Relationship Between Measures of Semen Quality and Fertility in Bulls Mated Under Natural Conditions

An evaluation of the relationship between several measures of semen quality and fertility of bulls mated under natural conditions has been conducted over a three-year period. Semen was collected three different times with an electro-ejaculator from bulls used in the fertility tests. Collections were made at weekly intervals. Fertility was evaluated by mating bulls to heifers starting about three days after the last collection. Semen quality refers to the percent abnormals and the percent motility evaluated at the time of collection. Fertility was measured by the proportion of heifers that settled at natural service. Several multiple regression equations were calculated. These data show that the average fertility of a group of bulls could be predicted with some accuracy but the prediction of fertility of individual bulls is subject to large error. One of the principal reasons for this error is the changes that occur in semen quality