Effect of Season and Year of Measurement on Testicular Growth and Consistency of Holstein Bulls1

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Abstract
Seasonal effects were found for 635 testicular size and consistency measurements taken in early spring and 644 in late summer on Holstein bulls 6 to 180 months of age. Regression equations were calculated for each season to best describe the data. The equation for early spring measurements of scrotal circumference was Ŷ = −9.54 + 53.93 log X − 13.63 (log X)2 where Ŷ is scrotal circumference in centimeters and X is bull age in months. The corresponding equation for late summer was Ŷ = .54 + 39.86 log X − 9.29 (log X)2. Tonometry measurements made in the early spring were described by the equation Ŷ = 25.10 − 14.63 log X + 4.24 (log X)2, where Ŷ is the average deflection in millimeters of weak and strong spring tonometers and X is bull age in months. The corresponding equation for late summer was Ŷ = 22.53 − 10.33 log X + 2.81 (log X)2. Bulls 6 to 48 months of age were analyzed more extensively. An effect of season and/or artificial insemination center on both testicular characteristics were found. Also, there was a significant year effect. Regression equations fitted to data for each of the 5 years were more accurate (P<.01) in describing the distribution of data than was a common equation for pooled data. These results emphasized that selection of bulls for these traits can best be made within the same age group, bull stud and year-season. As more data become available correction factors could be developed and indexes computed to make more general comparisons possible. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.

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