Effect of Age on Fertility of Bulls in Artificial Insemination

Abstract

The effect of age on changes in fertility of 91 Holstein and 66 Guernsey bulls was examined in the 60- 90 day nonreturn records of an artificial breeding stud. These bulls were used for 1000 or more services per year in the period from 1947 through 1959. Data from the terminal year of service were excluded for each bull culled either because of low fertility or poor semen quality. In each year, the deviation of the nonreturn rate of each bull from the stud average in percentage points for his breed was calculated. These deviations were used in the analysis of age effects with the intention of reducing the effect of time trends in fertility. Analysis of variance indicated a highly significant difference among age groups (P<0.01) in both breeds. Regression analysis showed a highly significant decline in nonreturn rate of 0.31 and 0.50% per year of age for Holsteins and Guernseys, respectively. Deviations from linear regression were nonsignificant. The age effect in the later age groups was actually underestimated, because the culling rate for fertility reasons was greater in these groups.