How heat stress affects dairy cows: Part 3

In this final instalment of our three-part series on the consequences of climate change on dairy cows, veterinarian Dr JH du Preez explores the negative impact of heat stress on these animals’ conception rates.

- Dr JH du Preez

In the second article in this series on the impact of climate change on South Africa’s dairy herd (see FW, 9 December 2022), I discussed the negative effects of heat stress on milk production. In this article, I focus on the impact that increased temperatures and humidity have on the conception rate of a dairy herd.

As mentioned before, South Africa’s average maximum temperature (MT) rose by 0,9°C between from 1933 to 2020. Between 1960 and 2020, the average conception rate in dairy cows dropped by 4,9%, which has obviously had a negative effect on local milk production and farmers’ profit margins.

LONG-TERM CONCEPTION RATES

In order to calculate the implications of heat stress on the conception rate (CR) of dairy cows raised under South African conditions, we use the following regression calculation: CR = - 890,2 + 31,15 THI - 0,253 THI2, where THI represents the temperature-humidity index.

The table shows a summary of data collected at various weather stations across seven of South Africa’s provinces. Included therein are the corresponding THI values, the CR of dairy cows in the relevant region, and the MT recorded at each station at 2pm on any given day.

The long-term averages have been taken over periods of between 26 and 87 years, depending on the data available.