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Abstract Twelve Charolais bulls were ejaculated once weekly (1×) and 10 bulls six times weekly (6×) from puberty to 2 years of age to determine the effect of age and ejaculation frequency on semen characteristics, sperm output and testes growth; 6× weekly sperm output at 2 and 3 years of age was determined for each of the 22 bulls. Significant increases as bulls aged were found in sperm concentration, sperm motility, weekly total sperm and total motile sperm output, scrotal circumference and width (P<.01) and ejaculate volume (P<.05). From 1 to 2 years of age, average weekly sperm output for 6× bulls increased 2.4 times from 14.0 × 109 at 53 to 56 weeks to 33.6 × 109 at 101 to 104 weeks of age and for 1× bulls increased 2.2 times from 5.0 × 109 to 11.2 × 109 at the same ages. For 6× as compared to 1× bulls, ejaculate volume was smaller (P<.01) but total sperm and total motile sperm per week were greater (P<.01); differences for sperm motility, sperm concentration and scrotal circumference and width were not significant. Scrotal circumference and width each increased by 32% from puberty (41 ± 1 weeks) to 2 years of age with slightly over three-fourths of the increase occurring between puberty and 65 weeks of age. Scrotal circumference at 1 and 1.5 years of age was correlated .44 and .52 (P<.05) with 6× weekly sperm output at 2 years of age and .62 and .75 (P<.01) with 6× weekly sperm output at 3 years of age. Testes growth rate, based on the percentage increase in scrotal circumference between 1 and 1.5 years of age, was not significantly correlated (.30) with 6× weekly sperm output at 3 years of age. The correlation increased to .61 (P<.01) when testes growth rate during the first 24 weeks after puberty was used. Thus these relationships accounted for up to 56% of the variation in sperm output. These results and other evidence discussed support the postulate that, except for early detection of bulls with a potential for severe testicular underdevelopment, scrotal measurements of young bulls cannot be used to predict subsequent sperm output precisely. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.