Selection of bulls for artificial breeding on basis of semen storage characteristics

Abstract

1. Analysis of spring mating data in 1955 indicated that the fall in conception rate associated with the use of older semen was a serious problem. 2. The fall in conception rate in 1955 was more marked in bulls of below average conception rate than in those with above average conception rate. 3. Results for groups used in two successive years indicate that this decline is highly repeatable. 4. Bulls which have ‘good keeping quality’ semen can be selected on the records of a previous season. Where there is no previous record, it has been shown that an individual can be classified as a ‘good keeper’ or a ‘poor keeper’ by measuring the fructolysis index of the semen on the day after collection. If the fructolysis index is above some minimum figure (1·05 mg. fructose/hr./109 sperm has been adopted in this case for second-day semen), the bull can be used for delayed inseminations with good prospects of success. 5. The best method of classification would appear to be: (i) initial classification by fructolysis index; (ii) subsequent rejection of bulls of low fertility in this group after fertility records are available.