Even Moderate Climates Cause Heat Stress

Source: Dairy Cattle Reproductive Council

A retrospective study published in the May 1 Journal of Theriogenology investigated the relationship between the temperature-humidity index (THI) and the conception rate (CR) of lactating dairy cows. The aim was to estimate a threshold for this relationship and to identify periods of exposure to heat stress relative to breeding in an area of moderate climate.

The researchers also compared three different heat load indices related to CR: mean THI, maximum THI and number of hours above the mean THI threshold. They found that the THI threshold for the influence of heat stress on CR was 73.

Negative effects of heat stress, however, were already apparent at lower levels of THI, and 1 hour of mean THI of 73 or more decreased the CR significantly. The CR of lactating dairy cows was negatively affected by heat stress both before and after the day of breeding.

• The greatest negative impact of heat stress on CR was observed 21 to 1 day before breeding. When the mean THI was 73 or more in this period, CR decreased from 31% to 12%.

• Compared with the average maximum THI and the total number of hours above a threshold of more than or 9 hours, the mean THI was the most sensitive heat-load index relating to CR.
These results indicated that the CR of dairy cows raised in moderate climates is highly affected by heat stress.

You can read the abstract here.