The economic return from individual cows is represented by the integration of several performance traits besides milk production. Specific traits such as the age at first service, age at first calving (AFC), number of days open, gestation length, conception rate (CR; pregnancy per artificial insemination (AI) service), calving interval for different parities, and length of herd life are important factors contributing to economic return and are partially determined by management policies. Lactating dairy cows prefer ambient temperatures (ATs) between −5°C and 25°C, which is known as the thermoneutral zone. Above this temperature, cows can no longer adequately cool themselves and experience heat stress [
1]. Milk production leads to metabolic heat production because of the metabolism of large amounts of nutrients; thus, cows with high milk productivity are more vulnerable to heat stress compared to those with lower yields [
2]. Heat stress, which is caused by a combination of seasonal influences such as air temperature, relative humidity (RH), solar radiation, and wind speed, depresses milk production and reproduction performance in lactating cows [
3]. Heat stress is the main environmental factor responsible for the lower fertility of cattle, especially during the summer season in many warm areas of the world [
4]. This worldwide problem inflicts considerable economic losses and affects about 60% of the world’s cattle population [
5]. For the US dairy industry, annual economic losses caused by heat stress alone have been estimated to be about $900 million [
6]. The climate of central Taiwan is defined as subtropical with concentrated rainfall during the hot season. Summers in Taiwan are extremely hot and humid, with temperatures and RH often exceeding 30°C (above 85°F) and 80% from June to November. Even though the environmental conditions suggest the possibility of the presence of heat stress, there has been no assessment of the influence of heat stress on the fertility of dairy cows in central Taiwan. The knowledge of these factors will make it easy to establish corrective measures to attain better fertility. In addition, AFC plays a critical role in achieving high profitability in dairy enterprises. Rearing replacement heifers needs large financial expenditure with no returns until the animals enter the milking herd [
7]. An earlier AFC can save rearing costs due to decreased feed, labor fee, and rearing space. A previous study showed that reducing AFC from 25 months to 24 or 21 months could reduce rearing costs by 4.3% or 18%, respectively [
7]. To maximize lactation performance and to reduce rearing costs, the mean AFC in Holstein heifers is recommended to be ≤24 months of age with a body weight of <560 kg [
8]. A majority of dairy producers in the United Kingdom aim to start breeding Holstein Friesian heifers from around 14 to 15 months of age, and thus, the target AFC is generally 24 months on their farms [
7]. No similar investigation has been conducted in central Taiwan. Therefore, the objective of the present study was to evaluate the effects of temperature, RH, and temperature–humidity index (THI) on the reproductive performance (AFC, number of days open, gestation length, CR, and calving interval) in Holstein cows of different parities. These results will help establish a management guide for dairy cattle in a subtropical region similar to central Taiwan.