The present invention relates to the use of 25-hydroxy vitamin D3 to improve animal fertility. More particularly, the invention relates to the use of 25-hydroxy vitamin D3 for improving the quality of semen and/or conception rate in animals including men. In a preferred embodiment the invention relates to the use of 25-hydroxy vitamin D3 for improving the quality of semen in male and the conception rate in female animals as for example cattle, dogs, cats and horses, preferably pigs.