Abstract
Summary The effect of age on changes in fertility of Holstein and Guernsey bulls was examined in the 60–90 day nonreturn records of an artificial breeding stud. Bulls used for 1,000 or more services per year in the period from 1947 through 1959 were included. Data from the terminal year of service were excluded for each bull culled either because of low fertility or poor semen quality. This exclusion eliminated records on bulls whose fertility dropped markedly in a short period of time. Ninety-one Holstein and 66 Guernsey bulls had one or more records which met the criteria established for inclusion in the analysis. The two groups contributed 322 and 218 bull-years, respectively. 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
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