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

摘要

Rising temperatures caused by climate change haveadverse effects on cattle physiology, welfare, health,and reproduction. Heat stress in cows affects the oocyteand embryo directly through heat shock on cellularfunction. Fewer data are available on the effect of hightemperatures on male fertility. Temperature-humidityindex (THI) is a measure for assessing the risk of heatstress that combines the effects of temperature andhumidity. The aim of this study was to determine therelationship between THI and fresh or frozen–thawedsperm quality of Holstein bulls kept in temperate climates.Bull sperm data of 29,170 ejaculates from 933bulls collected at 3 Dutch artificial insemination centersbetween 2015 and 2018 were evaluated. The assessedvariables included total sperm motility and morphologyof fresh semen, and total sperm motility, morphology,and progressive motility of frozen semen 0 and 3 h afterthawing. In addition, 56-d nonreturn rates were analyzed.The assessed effects were season and THI on theday of semen collection and during spermatogenesis (30d before collection), bull, age of bull, year, and location.Bulls were divided into 2 categories according totheir age: young (<36 mo) and older (>36 mo). Overallsperm quality of young bulls improved as age increased.No effect of THI on fresh sperm variables was observedin either young or older bulls. However, high THI atspermatogenesis negatively affected the cryotoleranceof sperm cells. Sperm cells from young and older bullsshowed a pronounced decrease (14–18%) of the assessedvariables 3 h after thawing after the increase of THIduring spermatogenesis in autumn. Remarkably, olderbulls were more sensitive to THI at spermatogenesiscompared with semen collection, showing up to a 3.8times higher negative effect on frozen sperm quality.However, an elevated THI at semen collection produceda tendency toward decreased 56-d nonreturn rates asthe age of the bull increased. Although this decreasewas up to 4%, rising temperatures may still cause importanteconomic losses in the future. For the first time,the present study confirmed that climate compromisesnot only sperm quality, but also dairy bull fertility.