Effect of age on testicular growth and consistency of Holstein and Angus bulls.

A total of 5,909 scrotal circumference and 5,373 tesricular consistency (tonometer) measurements were made on Holstein bulls in seven artificial insemination studs. Comparable measurements were made on 339Angus bulls. The multiple regression equation, Y = -4.67 + 47.26 log X 1 1 . 7 4 (log X)2), where ~f is scrotal circumference in cm and X is bull age in months, described the high rate of testicular growth in young Holstein bulls, limited growth in mature bulls, and a slight decrease in the oldest bulls. Angus tended to have larger testes than Holsteins up to 3 years of age, but they were surpassed after that age by Holstein bulls. Regression of testicular consistency on age of Holstein bulls resulted in the equation Y = 20.83 9.06 log X + 2.47 (log X) 2, where CZ is the mean deflection averaged for weak and strong spring tonometers in mm and X is bull age in months. This equation describes the decrease in firmness of testes of bulls from 6 to 12 months of age up to 48 to 54 months of age. Then consistency became slightly firmer with age, but in the few oldest bulls measured there was a decline. Differences among bull studs existed for both traits. Differences in testis size among bulls of the same age were large. Selection of bulls with large testes could decrease the number required to produce enough sperm to inseminate a specified population.