Summary. The relationships between scrotal circumference and live weight and sperm output were examined in 3 samples of bulls selected for use in AI. In Study 1 the linear correlation between live weight and scrotal circumference in 418 British Friesian bulls at about 400 days of age was +0·28, and the circumference of the scrotum was related to the live weight of the bull by +0·179 ± 0·060 mm/kg live weight. At 400 days of age the mean (± s.e.) live weight and scrotal circumference of 22 British Friesian bulls in Study 2 was 412 ± 7 kg and 332 ± 4 mm respectively. Rates of growth were 1·11 ± 0·023 kg/day and 0·426 ± 0·023 mm/day. The correlation (+0.42) between the size of the scrotum and the no. of spermatozoa/ ejaculate was not significant. British Friesian (average age 9 years) and Hereford (average age 6·5 years) bulls were examined in Study 3. For the 25 Herefords, the correlation between the number of usable straws over a 6-month period and the scrotal circumference was +0.43, compared with +0.21 among the 28 British Friesians. It is concluded that scrotal circumference is unlikely to be an accurate predictor of sperm output in AI bulls.
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