Actually the genetic material in the sperm does degrade over time. Which is why many of the high fenced places get rid or their deer at the 6 1/2 age mark. What the find is many of the sperm are dead or dying and the sperm tends to have problems with in its self. The same thing actually happens in humans males and females after a certain age begin having birth defects on a high scale than younger people. I don't know what causes this but it is well documented.