In 1995, in an effort to improve reproductive performance, a decision was made by Montana State Univ. scientists to begin selection for scrotal circumference (SC) in a herd of inbred Hereford cattle at the Northern Agricultural Research Center, Havre, Montana.

This herd was closed to outside breeding in 1963, and in 1976 selection began on an index of adjusted yearling weight minus 3.2 times adjusted birth weight.

The selection project continued until 1995, when the first bull calves were selected for larger SC. The primary objective of this study was to determine the effects of single-trait selection for SC on other economically important traits.

Data collected for both bulls and heifers included birth wt. (BWT), weaning wt. (WWT), yearling wt. (YWT), pelvic height (PH), and pelvic width (PW).

At a year of age, SC was measured on bulls and their sperm motility and morphology were evaluated. Genetic selection for increased SC resulted in correlated changes in growth traits, as demonstrated in the following genetic trends for increases in estimated breeding values (EBV) of 0.11, 1.10, and 3.26 lb/yr. for BWT, WWT and YWT, respectively.

Genetic trend for SC was 0.05 cm/yr. Genetic trends for EBV's of PH and PW were 0.005 and 0.01 cm/yr., respectively. There was also a very slight increase in sperm motility EBV.

The authors concluded these results may indicate that selecting solely for increased scrotal size may also increase the size of the cattle in the herd (Davis et al. 2004. Proc. Western Section, ASAS, 55:70).