One of the most important and often over-looked parameters is the quality of semen that is used.
Bull semen is commonly collected using an artificial vagina. This semen is then treated with cryoprotective agents and semen extenders before being stored in plastic straws for freezing.
Semen quality itself depends on several factors including the age of animal, stress, nutrition and genetics. However, some bulls just simply do not perform well in vitro, no matter how valuable the semen would be for use with in vivo techniques.