Pickett and Ionata 43 enumerated many of the factors that determine the number of mares a stallion can be used to inseminate in an AI program. The first assumption that must be made is that management is ideal and that there are sufficient facilities and personnel to perform the inseminations in a reasonable period of time (approximately one hour) after seminal collectio n, If a daily spermatozoal output (DSO) of 3.5 to 7.0 billion is assumed, depending on season of the year, the number of mares that can be bred can be predicted. It is currently recommended that a stallion be collected every other day 9 when his semen is to be used for artificial insemination. Thus, spermatozoal output per collection should be 7.0 to 14.0 billion, double the DSO of 3.5 or 7.0 billion when a stallion is collected daily. If one makes these assumptions, and utilizes the recommendation that mares in estrus for two days or longer be inseminated, then the number of mares that can be inseminated on an every-other-day seminal-collection schedule is shown in Table 5. Since progressive spermatozoal motility is the seminal characteristic most commonly used on the farm to predict fertility and estimate number of mares to inseminate, this must also be taken into consideration, as well as number of spermatozoa. Therefore, in Table 5, spermatozoal motilities ranging from 25 to 75%, and spermatozoal outputs of 7.0 and 14.0 billion per ejaculate, have been utilized.