hotter conditions.

This study examined the impact of heat stress during specific periods before and after insemination to identify the most critical time affecting insemination success in dairy cows. A total of 47,199 insemination records from 6751 Holstein-Friesian cows (2002–2024) were analysed using generalised linear mixed models (GLMMs) with a binomial distribution. The periods assessed were the day of insemination, 21–6 days before (P1), 5–2 days before (P2), 1 day before to 1 day after (P3), 2–7 days after (P4), and 8–30 days after insemination (P5). Mild heat stress significantly reduced insemination success on the day of insemination, with a critical THI threshold of 60. Among all periods, P1 was the most sensitive to heat stress, with even mild stress negatively affecting outcomes. Severe heat stress (THI ≥ 72) also impacted P3 and P5, whereas P2 and P4 showed no significant effect. Heritability estimates increased with heat stress intensity, suggesting genetic variability in resilience. These findings highlight the importance of managing heat stress from three weeks before to one month after insemination and support the potential for genetic selection to improve reproductive performance under heat stress conditions.