Discussions over possible declines in male sperm counts are hampered by the lack of human data. Data is readily available, however, on sperm counts from other mammals, and this can provide helpful insight to questions regarding male fertility rates. This study evaluated the long-term trends in sperm output from the ejaculates of over 5000 dairy bulls between 1962 and 1996. The mean sperm output per year of collection was calculated after adjusting the data for known effects (such as age and season of collection). There was no decline in sperm counts over this 34 year period, despite exposure of the dairy bulls to pesticides and other polychlorinated organic compounds.
Van Os JL, de Vries MJ, den Daas NH, Kaal Lansbergen LM. Long-term trends in sperm counts of dairy bulls. Journal of Andrology : Vol. 18, issue 6 - 725-31, November 1997.