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Summary. The 12- to 24-month-old Holstein bulls were electroejaculated twice on each of 3 days per week throughout the study. After a 2-week stabilization period and subsequent 2-week pre-treatment period, 7 bulls were given 50 i.u. oxytocin via the jugular vein 10 min before each first ejaculate for 10 weeks. The 7 control bulls were handled identically but did not receive oxytocin. All bulls were castrated at the end of the study. Oxytocin was without effect on spermatogenesis (P > 0·10). Oxytocin did not alter the total number of spermatozoa harvested per collection day (P > 0·10), but increased the number of spermatozoa in first ejaculates by an average of 34·2% (P < 0·025). Oxytocin did not affect sperm quality (P > 0·10) as judged by the motility of spermatozoa in fresh semen or by the motility or percentage of spermatozoa with intact acrosomes in thawed semen. It is concluded that 50 i.u. oxytocin enhanced sperm output in first ejaculates of electroejaculated bulls without altering daily sperm production or seminal quality.

Keywords: bull; spermatogenesis; sperm output; oxytocin; semen

Summary. Testes were obtained from 47 1–20-year-old stallions during the natural breeding season. Total testicular testosterone and testosterone/g testis increased with age (P < 0·005), and total testicular testosterone was associated with larger testis size (P < 0·05). Neither testosterone per gram nor per paired testes were related to total Sertoli cell number (P > 0·05), but greater testosterone per paired testes was associated with fewer Sertoli cells per unit of seminiferous tubule length (P < 0·005) or basement membrane area (P < 0·02) and with a higher number of germ cells supported per Sertoli cell (P < 0·05). Although values for testosterone per gram and per paired testes were unrelated (P > 0·10) to sperm production/g testis or to the yield of spermatids/spermatogonium, testosterone per paired testes was positively related to sperm production per paired testes (P < 0·05). It is concluded that intratesticular testosterone increases with age, is related in a positive manner to quantitative rates of sperm production, and can account for some of the differences in sperm production among individual stallions within a single breeding season.

The normal equine breeding season begins in the spring and extends through mid-summer. It is characterized by marked increases in sperm output and seminal volume, as well as behavioural changes manifested in decreased reaction time and the number of mounts per ejaculation (Pickett & Voss, 1972). The production of androgenic steroids by the equine testis has been investigated both in vivo (Lindner, 1961) and in vitro (Savard & Goldzieher, 1960; Bedrak & Samuels, 1969; Oh & Tamaoki, 1970) but the testosterone level in peripheral plasma has not been reported (Lindner, 1961) and seasonal influences upon testicular function have received little attention. The object of this study was to establish the influence of season on the concentration of testosterone in the peripheral plasma of the stallion.

Two ejaculates were collected from