High temperature-humidity index compromises sperm quality and fertility of Holstein bulls in temperate climates.

Rising temperatures caused by climate change have adverse effects on cattle physiology, welfare, health, and reproduction. Heat stress in cows affects the oocyte and embryo directly through heat shock on cellular function. Fewer data are available on the effect of high temperatures on male fertility. Temperature-humidity index (THI) is a measure for assessing the risk of heat stress that combines the effects of temperature and humidity. The aim of this study was to determine the relationship between THI and fresh or frozen-thawed sperm quality of Holstein bulls kept in temperate climates. Bull sperm data of 29,170 ejaculates from 933 bulls collected at 3 Dutch artificial insemination centers between 2015 and 2018 were evaluated. The assessed variables included total sperm motility and morphology of fresh semen, and total sperm motility, morphology, and progressive motility of frozen semen 0 and 3 h after thawing. In addition, 56-d nonreturn rates were analyzed. The assessed effects were season and THI on the day of semen collection and during spermatogenesis (30 d before collection), bull, age of bull, year, and location. Bulls were divided into 2 categories according to their age: young (<36 mo) and older (>36 mo). Overall sperm quality of young bulls improved as age increased. No effect of THI on fresh sperm variables was observed in either young or older bulls. However, high THI at spermatogenesis negatively affected the cryotolerance of sperm cells. Sperm cells from young and older bulls showed a pronounced decrease (14-18%) of the assessed variables 3 h after thawing after the increase of THI during spermatogenesis in autumn. Remarkably, older bulls were more sensitive to THI at spermatogenesis compared with semen collection, showing up to a 3.8 times higher negative effect on frozen sperm quality. However, an elevated THI at semen collection produced a tendency toward decreased 56-d nonreturn rates as the age of the bull increased. Although this decrease was up to 4%, rising temperatures may still cause important economic losses in the future. For the first time, the present study confirmed that climate compromises not only sperm quality, but also dairy bull fertility.