Two ejaculates were harvested by electroejamlation on each of 3 d per week for 14 wk from 14, 12-to 24-mo-old Holstein bulls. Ejaculates during the first 2 wk served to stabilize sperm output. Data for the remaining ejaculates were used in a components of variance approach to determine the number of bulls per treatment and ejaculates per bull that would be needed to provide adequate sensitivity and precision for assessing treatment effects on seminal characteristics. The latter included the

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... er of sperm (total and motile), volume and sperm concentrations of the sperm-rich fraction. and the percentage of progressively motile sperm in first and second ejaculates or per day. Replication requirements declined as the number of ejaculates per bull was increased to about 15 to 25 but declined minimally thereafter. The number of bulls needed per treatment varied in relation to the size of the treatment response to be detected but was much greater than the replication used in typical experiments. Replication for assessing sperm output in first ejaculates or per day was approximately one-half as great for experiments in which pretreatment information was used to adjust post-treatment data. Tables should be useful as a guide for designing efficient, cost-effective studies of known sensitivity and precision.